Journal of Animal Science, Vol 73, Issue 11 3421-3426, Copyright © 1995 by American Society of Animal Science

CLINICAL TRIAL

Mifepristone (RU 486) induces parturition in primiparous beef heifers and reduces incidence of dystocia

B. J. Dlamini, Y. Li and L. L. Anderson
Department of Animal Science, Iowa State University, Ames 50011-3150, USA.

Crossbred beef heifers (n = 21) were artificially inseminated and on d 268 (expected parturition = d 283) heifers were fitted with an indwelling jugular catheter for sequential blood sampling from d 270 to 278. Heifers were assigned randomly to three treatment groups (n = 7) on d 277: Group 1 received the synthetic steroid mifepristone (RU 486) alone, Group 2 received RU 486 plus relaxin, and Group 3 received placebo treatment. Parturition occurred 43 h after treatment in Group 1, 52 h in Group 2, and 182 h in Group 3, and hormone treatment reduced treatment-to-calving interval compared with diluent treatment (P < .01). Calf birth weights were lower in hormone-treated than in placebo-treated heifers (P < .025), but all calves were vigorous at birth. The incidence of dystocia was lower (P < .05) in heifers in Groups 1 and 2 than in controls. Conversely, placental retention longer than 24 h was observed in hormone-treated heifers, and none occurred in the control group. The RU 486 reduced circulating progesterone concentrations in treated heifers, whereas progesterone concentration remained increased in control (P < .01). The results show that RU 486 alone or combined with relaxin induces delivery in late-pregnant heifers. Hormone treatment promotes calving ease with a markedly reduced incidence of the detrimental effects of dystocia, which may be a serious problem in first-calf heifers.