This study investigated whether treatment with progestagen, which improves fertility after early weaning in postpartum cows, altered concentrations of gonadotrophins or development and function of follicles. Patterns of luteinizing hormone (LH), oestradiol and follicle-stimulating hormone (FSH) and of follicular growth before first postpartum ovulation were compared in two experiments. At 17 to 25 days post partum, suckled anoestrous beef cows received an ear-implant containing 6 mg progestagen (norgestomet) for 9 days or served as untreated controls. Calves were weaned from all cows 7 days after initiation of treatment. Cows were observed for oestrous behaviour twice a day until 10 days after weaning. As expected, the proportion of anoestrous cows that formed a corpus luteum with a normal lifespan was greater (P < 0.01) in cows treated with norgestomet (Expt 1, 17 of 24; Expt 2, 18 of 22) than in control cows (Expt 1, 2 of 16; Expt 2, 3 of 18). In general, patterns of secretion of LH and oestradiol and of final growth of the preovulatory follicle did not differ between control and norgestomet-treated cows. However, there was a transient rise in FSH in association with weaning in control cows that did not occur in norgestomet-treated cows. In addition, mean LH increased more rapidly and mean oestradiol was higher during the 3 days immediately before the preovulatory surge of LH in the norgestomet-treated cows (P < 0.01). These alterations in LH, FSH and follicular function, in the absence of any difference in rate of final growth of the preovulatory follicle, may mean that there was a lack of synchrony between follicular maturation and the LH surge in control cows which resulted in ovulation of a follicle that was not at an optimal stage of maturation. In comparison, treatment with norgestomet may partially synchronize follicular maturation and the LH surge.
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