Growth and sexual maturation in ewes: Dietary and seasonal effects modulating luteinizing hormone secretion and first ovulation

Abstract

A series of experiments were designed to examine the effects of diet and photoperiod upon the onset of sexual maturity in the ewe. In Experiment I, March-born ewe lambs (n=77) were reared from 9 to 28 weeks of age on two levels of dietary energy and three photoperiods consisting of ambient light (May-November), artificial long (16L:8D) and artificial short (8L:16D) days. Ambient temperature was not controlled. At 22 weeks of age, some of the lambs (n=22) reared in artificial photoperiods were induced to ovulate with progesterone and pregnant mare's serum gonadotropin (PMSG). Lambs originally given the low energy diet were switched to the high energy diet at 28 weeks of age. The onset of puberty was not influenced by photoperiods, but was affected by diet. By 28 weeks of age the number of lambs ovulating (24/26 vs. 9/29) and the body weight at first ovulation (43.7 ± 1.2 kg vs. 36.7 ± 1.4 kg) were greater (P<0.01) for lambs given the high energy diet. Following dietary switchover of lambs on the low energy diet, the percentage of lambs ovulating increased from 31% to 65% within 31 ± 10 days. Lambs ovulating after dietary switchover weighed 38.8 ± 2.0 kg, while lambs which remained nonpubertal weighed only 29.0 ± 1.5 kg (n=10) by the end of the study. A reduction in growth rate in lambs fed the low energy diet was associated with reduced (P<0.05) plasma luteinizing hormone (LH) concentrations. Neither diet nor photoperiod altered the number of lambs (20/22) ovulating following PMSG treatment. Collectively, these data demonstrate an effect of diet upon the attainment of puberty. The results suggest that the growth effect may be mediated via changes in LH secretion. Since first ovulation occurred rather synchronously in the fall, regardless of photoperiod control, other factors such as temperature or endogenous rhythms are implicated in timing sexual maturity.