Reproducibility of Heat Stress Susceptibility and Future Reproductive Success During Heat Stress in Pigs

Abstract Text: Identifying factors associated with susceptibility or resistance to heat stress (HS) is likely a prerequisite to developing mitigation strategies to improve pig reproductive efficiency. Study objectives were to determine if the HS response early in life predicts future reproductive success during HS. During phase I of the study, pre-pubertal gilts (n=235; 78±1.2 kg BW) were exposed to a TN period (24 h; 22±0.5°C, 62±13% RH; fed ad libitum) followed by a HS period (24 h; 30±1°C, 49±8% RH; fed ad libitum). Respiration rate (RR), skin temperature (T_S), and rectal temperature (T_R) were recorded at 16 regularly scheduled time points within each experimental period. Body weights (BW) and daily feed intake (FI) were also recorded during the experiment. Interestingly, HS T_R between gilts did not explain the variation in the decrease in FI during the acute phase of HS (R²<0.01, P<0.05).  Also, a low proportion of variability in the severity of BW loss during HS could be explained by T_R (R²=0.03, P<0.05) or FI (R²=0.09, P<0.01). Gilts deemed the most tolerant (T; n=48) and susceptible (S; n=48), as defined by their ability to maintain a minimal T_R during HS, were subjected to phase II after puberty.  During phase II, gilts were fed Matrix^® for 14 d in TN conditions (18°C; limit fed 2.7 kg/d). Following synchronization, estrus detection and artificial insemination were conducted over a period of 9 d during cyclical and progressive HS conditions (21 to 35°C for 9 d). Gilts were slaughtered after 43-48 d of gestation in TN conditions (21°C). Fetal weight and crown-rump length were increased by 7.4 and 2.8%, respectively, in gilts classified as S compared to T (P<0.01). Fetal count, corpus luteum count and size, and embryo survivability were not correlated with post-pubertal HS T_R whereas fetal weight (R²=0.07) and crown-rump length (R²=0.07) were positively correlated with HS T_R (P<0.05). Positive correlations existed between pre-pubertal and post-pubertal HS T_R (R²=0.40, P<0.05). Interestingly, pre-pubertal TN T_R was also correlated with post-pubertal HS T_R (R²=0.30, P<0.01) suggesting that pre-pubertal thermoregulatory responses to HS, despite variable between animals, were predictive of future responses to HS. Importantly, the thermoregulatory response (T_R, T_S, RR) and production response (decreased FI and BW) to HS appear to be only marginally related, indicating that production losses during HS are independent from the thermoregulatory response during HS. This project was supported by the National Pork Board and the Iowa Pork Producers Association.

Keywords: pigs, heat stress, reproduction