Effect of prepartum photoperiod and melatonin feeding on milk production and prolactin concentration in dairy heifers and cows

Abstract

Holstein multiparous cows (n=29) and primiparous heifers (n=32) calving over a 1-yr period were subjected to photoperiod-melatonin treatments according to a 2×3 factorial design. Starting 8wk before expected calving, all animals were subjected to 1 of the following treatments: 8h of light and 16h of dark (8L:16D), 16h of light and 8h of dark (16L:8D), or 16L:8D plus melatonin feeding (16L:8D-melatonin). Each day at 1355h, the animals in the melatonin treatment received orally a gelatin capsule containing 25mg of melatonin. The treatments ended at calving, when the animals were moved to the lactation barn; all animals were then subjected to about 16h of light per day. At the beginning and end of the treatment period before calving, blood samples were taken from 6heifers and 6cows through a jugular cannula for 24h at 30-min intervals to monitor serum melatonin and prolactin concentrations. Milk production in the heifers was not affected by the photoperiod treatments. Early-lactation milk production was higher in the cows exposed to the short-day photoperiod than in those exposed to a long-day photoperiod (16L:8D and 16L:8D-melatonin), with averages of 36.7±0.9, 33.1±0.8, and 34.1±0.9kg/d for 8L:16D, 16L:8D, and 16L:8D-melatonin, respectively. Photoperiod had no effect on late-lactation milk production in the cows. During lactation, the dry matter intake of heifers was not affected by the treatments, but dry matter intake of the cows exposed to a short-day photoperiod was greater than that of the cows exposed to a long-day photoperiod. Feed efficiency of heifers was improved by short-day photoperiod. During the treatment period, prolactin concentration was lower in the animals exposed to a short-day photoperiod than in those exposed to a long-day photoperiod, was lower with the 16L:8D-melatonin treatment than with the 16L:8D treatment, and tended to be lower with the 8L:16D treatment than with the 16L:8D-melatonin treatment, with averages of 3.5±0.8, 9.9±0.8, and 6.0±0.8ng/mL for 8L:16D, 16L:8D, and 16L:8D-melatonin, respectively. In early lactation, prolactin concentration was lower in the heifers exposed to the 16L:8D photoperiod during the dry period than in those exposed to the 8L:16D photoperiod or fed melatonin. In conclusion, a short-day photoperiod during the dry period transiently increases milk production of cows and the feed efficiency of heifers in the following lactation. However, melatonin cannot be used to mimic a short-day photoperiod during the dry period.