Influence of dietary intake and weight variation on LH release after a gonadotrophin-releasing hormone (GnRH) injection during the post-partum period of the nursing cow.

The experiment was conducted to measure the effects of restricting dietary intake on LH release following a GnRH injection during the post-partum period in nursing cows. Eighteen multiparous Charolais cows were fed a low ration from 45 days pre-partum to 45 days post-partum, and received an intravenous injection of synthetic GnRH (55 micrograms) at 5, 15 and 30 days post-partum. Plasma LH concentration was measured during the 4 hrs following each injection. The cows were separated into 2 groups depending on whether their mean daily gain was negative (group 1) or positive (group 2). The peak plasma LH concentration and the total release of LH after a GnRH injection tended to increase during the post-partum period irrespective of the group. The maximum LH value and the total LH release following GnRH were higher in Group 1 than in Group 2 (p less than 0.05), at 5, 15 and 30 days post-partum. Furthermore, the maximal plasma LH concentrations recorded after GnRH were highly correlated with the mean daily weight gain post-partum (day 15, r = -0.50; day 30, r = -0.54).