title: Response of body weight and clearance and secretion rates of growth hormone to photoperiod in Holstein heifers.
abstract: Effects of photoperiod on growth, eating patterns and clearance rate, secretion rate and half-life of growth hormone in serum were assessed in Holstein heifers. Sixty-four prepubertal heifers (avg body wt 102 kg) were blocked by weight into four groups of 16. Within block, each animal was randomly assigned to one of four photoperiod treatments: 16 h of cool-white fluorescent light:8 h dark (CW-16L:8D), CW-8L:16D, CW-6L:8D:2L:8D or 16 h of VitaLite fluorescent light:8D (V-16L:8D). Animals were fed ad libitum a complete mixed diet formulated for heifers to gain approximately .9 kg/d. Average daily body weight gains were .93, .96, .98 and 1.0 kg/d during 112 d of exposure to CW-8L:16D, CW-6L:8D:2L:8D, CW-16L:8D and V-16L:8D, respectively. Heifers exposed to V16L:8D and CW-16L:8D gained more weight (112 and 110 kg; P less than .05 and P less than .08, respectively) than heifers exposed to CW-8L:16D (104 kg). Body weight gain was not different (P greater than .10) between heifers exposed to CW-6L:8D:2L:8D (108 kg) or CW-8L:16D, or between heifers exposed to CW-16L:8D or V-16L:8D. Total number of eating events in 30 h and percentage of total events in the lighted period were 549 and 96% and 523 and 50% in heifers exposed to CW-16L:8D and CW-8L:16D, respectively. Clearance rates (620 vs 636 ml/min), secretion rates (3.0 vs 3.8 micrograms/min) and half lives (17.2 vs 17.9 min) of growth hormone in serum were similar in heifers exposed to CW-16L:8D and CW-8L:16D.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D000818:Animals; D001835:Body Weight; D002417:Cattle / Q000502:physiology*; D005247:Feeding Behavior / Q000502:physiology*; D005260:Female; D013006:Growth Hormone / Q000378:metabolism*; D008027:Light
chemical_list: D013006:Growth Hormone