Seasonal Effects and Hormonal Patterns Related to Puberty in Ewe Lambs

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Abstract
Ewe lambs born in March, July and August were studied in an effort to separate the effects of season from the ongoing maturational changes affecting serum luteinizing hormone (LH), folliclestimulating hormone (FSH), estradiol, prolactin and progesterone concentrations from early life through first ovulation. Blood samples were collected twice weekly and ovulation was assessed from increments in serum progesterone (>1.0 ng/ml) in two or more consecutive samples. Puberty was attained in 81% (21/26) of the lambs from October through January. A small transient rise in progesterone of 4–7 days duration was observed prior to ovulation in most (16/21) of the animals. At puberty, March lambs were older (220.9 ± 5.8 days, P<.01) than either July or August lambs (170.0 ± 12.9 and 153.0 ± 7.1 days) and also heavier (P<0.05, 40.8 ± 1.9 kg, 38 ± 1.5 and 35.7 ± 1.9, respectively). During the prepuberal developmental period, serum concentrations of LH, FSI-I and prolactin were higher in lambs born in March and July than in those born in August. Serum prolactin concentrations were uniformly lowest in all groups during the fall and winter when the lambs reached puberty. The pattern of circulating hormone concentrations was very similar in lambs which reached puberty and in those which failed to ovulate. The only exception noted was that equivalent increments in serum estradiol, detected in both puberal lambs and in some of their nonpuberal counterparts, were followed by increased progesterone only in some lambs. Collectively these data suggest that sexual maturity in ewe lambs can be attained as early as 150 days of age, but that season dominates the timing of first ovulation and delays its occurrence in spring-born lambs until the fall. Body weights of lambs at puberty suggest that a minimum threshold weight rather than a critical weight may be a factor. The decline in serum prolactin during the fall, coincident with decreasing photoperiod, may be one component of seasonal regulation of ovulation, but a direct causal relationship is yet to be established and other factors must be considered