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.