Melatonin suppression of mammary growth in heifers.

The objective of this study was to determine if melatonin fed in the middle of a long day (16L:8D) reduces mammary parenchymal growth and reduces serum prolactin in prepubertal heifers, thereby mimicking the effects of a short photoperiod. Sixteen prepubertal Holstein heifers were maintained under natural May to August environmental conditions of Michigan plus supplemental lighting to provide a photoperiod of 16L:8D (lights-on 0600-2200 h). At the midpoint of each day, 8 animals were individually fed melatonin (4 mg/100 kg body weight) and 8 were individually fed vehicle (95% ethanol) in 200 g of a grain concentrate mixture. Blood samples were collected at 1-h intervals for 25 h beginning on Day 67. On Day 70 or 72 heifers were slaughtered. No differences were found in body weight gain between melatonin- and vehicle-fed animals. Parenchyma of mammary glands from melatonin-fed heifers had a lower content (24%) and concentration (17%) of deoxyribonucleic acid but a greater concentration of triglyceride (24%) than that of controls. Mean serum prolactin concentration was 27% lower in melatonin-fed animals. In both groups, serum concentrations of prolactin varied throughout the day, with greatest values occurring between 1100 and 1800 h in positive association with changes in ambient temperature. We conclude that melatonin orally administered to prepubertal heifers reduced mammary parenchymal growth and concentration of prolactin in serum. The data support the hypothesis that melatonin mimics photoperiodic effects on mammary growth and prolactin secretion in cattle.