The Effects of in Utero Heat Stress on Fasting Heat Production in Growing Pigs

The study objective was to determine the effects of in utero heat stress (IUHS) on postnatal fasting heat production (FHP) in growing pigs. Based on our previous observation of increased postnatal core body temperature ‘set-point’ in IUHS pigs, we hypothesized that FHP would be greater during postnatal life. Pregnant gilts were exposed to thermoneutral (TN; n=4; cyclical 15 to 20⁰C) or heat stress (HS; n=4; cyclical 27 to 37⁰C) conditions from d 30 to 60 of gestation. At weaning, two median weight male pigs (1 barrow and 1 boar) were selected from each gilt [n=8 in utero thermoneutral (IUTN) and 8 IUHS pigs], and then housed in TN conditions (22.3±0.1⁰C). Pigs were acclimated to an indirect calorimeter one week prior to testing. At 84 d of age, pigs were fasted for 24 h and placed individually into an indirect calorimeter with ad libitum water access for an additional 24 h testing period (1 h acclimation + 23 h air collection). Body weight (BW) was determined before testing and was similar for all pigs (P=0.67; 30.8±0.3 kg BW). Air input and output were collected using gas bags in three intervals (0900-1700 h, 1700-0000 h, 0000-0800 h), and then analyzed for CO₂ and O₂ concentrations to calculate FHP and the respiratory quotient (RQ). During testing, pigs were video recorded to account for activity differences and this was included as a covariate in the final FHP and RQ analyses. Data were analyzed using PROC MIXED in SAS 9.4. No boar versus barrow differences were detected with any analysis. Overall, no differences (P=0.25) in FHP were detected between IUHS (149.1±10.6 kcal/h) and IUTN (137.4±8.8 kcal/h) pigs. However, FHP/kg BW was increased (P=0.03; 14.6%) in IUHS compared to IUTN pigs, and FHP/kg metabolic BW was increased (P=0.05; 9.7%) in IUHS versus IUTN pigs. A time effect was observed where FHP was increased (P<0.01; 19.8%) from 0900-1700 h compared to 1700-0000 h and 0000-0800h. Respiratory quotient was not effected by treatment, but RQ was increased (P<0.01; 15.3%) from 1700-0000 h compared to 0900-1700 h and 0000-0800 h. No other differences were detected for FHP and RQ. Animal activity was not different (P>0.19) with regard to time of day, treatment, or their interaction. In summary, IUHS pigs had increased postnatal FHP compared to IUTN pigs and this has implications towards future production efficiency in pigs gestated during hot summer months.