Effect of Days Postpartum and Endogenous Plasma Progesterone Concentrations on Estradiol Benzoate Induced LH Release in Dairy Cattle

Abstract

Fifty Holstein cows were divided into 5 groups (10 cows group) and received a single injection of 2mg EB (IM) 0-3. 10-13. 20-23. 30-33 or 40-43 days post-partum (PP). groups 1-5. respectively. Blood samples were collected prior to EB administration (Ohr). and every 2 hr from 9-43 hr., posttreatment. The objective of this experiment was to determine the time interval postpartum when the hypothalamo-hypophysial axis regains responsiveness to estrogen by releasing a surge of LH. Plasma LH and progesterone concentrations, prior to treatment increased with days PP. The number of animals that responded to EB treatment was significantly lower in group I than groups 2-5. The mean peak LH concentrations for groups 1-5 significantly increased with increasing days PP. These data suggest that diary cows can respond to EB and release LH by 10-13 days following calving and this response increases as plasma progesterone decreases and time postpartum increases.