Changing daily exposure of prepubertal bulls from 8 hr of light: 16 hr of dark (8L: 16D) to 16L:8D or 6L:8D:2L8D increased basal secretion of prolactin 418% 6 weeks later. When daily exposure was changed from 8 L 16D (6 weeks) to 6 L 14D:2L2D (6 weeks), basal secretion of prolactin increased only 173%. Among photoperiod exposures, prolactin released into blood after injection of 33 pg/lOO kg body weight of thyrotropin-releasing hormone paralleled the changes described for basal conditions. There was no repeatable diurnal secretory pattern for secretion of prolactin. The data support the hypothesis that cattle possess a photosensitive rhythm for secretion of prolactin. 478

In each of two experiments, 70 crossbred steers were blocked by BW and assigned to initial slaughter groups or to treatments in a 2 x 2 design. In Exp. 1, treatments were 168 d of photoperiod (8 h of light [L]:16 h of dark [D] or 16L:8D) and plane of nutrition (high energy [HPN] or low energy [LPN]). On d -22, 67 and 155, blood was sampled every 20 min for 8 h. Relative to LPN, HPN increased (P less than .01) ADG by 28%, carcass weight by 26% and accretion of carcass fat by 109% and carcass protein by 20%. On d 155, compared with LPN, HPN increased (P less than .01) serum insulin (INS; 1.09 vs .64 ng/ml) and lowered (P less than .05) growth hormone (GH; 2.14 vs 3.70 ng/ml), but prolactin was not affected. Photoperiod did not affect BW gains, carcass composition or serum hormones. In Exp. 2, treatments were 113 d of photoperiod (8L:16D or 16L:8D) and Synovex-S implant (presence [IMP] or absence [NONIMP]). On d 93, blood was sampled every 30 min for 10 h. Relative to NONIMP, IMP increased (P less than .01) ADG by 12% and accretion of carcass protein by 16%. Implants did not affect carcass weight or accretion of fat. Compared with NONIMP, IMP increased (P less than .05) GH (3.16 vs 2.39 ng/ml) and INS (.68 vs .46 ng/ml) but did not affect PRL. Photoperiod did not affect BW gain, carcass composition or serum hormones. We conclude that photoperiod fails to influence growth and carcass composition of steers.

Early temporal changes in concentrations of prolactin (PRL) in serum after a sudden change in photoperiod and daily responsiveness to PRL-releasing and inhibiting factors were investigated in prepubertal Holstein bull calves exposed to different photoperiods. In calves switched from 8-hr light16-hr dark to 16-hr lighk8-hr dark, there was no observable change in the daily pattern of serum concentrations of PRL after 1, 2, or 4 days. On the other hand, in animals switched from 16-hr lighk8-hr dark to 8-hr light:ls-hr dark, there was a consistent increase in serum PRL from 33.4 ng/ml on Day 0 to maximum values of 57.3, 62.7, and 78.9 ng/ml between 14 and 18 hr after onset of light on Days 1, 2, and 4, respectively. Thus, absence of light allowed expression of a daily rhythm in serum concentrations of PRL that persisted for at least 4 days after the photoperiod switch. There were no differences in L-dopa inhibition of PRL release in animals exposed to 16-hr lighk8-hr dark at 3 or 15 hr after onset of light. However, thyrotropin-releasing hormone-induced release of PRL was greater 3 hr after onset of light (11 hr after onset of dark) compared with release at 9, 15, and 21 hr after onset of light in animals exposed to 16-hr lighk8-hr dark, but not in bulls exposed to 8-hr light:16hr dark. The results provide evidence that the cue for the putative photosensitive period of PRL secretion in cattle may be more closely associated with onset of dark, not onset of light.