Effect of ambient temperature on duration of gestation and changes in rumen temperature at parturition and estrus in fall calving beef cow

Scope and method of study. Effect of ambient temperature on duration of gestation was evaluated in 24 fall calving Angus x Hereford cows during 2 yr. Cows were blocked according to age and AI to calve in August or October in yr 1. Cows were blocked according to age and AI to calve in mid August, late August, September, or October in yr 2. Plasma samples were taken for 2 wk prior to calving and concentrations of progesterone, cortisol and estradiol were assayed by RIA. Rumen temperature was recorded hourly at parturition and estrus in yr 2.;Findings and conclusions. Cows exposed to elevated ambient temperatures during the last 2 wk of gestation have a shorter (P < 0.05) duration of gestation compared with cows not exposed to elevated ambient temperatures at this time. Concentrations of progesterone and estradiol in plasma are not altered by elevated ambient temperature. Concentrations of cortisol in plasma may be altered by exposure to elevated ambient temperatures. Rumen temperature decreases ≥ 0.3 °C the day before parturition in fall calving beef cows. Rumen temperature increases ≥ 0.3 °C at onset of estrus compared to the same daily hours on the day before onset of estrus. The change in rumen temperature can be used to predict parturition and estrus in beef cows.