Growth, Carcass Composition and Plasma Melatonin in Postpubertal Beef Heifers Fed Melatonin2

Two experiments were conducted to determine if feeding melatonin alters plasma concentrations of melatonin, growth and carcass composition of postpubertal beef heifers exposed to 16 h light (L):8 h dark (D). In Exp. 1, 16 heifers were blocked by initial body weight (318 ± 5.6 kg). Four heifers were killed before starting the melatonin treatment to obtain initial carcass composition. Six heifers received vehicle (95% ethanol) and six were fed melatonin (4 mg/100 kg body weight) daily for 58 d at 1330 to coincide with the middle of the 16-h light period. On d 59 heifers were slaughtered. Melatonin feeding increased the percentage of fat in rib (P<.05) and longissimus muscle (LD; P<.10) and carcass fat accretion 28% (P<,09) but reduced the percentage of protein 8% in rib (P<.05) and carcass protein accretion 30% (P<.09). Other measures in the carcass and body weight gain were not affected (P>.10) by feeding melatonin. Plasma concentrations of melatonin increased (P<01) from 10 to 140 pg/ml within 30 min of feeding melatonin. In Exp. 2, 24 heifers were blocked by initial body weight (348 ± 13.7 kg). Eight heifers were killed initially, eight received vehicle and eight were fed melatonin for 63 d as described in Exp. 1. Melatonin did not influence (P>. 10) body weight gain or any measure in the carcass; however, these heifers were fatter (40.1%) than those in Exp. 1 (30.9%) at the beginning of the experiment. We conclude that feeding melatonin to heifers in the middle of the light period of a 16L:8D photoperiod can increase plasma melatonin and alter carcass composition if melatonin feeding begins before heifers approach maturity with respect to fatness.