Effect of ovariectomy on tonic gonadotrophin secretion in cyclic and post-partum dairy cows

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Abstract
Blood collections were taken every 20 min over 12 h on 6 occasions within 1 mo. before and after gonadectomy of 5 cyclic and 5 post-partum dairy cows and again on 1 occasion 3-5 mo. thereafter. By 4 days after ovariectomy of the cyclic cows (i.e., 4 days after estrus) average plasma LH [lutropin] values and the frequency and amplitude of pulsatile LH release had increased .apprx. 3-fold. In the cows ovariectomized 4 days after parturition, mean concentrations of LH, pulse frequency and amplitude rose only .apprx. 2-fold during the initial 4 days. Mean LH values were significantly lower than those of cyclic cows during the first 11 days after ovariectomy. The changes in FSH concentrations were similar to those of LH in the 2 groups of cows, with mean values rising .apprx. 2-fold within the first 4 days and the same frequency of pulsatile release, but the amplitude was reduced. By 3-5 mo. after gonadectomy, the frequency of pulsatile release in both groups of cows had decreased whereas the amplitude had increased, resulting in additionally enhanced average gonadotropin concentrations. The hypothalamo-pituitary axis of parturient dairy cows evidently is partly refractory within the first 15 days after calving and there is a slower acceleration of pulsatile gonadoliberin release in cows after parturition.