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 P GF2a given 8 or 24 h, apart on the number of cows observed in estrus w ithin 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) w hile cows in Group 2 ( n - 116 ) were treated only once (0 h). In Expe riment 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, b lood samples were obtained from cows in both groups on Days 0 and 2. H owever, in Experiment 2, blood samples were obtained from cows in both groups only on Day 0. Plasma progesterone concentration was determine d using radioimmunoassay. Cows in both experiments were observed for e strus 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 i n estrus within 7 d than in Group 2 ( P < 0.03 ). In Experiment 2, the re was no significant difference in the number of cows in both groups that were observed in estrus within 7 d. However, the interval from tr eatment to the first observed estrus for cows not observed in estrus w ithin 7 d was significantly longer in cows treated at 0 and 24 h compa red with the cows treated once at 0 h. In both experiments, the pregna ncy rate for cows inseminated within 7 d was similar for cows in all g roups. 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 re sulted in more cows being observed in estrus than at a 24-h interval.