title: Growth and hormonal response of heifers to various photoperiods.
abstract: 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.
mesh_terms: D000821:Animal Feed; D000818:Animals; D001835:Body Weight; D002417:Cattle / Q000097:blood / Q000254:growth & development*; D005260:Female; D005938:Glucocorticoids / Q000097:blood; D013006:Growth Hormone / Q000097:blood; D006728:Hormones / Q000097:blood*; D008027:Light; D011388:Prolactin / Q000097:blood
chemical_list: D005938:Glucocorticoids; D006728:Hormones; D011388:Prolactin; D013006:Growth Hormone