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Gonadotrophins and ovarian steroids in cattle. IV. Re-establishment of the stimulatory feedback action of oestradiol-17 beta on LH and FSH.

The re-establishment of the positive feedback action of oestradiol-17 beta on both gonadotrophins was assessed in 20 dairy cows receiving three challenges once every 20 days from parturition through day 55 post partum (pp). One mg oestradiol-17 beta-benzoate dissolved in 5 ml oil was given im every 5 days to groups of 5 cows each. Blood was collected from the jugular vein every 3-12 h from 2 days before until 2 days after the injection. No effect of oestradiol-17 beta on the LH release was exerted at parturition or day 5 pp. A stimulation of LH release started to occur in the majority of cows by day 10 pp. Two types of preovulatory like surges could be elicited: low magnitude LH releases lacking any concomitant FSH increase and parallel high magnitude LH and FSH surges. No cow expressed heat when exhibiting a weak LH surge before day 30 pp and only 30% thereafter. All cows showed oestrus when exhibiting parallel high magnitude LH/FSH surges after day 20 pp but only 40% on days 10 and 15 pp. Progesterone concentrations greater than 0.5 ng/ml prevented the positive feedback action of oestradiol on LH and FSH. An initial negative feedback on FSH was exerted throughout the entire post-partum period comprising the luteal phase. We conclude from these results that a certain minimum threshold frequency of endogenous LH secretion is necessary to allow oestradiol-17 beta to evoke its unimpaired stimulatory feedback action after an initial period of pituitary refractoriness during the early puerperal anoestrus and acyclic period.