Influence of Photoperiod and Protein Diet on Growth Hormone Secretion in Rams

Abstract

Abstract

Studies of sheep were undertaken to determine the effects of photoperiod and protein diet on growth hormone (GH) secretion. Rams were subjected to either a control (R1) or an inverted (R2) 6-month (semestral) light regime. In both light regimes day lengths varied gradually between 8 and 16 hr. Within each light regime group of animals, the rams received either a low (L) or a high (H) protein diet containing the same level of energy. Plasma GH profiles consisting of 13 hourly samples were determined at regular intervals corresponding to known day lengths. Analysis of variance indicated that (i) there was a significant effect of day length (P < 0.01) and protein diet (P < 0.05) on GH secretion, (ii) the two light regimes R1 and R2 were equivalent with respect to GH secretion, and (iii) there were no interactions among the three experimental factors. Although mean GH secretion was consistently higher in L groups than in H groups, there was a similar trend in all the animals of increasing GH secretion as day length was increased. GH secretion was maximum when the day length reached 13 hr 20 min in increasing photoperiods in L groups (15.6 ± 1.6 ng × h × ml^-1) and 16 hr in H groups (13.0 ± 1.2 ng × h × ml^-1). From these results we conclude that both an increasing day length and a deficiency in protein diet stimulate GH secretion in rams but the GH response to these two factors may involve different regulatory processes and may have different functions.