Milk yield, feed intake, prolactin, growth hormone, and glucocorticoid response of cows to supplemented light.

Milk production and dry matter intake of 21 cows subjected to 16 h of fluorescent light and 8 h dark and of 21 cows subjected to natural light 9 to 12 h daily between October 25 and March 14 were measured beginning in early (37 to 74 days postpartum) and late (94 to 204 days postpartum) lactation. Cows that received 16 h of fluorescent light produced 6.7% (1.4 kg) more milk per day (adjusted for parity and pretreatment production) than cows exposed to natural photoperiods. Increases of milk production with 16 h of fluorescent light were similar for early and late lactation. Photoperiod did not alter percent of fat in milk. Dry matter intake increased 6.1% for cows in 16 h of light, and this increase could account for increased milk yields. Basal prolactin in serum and that released by thyrotropin releasing hormone were 1.5 to 1.8 times greater for cows exposed to 16 h of light than for cows in 9 to 12 h of natural light daily. Photoperiod did not affect release of prolactin by milking. Cold ambient temperatures reduced basal prolactin and prolactin released by thyrotropin releasing hormone but had no effect on concentrations of growth hormone or glucocorticoids. Compared with cows in late lactation, cows in early lactation released 2.4 times more prolactin after milking, but they released similar amounts of prolactin after thyrotropin releasing hormone. Photoperiod did not affect concentrations of growth hormone or glucocorticoids in blood sera.