Growth and hormonal response of heifers to various photoperiods.

Body growth and dry matter intake were measured in three groups of 16 Holstein heifers exposed to (1) natural, (2) 16 hr light: 8 hr dark (16L:8D) or (3) 24L:0D photoperiods for November 11 to March 10. Average daily gain (ADG) of heifers subjected to 16L:8D was 11 (P less than .05) and 17% (P less than .02) greater than ADG of animals receiving 24L:0D or natural photoperiod, respectively. Heifers subjected to 16L:8D consumed 6.9 and 8.3% more (P less than .01) dry matter per day than did animals exposed to 24L:0D or natural photoperiod, respectively. Yet feed to gain ratio of heifers exposed to 16L:8D was lower than that of heifers exposed to 24L:0D or natural photoperiod. Serum prolactin (PRL) and growth hormone (GH) were measured in six heifers in each treatment group at 30-min intervals, and serum total glucocorticoids were measured at 2-hr intervals over a 6-hr period on December 26, February 6 and March 7. Concentrations of serum PRl remained low (less than 3 ng/ml) regardless of photoperiod treatment; this would be expected, however, because mean ambient temperatures were below 0 C on all sampling days. Serum GH and total glucocorticoids were likewise unaffected by photoperiod treatment. This study demonstrates that an 8-hr period of darkness is necessary to achieve increases in weight gain when Holstein heifers are supplemented with light during the autumn-winter season in Michigan, but the mechanisms whereby photoperiod affects body growth remain unknown.