Effect of ovariectomy on tonic gonadotrophin secretion in cyclic and post-partum dairy cows.
Blood collections were taken every 20 min over 12 h on 6 occasions within 1 month before and after gonadectomy of 5 cyclic and 5 post-partum dairy cows and again on one occasion 3--5 months thereafter. By 4 days after ovariectomy of the cyclic cows (i.e. 4 days after oestrus) average plasma LH values and the frequency and amplitude of pulsatile LH release had increased about 3-fold. In the cows ovariectomized 4 days after parturition, mean concentrations of LH, pulse frequency and amplitude rose only about 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 two groups of cows, with mean values rising about 2-fold within the first 4 days and the same frequency of pulsatile release, but the amplitude was reduced. By 3--5 months after gonadectomy, the frequency of pulsatile release in both groups of cows had decreased whereas the amplitude had increased, resulting in additionally enhanced average gonadotrophin concentrations. These findings indicate that the hypothalamo-pituitary axis of parturient dairy cows is partly refractory within the first 15 days after calving and that there is a slower acceleration of pulsatile GnRH release in cows after parturition.