| Summary: | The objective of this study was to determine the effect of vaginal temperature on levels of physical activity expressed by lactating Holstein cows following induced estrus. Lactating Holstein cows (n = 641; 41.5 9.4 kg milk/d) were fitted with a leg-mounted pedometer resulting in 843 evaluated activity episodes of estrus. Vaginal temperature was monitored using thermometers, attached to an intravaginal device that recorded vaginal temperature every 10 min for 3 d. Ambient temperature and relative humidity were monitored using an external thermometer placed in the center of each pen. Milk production and BCS were collected at the time of thermometer insertion. All statistical analysis was performed in R. Heat stress (HS) was calculated based on the percentage of time the cow spent with a vaginal temperature ≥39.1°C (PCT39) 9-11 d prior to Timed Artificial Insemination (TAI), and was classified as high or low (median: 22.9%). The mean vaginal temperature was 38.9 ± 0.2°C, whereas the mean maximum and minimum vaginal temperatures were 39.7 ± 0.5°C and 38.0 ± 0.8°C, respectively, with an average amplitude 1.71 ± 0.9°C. Mean relative increase (RI) of steps/hr at estrus was 237.0 ± 160 %. Animals with low BCS were associated with lower RI compared to cows with medium BCS (260.31 ± 17.45% vs. 296.42 ± 6.62%). Lower temperature and humidity (THI) values (≤ 65) were associated with greater RI compared with medium (> 65 - < 70) and high conditions (≥ 70). There was no association between PCT39 on RI. Cows who displayed greater RI at estrus had stronger associations with pregnancy per artificial insemination (P/AI) compared with low RI (27% vs. 20%) and no RI (27% vs. 12%). An interaction was observed between PCT39 and THI on P/AI, where a subpopulation of cows with high PCT39 were associated with decreased P/AI under high THI conditions, but no differences in P/AI were observed for high PCT39 cows under medium and low THI conditions (13% vs. 24% vs. 26%). Future research should aim to refine variables related to hyperthermia as well as understanding the effects of body temperature on estrus expression and fertility. === Land and Food Systems, Faculty of === Graduate
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