The preovulatory follicle in oestrous cycling Angus cow was larger (P<0.01) and contained more granulosa cells (P<0.01) in May and June than in October. Also in May and June, the corpora lutea were heavier (P<0.05) and secreted more progesterone (P<0.01) than in October. In contrast, the luteinizing hormone peak frequency and plasma prolactin concentrations were higher (P<0.01) in October than in May and June. Seasonal differences in ovarian activity are probably a direct consequence of seasonal differences in pituitary gonadotrophin secretion.
Proceedings of the New Zealand Society of Animal Production, Volume 44, , 11-14, 1984
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