Growth and sexual maturation in ewes: the role of photoperiod, diet and temperature on growth rate and the control of prolactin, thyroxine and luteinizing hormone secretion.

Abstract

The relationship of photoperiod, temperature and diet on growth rate and serum concentrations of thyroxine (T4), prolactin (PRL) and luteinizing hormone (LH) were studied in ewe lambs from 9 to 32 wk of age. Lambs reared on a diet supplying 1.78 Kcal/g net energy had higher (P less than .025) serum T4 concentrations and greater average daily gain (ADG) than lambs fed a lower energy ration (1.5 Kcal/g). Long artificial photoperiod of 16 h of light neither improved ADG nor altered T4 concentrations compared with ambient (natural light) or a short artificial photoperiod of 8 h light. Serum PRL concentrations were correlated (r = .83 to .74; P less than .01) to ambient temperature during the seasonal transition from summer to fall and temperature also determined the magnitude of photoperiodic stimulation of PRL levels in winter. Temperature had no effect on basal LH concentrations for lambs kept in warm (15 to 20 C) vs cold (0 to 15 C) environments for 6 wk, but the LH surge induced by estradiol was delayed (P less than .05) by the warm environment. The correlation (r = .015) between serum PRL levels and ADG was not significant, although serum PRL concentrations were dependent upon dietary energy levels. These observations do not support a positive relationship of PRL or photoperiod to ADG in the ewe lamb.