315 RETURN TO ESTRUS AND OVULATION FOLLOWING ADMINISTRATION OF PROSTAGLANDIN IN SUPEROVULATED BEEF COWS

Two experiments were designed to evaluate the effect of administering 1 or 2 doses of prostaglandin F2α (PGF) at the time of ova/embryo collection on luteal regression and return to oestrus and ovulation in donor cows. In Experiment 1, 14 Angus cows were blocked at the time of embryo collection by the number of corpora lutea (CL) determined by rectal palpation [6 to 10 CL (n = 6), 11 to 15 CL (n = 4), >15 CL (n = 4)] and were randomly assigned to 2 groups to receive a single injection of 500 µg of sodium cloprostenol (PGF, Ciclase, Syntex S.A., Buenos Aires, Argentina) on the day of embryo collection (Day 0) or a single injection of PGF on Day 0 and a second injection of PGF 24 h later. All cows were bled daily to determine progesterone concentrations by RIA and ultrasonically scanned daily (Falco 100, Pie Medical, Maastricht, the Netherlands) for 19 days after embryo collection, to determine ovulation of a newly recruited dominant follicle. In Experiment 2, 47 superovulated Angus cows were blocked by CL numbers [1 to 4 CL (n = 5), 5 to 10 (n = 11), 11 to 15 (n = 13), >15 CL (n = 18)] and treated as in Experiment 1. In this experiment, cows were bled and scanned on Days 0 (i.e. embryo collection), 4, 7, 10, 13, 16, and 19. The interval from embryo collection to luteal regression (i.e. plasma progesterone <1 ng mL–1) and to ovulation was analysed using ANOVA. Proportions were compared by Chi-square test. In Experiment 1, the mean (± SEM) number of CL in donor cows was 11.9 ± 2.0, and the effect of CL numbers on luteal regression, oestrus, and ovulation was not significant (P > 0.5). The interval from treatment to luteal regression was not affected by PGF treatment (1 PGF: 2.0 ± 1.2 days; 2 PGF: 2.4 ± 1.0 days). Furthermore, 6/7 animals (85.7%) were observed in oestrus in both groups, and 6/7 (85.7%) and 4/7 (57.1%) ovulated within 19 days in the 1 PGF and 2 PGF groups, respectively (P > 0.2). The interval to oestrus and ovulation did not differ among groups (7.0 ± 0.7 days and 8.5 ± 0.6 days v. 9.7 ± 0.5 days and 10.0 ± 0.7 days, for 1 PGF and 2 PGF, respectively; P > 0.1). In Experiment 2, the mean number of CL in donor cows did not differ from that in Experiment 1 (13.9 ± 1.1; P > 0.37), and the effect of CL numbers on luteal regression and ovulation was not significant (P > 0.7). Although luteal regression occurred earlier (P = 0.03) in the 2 PGF group (4.1 ± 0.6 days) compared with the 1 PGF group (5.3 ± 2.2 days), no differences were detected between PGF groups in any of the other parameters. The proportion of cows in oestrus and the interval to oestrus was 19/24 (79.2%) and 7.2 ± 0.4 days v. 18/23 (78.3%) and 7.2 ± 0.3 days for cows in the 1 PGF and 2 PGF groups, respectively (P > 0.7). Finally, the proportion of cows ovulating within 19 days after collection and the interval to ovulation were 20/24 (83.3%) and 10.3 ± 0.4 days v. 21/23 (91.3%) and 10.1 ± 0.4 days for cows in the 1 PGF and 2 PGF groups, respectively (P > 0.4). In summary, treatment with PGF at the time on embryo collection results in luteal regression in 4 to 5 days and ovulation of a new dominant follicle in 80 to 90% of the donors within 19 days. The addition of a second PGF treatment did not shorten the interval to oestrus and ovulation.