The Philippine dairy sector is highly reliant on imports, producing less than 1% of the national demand(Agriculture Press Office, 2024). To bridge this gap, upgrading programs have extensively utilized Holstein x Jersey (HxJ) (Bos taurus taurus) crosses to improve milk yields(Lagare, 2026). However, the introduction of temperate genetics into tropical environments often compromises herd health and milk performance(Polsky & Keyserlingk, 2017). This causes metabolic instability, exacerbated by heat stress and vector-borne diseases(Xiong et al., 2025). Addressing these failures requires a critical understanding between the concepts of adaptation and resilience.

Adaptation refers to the accumulation of fixed genetic traits over evolutionary time, which allows a population to survive in a specific environment(Henry et al., 2018). Less-dense coat surface is an example of an evolutionary morphological feature that facilitates rapid heat dissipation through sweating from the animal’s body back to its environment(Santos et al., 2022). Whereas resilience is defined as the dynamic capacity of an animal to minimize the impact of disturbance(Berghof et al., 2019). This could manifest via high heat load or a quick return to homeostasis(Colditz & Hine, 2016). In other words, while adaptation is a passive state of being fit to an environment, resilience is the active response to environmental adjustments. Tropical resilience, in this context, refers to the ability to maintain functional physiological stability even with high Temperature-Humidity indices or disease-related challenges.

Indigenous or native cattle populations, such as Panay Native (PN) (Bos taurus indicus) cattle are perceived to exhibit tropical resilience. Previous breeding programs in the Philippines tend to neglect the underlying mechanisms that made these cattle exceptional under the tropics(Ortega et al., 2021). Recently, a study by Dominguez et al. (2026)(Dominguez et al., 2026) confirmed that these populations have specific selection signatures associated with tropical insular adaptation and resilience. Thereby, an exploration of their physiological processes is of the essence. A robust putative biomarker for measuring qualities of resiliency is the hematological profile. Observed variations in erythrograms and leukocyte differentials provide insights into the animal’s thermoregulatory capacity(Sejian et al., 2021) and immunocompetence(Rashamol et al., 2018). Sejian et al. (2018)(Sejian et al., 2018) reported indicators including erythrocyte osmotic fragility, Neutrophil:Lymphocyte (N:L) ratio, and the maintenance of Packed Cell Volume (PCV) without hemoconcentration.

To maximize the usage of these biomarkers, accurate veterinary diagnostics is paramount. At its core, is a Reference Intervals (RIs) that reflects the specific genetic, environmental, and management factors of the population(Quagliardi et al., 2024). Unfortunately, the Philippines continues to rely on antiquated, foreign-derived RIs such as those founded by Oscar William Schalm in 1965. Clinical misdiagnosis may potentially emanate from the continued use of these outdated, non-contextual, and non-native intervals to tropical herds(Chantip et al., 2025). This mismanagement may mask anemia in susceptible crossbreds or misinterpret distinct adaptive physiological baselines of indigenous breeds. Currently, no published hematological standards exist for Philippine native animals and temperate breeds under Philippine conditions. This creates a strategic gap in the evidence needed to support selection for tropical-resilient traits and biomarkers.

Therefore, this study intended to assess hematological values specific to each breed in PN and HxJ heifers managed under tropical conditions. It sought to quantify differences in erythropoietic and immunological profiles to identify putative biomarkers of tropical resilience. This pioneering data will guide the establishment of local RIs, which are essential for correcting diagnostic standards. Ultimately, this provides the biological rationale for developing composite breeds combining the metabolic efficiency of native cattle with the production potential of exotic breeds.

The fundamental hematological divergence realized in this study is a direct reflection of the genetic potential between the breeds Panay Native (PN) and Holstein x Jersey (HxJ) crossbreds and their environment. With a mean THI of 78.71 in the DTRI farm, experimental animals experienced continuous thermal pressure. This accounted for mild-to-moderate heat stress, especially to the temperate-derived breeds(Yan et al., 2021). The thermoneutral zone (TNZ) is also an indispensable aspect of the functional health of the heifers. Yadav et al. (2025)(Yadav et al., 2025) reported that tropical breeds, much like the PN, can easily tolerate a thermal load within the 18-30 °C range. Given that farm conditions reached an average ambient temperature of 27 °C, this jeopardized our HxJ temperate crosses, as it exceeded their 16-25 °C optimum TNZ(Dimov et al., 2020). As a result, significant physiological shifts in the erythropoietic and leukocytic pathways in the HxJ group occurred, altering their blood profiles. Recent comprehensive studies published by Habimana et al. (2023)(Habimana et al., 2023) and Velayudhan et al. (2022)(Velayudhan et al., 2022) corroborate these findings. They noted that the high heat load of tropical microclimates is deleterious to sustaining homeostasis in temperate-derived crossbreds.

The clusters identified on the PCA biplot (Figure 1) are proof that these hematological variations are not random. The clusters represent precise physiological strategies illustrating a biological gradient. The HxJ profile is heavily pulled by indices of cellular turnover (MCV, MCH), which predominantly marked Cluster 1. This reflects chronic physiological response to environmental thermal pressure(Koch et al., 2025). In contrast, the PN breed dominated the physiological space of Cluster 3. This cluster is compelled by superior oxygen-carrying capacity (RBC, HGB, PCV), which preserves homeostatic stability amidst tropical conditions(Slayi & Jaja, 2025). The presence of Cluster 2 implies a transition zone. This presumes that even if a small percentage of crossbreds may achieve a basal coping level, they still fail to reach the superior physiological durability of the adapted native breed. Moreover, these results provide valid phenotypic evidence for the recent genomic findings of Dominguez et al. in 2026(Dominguez et al., 2026). Their study identified various selection signatures linked to tropical insular adaptation and resilience in Philippine Visayan native cattle. The hematological baselines of PN cattle are therefore a functional manifestation of these fixed evolutionary traits.

The limitations of using temperate-derived, non-contextual RIs emerged as a vital gap in evaluating the diagnostic health of tropical cattle. Results showed key blood components having significant variations between breeds. The HxJ group had HGB and PCV values constantly below the lower limits of all tested RIs(Schalm, 1965);(George et al., 2010);(González-Garduño et al., 2023). As a consequence, at least 60% of HxJ heifers were scored as having Absolute anemia despite being apparently healthy. This is in stark comparison to only ~12% of the PN group. The PN breed exhibited a superior erythroid profile, even exceeding the upper limits of temperate standards. This tendency should not be misinterpreted as pathological dehydration. Supposably, it reflects the evolutionary upregulation of erythropoiesis adhered to by the native cattle(Constable et al., 2016). This relates to the specific adaptation of maintaining tissue oxygenation under high thermal load in tropical environments.

Nonetheless, these hematological variations monitored between the breeds must be construed in the context of their respective production systems(Rubio Lozano et al., 2021). In this study, the two genetic groups were managed in accordance with the practical realities of tropical dairy production in the Philippines. Each system was optimized to satisfy its physiological requirements. The HxJ crossbred, being a high-yielding exotic composite, naturally requires nutrient-dense supplementation and confinement(Cooke et al., 2020) to support metabolic demands and mitigate exposure to environmental stressors(Brito et al., 2021). On the contrary, the evolution of the indigenous breed made them accustomed to forage-based grazing systems(Slayi & Jaja, 2025). In the Philippine setting, the PN cattle are traditionally managed in this scheme. Hartwiger et al. (2019)(Hartwiger et al., 2019) framed that transitioning cattle between indoor and pasture-based systems may influence their immunological and hematological baselines. So, while evaluating these breeds within their production settings provides grounded validity for local farmers, it introduces management as a baseline factor(Jongbo et al., 2026).

This study is limited to presenting profiles under the cohorts’ typical field conditions. Albeit the results signified that blood inconstancies are rooted in breed divergence, the impact of management vs. genetics cannot be overstated. Case in point, the elevated HGB and PCV observed in PN heifers possibly reflect a combination of genetic resilience and the effects of active foraging. Additionally, the HxJ heifers exhibited a higher prevalence of regenerative anemia when the thermal load challenged their intensive housing system. To thoroughly isolate the genetic effect, it is advised to use a controlled, uniform protocol across breeds. Nevertheless, the results undeniably demonstrated that PN cattle maintain different physiological processes compared with HxJ crosses following standard Philippine dairy practices.

Clinical diagnostics made on differing breeds must align with their respective production systems. In the Philippines, a one-size-fits-all approach is scientifically flawed. According to Abramowicz et al. (2019)(Abramowicz et al., 2019), accurate clinical diagnosis in bovine medicine accounts for the specificity of animal nutrition, genetics, and herd management. These requisite actions for localized, system-specific data are further magnified by recent revelations in neighboring Southeast Asian tropical production systems. Nguyen & Bui in 2026(Nguyen & Bui, 2026) confirmed that the establishment of age-specific hematological baselines for Holstein-Friesian cattle proved essential for effective diagnosis and health assessment. The PN cattle require a threshold to validate their adaptive polycythemia-leaning traits. Temperate and crossbred stocks equivalently warrant RIs that mirror their specific physiological realities. Therefore, designing environment-specific RIs for native and crossbred cattle is not merely an academic exercise but a clinical obligation to prevent the misdiagnosis of healthy native stock from vulnerable crossbreds.

Beyond the diagnostics, this study also unraveled hemodynamic mechanisms notably promoting tropical resilience. Correlation analysis revealed that the PN group possesses an active viscosity clamp. This is supported by a highly significant negative correlation between RBC and MCV. The uncoupling averts cardiac strain in the animals during heat stress(Morshedi et al., 2022) by increasing the oxygen-carrying capacity while maintaining blood viscosity. It appears that indigenous cattle vigorously reduce cell size in response to erythrocytosis to uphold homeostasis.

In contrast, the HxJ group lacks this homeostatic control. The crossbred showed a strong positive coupling between RBC and PCV variables without appropriate regulation of cell size reduction. The HxJ group has a high prevalence of regenerative anemia with microcytosis, as indicated by anemia scores. Results suggest that HxJ cattle are suffering from compensated hemolysis. Rajashekaraiah et al. (2023)(Rajashekaraiah et al., 2023) narrated that the process begins when the chronic thermal load seemingly accelerates the accumulation of reactive oxygen species (ROS) within the cell’s membrane. This consequently shortens the lifespan of circulating red blood cells. The cascade triggers a compensatory erythropoietic response from the bone marrow. At the same time, chronic thermal load and oxidative stress impair cell maturation. This limits the cell volume resulting in the observed microcytosis. Compensated hemolysis thereby diverts energy away from growth and lactation(Brooks et al., 2022) and becomes a significant production constraint. The dichotomy between stable (PN) and high-turnover (HxJ) biological strategies affirms the utility of selected erythroid indices as putative biomarkers of tropical resilience.

The interaction of immunological parameters underscored the cost of this metabolic strain. While both groups supported an N:L inverse relationship, the PN breed remarkably demonstrated pan-myeloid coherence. This is achieved by a significant positive correlation between RBC, N, and PLT levels(Bagath et al., 2019). This conformity suggests that adapted cattle can synchronize oxygen transport and immune surveillance without resource trade-offs(Chauhan et al., 2021). The proposed biological balance aligns with current genomic evidence from Dominguez et al. (2026)(Dominguez et al., 2026) on immune-regulating genes (i.e., MHC complex) in native Visayan cattle. Their study asserted that the breed’s versatile immune function is a genetically inherent trait rather than just temporary environmental acclimation.

The HxJ group plainly missed this synergy. The significantly higher L counts, paired with a lack of PLT correlation in HxJ heifers, exposed an immune baseline alarming from that of the native breed. This system corresponds with Gupta et al.’s (2022)(Gupta et al., 2022) theory on resource allocation, implying that unadapted animals under chronic stress prioritize immediate robustness.

Finally, these hematological insights serve as a rationale for developing composite breeds that can withstand the Philippine climate. While valued for their superior milk production, the data in this present study indicated that HxJ animals are hematologically fragile under a tropical environment. This group is surviving at a physiological deficit induced by compensated hemolysis(Brooks et al., 2022). Conversely, the PN breed possesses specific physiological strengths, such as efficient erythropoiesis, immune stability, and a viscosity-clamp regulatory mechanism, that the HxJ lacks. These findings suggest that future breeding programs should not focus solely on yield but systematically introgress these PN-specific putative resilience traits into dairy lines. Selecting for the identified putative biomarkers, such as high erythroid indices and hemodynamic stability, could effectively reduce the productivity gap caused by environmental stress.

This study confirms that Panay Native and Holstein x Jersey heifers exhibit fundamentally distinct hematological profiles when managed under tropical conditions. The Panay Native cattle display a highly adapted erythropoietic profile, whereas the crossbreds show evidence of physiological strain. Consequently, current temperature-derived reference intervals are diagnostically inaccurate for these populations. Establishing local, environment-specific baselines is essential, and the unique erythropoietic traits of Panay Native stock should be considered as valuable putative biomarkers for breeding climate-resilient tropical dairy composites.