163. A Preliminary Analysis of Physiological Factors Influencing Fetal Development in Gilts Exposed to Varying Heat Stress Intensities During Early and Late Gestation.

Abstract The study objective was to understand the relationship between the physiological response to gestational heat stress (HS) and measures of sow body condition and fetal development. Twenty-seven nulliparous sows were bred and randomly assigned to early gestation (EG; n = 13; d 0-52 of gestation) or late gestation (LG; n = 14; d 52-105 of gestation) groups. Gilts were assigned to one of four environmental treatments: thermoneutral (TN; n = 7; 17 to 21 °C), mild (MILD; n = 6; 24-26 °C), moderate (MOD; n = 7; 26-32 °C), or severe (SEV; n = 7; 28-36 °C). Respiration rate [RR; breaths per minute (bpm)] was measured daily (0800, 1200, 1600, and 2000 h). Vaginal temperature (Tv) was measured in 15-min intervals using implanted data loggers. Body weight (BW), body condition score (BCS), and back fat depth (BF) were measured weekly. At the conclusion of EG and LG, the reproductive tract, placentas, and fetuses were collected to determine weights, fetal counts and viability, and conceptus loss (corpora lutea-viable fetuses). Data were analyzed using PROC GLIMMIX, with the individual sow or fetus serving as the experimental unit. Pearson’s correlations were done to evaluate associations among sow physiological variables related to sow body condition and fetal development. During EG, RR increased (P < 0.01) in SEV (74±1 bpm) and MOD (75±1 bpm) sows relative to MILD (55±1 bpm) and TN (30±1 bpm) sows but was similar between SEV and MOD sows. In LG, RR was greatest (P < 0.01) in SEV (97±2 bpm), followed by MOD (85±2 bpm), MILD (63±2 bpm), and TN (31±2 bpm) sows. Sow BW in EG was greater (P < 0.01) in SEV (149.05±1.93 kg) and MOD (148.60±1.86 kg) sows versus TN sows (139.95±1.91 kg), but similar between MILD (143.35±2.02 kg), SEV, MOD, and TN sows. Backfat was greater in EG (P < 0.01) for SEV (23.95±0.45 mm) and MOD (23.61±0.37 mm) versus MILD (21.05±0.43 mm) and TN (20.56±0.50 mm) sows, but no differences were observed between SEV and MOD sows. During LG, BF was greater (P < 0.01) in SEV (20.25±0.51 mm) and MOD (19.07±0.58 mm) sows versus TN (17.06±0.50 mm) sows, and greater in SEV versus MILD (17.06±0.50 mm) sows. Viable fetuses were greater (P = 0.02; +2.5 fetuses) in MOD versus SEV and MILD litters in LG. Positive correlations were observed between Tv and BF (P = 0.01; r = 0.67), BCS and conceptus loss (P = 0.01; r = 0.66) in EG sows, and RR and BF (P = 0.04; r = 0.56) in LG sows. Negative correlations were observed between Tv and BW (P = 0.02; r=-0.63), RR and placental weight (P < 0.01; r=-0.73), and BF and fetal weight (P = 0.05; r=-0.54) in LG sows. No differences were observed with any other comparison (P > 0.05). These relationships suggest that increasing heat load may alter partitioning of maternal resources during pregnancy, with implications for fetal development and viability.