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Evaluation of physiological indicators of heat stress and their association with seasonal infertility in first parity sows
Understanding the biological mechanisms contributing to seasonal infertility in swine is essential for developing mitigation strategies to improve reproductive efficiency. Seasonal infertility is associated with increased wean-to-estrus-interval (WEI) and reduced reproductive efficiency. Study objectives were conducted to retrospectively analyze the relationship between physiological responses to heat stress during the WEI and phenotypes associated with seasonal infertility. Rectal temperature (Tr), skin temperature (Ts), and respiratory rate (RR) were collected five times daily for seven days following weaning, during two 4-week periods of heat detection and insemination associated with peak reproductive performance (Spring; n=424 P1 sows) and the nadir of reproductive performance (Summer; n=445 P1 sows). Plasma was collected on d 1 and 3 during the WEI and used to measure circulating insulin and lipopolysaccharide binding protein (LBP) levels on a subset of 80 sows representing three reproductive outcomes for each season: farrowed (serviced within 7 d of weaning), not farrowed (serviced within 7 d of weaning), and sows with a WEI greater than 15d (>15WEI). Data were analyzed using SAS, either PROC MIXED for reproductive parameters or PROC GLIMMIX for farrowing rate. Compared to Spring, a substantial reduction in sows expressing estrus by 7 d post weaning was observed in the Summer (89.1 vs. 79.5 ± 1.7%, P < 0.01), and among these sows, farrowing rate was decreased in the Summer (91.1 vs. 82.3 ± 1.8%, P < 0.01). Still, of all litters produced total born, born alive, still-born, and mummies per litter were not different between seasons (P > 0.10), even though Summer-weaned sows tended to have an increased WEI (P = 0.06). The relationship between reproductive, physiological, and environmental parameters were analyzed using PROC CORR. No effect of season was detected for Tr and RR, although Ts was increased in Spring-weaned sows (P < 0.01). Correlations across season were identified between WEI and Tr (r = 0.07, P = 0.03), Ts (r = -0.12, P < 0.01), and RR (r = -0.12, P< 0.01). Insulin and LBP were similar across seasons and were not different by reproductive status or day of WEI. These data indicate that thermal indices of heat stress during the WEI do not fully explain decreased reproductive efficiency observed during seasonal infertility in P1 sows. Future investigations are needed understand the underlying mechanisms of heat stress during the WEI transition. This project was supported by the National Pork Board and Iowa Pork Producers Association.
Keywords: heat stress, infertility, swine