Day–night variation of thermoregulatory responses of heifers exposed to high environmental temperatures

Abstract

In order to assess relative thermal sensitivity between the day and the night, vaginal temperature (T_v), heat production (HP), heart rate (HR), respiration rate (RR), skin surface temperatures (from which mean skin temperature (T_s) was calculated) and standing time were measured at environmental temperatures (T_e) of 23, 28, 33 and 38°C during the day (11.00–15.00 h) and during the night (23.00–03.00 h) using four Holstein heifers. Both T_v and mean body temperature (T_b) were greater during the night than during the day, increased with increased T_e, and the rate of increase of both T_v and T_b with increased T_e was greater during the night (P<0·05). Estimated mean HP was similar during the night and during the day, and HP did not increase with increased T_e. Respiration rate was greater but not significantly different at night compared to during the day, and increased with increased T_e. Mean skin temperature was similar between day and night, and increased with increased T_e. The amount of time engaged in standing activity was greater but not significantly different during the night than during the day and standing activity increased with increased T_e. The results suggest that thermal sensitivity is lower during the night than during the day, and consequently the greater night responses of T_v and T_b, over day responses, are a requirement for the maintenance of heat balance.