Dietary betaine supplementation improves oxidative status and ameliorates the negative effects of heat stress in lambs

Heat stress challenges livestock welfare, production and sustainability and is expected to intensify with climate change. Betaine has been shown to alleviate heat stress, but its efficacy under sustained heat load with restricted nocturnal cooling remains unexplored. This study is among the first to evaluate dietary betaine when warm nights prevent thermal recovery. Forty-eight crossbred [(Merino × Border Leicester) × Dorset] lambs (6–10 months) were assigned to a 2 × 2 factorial design: thermoneutral (TN: 18–21 °C, 45–55% relative humidity (RH)) or heat stress (HS: 28–38 °C, 40–60% RH daytime, 26–28 °C and 45–55% RH overnight) for 14 days, with a control (CON) or betaine (BET; 2 g/day, previously shown effective in heat-stressed sheep) diets. Rectal temperature (RT) and respiration rate (RR) were measured thrice daily; blood and muscle samples were collected for plasma metabolites and antioxidant enzyme activities. Under HS, BET lambs had lower RT at 0900h (39.7 vs. 39.6 °C; p = 0.014), and 1300h (40.2 vs. 40.1°C; p = 0.043), and lower RR at 1300h (194 vs. 181 bpm; p = 0.013) compared with CON lambs. Plasma cortisol was lower in BET lambs on day 7 (20.3 vs. 15.8 ng/mL; p = 0.016). Muscle glutathione peroxidase (GPX) (54.1 vs. 75.6 nmol/min/mg of protein; p = 0.047), and catalase (CAT) (12.4 vs. 33.2 nmol/min/mg of protein; p = 0.02) were higher with BET under HS. Betaine ameliorated physiological stress and improved antioxidant capacity in lambs under sustained heat stress with elevated night-time temperatures, suggesting a cost-effective strategy to safeguard lamb welfare and productivity.