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Growth, Carcass Composition and Plasma Melatonin in Postpubertal Beef Heifers Fed Melatonin^1,2,

Michigan State University³, East Lansing 48824-1225

Abstract

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.

¹ Michigan Agric. Exp. Sta. Journal Article. No. 12364. This research was supported by USPHS Grant HD 17738, USDA Grant 84-CRSR-2-2340, and a MSU-A11 University Research Grant.

² The authors gratefully acknowledge the technical assistance of J. E. VanOver.

³ Dept. of Anim. Sci.