PS3-15. Physiological Responses and Thermoregulatory Mechanisms of Beef Cows Under Heat-stress Conditions Post-breeding.

Abstract Reproductive performance of cows exposed to high environmental temperatures can be compromised. Therefore, our objective was to determine the effects of pasture microclimate on bovine physiological and reproductive parameters under heat-stress conditions. Twenty-four cows were synchronized using a 7-day CO-synch+CIDR protocol and received timed artificial insemination followed by an intravaginal temperature datalogger on d 0. After the breeding, cows were stratified by body weight and body condition score and assigned to 10 paddocks housing 2 to 3 cows. Paddocks either received 1) HEAT: unshaded open pasture (no trees) or 2) COOL: integrated silvopasture. Cows in the heat treatment had access to artificial shade until d 7, and from d 8 to d 16, artificial shades were removed. Temperature-humidity index (THI), cow's body temperature, and respiration rate (breaths per minute) were measured daily (d 0 to d 14). On d 14, the corpus luteum area was measured, and on d 16, cows received uterine flushing for conceptus collection. Data were analyzed by repeated measures ANOVA for the fixed effects of treatment, day, hour, and their interactions. There was no effect of treatment on the temperature-humidity index (THI, P = 0.94). In contrast, there was an interaction between day and hour (P < 0.0001), as THI increased as the day advanced, peaking during the afternoon and ranging from 60.85 to 87.47. Despite no treatment effect in THI, there was a significant interaction between treatment × day × hour (P < 0.0001) on cow's body temperature and respiration rate, as body temperatures were greater in cows exposed to HEAT during the warmest hours of the day, as well as the respiration rate. Body temperature differences between COOL and HEAT ranged from 0.5 to 2 ºC. There were no effects of treatment on corpus luteum area on d 14 (P = 0.27). Ongoing analyses focus on the metabolite concentration in the uterine luminal fluid and concentration of progesterone on d 14 and gene expression on conceptus and uterine tissues on d 16, as well as the concentration of interferon tau on uterine flushing and progesterone. Altogether, these preliminary data show that, under conditions of elevated body temperature, heat-dissipation mechanisms, such as increased respiration rates, were enhanced as a homeostatic response to maintain core body temperature within physiological limits.