EFFECT OF BOVINE SOMATOTROPIN ON REPRODUCTIVE FUNCTION IN LACTATING DAIRY-COWS

Several recent experiments have reported that chronic treatment with b ovine somatotropin (bST) increased the number of days open without aff ecting the services per conception. The physiological basis for these effects was examined. Eleven lactating Holstein cows received daily in jections of bST (40 mg) and 10 received daily injections of vehicle. T reatment was initiated between 32 and 85 d post partum and continued f or up to 180 d. Eight of 11 bST-treated cows experienced at least one period of extended ovarian acyclicity during treatment. Only 3 of 10 c ontrol cows did so (P = 0.05). Concentrations of progesterone during l uteal phases were lower in bST-treated cows than in controls (P = 0.06 ). Baseline concentrations of LH were suppressed in bST-treated cows c ompared with those of controls (P < 0.04). Neither the pulse frequency of LH nor the expression of estrous behavior was affected by bST (P > 0.30). These results indicate that chronic administration of a high d ose of bST can reduce reproduction performance by promoting ovarian ac yclicity.