Commonly, the number of piglets born alive (NBA) in the second parity is higher than that in the first parity (Gruhot et al., 2017)(M. et al., 2022)(L. et al., 2022)(J. et al., 2024)(Nam et al., 2024). However, many sows in parity 2 have an NBA equal to or lower than those in parity 1 (L. et al., 2018)(L. et al., 2024), a condition known as second litter syndrome (SLS) (Segura Correa et al., 2013). Previous studies have reported high incidences of SLS in many countries. In the United States, a study documented that 54% of sows acquired SLS (Morrow et al., 1992), while research in Japan showed that 49.5% of sows acquired SLS (Saito et al., 2010). Similar values have also been reported in Mexico (Segura-Correa et al., 2014); (Segura Correa et al., 2013) and Brazil (Rabelo et al., 2016), ranging from 48.1% to 55.8%. In Europe, the incidence of SLS varied significantly, ranging from 33.3% in the Netherlands (Sell-Kubiak et al., 2021) to 56.6% in Spain (Sanz-Fernández et al., 2022). Recently, research conducted under tropical conditions has demonstrated a high incidence of 47.8% in Vietnam (Nam et al., 2024). These findings suggest that SLS remains a common reproductive issue in commercial sow herds worldwide.

SLS has been shown to be negatively associated with sow productivity. Compared to sows without SLS, sows with SLS had 0.3 fewer NBA in the first two parities (Saito et al., 2010). Another study also reported that SLS reduced NBA in the first two parities by 1.34 piglets, and by 0.11-0.47 piglets in later parities (Sell-Kubiak et al., 2021). In a recent study, we found that SLS reduced the cumulative NBA in the first 2 parities by 0.8 piglets, although no significant difference was detected when considering the total NBA from parity one to five (Nam et al., 2024). However, sows with a small litter size in the second parity had a higher odds of having a small litter size in subsequent parities and were associated with an elevated culling risk (Hoving et al., 2010). These findings highlight the economic and biological significance of preventing SLS from maintaining herd productivity and profitability.

Previous studies have found several factors associ- ated with SLS in sows. NBA in the first parity (NBA1) has a strong association with SLS. Increased NBA1 el- evated the likelihood of SLS (Morrow et al., 1992); (Segura Correa et al., 2013); (Segura-Correa et al., 2014); (Sanz-Fernández et al., 2022). The weaning-to-service interval (WSI) is another significant determinant. A shorter WSI was associated with an increased risk of SLS (Morrow et al., 1992); (Segura Correa et al., 2013); (Segura-Correa et al., 2014). However, the role of seasons is less consistent. While (Segura Correa et al., 2013) reported higher odds during rainy (OR=1.20) and dry (OR=1.24) seasons compared to the windy season, later research found no significant seasonal effect (Segura-Correa et al., 2014); (Sell-Kubiak et al., 2021). Lactation length also appeared relevant, as longer lactations were associated with a reduced risk of SLS in European herds (Sell-Kubiak et al., 2021), and distinct lactation lengths were linked to changes in the difference in NBA between parity 1 and 2 (Sanz-Fernández et al., 2022). Additionally, sows first farrowed at an average of 337 days lost nearly twice as many piglets at parity 2 compared to those farrowed at an average of 474 days (Sanz-Fernández et al., 2022).

Taken together, these findings suggest that SLS involves both physiological and management-related factors in several production systems. However, most of these studies were conducted in temperate regions. Therefore, there is a need to evaluate these factors under tropical conditions, where different environmental and managerial situations may exert distinct impacts on SLS occurrence. To address this knowledge gap, the present study aimed to evaluate the influence of age at first artificial insemination, age at first farrowing, number of piglets born alive at first parity, litter birth weight at first parity, number of piglets weaned, litter weaning weight, weaning-to-service interval, age at insemination after weaning, month at insemination after weaning, and month of second farrowing on the incidence of SLS in a commercial sow herd under tropical conditions in Vietnam.

The incidence of SLS in the present study was 56.8%, which is comparable to the 56.6% reported in Spain (Sanz-Fernández et al., 2022). However, it was higher than 48.1% in Mexico (Segura-Correa et al., 2014), 47.8% in Vietnam (Nam et al., 2024), and 33.3% in the Netherlands (Sell-Kubiak et al., 2021). These differences may reflect variations in genetics, management practices, and environmental conditions across production systems.

The positive association between NBA1 and SLS observed in the present study aligns with previous reports (Morrow et al., 1992); (Segura Correa et al., 2013); (Segura-Correa et al., 2014); (Sanz-Fernández et al., 2022). Specifically, compared with sows that had NBA1<9 piglets, those with NBA1=9-10, 11-12, and >12 piglets had increased odds of SLS, with ORs of 3.6, 9.5, and 33.2, respectively (Segura Correa et al., 2013). NBA has been shown to be positively associated with piglet birth weight (Nam & Sukon, 2020); (Nam & Sukon, 2021); (Lanh & Nam, 2022); (Junior et al., 2023). Furthermore, sows with moderate body condition scores produced larger NBA compared with those with low or high scores (Ajay et al., 2023); (Authement & Knauer, 2023). The associations between NBA and piglet birth through optimizing body condition after weaning and improving piglet birth weight may increase NBA in the second parity and consequently reduce the risk of SLS.

The effect of ASAI on SLS in this study is consistent with (Sanz-Fernández et al., 2022), who showed that sows with an average AFF of 337 days lost twice as many piglets in the second litter compared to those with an average AFF of 474 days. In contrast, (Sell-Kubiak et al., 2021) reported no effect of AFAI on SLS. As primiparous sows are usually not fully mature at the time of their second insemination (Dumniem et al., 2025), a higher ASAI provides additional time for development, which contributes to a greater NBA in the second litter and a lower risk of SLS (Morrow et al., 1992).

This study is among the first to report a negative association between LSW/LWW and SLS in Landrace×Yorkshire sows raised under tropical conditions. Previous studies have shown that smaller LSW improves the quality and survival rates of embryos (Weaver et al., 2024); (Weaver et al., 2024). In contrast, a larger LSW increases lactational weight loss (Guo et al., 2019) and impairs post-weaning follicular development (Liu et al., 2025). These findings suggest that sows producing higher LSW/LWW are more likely to experience greater body reserve depletion, which could impair embryo quality and reduce NBA in the subsequent litter, or increase the risk of SLS. Practically, these findings suggest a need to control LSW and adjust nutritional management to reduce excessive lactational body weight loss. Future research should investigate suitable levels of LSW and nutritional strategies that promote body reserve recovery and maintain reproductive efficiency in primiparous sows reared in tropical environments.

The lowest odds of SLS found in sows with WSI >17 days in the present study are consistent with the findings of (Segura-Correa et al., 2014). Previous research has reported that shorter intervals of WSI, such as WSI <22 days (Morrow et al., 1992), <12 days (Segura Correa et al., 2013), or <21 days (Hoving et al., 2010), increased the risk of SLS. This relationship may be partly due to the longer recovery time available to sows with extended WSI (>17 days), which allows them to regain body reserve and achieve better reproductive performance in the next cycle (Hoving et al., 2010). Nevertheless, the higher odds of SLS observed in sows with WSI of 0-6 days are difficult to explain, as this short interval has been shown to be associated with favorable outcomes such as higher farrowing rates and larger litter sizes compared to longer WSIs (Hoshino & Koketsu, 2008); (Nam & Sukon, 2024).

The effect of season on SLS appears inconsistent, as (Segura Correa et al., 2013) reported a slightly higher risk during the rainy (OR=1.20) and dry seasons (OR=1.24) compared with the windy season, whereas other studies found no significant association between season and SLS (Segura-Correa et al., 2014); (Sell-Kubiak et al., 2021). In this study, the lower incidence of SLS in sows inseminated between May and August and those farrowing between August and December is likely the result of multiple interacting factors. Seasonal variation in NBA2 may partly explain this pattern, as higher NBA in autumn and winter compared with summer and spring has been reported (Piñán et al., 2021), along with increased litter size in winter relative to summer (Chokoe & Siebrits, 2009). In the present study, NBA2 of sows inseminated between May and August (12.7) and those farrowing between August and December (12.6) were higher than those of sows inseminated during September-November (12.4) and farrowing during January-March (12.4), respectively. However, the lower SLS incidence may also be related to their smaller NBA1 (12.5) values compared to the NBA1 values in other groups, which ranged from 12.6 to 12.9. These findings suggest that the seasonal variation in SLS incidence is influenced not only by seasonal changes in NBA2 but also by the baseline established by NBA1 in the first parity. Therefore, breeding schedules should be planned to align farrowings with favorable periods, and special attention should be given to sows with a large NBA1, as they may require additional management strategies, such as optimized nutrition or extended recovery time, to reduce metabolic stress and minimize the likelihood of SLS in subsequent parities.

In the present study, LL did not affect the incidence of SLS, which aligns with the findings of (Segura Correa et al., 2013). In contrast, (Sell-Kubiak et al., 2021) reported that a longer LL decreased the risk of SLS, and (Sanz-Fernández et al., 2022) observed that a very short LL (15.8 days) reduced NBA2 compared with a longer LL (>21.7 days). The discrepancies among the studies could be attributable to variations in genetic background, management practices, and environmental conditions. Based on the present findings, extending LL does not appear to reduce the incidence of SLS.

This study has some limitations. First, because information on body condition score and body weight loss during lactation was not available, it was impossible to assess the impact of energy balance on the risk of SLS. Second, this study did not measure hormonal profiles or indicators of follicular and oocyte quality/quantity, which restricted the ability to explore the underlying physiological mechanisms. These limitations should be taken into account when interpreting the results, highlighting the need for future studies that incorporate both metabolic and reproductive biomarkers.

This study indicated that the incidence of SLS in Landrace × Yorkshire sows under tropical conditions was high. Major risk factors included large NBA1, short WSI, high LSW, and low ASAI, while insemination in May-August and farrowing in August-December reduced the risk. These findings suggest that optimizing recovery intervals, managing litter size at weaning, and adjusting breeding schedules to favorable seasons can help reduce SLS if productivity trade-offs are carefully balanced.