Seasonal Variation in Sperm Quality of Brahman Bulls under Consistently High Temperature-Humidity Index Conditions

I. Araya-Zúñiga; F. Sevilla; K. Matamoros; A. Saborío-Montero; H. Cucho; A. Salamanca-Carreño; A. Valverde

doi:10.5398/tasj.2026.49.6.512

I. Araya-Zúñiga ⁽¹⁾ , F. Sevilla ⁽¹⁾ , K. Matamoros ⁽¹⁾ , A. Saborío-Montero ⁽²⁾ , H. Cucho ⁽³⁾ , A. Salamanca-Carreño ⁽⁴⁾ , A. Valverde ⁽¹⁾

(1) Costa Rica Institute of Technology, Costa Rica,

(2) University of Costa Rica, Costa Rica,

(3) Universidad Nacional de San Antonio Abad del Cusco, Peru,

(4) Universidad Cooperativa de Colombia, Colombia

https://doi.org/10.5398/tasj.2026.49.6.512

Issue
Vol. 49 No. 6 (2026): Tropical Animal Science Journal (Issue in progress)

Submitted
March 12, 2026

Accepted
June 25, 2026

Published
September 3, 2026

Keywords:

heat stress, climate change, fresh semen, reproduction, temperature-humidity index

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Abstract

Bulls in humid tropical climates are constantly exposed to heat stress, affecting their reproductive performance. This study evaluated the effect of variables associated with season, especially temperature and humidity, on semen quality of Brahman bulls. A total of 39 ejaculates were analyzed from 12 Brahman bulls with ages at collection between 24 and 30 months. The temperature-humidity index (THI) was modeled as a continuous predictor variable; whereas, ejaculates were grouped into two THI cohorts for secondary exploratory analyses. These were based on sample medians of ≤80.1 and >80.1 points in a consistently high-THI tropical environment. Season was associated with variation in total motility (TM), progressive motility (PM), and sperm morphology. TM and PM were highest during the transitional season, whereas the percentage of normal sperm morphology was highest during the rainy season. Sperm curvilinear velocity (VCL), straight-line velocity (VSL), average path velocity (VAP), linearity index (LIN), straightness index (STR), and wobble (WOB) were significantly higher (p<0.05) during the dry season. Sperm viability did not differ significantly among seasons. Under the THI cohort comparison, significant differences (p<0.05) were observed in VCL, VAP, LIN, STR, and WOB, whereas TM, PM, viability, normal morphology, and VSL were not significantly affected. In conclusion, season was associated with variation in sperm motility, kinematic patterns, and morphology in Brahman bulls, whereas sperm viability remained relatively stable across seasons. The results do not suggest a beneficial effect of heat stress, but rather highlight the complexity of interpreting seminal quality responses under tropical conditions. In the continuous THI analysis, THI showed significant positive associations with TM and PM, whereas no significant associations were detected for the remaining kinematic variables, viability, or normal morphology.

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References

Abd El-Emam, M. M., Ray, M. N., Ozono, M., & Kogure, K. (2023). Heat stress disrupts spermatogenesis via modulation of sperm-specific calcium channels in rats. Journal of Thermal Biology, 112, 103465. https://doi.org/10.1016/j.jtherbio.2023.103465

Alves, L. A., Cooke, R. F., Sant’Ana Borges, M., Souza, F. F., Kastelic, J. P., & Crespilho, A. M. (2025). Season and temperature-humidity index affect sperm quality and in vivo fertility of beef bulls. Journal of Thermal Biology, 133, 104271. https://doi.org/10.1016/j.jtherbio.2025.104271

Amini, A., & Behnam, B. (2026). Global warming, fertility, and spermatogenesis decline: global and regional evidence from 195 countries and implications for climate adaptation policy. International Journal of Environmental Research and Public Health, 23(3), 331. https://doi.org/10.3390/ijerph23030331

Araya-Zúñiga, I., Sevilla, F., Molina-Montero, R., Roldan, E. R. S., Barrientos-Morales, M., Silvestre, M. A., & Valverde, A. (2024). Kinematic and morphometric assessment of fresh semen, before, during and after mating period in Brahman bulls. Animals, 14(1), 132. https://doi.org/10.3390/ani14010132

Arias, R. A., & Mader, T. L. (2023). Evaluation of four thermal comfort indices and their relationship with physiological variables in feedlot cattle. Animals, 13(7), 1169. https://doi.org/10.3390/ani13071169

Belhadj Slimen, I., Najar, T., Ghram, A., & Abdrrabba, M. (2016). Heat stress effects on livestock: molecular, cellular and metabolic aspects, a review. Journal of Animal Physiology and Animal Nutrition, 100(3), 401–412. https://doi.org/10.1111/jpn.12379

Boe‐Hansen, G. B., Rêgo, J. P. A., Satake, N., Venus, B., Sadowski, P., Nouwens, A., Li, Y., & McGowan, M. (2020). Effects of increased scrotal temperature on semen quality and seminal plasma proteins in Brahman bulls. Molecular Reproduction and Development, 87(5), 574–597. https://doi.org/10.1002/mrd.23328

Boonkum, W., Chankitisakul, V., & Juiputta, J. (2025). A novel temperature-humidity index-based model for evaluating semen characteristics in Thai native roosters under tropical conditions. Poultry Science, 104(8), 105321. https://doi.org/10.1016/j.psj.2025.105321

Brito, L. F. C., Silva, A. E. D. F., Barbosa, R. T., & Kastelic, J. P. (2004). Testicular thermoregulation in Bos indicus, crossbred and Bos taurus bulls: relationship with scrotal, testicular vascular cone and testicular morphology, and effects on semen quality and sperm production. Theriogenology, 61(2–3), 511–528. https://doi.org/10.1016/S0093-691X(03)00231-0

Capela, L., Leites, I., Romão, R., Lopes-da-Costa, L., & Pereira, R. M. L. N. (2022). Impact of heat stress on bovine sperm quality and competence. Animals, 12(8), 975. https://doi.org/10.3390/ani12080975

Castillo, R., & Amador, J. A. (2020). Precipitation and temperature in Costa Rica at the end of the century based on NEX-GDDP projected scenarios. Atmosphere, 11(12), 1323. https://doi.org/10.3390/atmos11121323

Celeghini, E. C. C., Baatsch-Nascimento, F., Bozzi, A. da R., Garcia-Oliveros, L. N., & Arruda, R. P. (2024). Bovine testicular heat stress: From climate change to effects on microRNA profile. Animal Reproduction Science, 270, 107620. https://doi.org/10.1016/j.anireprosci.2024.107620

Chacón, L., Navarro, O., Ladino, C., Martins, J., Perez, J., & Ardila, A. (2022). Sexual behavior and seminal characteristics of Brahman bulls in the Colombian tropical flooded savanna: effects of reproductive management systems and climatic periods. Tropical Animal Health and Production, 54(1), 81. https://doi.org/10.1007/s11250-022-03087-w

Chitkara, M., Gurao, A., Kaur, H., Dige, M. S., Sodhi, M., Mukesh, M., & Kataria, R. S. (2025). Omics approaches to understand impact of heat stress on semen quality and fertility in bovines. Animal Reproduction Science, 274, 107786. https://doi.org/10.1016/j.anireprosci.2025.107786

Corte Pause, F., Crociati, M., Urli, S., Monaci, M., Degano, L., & Stradaioli, G. (2022). Environmental factors affecting the reproductive efficiency of Italian Simmental young bulls. Animals, 12(18), 2476. https://doi.org/10.3390/ani12182476

Díaz, R. F., Galina, C. S., Aranda, E. M., Aceves, L. A., Sánchez, J. G., & Pablos, J. L. (2020). Effect of temperature humidity index on the onset of post- partum ovarian activity and reproductive behavior in Bos indicus cows. Animal Reproduction, 17(1), e20190074. https://doi.org/10.21451/1984-3143-AR2019-0074

Dunn, R. J. H., Mead, N. E., Willett, K. M., & Parker, D. E. (2014). Analysis of heat stress in UK dairy cattle and impact on milk yields. Environmental Research Letters, 9(6), 064006. https://doi.org/10.1088/1748-9326/9/6/064006

Fraczek, M., Lewandowska, A., Budzinska, M., Kamieniczna, M., Wojnar, L., Gill, K., Piasecka, M., Kups, M., Havrylyuk, A., Chopyak, V., Nakonechnyy, J., Nakonechnyy, A., & Kurpisz, M. (2022). The role of seminal oxidative stress scavenging system in the pathogenesis of sperm DNA damage in men exposed and not exposed to genital heat stress. International Journal of Environmental Research and Public Health, 19(5), 2713. https://doi.org/10.3390/ijerph19052713

Freitas, A. de P., dos Santos, G. F. F., Fernandes, A. R., Mendonça, G. G., de Paz, C. C. P., Vercesi Filho, A. E., & El Faro, L. (2020). Effect of thermal stress on basic seminal characteristics of Gyr bulls. International Journal of Biometeorology, 64(10), 1649–1656. https://doi.org/10.1007/s00484-020-01945-6

Garcia-Oliveros, L. N., Arruda, R. P. de, Batissaco, L., Gonzaga, V. H. G., Nogueira, V. J. M., Florez-Rodriguez, S. A., Almeida, F. dos S., Alves, M. B. R., Pinto, S. C. C., Nichi, M., Losano, J. D. de A., Kawai, G. K. V., & Celeghini, E. C. C. (2022). Chronological characterization of sperm morpho-functional damage and recovery after testicular heat stress in Nellore bulls. Journal of Thermal Biology, 106, 103237. https://doi.org/10.1016/j.jtherbio.2022.103237

Gloria, A., Candeloro, L., Wegher, L., Robbe, D., Carluccio, A., & Contri, A. (2021). Environmental temperature and relative humidity differently affect the sperm characteristics in Brown Swiss and Belgian Blue bulls. International Journal of Biometeorology, 65(12), 2189–2199. https://doi.org/10.1007/s00484-021-02184-z

Habeeb, A. A. M., Osman, S. F., & Gad, A. E. (2020). Signs of heat stress and some steps to reduce the negative effects on animals. GSC Advanced Research and Reviews, 4(1), 046–058. https://doi.org/10.30574/gscarr.2020.4.1.0058

Hamilton, T. R. dos S., Mendes, C. M., Castro, L. S. de, Assis, P. M. de, Siqueira, A. F. P., Delgado, J. de C., Goissis, M. D., Muiño-Blanco, T., Cebrián-Pérez, J. Á., Nichi, M., Visintin, J. A., & Assumpção, M. E. O. D. (2016). Evaluation of lasting effects of heat stress on sperm profile and oxidative status of ram semen and epididymal sperm. Oxidative Medicine and Cellular Longevity, 2016, 1687657. https://doi.org/10.1155/2016/1687657

Kastelic, J. P., Rizzoto, G., Shahat, A. M., Ferreira, J. C. P., & Thumdathil, J. (2021). Pathogenesis and mitigation of the deleterious effects of heat stress on bull reproduction. Revista Brasileira de Reprodução Animal, 45(4), 518–524. https://doi.org/10.21451/1809-3000.RBRA2021.070

Koivisto, M., Costa, M., Perri, S., & Vicente, W. (2009). The effect of season on semen characteristics and freezability in Bos indicus and Bos taurus bulls in the southeastern region of Brazil. Reproduction in Domestic Animals, 44(4), 587–592. https://doi.org/10.1111/j.1439-0531.2008.01023.x

Kumar, G., Gurao, A., Vasisth, R., Chitkara, M., Singh, R., Ranganatha Sriranga, K., Shivanand Dige, M., Mukesh, M., Singh, P., & Singh Kataria, R. (2024). Genome-wide 5′-C-phosphate-G-3′ methylation patterns reveal the effect of heat stress on the altered semen quality in Bubalus bubalis. Gene, 906, 148233. https://doi.org/10.1016/j.gene.2024.148233

Kumar, N., Ghosh, S. K., Mohanty, T. K., Prasad, J. K., Singh, R. K., & Patel, B. (2019). Effect of heat stress on physico-morphological characteristics and sperm functions in Murrah buffalo semen. The Indian Journal of Animal Sciences, 89(4), 390–393. https://doi.org/10.56093/ijans.v89i4.89123

Landaeta-Hernández, A. J., Gil-Araujo, M. A., Ungerfeld, R., Owen Rae, D., Urdaneta-Moyer, A., Parra-Núñez, A., Kaske, M., Bollwein, H., & Chenoweth, P. J. (2020). Effect of season and genotype on values for bull semen variables under tropical conditions. Animal Reproduction Science, 221, 106592. https://doi.org/10.1016/j.anireprosci.2020.106592

Livernois, A., Mallard, B., Miglior, F., & Cánovas, A. (2018). PSVI-28 Influence of ambient temperature and temperature-humidity index on a measure of semen quality in crossbreed beef bulls. Journal of Animal Science, 96(suppl_3), 374. https://doi.org/10.1093/jas/sky404.821

Llamas-Luceño, N., Hostens, M., Mullaart, E., Broekhuijse, M., Lonergan, P., & Van Soom, A. (2020). High temperature-humidity index compromises sperm quality and fertility of Holstein bulls in temperate climates. Journal of Dairy Science, 103(10), 9502–9514. https://doi.org/10.3168/jds.2019-18089

Mader, T. L., Davis, M. S., & Brown-Brandl, T. (2006). Environmental factors influencing heat stress in feedlot cattle. Journal of Animal Science, 84(3), 712–719. https://doi.org/10.2527/2006.843712x

Morrell, J. M. (2020). Heat stress and bull fertility. Theriogenology, 153, 62–67. https://doi.org/10.1016/j.theriogenology.2020.05.014

Netherton, J. K., Robinson, B. R., Ogle, R. A., Gunn, A., Villaverde, A. I. S. B., Colyvas, K., Wise, C., Russo, T., Dowdell, A., & Baker, M. A. (2022). Seasonal variation in bull semen quality demonstrates there are heat-sensitive and heat-tolerant bulls. Scientific Reports, 12(1), 15322. https://doi.org/10.1038/s41598-022-17708-9

Nichi, M., Bols, P. E. J., Züge, R. M., Barnabe, V. H., Goovaerts, I. G. F., Barnabe, R. C., & Cortada, C. N. M. (2006). Seasonal variation in semen quality in Bos indicus and Bos taurus bulls raised under tropical conditions. Theriogenology, 66(4), 822–828. https://doi.org/10.1016/j.theriogenology.2006.01.056

Perry, V. E. A. (2021). The role of sperm morphology standards in the laboratory assessment of bull fertility in Australia. Frontiers in Veterinary Science, 8, 672058. https://doi.org/10.3389/fvets.2021.672058

Poclín-Rojas, A. Y., Arbaiza Barnechea, M. D., Segura Portocarrero, G. T., Ampuero-Trigoso, G., Bernilla Carrillo, D., Depaz-Hizo, B. A., Vásquez-Tarrillo, R. W., Diaz-Quevedo, C., & Quispe-Ccasa, H. A. (2025). Motility performance of thawed spermatozoa of bulls from the tropics throughout the year. Animals, 15(16), 2451. https://doi.org/10.3390/ani15162451

Popović, N., Djedović, R., Stojić, P., Stanojević, D., Jovanović, R., & Beskorovajni, R. (2024). Influence of THI index, age of bulls, year, and season on production and quality of native sperm of Holstein-Friesian bulls. Revista Brasileira de Zootecnia, 53, e20240011. https://doi.org/10.37496/RBZ5320240011

Rahbar, M., Behgar, M., Khoshniyat, M. R., Motamedi, F. F., & Shawrang, P. (2025). The effect of temperature-humidity index on sperm motility parameters in Holstein bulls. Journal of Animal Production, 27(2), 223–233. https://doi.org/10.22059/jap.2025.389731.623831

Rodriguez-Venegas, R., Meza-Herrera, C. A., Robles-Trillo, P. A., Angel-Garcia, O., Legarreta-Gonzalez, M. A., Sánchez-Vocanegra, H. F., & Rodriguez-Martinez, R. (2023). Effect of THI on milk production, percentage of milking cows, and time lying in Holstein cows in Northern-arid Mexico. Animals, 13(10), 1715. https://doi.org/10.3390/ani13101715

Sabés-Alsina, M., Lundeheim, N., Johannisson, A., López-Béjar, M., & Morrell, J. M. (2019). Relationships between climate and sperm quality in dairy bull semen: A retrospective analysis. Journal of Dairy Science, 102(6), 5623–5633. https://doi.org/10.3168/jds.2018-15837

Santos, T. de S., Contrim, I. S., da Silva, D. F., Assumpção, M. E. O. D., Paula-Lopes, F. F. de, & Feitosa, W. B. (2025). Heat shock affects the Ca2+/calmodulin-dependent protein kinase II dynamic during bovine sperm capacitation and acrosome reaction. Frontiers in Cell and Developmental Biology, 13, 1552282. https://doi.org/10.3389/fcell.2025.1552282

Savvulidi, F. G., Ducháček, J., Ptáček, M., Gašparík, M., Kovářová, V., Codl, R., & Stádník, L. (2025). Impact of microclimatic conditions on sperm production in Czech Holstein bulls: A correlational study. Czech Journal of Animal Science, 70(4), 121–129. https://doi.org/10.17221/21/2025-CJAS

Seifi-Jamadi, A., Kohram, H., Zhandi, M., Llamas Luceño, N., Leemans, B., Henrotte, E., Latour, C., Demeyere, K., Meyer, E., & Van Soom, A. (2019). Heat stress has a deleterious effect on bull semen quality and subsequent embryo development. Reproduction, Fertility and Development, 32(2), 196. https://doi.org/10.1071/RDv32n2Ab139

Serrano-Jara, D., Gacem, S., Argente, MJ., Agea, I., Silvestre, MA., & García, ML. (2026). Impact of cold storage on semen quality in two divergent rabbit lines selected for resilience under heat stress condition. Veterinary and Animal Science, 32, 100602. https://doi.org/10.1016/j.vas.2026.100602

Sevilla, F., Araya-Zúñiga, I., Salamanca-Carreño, A., Silvestre, M. A., Rodríguez, J., Matamoros, K., Molina-Montero, R., Carranza-Rojas, L. C., Roldan, E. R. S., & Valverde, A. (2025). Effect of age and scrotal circumference on sperm morphology in Brahman bulls using a modified fixation technique. Frontiers in Veterinary Science, 12, 1626425. https://doi.org/10.3389/fvets.2025.1626425

Slayi, M., & Jaja, I. F. (2025). Strategies for mitigating heat stress and their effects on behavior, physiological indicators, and growth performance in communally managed feedlot cattle. Frontiers in Veterinary Science, 12, 1513368. https://doi.org/10.3389/fvets.2025.1513368

Solís, J. M., Sevilla, F., Silvestre, M. A., Araya-Zúñiga, I., Roldan, E. R. S., Saborío-Montero, A., & Valverde, A. (2024). Effect of thawing procedure and thermo-resistance test on sperm motility and kinematics patterns in two bovine breeds. Animals, 14(19), 2768. https://doi.org/10.3390/ani14192768

Souto, P. L. G., Barbosa, E. A., da Silva, F. I. C., Martins, V. M. V., Hatamoto-Zervoudakis, L. K., McManus, C., de Alencar, E. R., & Ramos, A. F. (2022). Seasonal semen quality of a local and commercial taurine cattle breeds, raised in a subtropical climate: Relationship between external morphology of the animals and climate. Animal Reproduction Science, 240, 106974. https://doi.org/10.1016/j.anireprosci.2022.106974

Swain, D. K., Vergara, C., Castro-Arnau, J., & Lishko, P. V. (2025). The essential calcium channel of sperm CatSper is temperature-gated. Nature Communications, 16(1), 3657. https://doi.org/10.1038/s41467-025-58824-0

Umar, S. I. U., Konwar, D., Khan, A., Bhat, M. A., Javid, F., Jeelani, R., Nabi, B., Najar, A. A., Kumar, D., & Brahma, B. (2021). Delineation of temperature-humidity index (THI) as indicator of heat stress in riverine buffaloes (Bubalus bubalis) of a sub-tropical Indian region. Cell Stress and Chaperones, 26(4), 657–669. https://doi.org/10.1007/s12192-021-01209-1

Vanselow, J., Wesenauer, C., Eggert, A., Sharma, A., & Becker, F. (2024). Summer heat during spermatogenesis reduces in vitro blastocyst rates and affects sperm quality of next generation bulls. Andrology, 12(8), 1897–1906. https://doi.org/10.1111/andr.13627

Vargas-Leitón, B., Núñez-Cárdenas, J., Valverde-Abarca, A., León-Rodríguez, B., & Saborío-Montero, A. (2025). Unveiling the genetic structure of Costa Rican bovines through genetic admixture models. Agronomía Mesoamericana, 36, jh40ks60. https://doi.org/10.15517/jh40ks60

Vince, S., Žura Žaja, I., Samardžija, M., Majić Balić, I., Vilić, M., Đuričić, D., Valpotić, H., Marković, F., & Milinković-Tur, S. (2018). Age-related differences of semen quality, seminal plasma, and spermatozoa antioxidative and oxidative stress variables in bulls during cold and warm periods of the year. Animal, 12(3), 559–568. https://doi.org/10.1017/S1751731117001811

Wang, J., Luo, Y., He, Y., Li, W., Qin, Y., & Wu, Y. (2025). Astaxanthin alleviates the decline of sperm quality caused by heat stress in mice via reducing oxidative stress. Life, 15(6), 851. https://doi.org/10.3390/life15060851

Authors

I. Araya-Zúñiga

Costa Rica Institute of Technology, Costa Rica

https://orcid.org/0000-0002-4292-2287

F. Sevilla

Costa Rica Institute of Technology, Costa Rica

https://orcid.org/0000-0003-1480-4141

K. Matamoros

Costa Rica Institute of Technology, Costa Rica

https://orcid.org/0000-0002-0827-9726

A. Saborío-Montero

University of Costa Rica, Costa Rica

https://orcid.org/0000-0002-9840-0058

H. Cucho

Universidad Nacional de San Antonio Abad del Cusco, Peru

https://orcid.org/0000-0001-7170-9795

A. Salamanca-Carreño

Universidad Cooperativa de Colombia, Colombia

https://orcid.org/0000-0002-5416-5906

A. Valverde

Costa Rica Institute of Technology, Costa Rica

https://orcid.org/0000-0002-3191-6965

anvalverde@tec.ac.cr (Primary Contact)

Araya-Zúñiga, I., Sevilla, F., Matamoros, K., Saborío-Montero, A., Cucho, H., Salamanca-Carreño, A., & Valverde, A. (2026). Seasonal Variation in Sperm Quality of Brahman Bulls under Consistently High Temperature-Humidity Index Conditions. Tropical Animal Science Journal, 49(6), 512. https://doi.org/10.5398/tasj.2026.49.6.512

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INTRODUCTION

For bulls, heat stress has primarily been linked to the inability to regulate body temperature as well as testicular dysfunction (Gloria et al., 2021). Thermal stress furthermore affects spermatogenesis, which can limit sperm quality and fertility potential (Corte Pause et al., 2022); (Freitas et al., 2020); (Netherton et al., 2022). The inability to regulate body temperature can provoke reactive oxygen species production, which can exceed the natural antioxidant capacity, resulting in cellular oxidative stress (Fraczek et al., 2022); (Hamilton et al., 2016); (Wang et al., 2025). Likewise, these conditions have been shown to decrease sperm concentration, motility, and integrity (Freitas et al., 2020), as well as cellular DNA chromatin (Fraczek et al., 2022), microRNA (Celeghini et al., 2024), mitochondrial activity (Serrano-Jara et al., 2026), and plasma antioxidant capacity in animal ejaculates (Garcia-Oliveros et al., 2022); (Sabés-Alsina et al., 2019). For this reason, the possibility of using bull ejaculates under these conditions for conservation and artificial insemination programs could be risky (Landaeta-Hernández et al., 2020). Therefore, it is necessary to study the effect of environmental factors such as temperature and humidity on animal stress, which can limit reproductive performance (Morrell, 2020).

The temperature-humidity index (THI) is a proven tool to calculate animal stress(Dunn et al., 2014); (Habeeb et al., 2020); (Popović et al., 2024) because it directly relates to bull sperm function (Landaeta-Hernández et al., 2020); (Llamas-Luceño et al., 2020); (Savvulidi et al., 2025). In the tropics, Bos indicus dominates breeding programs, as it can adapt to climatic factors better than Bos taurus (Alves et al., 2025). Within B. indicus breeds, nonetheless, there may be individual variations that compromise reproductive efficiency (Netherton et al., 2022). Because these climatic adaptation variations might affect reproductive performance, their study may still improve animal management and selection programs (Morrell, 2020); (Poclín-Rojas et al., 2025).

High THI can impact bull reproductive behavior; there is no consensus, however, on how high THI must go to define an animal as under stress (Savvulidi et al., 2025). Some researchers have used THI values greater than 72 points as the threshold (Popović et al., 2024). Other studies of zebu breeds indicate that this value can rise above 74 points (Díaz et al., 2020). For buffalo, heat stress is defined as values greater than 68 (Umar et al., 2021). A recent study has proposed a categorical heat stress scale for B. indicus bulls under tropical conditions, where an animal is beginning to suffer heat stress with THI under 76 points, on alert from 76.2–79.2, in danger from 79.2–84.1, and in an emergency at greater than 84.1 points (Alves et al., 2025). Despite extensive research on the relationship between THI and semen quality, it is necessary to evaluate heat stress in Costa Rica’s humid environment for the predominant Brahman breed (Vargas-Leitón et al., 2025), especially given changing climatic conditions.

This study used season and THI as complementary indicators of the thermal environment rather than interchangeable variables. Season reflects the overall climatic context, including concurrent changes in rainfall, solar radiation, and other environmental conditions that may affect thermoregulation and spermatogenesis (Amini & Behnam, 2026), whereas THI provides a quantitative estimate of combined heat and humidity load. Thus, season was treated as a broader ecological framework, while THI was used as a specific climatic proxy to examine whether variation in heat load was associated with changes in semen quality (Boonkum et al., 2025); (Popović et al., 2024). We hypothesized that, within a consistently warm tropical environment, variation in season and THI would be associated mainly with changes in sperm motility and kinematic traits rather than viability or morphology. The study then evaluated the effect of season and THI on Brahman semen quality under tropical conditions.

MATERIALS AND METHODS

Data Availability Statement

The data for this study are included in this article and are available from the TECdatos Repository, Costa Rica Institute of Technology—data identification number: https://doi.org/10.18845/RDA/JBLPNT.

Ethical Statement

This study used bull sperm samples obtained in collaboration with the Agricultural Production Program (PPASC-026-2025) of the Costa Rica Institute of Technology. The collection was conducted in accordance with the laws and regulations for live animal experimentation in Costa Rica. Throughout the study, animals were handled to ensure their well-being and avoid unnecessary stress. The study adhered to ethical research principles, including the three Rs, and all procedures were strictly followed according to relevant guidelines and regulations to ensure ethical integrity and animal welfare. This research was approved by the Center for Research and Development of Sustainable Agriculture for the Humid Tropics of the Costa Rica Institute of Technology in session 20/2023 Article 1.0, DAGSC-188-2023 and CIE-206-2023. The study also adhered to the ARRIVE guidelines (https://arriveguidelines.org/).

Animals and Ejaculates Collection

The study was conducted at La Balsa Farm, owned by the Costa Rica Institute of Technology in Alajuela Province. A total of 12 Brahman (B. indicus) bulls were used, with an average age of 25.97±1.45 months and an average scrotal circumference of 34.77±2.61 cm at the start of the study. The animals were pastured on Ratana grass (Ischaemum indicum) and Mombasa grass (Megathyrsus maximus). Forage intake was not restricted, and salt, minerals, and water were provided ad libitum. The animals were kept in a rotational grazing system of approximately 18 ha, divided into seven paddocks with a grazing period of six days and a pasture rest period of 42 days. Sanitary management included deworming (e.g., Doramectin), vitamin supplementation (Catosal, Elanco, USA), and mineral supplementation (Matsuda Breed Top Line, Brasília, Brazil).

Prior to the study, the animals passed a breeding soundness evaluation test. A total of 39 ejaculates were collected at seven-day intervals using the electroejaculation technique described in (Araya-Zúñiga et al., 2024). The prepuce area was cleaned, and excess hair was trimmed to avoid semen sample contamination. A gloved hand entered the animal’s rectum to remove fecal matter and massage the accessory glands and prostate. A 75 mm Pulsator V® electroejaculator probe (Lane Manufacturing, Denver, CO, USA) was inserted into the animal’s rectum with automatic mode on. The samples were collected directly from the animal’s penis and transferred to sterile 15 mL Falcon tubes to be placed in a thermoregulated water bath (Cito Products Inc., Watertown, Wisconsin, USA) at 37 °C. Upon completion, all samples were transported in the thermoregulated water bath for less than 15 minutes to the animal reproduction laboratory of the Costa Rica Institute of Technology (AndroTEC) at the San Carlos campus. Figure 1 describes the experimental design.

Figure 1.Experimental design used in the study

Season, Climate Data, and Estimation of the Temperature-Humidity Index

Seasons were classified as dry (November to February), transitional (March to April), and rainy (May to October) (Castillo & Amador, 2020). To estimate THI, data were taken from the National Meteorological Institute of Costa Rica station located on the San Carlos campus (10°21′51.22″ N, 84°30′28.27″ W). The station is 1.58 km from the study farm. To determine THI, the average daily temperature (°C) and relative humidity (%) from the 60 days prior to each semen collection were recorded. The 60-day window corresponds to bull spermatogenesis of the same duration, relevant for assessing the cumulative effect of thermal stress on semen quality. Because ejaculates were collected at seven-day intervals, the 60-day pre-collection THI windows partially overlapped across consecutive samples, which may reduce the independence of thermal exposure estimates. This was addressed by including bull identity as a random effect, although residual temporal dependence cannot be excluded. Thus, mean THI during the 60 days before ejaculation was used as a proxy for climatic load, although it reflects ambient conditions from a nearby weather station rather than each bull’s exact thermal exposure. The THI was calculated using the formula described in (Rodriguez-Venegas et al., 2023), where T is the temperature in °C and RH is relative humidity (%):

THI = (1.8 × T + 32) – [(0.55 - 0.0055 × RH) × (1.8 × T – 26)]

THI values were higher than 76, the limit reported for describing an animal under thermal stress (Alves et al., 2025). A more detailed analysis of semen quality variables according to observed THI values was carried out by grouping ejaculates obtained using the median as a discrimination factor, resulting in the following groups: ≤80.1 points (n = 20 ejaculates) and >80.1 points (n = 19 ejaculates), within a consistently high-THI tropical environment. Because the THI range was narrow, cohort-based comparisons were treated as secondary exploratory analyses rather than the primary basis for biological inference.

Motility and Sperm Kinematic Assessment

The analysis of sperm motility and kinematics was performed using an AI Station v1.2 system (SPERM.TECH®, Sperm Analysis Technologies S.L., Buñol, Spain). The bovine module had the following configuration: the cellular size between 6 and 69 μm2, progressive sperm when STR > 15%, average path velocity (VAP) as 20 µm s-1 < slow < 25 < medium < 46 < rapid. Immotile sperm were defined as sperm swimming below 20 µm s-1 of VAP. Connectivity setting was fixed at 12 μm. For each analysis, 3 µL of semen was placed in a Spermtrack® counting chamber to a 20 µm depth (Proiser I+D., Paterna, Spain) preheated to 37 °C using a hot plate (Proiser R+D., Paterna, Spain) as described by Solís et al. (2024). A video camera (Ximea xiC, Marianka, Slovakia) was connected to a UB203 microscope (UOP/Proiser R+D) with a 1× eyepiece and a 10× negative phase contrast objective (NA 0.25). The camera captured images at 50 fps. The microscope’s integrated stage was maintained at 37.0±0.5 °C when placing and analyzing semen samples. Two replicates were taken for each sample, and at least eight fields were captured for each replicate. The response variables obtained from the CASA analysis were total motility (TM, %) and progressive motility (PM, %). Additionally, the sperm kinematic variables of curvilinear velocity (VCL, µm s⁻¹), straight-line velocity (VSL, µm s⁻¹), and VAP (µm s⁻¹) were also measured, as well as the percentages of sperm straightness (STR) and linearity (LIN). Wobble (WOB) was calculated as a derived kinematic descriptor using the formula WOB = (VAP/VCL) × 100.

Sperm Viability and Morphology

The analysis of viability was performed by placing 5 µL of semen on a slide and then adding 5 µL of the Eosin-Nigrosin stain (Mortimer® Formula, Humeco, Huesca, Spain). A sample smear on a microscope slide was dried at 25 °C for 10 minutes. Afterward, a coverslip was placed over the sample, and analyzed using a UB203 phase-detection microscope (UOP / Proiser R + D) at 400X. A total of 200 sperm cells were evaluated for each sample, and the percentages of viable and non-viable cells were reported. For sperm morphology assessment, 5 μL of the previously diluted semen sample was first placed on a slide. A coverslip was then placed over it and fixed using Trumorph® (Proiser R + D, SL, Paterna, Spain), following protocols described by (Sevilla et al., 2025). Morphologically normal sperm were defined according to (Perry, 2021) as cells showing normal head, midpiece, and tail morphology and the absence of cytoplasmic droplets. Sperm abnormalities were classified as head, midpiece, tail, or cytoplasmic droplet defects. Morphology assessment was performed by the same trained evaluator, blind to season and THI classifications. Once samples were fixed, they were evaluated in a UB203 negative phase microscope at 400X.

Statistical Analysis

For each continuous response, model residuals were evaluated for normality and homoscedasticity using Q-Q plots and residual-vs-fitted plots; Shapiro-Wilk and Levene-type assessments were supporting diagnostics. The assumption of normal distribution was verified by using a normal probability plot. Continuous semen traits, including TM, PM, VCL, VSL, VAP, LIN, STR, and WOB, were analyzed using linear mixed-effects models. Because repeated ejaculates were obtained from the same bull, bull identity was included as a random intercept to account for within-animal correlation. Age at collection was included as a covariate in the season and season × THI models. Although bull was included as a random effect, the weekly sampling design and overlapping 60-day THI windows may also have introduced temporal autocorrelation not explicitly modeled.

For the evaluation of the season, the following model was fitted for each continuous response variable:

y_ij = μ + Si + β₁ THI_ij + β₂ Age_ij + uj + ε_ij

where y_ij is the observed semen trait for ejaculate i from bull j, μ is the overall mean, Si is the fixed effect of season (dry, transitional, or rainy), uj is the random effect of bull (uj ~ N(0, σ_bull²)), Age_ij is age at collection for ejaculate i from bull j, and ε_ij is the residual error (ε_ij ~ N(0, σ²)).

For THI, a separate model was fitted using THI as a continuous covariate:

y_ij = μ + β₁ THI_ij + u_j + ε_ij

This model was intentionally fitted without adjustment for age at collection because its objective was to characterize the marginal association between continuous THI and semen-quality traits within the relatively narrow age range represented in the study, rather than to estimate an age-adjusted independent effect of THI. Age was retained as a covariate in the season and season × THI models because those analyses were intended to evaluate seasonal differences while accounting for potential age-related variation across the sampling period. Accordingly, results from the continuous THI model were interpreted as exploratory associations rather than causal effects.

A season × THI interaction was explored as:

y_ij = μ + Si + β₁ THI_ij + β₂ Age_ij + (Si×THI_ij) + u_j + ε_ij

Because THI represents temperature and relative humidity, these variables were not included simultaneously with THI in the same model. As a secondary descriptive analysis, ejaculates were also grouped into two THI cohorts according to the sample median (≤80.1 and >80.1). Viability and normal morphology were analyzed as binomial responses using the numbers of viable/non-viable and normal/abnormal sperm out of 200 counted cells per sample. When effects were significant, mean comparisons were carried out using the Bonferroni test. The results were expressed as mean ± standard error of the mean. Statistical significance was defined as p<0.05. All data were analyzed using IBM SPSS, version 29.0.0.0 for Windows (SPSS Inc., Chicago, IL, USA).

RESULTS

Table 1 shows the effect of season on sperm TM, PM, viability, and morphology. A significant effect of season was found on TM, which was higher during the transition. A similar effect was determined for PM, which was lower during the dry and rainy seasons (p<0.05). Sperm viability did not differ significantly among seasons. Season was significantly associated with sperm morphology, with the rainy season showing a higher percentage of normal sperm morphology and a correspondingly lower percentage of abnormal sperm than the dry and transitional seasons (p<0.05). THI behavior was analyzed for each season. Mean THI increased significantly from the dry to the rainy season (p<0.05). The mean during the dry season was 79.51±0.03 points, the transitional 80.17±0.10 points, and the rainy season was 80.64±0.08 points.

Table 1.Model-adjusted means (± SEM) for bull sperm total motility (TM), progressive motility (PM), viability, and normal sperm morphology according to seasonSEM=standard error of the mean; TM=total motility; PM=progressive motility. * The dry season (n = 12 ejaculates) from November to February, the transitional season (n = 13 ejaculates) from March to April, and the rainy season (n = 14 ejaculates) from May to October were considered. a-c Means in the same row with different superscripts differ significantly (p<0.05).
Percentage	Season^*
Percentage	Dry	Transitional	Rainy
TM	64.98±2.82^c	74.84±1.41^a	60.52±2.78^b
PM	62.65±2.82^b	70.85±1.41^a	57.34±2.78^c
Viability	70.93±6.47	84.57±3.76	83.72±5.48
Normal sperm	80.30±5.84^b	77.66±3.37^b	95.04±4.83^a
Abnormal sperm	19.70±5.84^a	22.34±3.37^a	4.96±4.83^b

In the continuous THI mixed model, fitted at the ejaculate level with bull as a random effect, THI showed significant positive associations with TM (β = 9.674, SE = 4.864, p = 0.047) and PM (β = 9.365, SE = 4.767, p = 0.049). In contrast, THI was not significantly associated with VCL (β = 13.302, SE = 17.830, p = 0.456), VSL (β = 1.605, SE = 10.686, p = 0.881), VAP (β = 11.089, SE = 11.395, p = 0.330), LIN (β = -1.383, SE = 2.433, p = 0.570), STR (β = -3.309, SE = 1.971, p = 0.093), WOB (β = 1.571, SE = 1.633, p = 0.336), viability (β = -0.172, SE = 3.918, p = 0.965), or normal morphology (β = 2.941, SE = 4.281, p = 0.492) (Table 2).

Table 2. Estimated fixed effects of the continuous temperature-humidity index (THI) on semen quality traits in Brahman bulls from mixed-effects models fitted at the ejaculate level with bull included as a random effectNote: Continuous THI models were fitted without adjustment for age at sampling to estimate the marginal association between THI and each semen-quality trait. Bull was included as a random effect to account for repeated measurements within animals. SE=standard error; TM=total motility; PM=progressive motility; VCL=curvilinear velocity (µm s^-1); VSL=straight-line velocity (µm s^-1); VAP=average path velocity; LIN=linearity of forward progression (%); STR=straightness index (%); WOB=wobble (%). THI: temperature-humidity index.
Variables	β THI	SE	p-value
TM	9.674	4.864	0.047
PM	9.365	4.767	0.049
VCL	13.302	17.830	0.456
VSL	1.605	10.686	0.881
VAP	11.089	11.395	0.330
LIN	-1.383	2.433	0.570
STR	-3.309	1.971	0.093
WOB	1.571	1.633	0.336
Viability	-0.172	3.918	0.965
Normal morphology	2.941	4.281	0.492

Analysis of sperm kinematic variables across different seasons revealed a significant effect (p<0.05). Sperm had higher velocities during the dry season (Table 3). Regarding sperm kinematic patterns, the variables LIN, STR, and WOB were significantly higher during the dry season (p<0.05).

Figure 2 reports the behavior of relative humidity, temperature, and THI in each ejaculate analyzed. The temperature during the study remained between 26 °C and 28 °C, while the relative humidity ranged from 80% to 94%. THI fluctuated from 79.44 to 81.01. Based on THI data for the 39 ejaculates, two cohorts emerged: ≤80.1 (n = 20 ejaculates) and >80.1 (n = 19 ejaculates). No significant differences between THI cohorts were detected for TM, PM, viability, or normal sperm morphology (p>0.05) (Figure 3).

Table 3.Model-adjusted means (± SEM) for sperm kinematic parameters according to season in Brahman bullsNote: SEM=standard error of the mean; VCL=curvilinear velocity (µm s^-1); VSL=straight-line velocity (µm ^s-1); VAP=average path velocity; LIN=linearity of forward progression (%); STR=straightness index (%); WOB=wobble (%). * The dry season (n = 12 ejaculates) from November to February, the transitional season (n = 13 ejaculates) from March to April, and the rainy season (n = 14 ejaculates) from May to October were considered. ^a-c Means in the same row with different superscripts differ significantly (p<0.05).
Variables	Season^*
Variables	Dry	Transitional	Rainy
VCL	343.37±16.54^a	321.75±5.32^b	322.07±14.97^b
VSL	208.53±12.65^a	157.06±4.07^b	139.35±11.45^b
VAP	261.25±12.30^a	208.20±3.96^b	190.91±11.14^b
LIN	58.92±2.62^a	45.38±0.84^b	39.97±2.37^c
STR	69.32±2.22^a	66.02±0.71^b	65.18±2.01^b
WOB	77.64±1.82^a	64.71±0.59^b	59.10±1.65^c

Figure 2.Mean temperature-humidity index (THI), relative humidity (RH, %), and daily temperature (°C) of the 60 days prior to the collection of each of the thirty-nine ejaculates analyzed during the study

Figure 3.Descriptive comparison of sperm total motility (TM), progressive motility (PM), viability, and normal sperm morphology between temperature-humidity index (THI) cohorts [≤80.1 (n = 20 ejaculates) and >80.1 (n = 19 ejaculates)] in Brahman bulls.

Table 4 shows the effect of THI on sperm cell kinematic variables. Comparing kinematic variables between THI cohorts revealed significant differences (p<0.05) in VCL, VAP, LIN, STR, and WOB. No significant effect of the THI cohort was detected for VSL (p>0.05). LIN and WOB were significantly greater in the ≤80.1 cohort, whereas STR was higher in the >80.1 cohort (p<0.05).

The interaction between season and THI was also explored. It was not significant for TM, PM, viability, and normal sperm morphology (p>0.05). On the other hand, when analyzing kinematic sperm variables, a non-significant effect on their behavior was also seen (p>0.05).

Table 4.Model-adjusted means (± SEM) for sperm kinematic variables according to temperature-humidity index (THI) cohort in Brahman bullsNote: SEM= standard error of the mean; THI= temperature-humidity index; VCL= curvilinear velocity (μm s^-1); VSL= straight-line velocity (μm s^-1); VAP= average path velocity; LIN= linearity of forward progression (%); STR= straightness index (%); WOB= wobble (%). * The ≤80.1 cohort included 20 ejaculates and the >80.1 cohort included 19 ejaculates. ^a-b Means in the same row with different superscript differ significantly (p<0.05).
	≤80.1	>80.1
VCL	317.62±6.15ᵃ	339.08±10.21ᵇ
VSL	160.16±7.81ᵃ	160.34±4.70ᵃ
VAP	208.00±7.59ᵃ	214.49±4.57ᵇ
LIN	47.12±0.97ᵃ	44.14±1.62ᵇ
STR	64.49±0.83ᵇ	69.12±1.37ᵃ
WOB	67.97±0.68ᵃ	60.67±1.12ᵇ

DISCUSSION

The effect of tropical environmental conditions on bull semen quality has been previously documented (Chacón et al., 2022); (Poclín-Rojas et al., 2025); (Sevilla et al., 2025). Our results indicate that season was associated with variation in several semen-quality traits in Brahman bulls. Total and progressive motility were highest during the transitional season, whereas sperm kinematic variables were generally higher during the dry season. In contrast, the percentage of morphologically normal sperm was highest during the rainy season, while sperm viability did not differ significantly among seasons. Therefore, seasonal variation was not restricted to sperm motility and kinematics but also involved sperm morphology. These findings partially support our initial hypothesis, as viability remained relatively stable across seasons, whereas morphology showed significant seasonal variation. Importantly, the seasonal variation observed in sperm morphology should not be interpreted as a direct THI-associated response. Normal morphology was not significantly associated with THI in the continuous analysis and did not differ significantly between the two THI cohorts. This distinction suggests that the seasonal pattern in sperm morphology may reflect broader seasonal conditions not fully captured by THI alone, which integrates temperature and relative humidity but does not account for other potentially relevant environmental factors. This pattern is consistent with previous reports in bulls under tropical conditions (Alves et al., 2025). It has been found that although some B. indicus animals exhibit adaptation to tropical conditions, a marked individual effect on semen production and quality can be observed (Alves et al., 2025); (Koivisto et al., 2009); (Netherton et al., 2022). It has been shown, however, that even when animals are under the same exposure scenarios to adverse weather conditions, heat stress can manifest differently (Arias & Mader, 2023); (Morrell, 2020). High temperatures alter spermatogenesis and morphology and motility during the spermatozoan journey through the epididymis (Boe‐Hansen et al., 2020); (Freitas et al., 2020). These effects that occur during cell formation, development, and maturation relate to the presence of oxidative stress resulting from an imbalance between antioxidant capacity and free radical production (Hamilton et al., 2016); (Morrell, 2020); (Sabés-Alsina et al., 2019).

Previous studies have shown that these conditions can be more influential on testicles according to the blood irrigation capacity via the pampiniform plexus, which varies between subspecies and individuals (Brito et al., 2004). For spermatogenesis to occur normally, the animal needs to maintain the testicular temperature 2–6 °C below that of the rest of the body (Kastelic et al., 2021). Failure to maintain such homeostasis, therefore, can alter semen quality, including variations in seminal plasma composition (Vince et al., 2018). This can also cause problems when the sperm and egg recognize each other, resulting in decreased potential fertility of the male (Llamas-Luceño et al., 2020). On the other hand, heat can also affect normal mitochondrial functioning, specifically by altering oxidation of the electron transport chain complexes responsible for normal ATP production (Belhadj Slimen et al., 2016); (Capela et al., 2022); (Nichi et al., 2006).

Our results generally reveal THI values higher than the 76-point threshold traditionally reported in the literature as heat stress (Alves et al., 2025); (Rodriguez-Venegas et al., 2023). Overall, temperatures and relative humidity were relatively similar across the 39 ejaculates analyzed. By observing their behavior during the 60 days prior to each semen collection, we gathered as much information as possible related to the hypothetical conditions during spermatogenesis. Study results show that, regardless of season, THI values were consistently high and varied within a limited interval, which restricted our ability to compare thermoneutral versus clearly heat-stressed conditions. Because THI integrates temperature and relative humidity, the persistently high humidity observed during the study may have contributed to the relatively narrow THI range despite modest variation in ambient temperature (Dunn et al., 2014).

In the tropics, particular conditions of high relative humidity have also been noted as associated with higher THI values during the summer (Kumar et al., 2019). Our results show that, despite the effect of season, the relative humidity during the study was almost constant and considerably high, ranging from 80%–94%. Other studies in Zebu breeds have demonstrated a trend whereby, as the THI increases, the quality of sperm decreases under tropical conditions (Alves et al., 2025). This has been observed with variables such as sperm morphology, motility, volume, and ejaculate concentration(Freitas et al., 2020); (Garcia-Oliveros et al., 2022); (Seifi-Jamadi et al., 2019). The differences from previous studies may reflect breed-specific adaptation, the narrow THI variation here, exposure quantification variations, and the modest sample size, rather than a true reversal of the general heat-stress pattern. One possible explanation is that unmeasured nocturnal cooling may have partially mitigated the impact of daytime heat load (Freitas et al., 2020); however, this remains speculative because nighttime thermal indices were not analyzed in the present study.

We also explored sperm quality characteristics in both cohorts. Our results show that within the THI cohorts, no significant differences were observed in TM, PM, viability, normal sperm morphology, and VSL, while there were significant variations in VCL, VAP, LIN, STR, and WOB. These differences suggest changes in sperm motile patterns rather than uniform improvement or deterioration in semen quality. Previous studies have shown that sperm motility and velocity tend to decrease with higher THI (Landaeta-Hernández et al., 2020); (Rahbar et al., 2025). This phenomenon could be related to the role of Ca²⁺ ions in the sperm flagellum (Abd El-Emam et al., 2023). During their movement, high thermal stress can prematurely activate the sperm cell (Swain et al., 2025). Because all ejaculates were collected under consistently high THI conditions, corresponding to a heat-stress range previously proposed for B. indicus bulls under tropical conditions (Alves et al., 2025), and because the THI range was narrow, these findings reflect variation within a generally heat-challenged tropical environment, rather than contrasts between thermoneutral and severely heat-stressed conditions. These findings further suggest that higher THI was associated with altered sperm motility patterns rather than a uniform increase in all kinematic traits. A recent study has shown that these heat-induced effects can alter the function of Ca²⁺/calmodulin-dependent protein kinase II, which has been linked to early sperm acrosome reaction and fertility problems (Santos et al., 2025). Previous studies conducted in European bull breeds have reported reductions in membrane integrity and sperm morphology with increasing THI (Gloria et al., 2021). In the present study, however, neither viability nor normal morphology was significantly associated with continuous THI or differed significantly between THI cohorts. Therefore, no clear THI-related pattern could be established for these traits under the relatively narrow and consistently high THI range evaluated. In this context, although breeds with some degree of adaptability to tropical conditions are already used, selection for animals with greater reproductive resilience to heat stress warrants further investigation (Capela et al., 2022); (Livernois et al., 2018); (Slayi & Jaja, 2025). Because semen-quality traits may have a heritable component, selection for heat-resilient reproductive phenotypes warrants further investigation (Vanselow et al., 2024).

In the continuous THI analysis, thermal load was not consistently associated with semen quality traits. Although TM and PM showed statistically significant positive associations with THI, no significant relationships were found for the remaining kinematic variables, viability, or normal morphology. This pattern indicates that, under the relatively homogeneous and generally high THI conditions recorded in this study, continuous THI variation had limited explanatory value for most semen traits.

For this reason, indicators of thermal comfort, such as THI, provide more information for decision makers (Mader et al., 2006); (Souto et al., 2022), whether for natural mating or germplasm conservation and reproductive biotechnologies. One limitation of this work, however, is the need to complement conventional semen analysis with more complex techniques such as omics science, which provide a broader view of what might be happening inside the sperm cell (Celeghini et al., 2024); (Chitkara et al., 2025); (Kumar et al., 2024).

CONCLUSION

This study indicates that season is associated with variation in sperm motility, kinematic patterns, and morphology in Brahman bulls raised under humid tropical conditions. Total and progressive motility were highest during the transitional season, whereas the percentage of morphologically normal sperm was highest during the rainy season; sperm viability remained relatively stable across seasons. Continuous THI analysis showed positive associations with total and progressive motility; however, these findings should be interpreted cautiously because they occurred within a narrow and consistently high THI range, without confirmation by the categorical THI-cohort comparison. Moreover, neither viability nor normal morphology was significantly associated with continuous THI, indicating that the seasonal variation observed in morphology should not be attributed directly to THI. The results do not suggest a beneficial effect of heat stress, but rather highlight the complexity of interpreting semen quality responses under tropical conditions in adapted B. indicus bulls. The modest sample size, high interindividual variability, and limited environmental contrast influence the final interpretation. Further studies integrating larger datasets, fertility outcomes, and cellular or molecular indicators of sperm function are needed before these traits can be used to select breeding bulls under humid tropical conditions.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interest.

ACKNOWLEDGEMENT

The authors thank the Costa Rica Institute of Technology for financing this study. This work was part of the research project VIE-2151083, Optimización de la conservación y búsqueda de parámetros de la fertilidad en espermatozoides de animales de interés productivo and Postgraduate Office of ITCR. The APC was funded by the Costa Rica Institute of Technology´s Vice-Chancellor’s office of Research and Extension. I.A.-Z. and F.S. thank the Postgraduate Office of the Costa Rica Institute of Technology.

DECLARATION OF GENERATIVE AI AND AI-ASSISTED TECHNOLOGIES IN THE WRITING PROCESS

During the writing process, none of the authors used generative AI or AI-assisted technologies.

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References

Abd El-Emam M.M., Ray M.N., Ozono M., Kogure K.. Heat stress disrupts spermatogenesis via modulation of sperm-specific calcium channels in rats. Journal of Thermal Biology. 2023; 112DOI
Alves L.A., Cooke R.F., Sant’Ana Borges M., Souza F.F., Kastelic J.P., Crespilho A.M.. Season and temperature-humidity index affect sperm quality and in vivo fertility of beef bulls. Journal of Thermal Biology. 2025; 133DOI
Amini A., Behnam B.. Global warming, fertility, and spermatogenesis decline: global and regional evidence from 195 countries and implications for climate adaptation policy. International Journal of Environmental Research and Public Health. 2026; 23(3)DOI
Araya-Zúñiga I., Sevilla F., Molina-Montero R., Roldan E.R.S., Barrientos-Morales M., Silvestre M.A., Valverde A.. Kinematic and morphometric assessment of fresh semen, before, during and after mating period in Brahman bulls. Animals. 2024; 14(1)DOI
Arias R.A., Mader T.L.. Evaluation of four thermal comfort indices and their relationship with physiological variables in feedlot cattle. Animals. 2023; 13(7)DOI
Belhadj Slimen I., Najar T., Ghram A., Abdrrabba M.. Heat stress effects on livestock: molecular, cellular and metabolic aspects, a review. Journal of Animal Physiology and Animal Nutrition. 2016; 100(3):401-412. DOI
Boe‐Hansen G.B., Rêgo J.P.A., Satake N., Venus B., Sadowski P., Nouwens A., Li Y., McGowan M.. Effects of increased scrotal temperature on semen quality and seminal plasma proteins in Brahman bulls. Molecular Reproduction and Development. 2020; 87(5):574-597. DOI
Boonkum W., Chankitisakul V., Juiputta J.. A novel temperature-humidity index-based model for evaluating semen characteristics in Thai native roosters under tropical conditions. Poultry Science. 2025; 104(8)DOI
Brito L.F.C., Silva A.E.D.F., Barbosa R.T., Kastelic J.P.. Testicular thermoregulation in Bos indicus, crossbred and Bos taurus bulls: relationship with scrotal, testicular vascular cone and testicular morphology, and effects on semen quality and sperm production. Theriogenology. 2004; 61(2–3):511-528. DOI
Capela L., Leites I., Romão R., Lopes-da-Costa L., Pereira R.M.L.N.. Impact of heat stress on bovine sperm quality and competence. Animals. 2022; 12(8)DOI
Castillo R., Amador J.A.. Precipitation and temperature in Costa Rica at the end of the century based on NEX-GDDP projected scenarios. Atmosphere. 2020; 11(12)DOI
Celeghini E.C.C., Baatsch-Nascimento F., Bozzi A.da R., Garcia-Oliveros L.N., Arruda R.P.. Bovine testicular heat stress: From climate change to effects on microRNA profile. Animal Reproduction Science. 2024; 270DOI
Chacón L., Navarro O., Ladino C., Martins J., Perez J., Ardila A.. Sexual behavior and seminal characteristics of Brahman bulls in the Colombian tropical flooded savanna: effects of reproductive management systems and climatic periods. Tropical Animal Health and Production. 2022; 54(1)DOI
Chitkara M., Gurao A., Kaur H., Dige M.S., Sodhi M., Mukesh M., Kataria R.S.. Omics approaches to understand impact of heat stress on semen quality and fertility in bovines. Animal Reproduction Science. 2025; 274DOI
Corte Pause F., Crociati M., Urli S., Monaci M., Degano L., Stradaioli G.. Environmental factors affecting the reproductive efficiency of Italian Simmental young bulls. Animals. 2022; 12(18)DOI
Díaz R.F., Galina C.S., Aranda E.M., Aceves L.A., Sánchez J.G., Pablos J.L.. Effect of temperature humidity index on the onset of post- partum ovarian activity and reproductive behavior in Bos indicus cows. Animal Reproduction. 2020; 17(1)DOI
Dunn R.J.H., Mead N.E., Willett K.M., Parker D.E.. Analysis of heat stress in UK dairy cattle and impact on milk yields. Environmental Research Letters. 2014; 9(6)DOI
Fraczek M., Lewandowska A., Budzinska M., Kamieniczna M., Wojnar L., Gill K., Piasecka M., Kups M., Havrylyuk A., Chopyak V., Nakonechnyy J., Nakonechnyy A., Kurpisz M.. The role of seminal oxidative stress scavenging system in the pathogenesis of sperm DNA damage in men exposed and not exposed to genital heat stress. International Journal of Environmental Research and Public Health. 2022; 19(5)DOI
Freitas A.de P., Santos G.F.F., Fernandes A.R., Mendonça G.G., Paz C.C.P., Vercesi Filho A.E., El Faro L.. Effect of thermal stress on basic seminal characteristics of Gyr bulls. International Journal of Biometeorology. 2020; 64(10):1649-1656. DOI
Garcia-Oliveros L.N., Arruda R.P.de, Batissaco L., Gonzaga V.H.G., Nogueira V.J.M., Florez-Rodriguez S.A., Almeida F.dos S., Alves M.B.R., Pinto S.C.C., Nichi M., Losano J.D.de A., Kawai G.K.V., Celeghini E.C.C.. Chronological characterization of sperm morpho-functional damage and recovery after testicular heat stress in Nellore bulls. Journal of Thermal Biology. 2022; 106DOI
Gloria A., Candeloro L., Wegher L., Robbe D., Carluccio A., Contri A.. Environmental temperature and relative humidity differently affect the sperm characteristics in Brown Swiss and Belgian Blue bulls. International Journal of Biometeorology. 2021; 65(12):2189-2199. DOI
Habeeb A.A.M., Osman S.F., Gad A.E.. Signs of heat stress and some steps to reduce the negative effects on animals. GSC Advanced Research and Reviews. 2020; 4(1):046-058. DOI
Hamilton T.R.dos S., Mendes C.M., Castro L.S.de, Assis P.M.de, Siqueira A.F.P., Delgado J.de C., Goissis M.D., Muiño-Blanco T., Cebrián-Pérez J.Á., Nichi M., Visintin J.A., Assumpção M.E.O.D.. Evaluation of lasting effects of heat stress on sperm profile and oxidative status of ram semen and epididymal sperm. Oxidative Medicine and Cellular Longevity. 2016. DOI
Kastelic J.P., Rizzoto G., Shahat A.M., Ferreira J.C.P., Thumdathil J.. Pathogenesis and mitigation of the deleterious effects of heat stress on bull reproduction. Revista Brasileira de Reprodução Animal. 2021; 45(4):518-524. DOI
Koivisto M., Costa M., Perri S., Vicente W.. The effect of season on semen characteristics and freezability in Bos indicus and Bos taurus bulls in the southeastern region of Brazil. Reproduction in Domestic Animals. 2009; 44(4):587-592. DOI
Kumar G., Gurao A., Vasisth R., Chitkara M., Singh R., Ranganatha Sriranga K., Shivanand Dige M., Mukesh M., Singh P., Singh Kataria R.. Genome-wide 5′-C-phosphate-G-3′ methylation patterns reveal the effect of heat stress on the altered semen quality in Bubalus bubalis. Gene. 2024; 906DOI
Kumar N., Ghosh S.K., Mohanty T.K., Prasad J.K., Singh R.K., Patel B.. Effect of heat stress on physico-morphological characteristics and sperm functions in Murrah buffalo semen. The Indian Journal of Animal Sciences. 2019; 89(4):390-393. DOI
Landaeta-Hernández A.J., Gil-Araujo M.A., Ungerfeld R., Owen Rae D., Urdaneta-Moyer A., Parra-Núñez A., Kaske M., Bollwein H., Chenoweth P.J.. Effect of season and genotype on values for bull semen variables under tropical conditions. Animal Reproduction Science. 2020; 221DOI
Livernois A., Mallard B., Miglior F., Cánovas A.. PSVI-28 Influence of ambient temperature and temperature-humidity index on a measure of semen quality in crossbreed beef bulls. Journal of Animal Science. 2018; 96(suppl_3)DOI
Llamas-Luceño N., Hostens M., Mullaart E., Broekhuijse M., Lonergan P., Soom A.. High temperature-humidity index compromises sperm quality and fertility of Holstein bulls in temperate climates. Journal of Dairy Science. 2020; 103(10):9502-9514. DOI
Mader T.L., Davis M.S., Brown-Brandl T.. Environmental factors influencing heat stress in feedlot cattle. Journal of Animal Science. 2006; 84(3):712-719. DOI
Morrell J.M.. Heat stress and bull fertility. Theriogenology. 2020; 153:62-67. DOI
Netherton J.K., Robinson B.R., Ogle R.A., Gunn A., Villaverde A.I.S.B., Colyvas K., Wise C., Russo T., Dowdell A., Baker M.A.. Seasonal variation in bull semen quality demonstrates there are heat-sensitive and heat-tolerant bulls. Scientific Reports. 2022; 12(1)DOI
Nichi M., Bols P.E.J., Züge R.M., Barnabe V.H., Goovaerts I.G.F., Barnabe R.C., Cortada C.N.M.. Seasonal variation in semen quality in Bos indicus and Bos taurus bulls raised under tropical conditions. Theriogenology. 2006; 66(4):822-828. DOI
Perry V.E.A.. The role of sperm morphology standards in the laboratory assessment of bull fertility in Australia. Frontiers in Veterinary Science. 2021; 8DOI
Poclín-Rojas A.Y., Arbaiza Barnechea M.D., Segura Portocarrero G.T., Ampuero-Trigoso G., Bernilla Carrillo D., Depaz-Hizo B.A., Vásquez-Tarrillo R.W., Diaz-Quevedo C., Quispe-Ccasa H.A.. Motility performance of thawed spermatozoa of bulls from the tropics throughout the year. Animals. 2025; 15(16)DOI
Popović N., Djedović R., Stojić P., Stanojević D., Jovanović R., Beskorovajni R.. Influence of THI index, age of bulls, year, and season on production and quality of native sperm of Holstein-Friesian bulls. Revista Brasileira de Zootecnia. 2024; 53DOI
Rahbar M., Behgar M., Khoshniyat M.R., Motamedi F.F., Shawrang P.. The effect of temperature-humidity index on sperm motility parameters in Holstein bulls. Journal of Animal Production. 2025; 27(2):223-233. DOI
Rodriguez-Venegas R., Meza-Herrera C.A., Robles-Trillo P.A., Angel-Garcia O., Legarreta-Gonzalez M.A., Sánchez-Vocanegra H.F., Rodriguez-Martinez R.. Effect of THI on milk production, percentage of milking cows, and time lying in Holstein cows in Northern-arid Mexico. Animals. 2023; 13(10)DOI
Sabés-Alsina M., Lundeheim N., Johannisson A., López-Béjar M., Morrell J.M.. Relationships between climate and sperm quality in dairy bull semen: A retrospective analysis. Journal of Dairy Science. 2019; 102(6):5623-5633. DOI
Santos T.de S., Contrim I.S., Silva D.F., Assumpção M.E.O.D., Paula-Lopes F.F.de, Feitosa W.B.. Heat shock affects the Ca2+/calmodulin-dependent protein kinase II dynamic during bovine sperm capacitation and acrosome reaction. Frontiers in Cell and Developmental Biology. 2025; 13DOI
Savvulidi F.G., Ducháček J., Ptáček M., Gašparík M., Kovářová V., Codl R., Stádník L.. Impact of microclimatic conditions on sperm production in Czech Holstein bulls: A correlational study. Czech Journal of Animal Science. 2025; 70(4):121-129. DOI
Seifi-Jamadi A., Kohram H., Zhandi M., Llamas Luceño N., Leemans B., Henrotte E., Latour C., Demeyere K., Meyer E., Soom A.. Heat stress has a deleterious effect on bull semen quality and subsequent embryo development. Reproduction, Fertility and Development. 2019; 32(2)DOI
Serrano-Jara D., Gacem S., Argente M.J., Agea I., Silvestre M.A., García M.L.. Impact of cold storage on semen quality in two divergent rabbit lines selected for resilience under heat stress condition. Veterinary and Animal Science. 2026; 32DOI
Sevilla F., Araya-Zúñiga I., Salamanca-Carreño A., Silvestre M.A., Rodríguez J., Matamoros K., Molina-Montero R., Carranza-Rojas L.C., Roldan E.R.S., Valverde A.. Effect of age and scrotal circumference on sperm morphology in Brahman bulls using a modified fixation technique. Frontiers in Veterinary Science. 2025; 12DOI
Slayi M., Jaja I.F.. Strategies for mitigating heat stress and their effects on behavior, physiological indicators, and growth performance in communally managed feedlot cattle. Frontiers in Veterinary Science. 2025; 12DOI
Solís J.M., Sevilla F., Silvestre M.A., Araya-Zúñiga I., Roldan E.R.S., Saborío-Montero A., Valverde A.. Effect of thawing procedure and thermo-resistance test on sperm motility and kinematics patterns in two bovine breeds. Animals. 2024; 14(19)DOI
Souto P.L.G., Barbosa E.A., Silva F.I.C., Martins V.M.V., Hatamoto-Zervoudakis L.K., McManus C., Alencar E.R., Ramos A.F.. Seasonal semen quality of a local and commercial taurine cattle breeds, raised in a subtropical climate: Relationship between external morphology of the animals and climate. Animal Reproduction Science. 2022; 240DOI
Swain D.K., Vergara C., Castro-Arnau J., Lishko P.V.. The essential calcium channel of sperm CatSper is temperature-gated. Nature Communications. 2025; 16(1)DOI
Umar S.I.U., Konwar D., Khan A., Bhat M.A., Javid F., Jeelani R., Nabi B., Najar A.A., Kumar D., Brahma B.. Delineation of temperature-humidity index (THI) as indicator of heat stress in riverine buffaloes (Bubalus bubalis) of a sub-tropical Indian region. Cell Stress and Chaperones. 2021; 26(4):657-669. DOI
Vanselow J., Wesenauer C., Eggert A., Sharma A., Becker F.. Summer heat during spermatogenesis reduces in vitro blastocyst rates and affects sperm quality of next generation bulls. Andrology. 2024; 12(8):1897-1906. DOI
Vargas-Leitón B., Núñez-Cárdenas J., Valverde-Abarca A., León-Rodríguez B., Saborío-Montero A.. Unveiling the genetic structure of Costa Rican bovines through genetic admixture models. Agronomía Mesoamericana. 2025; 36DOI
Vince S., Žura Žaja I., Samardžija M., Majić Balić I., Vilić M., Đuričić D., Valpotić H., Marković F., Milinković-Tur S.. Age-related differences of semen quality, seminal plasma, and spermatozoa antioxidative and oxidative stress variables in bulls during cold and warm periods of the year. Animal. 2018; 12(3):559-568. DOI
Wang J., Luo Y., He Y., Li W., Qin Y., Wu Y.. Astaxanthin alleviates the decline of sperm quality caused by heat stress in mice via reducing oxidative stress. Life. 2025; 15(6)DOI

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[BIBR-1] Abd El-Emam M.M., Ray M.N., Ozono M., Kogure K.. Heat stress disrupts spermatogenesis via modulation of sperm-specific calcium channels in rats. Journal of Thermal Biology. 2023; 112DOI

[BIBR-2] Alves L.A., Cooke R.F., Sant’Ana Borges M., Souza F.F., Kastelic J.P., Crespilho A.M.. Season and temperature-humidity index affect sperm quality and in vivo fertility of beef bulls. Journal of Thermal Biology. 2025; 133DOI

[BIBR-3] Amini A., Behnam B.. Global warming, fertility, and spermatogenesis decline: global and regional evidence from 195 countries and implications for climate adaptation policy. International Journal of Environmental Research and Public Health. 2026; 23(3)DOI

[BIBR-4] Araya-Zúñiga I., Sevilla F., Molina-Montero R., Roldan E.R.S., Barrientos-Morales M., Silvestre M.A., Valverde A.. Kinematic and morphometric assessment of fresh semen, before, during and after mating period in Brahman bulls. Animals. 2024; 14(1)DOI

[BIBR-5] Arias R.A., Mader T.L.. Evaluation of four thermal comfort indices and their relationship with physiological variables in feedlot cattle. Animals. 2023; 13(7)DOI

[BIBR-6] Belhadj Slimen I., Najar T., Ghram A., Abdrrabba M.. Heat stress effects on livestock: molecular, cellular and metabolic aspects, a review. Journal of Animal Physiology and Animal Nutrition. 2016; 100(3):401-412. DOI

[BIBR-7] Boe‐Hansen G.B., Rêgo J.P.A., Satake N., Venus B., Sadowski P., Nouwens A., Li Y., McGowan M.. Effects of increased scrotal temperature on semen quality and seminal plasma proteins in Brahman bulls. Molecular Reproduction and Development. 2020; 87(5):574-597. DOI

[BIBR-8] Boonkum W., Chankitisakul V., Juiputta J.. A novel temperature-humidity index-based model for evaluating semen characteristics in Thai native roosters under tropical conditions. Poultry Science. 2025; 104(8)DOI

[BIBR-9] Brito L.F.C., Silva A.E.D.F., Barbosa R.T., Kastelic J.P.. Testicular thermoregulation in Bos indicus, crossbred and Bos taurus bulls: relationship with scrotal, testicular vascular cone and testicular morphology, and effects on semen quality and sperm production. Theriogenology. 2004; 61(2–3):511-528. DOI

[BIBR-10] Capela L., Leites I., Romão R., Lopes-da-Costa L., Pereira R.M.L.N.. Impact of heat stress on bovine sperm quality and competence. Animals. 2022; 12(8)DOI

[BIBR-11] Castillo R., Amador J.A.. Precipitation and temperature in Costa Rica at the end of the century based on NEX-GDDP projected scenarios. Atmosphere. 2020; 11(12)DOI

[BIBR-12] Celeghini E.C.C., Baatsch-Nascimento F., Bozzi A.da R., Garcia-Oliveros L.N., Arruda R.P.. Bovine testicular heat stress: From climate change to effects on microRNA profile. Animal Reproduction Science. 2024; 270DOI

[BIBR-13] Chacón L., Navarro O., Ladino C., Martins J., Perez J., Ardila A.. Sexual behavior and seminal characteristics of Brahman bulls in the Colombian tropical flooded savanna: effects of reproductive management systems and climatic periods. Tropical Animal Health and Production. 2022; 54(1)DOI

[BIBR-14] Chitkara M., Gurao A., Kaur H., Dige M.S., Sodhi M., Mukesh M., Kataria R.S.. Omics approaches to understand impact of heat stress on semen quality and fertility in bovines. Animal Reproduction Science. 2025; 274DOI

[BIBR-15] Corte Pause F., Crociati M., Urli S., Monaci M., Degano L., Stradaioli G.. Environmental factors affecting the reproductive efficiency of Italian Simmental young bulls. Animals. 2022; 12(18)DOI

[BIBR-16] Díaz R.F., Galina C.S., Aranda E.M., Aceves L.A., Sánchez J.G., Pablos J.L.. Effect of temperature humidity index on the onset of post- partum ovarian activity and reproductive behavior in Bos indicus cows. Animal Reproduction. 2020; 17(1)DOI

[BIBR-17] Dunn R.J.H., Mead N.E., Willett K.M., Parker D.E.. Analysis of heat stress in UK dairy cattle and impact on milk yields. Environmental Research Letters. 2014; 9(6)DOI

[BIBR-18] Fraczek M., Lewandowska A., Budzinska M., Kamieniczna M., Wojnar L., Gill K., Piasecka M., Kups M., Havrylyuk A., Chopyak V., Nakonechnyy J., Nakonechnyy A., Kurpisz M.. The role of seminal oxidative stress scavenging system in the pathogenesis of sperm DNA damage in men exposed and not exposed to genital heat stress. International Journal of Environmental Research and Public Health. 2022; 19(5)DOI

[BIBR-19] Freitas A.de P., Santos G.F.F., Fernandes A.R., Mendonça G.G., Paz C.C.P., Vercesi Filho A.E., El Faro L.. Effect of thermal stress on basic seminal characteristics of Gyr bulls. International Journal of Biometeorology. 2020; 64(10):1649-1656. DOI

[BIBR-20] Garcia-Oliveros L.N., Arruda R.P.de, Batissaco L., Gonzaga V.H.G., Nogueira V.J.M., Florez-Rodriguez S.A., Almeida F.dos S., Alves M.B.R., Pinto S.C.C., Nichi M., Losano J.D.de A., Kawai G.K.V., Celeghini E.C.C.. Chronological characterization of sperm morpho-functional damage and recovery after testicular heat stress in Nellore bulls. Journal of Thermal Biology. 2022; 106DOI

[BIBR-21] Gloria A., Candeloro L., Wegher L., Robbe D., Carluccio A., Contri A.. Environmental temperature and relative humidity differently affect the sperm characteristics in Brown Swiss and Belgian Blue bulls. International Journal of Biometeorology. 2021; 65(12):2189-2199. DOI

[BIBR-22] Habeeb A.A.M., Osman S.F., Gad A.E.. Signs of heat stress and some steps to reduce the negative effects on animals. GSC Advanced Research and Reviews. 2020; 4(1):046-058. DOI

[BIBR-23] Hamilton T.R.dos S., Mendes C.M., Castro L.S.de, Assis P.M.de, Siqueira A.F.P., Delgado J.de C., Goissis M.D., Muiño-Blanco T., Cebrián-Pérez J.Á., Nichi M., Visintin J.A., Assumpção M.E.O.D.. Evaluation of lasting effects of heat stress on sperm profile and oxidative status of ram semen and epididymal sperm. Oxidative Medicine and Cellular Longevity. 2016. DOI

[BIBR-24] Kastelic J.P., Rizzoto G., Shahat A.M., Ferreira J.C.P., Thumdathil J.. Pathogenesis and mitigation of the deleterious effects of heat stress on bull reproduction. Revista Brasileira de Reprodução Animal. 2021; 45(4):518-524. DOI

[BIBR-25] Koivisto M., Costa M., Perri S., Vicente W.. The effect of season on semen characteristics and freezability in Bos indicus and Bos taurus bulls in the southeastern region of Brazil. Reproduction in Domestic Animals. 2009; 44(4):587-592. DOI

[BIBR-26] Kumar G., Gurao A., Vasisth R., Chitkara M., Singh R., Ranganatha Sriranga K., Shivanand Dige M., Mukesh M., Singh P., Singh Kataria R.. Genome-wide 5′-C-phosphate-G-3′ methylation patterns reveal the effect of heat stress on the altered semen quality in Bubalus bubalis. Gene. 2024; 906DOI

[BIBR-27] Kumar N., Ghosh S.K., Mohanty T.K., Prasad J.K., Singh R.K., Patel B.. Effect of heat stress on physico-morphological characteristics and sperm functions in Murrah buffalo semen. The Indian Journal of Animal Sciences. 2019; 89(4):390-393. DOI

[BIBR-28] Landaeta-Hernández A.J., Gil-Araujo M.A., Ungerfeld R., Owen Rae D., Urdaneta-Moyer A., Parra-Núñez A., Kaske M., Bollwein H., Chenoweth P.J.. Effect of season and genotype on values for bull semen variables under tropical conditions. Animal Reproduction Science. 2020; 221DOI

[BIBR-29] Livernois A., Mallard B., Miglior F., Cánovas A.. PSVI-28 Influence of ambient temperature and temperature-humidity index on a measure of semen quality in crossbreed beef bulls. Journal of Animal Science. 2018; 96(suppl_3)DOI

[BIBR-30] Llamas-Luceño N., Hostens M., Mullaart E., Broekhuijse M., Lonergan P., Soom A.. High temperature-humidity index compromises sperm quality and fertility of Holstein bulls in temperate climates. Journal of Dairy Science. 2020; 103(10):9502-9514. DOI

[BIBR-31] Mader T.L., Davis M.S., Brown-Brandl T.. Environmental factors influencing heat stress in feedlot cattle. Journal of Animal Science. 2006; 84(3):712-719. DOI

[BIBR-32] Morrell J.M.. Heat stress and bull fertility. Theriogenology. 2020; 153:62-67. DOI

[BIBR-33] Netherton J.K., Robinson B.R., Ogle R.A., Gunn A., Villaverde A.I.S.B., Colyvas K., Wise C., Russo T., Dowdell A., Baker M.A.. Seasonal variation in bull semen quality demonstrates there are heat-sensitive and heat-tolerant bulls. Scientific Reports. 2022; 12(1)DOI

[BIBR-34] Nichi M., Bols P.E.J., Züge R.M., Barnabe V.H., Goovaerts I.G.F., Barnabe R.C., Cortada C.N.M.. Seasonal variation in semen quality in Bos indicus and Bos taurus bulls raised under tropical conditions. Theriogenology. 2006; 66(4):822-828. DOI

[BIBR-35] Perry V.E.A.. The role of sperm morphology standards in the laboratory assessment of bull fertility in Australia. Frontiers in Veterinary Science. 2021; 8DOI

[BIBR-36] Poclín-Rojas A.Y., Arbaiza Barnechea M.D., Segura Portocarrero G.T., Ampuero-Trigoso G., Bernilla Carrillo D., Depaz-Hizo B.A., Vásquez-Tarrillo R.W., Diaz-Quevedo C., Quispe-Ccasa H.A.. Motility performance of thawed spermatozoa of bulls from the tropics throughout the year. Animals. 2025; 15(16)DOI

[BIBR-37] Popović N., Djedović R., Stojić P., Stanojević D., Jovanović R., Beskorovajni R.. Influence of THI index, age of bulls, year, and season on production and quality of native sperm of Holstein-Friesian bulls. Revista Brasileira de Zootecnia. 2024; 53DOI

[BIBR-38] Rahbar M., Behgar M., Khoshniyat M.R., Motamedi F.F., Shawrang P.. The effect of temperature-humidity index on sperm motility parameters in Holstein bulls. Journal of Animal Production. 2025; 27(2):223-233. DOI

[BIBR-39] Rodriguez-Venegas R., Meza-Herrera C.A., Robles-Trillo P.A., Angel-Garcia O., Legarreta-Gonzalez M.A., Sánchez-Vocanegra H.F., Rodriguez-Martinez R.. Effect of THI on milk production, percentage of milking cows, and time lying in Holstein cows in Northern-arid Mexico. Animals. 2023; 13(10)DOI

[BIBR-40] Sabés-Alsina M., Lundeheim N., Johannisson A., López-Béjar M., Morrell J.M.. Relationships between climate and sperm quality in dairy bull semen: A retrospective analysis. Journal of Dairy Science. 2019; 102(6):5623-5633. DOI

[BIBR-41] Santos T.de S., Contrim I.S., Silva D.F., Assumpção M.E.O.D., Paula-Lopes F.F.de, Feitosa W.B.. Heat shock affects the Ca2+/calmodulin-dependent protein kinase II dynamic during bovine sperm capacitation and acrosome reaction. Frontiers in Cell and Developmental Biology. 2025; 13DOI

[BIBR-42] Savvulidi F.G., Ducháček J., Ptáček M., Gašparík M., Kovářová V., Codl R., Stádník L.. Impact of microclimatic conditions on sperm production in Czech Holstein bulls: A correlational study. Czech Journal of Animal Science. 2025; 70(4):121-129. DOI

[BIBR-43] Seifi-Jamadi A., Kohram H., Zhandi M., Llamas Luceño N., Leemans B., Henrotte E., Latour C., Demeyere K., Meyer E., Soom A.. Heat stress has a deleterious effect on bull semen quality and subsequent embryo development. Reproduction, Fertility and Development. 2019; 32(2)DOI

[BIBR-44] Serrano-Jara D., Gacem S., Argente M.J., Agea I., Silvestre M.A., García M.L.. Impact of cold storage on semen quality in two divergent rabbit lines selected for resilience under heat stress condition. Veterinary and Animal Science. 2026; 32DOI

[BIBR-45] Sevilla F., Araya-Zúñiga I., Salamanca-Carreño A., Silvestre M.A., Rodríguez J., Matamoros K., Molina-Montero R., Carranza-Rojas L.C., Roldan E.R.S., Valverde A.. Effect of age and scrotal circumference on sperm morphology in Brahman bulls using a modified fixation technique. Frontiers in Veterinary Science. 2025; 12DOI

[BIBR-46] Slayi M., Jaja I.F.. Strategies for mitigating heat stress and their effects on behavior, physiological indicators, and growth performance in communally managed feedlot cattle. Frontiers in Veterinary Science. 2025; 12DOI

[BIBR-47] Solís J.M., Sevilla F., Silvestre M.A., Araya-Zúñiga I., Roldan E.R.S., Saborío-Montero A., Valverde A.. Effect of thawing procedure and thermo-resistance test on sperm motility and kinematics patterns in two bovine breeds. Animals. 2024; 14(19)DOI

[BIBR-48] Souto P.L.G., Barbosa E.A., Silva F.I.C., Martins V.M.V., Hatamoto-Zervoudakis L.K., McManus C., Alencar E.R., Ramos A.F.. Seasonal semen quality of a local and commercial taurine cattle breeds, raised in a subtropical climate: Relationship between external morphology of the animals and climate. Animal Reproduction Science. 2022; 240DOI

[BIBR-49] Swain D.K., Vergara C., Castro-Arnau J., Lishko P.V.. The essential calcium channel of sperm CatSper is temperature-gated. Nature Communications. 2025; 16(1)DOI

[BIBR-50] Umar S.I.U., Konwar D., Khan A., Bhat M.A., Javid F., Jeelani R., Nabi B., Najar A.A., Kumar D., Brahma B.. Delineation of temperature-humidity index (THI) as indicator of heat stress in riverine buffaloes (Bubalus bubalis) of a sub-tropical Indian region. Cell Stress and Chaperones. 2021; 26(4):657-669. DOI

[BIBR-51] Vanselow J., Wesenauer C., Eggert A., Sharma A., Becker F.. Summer heat during spermatogenesis reduces in vitro blastocyst rates and affects sperm quality of next generation bulls. Andrology. 2024; 12(8):1897-1906. DOI

[BIBR-52] Vargas-Leitón B., Núñez-Cárdenas J., Valverde-Abarca A., León-Rodríguez B., Saborío-Montero A.. Unveiling the genetic structure of Costa Rican bovines through genetic admixture models. Agronomía Mesoamericana. 2025; 36DOI

[BIBR-53] Vince S., Žura Žaja I., Samardžija M., Majić Balić I., Vilić M., Đuričić D., Valpotić H., Marković F., Milinković-Tur S.. Age-related differences of semen quality, seminal plasma, and spermatozoa antioxidative and oxidative stress variables in bulls during cold and warm periods of the year. Animal. 2018; 12(3):559-568. DOI

[BIBR-54] Wang J., Luo Y., He Y., Li W., Qin Y., Wu Y.. Astaxanthin alleviates the decline of sperm quality caused by heat stress in mice via reducing oxidative stress. Life. 2025; 15(6)DOI

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