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 greater than or equal to 17 mm in diameter were randomly allocated to receive cloprostenol as follows: 100 mug sc, 250 mug sc, or 500 mug im. Heifers given 100 mug 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 Lg sc or 500 jig 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 greater than or equal to1.0 ng/ml) received 500 or 125 mug 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 mug im (58.5 h) than in the other three groups (500 mug sc, 75.0 h; 125 mug im, 78.0 h; and 125 mug 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 mug sc (33 and 44%, respectively) or 125 mug im (55 and 55%) were lower (P < 0.05) than in those that received 500 mug sc (100 and 100%), but not different from those receiving 500 mug im (78 and 89%, respectively). Overall, ovulation was detected in 9/18 heifers given 125 mug and 17/18 heifers given 500 mug of cloprostenol, on Day 7 (P < 0.01) and was detected in 17/20 heifers given 125 mug and 18/18 heifers given 500 mug 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 mug) of cloprostenol, variability in responsiveness to a reduced dose depended upon CL sensitivity, therefore, reduced doses cannot be recommended for routine use. (C) 2002 Elsevier Science B.V. All rights reserved.