PROSTAGLANDIN-F2-ALPHA AND NALOXONE THERAPY IN THE ANESTROUS POSTPARTUM BEEF COW

Three experiments were conducted, using multiparous crossbred beef cow s, to test the ability of F-2 alpha, (PGF) and/or naloxone to reduce t he duration of the postpartum interval exogenous prostaglandin F to es trus and to improve subsequent reproductive performance. In each exper iment, postpartum cows were assigned to treatments by calving date. In Experiment I, cows (n=44) were assigned to 1 of 4 treatment groups: 1 ) control, 2) PGF on Day 25 post partum, 3) 400 mg naloxone (3 doses) at 12-h intervals on Day 30 post partum, and 4) PGF on Day 25 followed by 3 400-mg doses naloxone at 12-h intervals on Day 30 post partum In Experiment 2, cows (n=126) were assigned either to 1) control or 2) P GF on Day 30 post partum. In Experiment 3, cows (n=67) were again assi gned to 1 of 4 treatments 1) control, 2) PGF on Day 30 post partum, 3) PGF on Day 40 post partum, and 4) PGF on Day 30 and 40 post partum. S erum progesterone was used to determine the postpartum interval to est rus in Experiments 1 and 3. In all 3 experiments, serum progesterone w as used to determine the proportion of cows that had reestablished est rous cycles at the start of breeding. Pregnancy rate and calving inter val were analyzed for all trials. Naloxone had no effect (P>0.02) on a ny reproductive variable measured. The postpartum interval to estrus w as similar (P>0.30) for PGF-treated and control cows in Experiments 1 and 3. The proportion of cows cycling at the start of breeding and the calving interval were not affected (P>0.20) by PGF treatment in any o f the experiments. Only the administration of PGF on Day 40 post partu m in Experiment 3 improved (P=0.04) the subsequent pregnancy rate. Ana lysis of data pooled across experiments showed that the pregnancy rate was higher (P=0.03) for cows treated with PGF than for control cows ( 91.4 and 72.9%, respectively). It was concluded that administration of PGF during the early postpartum period improves subsequent reproducti ve function in beef cows.