Beginning at 128 d of pregnancy, Holstein heifers were exposed to 16 h light, 8 h dark (long days; n = 10) or 8 h light, 16 h dark (short days; n = 10) until 35 d before calving when they were killed. Photoperiod had no effect on weight or proportion of extraparenchymal fat or parenchyma in the mammary gland or on amount of fat, DNA, or RNA in mammary parenchyma. Serum prolactin concentration was 1.7-fold greater under long than under short days. Concentration of melatonin in serum was 2.4-fold greater during dark than light periods. Duration of elevated serum melatonin concentration in the dark period was longer in heifers given short days, but magnitude of this increase was lower than that in heifers exposed to long days. In a second experiment, peak amplitude and area of the periparturient surge of serum concentration of prolactin were 1.8-fold and 1.7-fold greater, respectively, in six Holstein heifers exposed to long days than in six heifers exposed to short days. We conclude that photoperiod had no effect on mammary development during pregnancy, but relative to short days, long days increased serum concentration of prolactin during pregnancy, including the period of the periparturient surge of prolactin.
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