Effects of dose and route of administration of cloprostenol on luteolysis, estrus and ovulation in beef heifers

Four experiments were conducted (with crossbred beef heifers) to determine the effects of dose and route of administration of cloprostenol on luteolysis, estrus and ovulation. In Experiment 1, 19 heifers with a CL ≥17 mm in diameter were randomly allocated to receive cloprostenol as follows: 100 μg sc, 250 μg sc, or 500 μg im. Heifers given 100 μg sc had a longer (P<0.03) interval (120.0 h±10.7 h; mean±S.E.M.) from treatment to ovulation than those given either 250 μg sc or 500 μg im (92.0 h±7.4 h and 84.0 h±8.2 h, respectively). In Experiment 2, 28 heifers were given porcine LH (pLH), followed in 7 days by cloprostenol (same doses and routes as in Experiment 1), and a second dose of pLH 48 h after cloprostenol. Luteolysis occurred in all heifers, and no difference was detected among treatment groups in the interval from cloprostenol treatment to ovulation (mean, 101 h; P<0.9). In Experiment 3, 38 heifers at random stages of the estrous cycle (but with plasma progesterone concentrations ≥1.0 ng/ml) received 500 or 125 μg cloprostenol by either im or sc injection (2/2 factorial design). There was no difference (P<0.4) among groups in the proportions of heifers that were detected in estrus or that ovulated. However, the interval from cloprostenol treatment to estrus was shorter (P<0.02) in the group that received 500 μg im (58.5 h) than in the other three groups (500 μg sc, 75.0 h; 125 μg im, 78.0 h; and 125 μg sc, 82.3 h). In Experiment 4, 36 heifers were treated (as in Experiment 3) on Day 7 after ovulation. The proportions of heifers detected in estrus and ovulating after 125 μg sc (33 and 44%, respectively) or 125 μg im (55 and 55%) were lower (P<0.05) than in those that received 500 μg sc (100 and 100%), but not different from those receiving 500 μg im (78 and 89%, respectively). Overall, ovulation was detected in 9/18 heifers given 125 μg and 17/18 heifers given 500 μg of cloprostenol, on Day 7 (P<0.01) and was detected in 17/20 heifers given 125 μg and 18/18 heifers given 500 μg of cloprostenol, at random stages of the estrous cycle (P>0.05). Although there was no significant difference in luteolytic efficacy between im and sc injections of the recommended dose (500 μg) of cloprostenol, variability in responsiveness to a reduced dose depended upon CL sensitivity, therefore, reduced doses cannot be recommended for routine use.