Effect of elevated ambient temperature at parturition on duration of gestation, ruminal temperature, and endocrine...

Fall-calving Angus cows were used to evaluate the effect of ambient temperature on duration of gestation. In Exp. 1, cows were AI and calved in August (n = 14) or October (n = 10). Cows grazed native prairie pasture in Oklahoma and had a BCS of 6.0 ± 0.5 (1 = emaciated, and 9 = obese) at parturition. Commencing 2 wk before the expected calving date, blood samples were taken from the coccygeal vein every 2 to 3 d until calving. Cows that calved in August tended to have shorter gestations (P = 0.07) compared with cows that calved in October. Maximum daily ambient temperature during the last 14 d of gestation was greater for August-calving cows (P < 0.001) compared with October cows. Concentrations of cortisol in plasma during the last 4 d of gestation were greater in cows that calved in August (P < 0.04) compared with cows that calved in October. In Exp. 2, cows were AI and calved in either mid-August (n = 7), late-August (n = 6), September (n = 6), or October (n = 8) to evaluate the effects of elevated ambient temperature on duration of gestation, ruminal temperature at parturition, and plasma cortisol, progesterone, and estradiol. Temperature boluses (SmartStock, LLC, Pawnee, OK) programmed to transmit temperature every hour were place in the rumen at 255 d of gestation. Cows grazed native prairie pasture in Oklahoma and had a BCS of 6.5 ± 0.4 at calving. Maximum ambient temperatures during d 263 to 273 of gestation were influenced by month of calving × day (P < 0.001). Duration of gestation was shorter for mid-August cows (P < 0.05) compared with October cows, but did not differ compared with late-August (P = 0.29) and September (P = 0.50) cows. Ruminal temperature during the 4 d before calving was not influenced by month of calving (P = 0.76). Ruminal temperature was decreased during the 24 h before parturition for cows in all months (P < 0.01) compared with 2 to 4 d before parturition. Concentrations of cortisol in plasma during d 271 to 276 of gestation were less (P < 0.05) for late-August compared with cows that calved during the other months. Concentrations of progesterone were greater during 7 d before parturition in October compared with cows that calved in September. Estradiol in plasma of cows during late gestation was not affected by month of calving (P = 0.76). Exposure of beef cows to elevated ambient temperature resulted in shorter gestations. Ruminal temperature in cows decreased ≥ 0.3°C the day before parturition.