Influence of Prenatal Melatonin on Postnatal Growth and Behavioral Reactivity in Angus Calves

Abstract Melatonin is a versatile hormone with antioxidant and neuromodulatory properties. Maternal melatonin supplementation has been shown to alter calf growth at birth and modulate neurotransmitter levels of fetuses; however, its long-term effects and behavioral outcomes remain unexplored. Our study aims to assess the effects of third-trimester melatonin supplementation on the growth, development, and behavior of Angus beef calves. Pregnant Angus beef cows (n = 26) in late gestation were trained to acquire feed from a Calan gate electronic feeding system and randomly assigned to either the control group (CON) or the melatonin group (MEL). MEL cows received melatonin (200 µg/kg body weight) dissolved in ethanol and applied to supplemental feed, while CON cows received ethanol alone. Treatments were adjusted weekly based on the dam's body weight. Treatments started at approximately day 209 of gestation and continued for 54 days until parturition, calf morphometrics (n = 24), including body weight (BW), heart girth (HG), abdominal girth (AG), hip height (HH), curved-crown rump length (CCR), and ponderal index (BW kg/CCR m3), were collected at birth, 30, 60, 90, 120, 150 and 180 days. Calves were abruptly weaned at approximately 180 days of age. Pre- and post-weaning ADG were determined. Behavioral metrics included chute score(CS), exit score (ES), pen score(PS), and temperament score (TS); were collected on the day of weaning, 5, 10, and 15 days post-weaning. Plasma melatonin concentrations were measured at birth. BW, calf morphometrics, pre- and post-weaning ADG were analyzed with the MIXED procedures in SAS; behavioral metrics and melatonin concentrations were analyzed with either PROC GENMOD or PROC GLIMMIX. All analyses included fixed effects of treatment (TRT), time, and TRT x time interactions. No differences were detected for BW (P ≥ 0.10), CCR (P ≥ 0.30), PI (P ≥ 0.28), HH (P = 0.57), pre-weaning ADG (P ≥ 0.29), CS (P ≥ 0.20), PS (P ≥ 0.09), TS (P ≥ 0.34), or plasma melatonin at birth (P = 0.12). CON calves exhibited greater HG (116.56 ± 1.22 vs. 112.47 ± 1.22; P = 0.02) and AG (127.7 ± 1.55 vs. 122.58 ± 1.55 cm; P = 0.01) compared to MEL calves. A TRT×time tendency for post-weaning ADG (P = 0.06) indicated MEL calves had higher ADG at day 5 (P = 0.01), but not thereafter; time effects were significant (P 0.01). ES showed significant TRT differences (P = 0.03), with CON calves having a 99% probability of ES = 1 (walk), while MEL calves displayed greater variability (65% ES = 1, 22% ES = 2, 11% ES = 3). Melatonin supplementation in late gestation decreased heart and abdominal girths without altering other growth metrics, confirming localized changes, but increased post-weaning behavioral reactivity, suggesting stress response alterations requiring further research.