314 Award Talk: Is mitigating heat stress in beef cattle adapted to tropical/subtropical environments necessary?

Abstract Approximately half of the beef cattle in the United States are in southern states, where Bos indicus influence and heat stress predominate. Preliminary data demonstrated the opposite responses to heat stress in Bos indicus-influenced vs. Bos taurus cattle, limiting the implications of the existing literature. Thus, there is a critical need to determine the effects of heat stress mitigation on performance of grazing Bos indicus-influenced beef cattle. The objective of this presentation is to discuss how heat stress mitigation strategies impact the physiology, and performance of Bos indicus-influenced cow-calf pairs. Study 1 evaluated the impacts of access to artificial shade during pre- and postpartum periods on body temperature, body condition score (BCS), and physiological responses of heat-stressed Bos indicus-influenced beef heifers, and its consequence to offspring growth and immunity. Removing maternal access to shade increased respiratory rates and intravaginal temperature (P ≤ 0.05) and reduced BCS (P ≤ 0.01) of heifers from pre-calving to breeding. Calves born from dams with no access to shade were lighter (P = 0.05) at birth but had greater humoral immune response before vaccination (P = 0.02) and body weight (BW) gain (P = 0.05) during a 60-day drylot period. Study 2 evaluated the long-term effects of access to artificial shade during late gestation and post-weaning (2 × 2 factorial design) on performance and metabolism of multiparous Bos indicus-influenced beef cows and their offspring. Removing access to shade during late gestation increased respiration rates and intravaginal temperature (P ≤ 0.05) and led to reduced BCS (P ≤ 0.01) of cows from pre-calving to breeding. Heifers born from cows without access to shade had greater average daily gain (ADG) during the first 43 days post-weaning (P = 0.01), whereas overall post-weaning ADG did not differ (P = 0.71) between gestational treatments. Heifers born from cows with no access to shade had reduced (P ≤ 0.01) conception percentage to AI (26 vs. 53%) but similar final pregnancy percentage (80 vs. 83%) compared to cohorts born from cows provided access to shade during late gestation. Heifers provided no access to shade during post-weaning had lower (P = 0.01) during the first 43 days, but greater ADG (P = 0.04) in the subsequent 109 days, leading to similar (P ≥ 0.39) overall ADG compared with cohorts provided access to shade. Post-weaning treatment did not (P ≥ 0.39) impact any heifer reproductive variable. These findings highlight how heat stress mitigation strategies during gestation can modulate the physiology, growth, immune response, and reproduction of Bos indicus-influenced beef offspring.