ESTRUS AND PREGNANCY RATE OF DAIRY-COWS GIVEN ONE OR 2 DOSES OF PROSTAGLANDIN-F2 ALPHA-8 OR 24 HOURS APART

A total of 329 cows was used in 2 experiments to study the effect of PGF2a given 8 or 24 h, apart on the number of cows observed in estrus within 7 d and the pregnancy rate to a single insemination at this time. In Experiment 1, 233 cows were divided into 2 groups. Cows in Group 1 ( n = 117 ) were treated twice with 25 mg, im of PGF2a (0 and 8 h) while cows in Group 2 ( n - 116 ) were treated only once (0 h). In Experiment 2, 96 cows were divided into 2 groups. Cows in Group 1 ( n = 49 ) were treated twice with 25 mg, im of PGF2a (0 and 24 h) while cows in Group 2 ( n = 47 ) were treated only once (0 h). In Experiment 1, blood samples were obtained from cows in both groups on Days 0 and 2. However, in Experiment 2, blood samples were obtained from cows in both groups only on Day 0. Plasma progesterone concentration was determined using radioimmunoassay. Cows in both experiments were observed for estrus within 7 d of treatment and were inseminated within 12 h of the observed estrus. In Experiment 1, more cows in Group 1 were observed in estrus within 7 d than in Group 2 ( P < 0.03 ). In Experiment 2, there was no significant difference in the number of cows in both groups that were observed in estrus within 7 d. However, the interval from treatment to the first observed estrus for cows not observed in estrus within 7 d was significantly longer in cows treated at 0 and 24 h compared with the cows treated once at 0 h. In both experiments, the pregnancy rate for cows inseminated within 7 d was similar for cows in all groups. From the results of this study, it is concluded that treatment of dairy cows with 2 luteolytic dosages of PGF2a at an 8-h interval resulted in more cows being observed in estrus than at a 24-h interval.