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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 Secretion¹

Cornell University, Ithaca, NY 14853

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<.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<.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<.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.

¹ The authors wish to express thanks to Dr. Hugh Travis and Dr. Doug Hogue for providing facilities and animals for these studies. We also wish to aknowledge Bill Gardner and Jay Miller for technical assistance. We thank Ari van Tienhoven, J. Apgar and R. J. Wall for review of this manuscript and finally wish to express our gratitude to Harriette Polan, Gail Donnen and Lesley Keane for manuscript preparation. Reagents for LH and prolactin radioimmunoassay were generously provided by NIAMDD, Dr. Gordon Niswender and Dr. L. E. Reichert, Jr.

² US DA Northeast Sheep Research, Dept. of Anim. Sci.

³ Dept. of Anim. Sci.