Induction of parturition, progesterone secretion, and delivery of placenta in beef heifers given relaxin with cloprostenol or dexamethasone.

Sixty primiparous beef heifers from a crossbreeding study were used to examine the effects of inducing parturition with relaxin (3,000 U/mg) combined with cloprostenol (500 micrograms, i.m., n = 30) or dexamethasone (20 mg, i.m., n = 30) at Day 273, 10 +/- 1 days before expected parturition (Day 283). Heifers were assigned at random within cloprostenol and dexamethasone groups to receive relaxin (1 mg, n = 5/treatment), i.m. or in the cervical os (OS), at 0 h (the same time as cloprostenol and dexamethasone) or 24 h later. Eleven and six first-calving heifers and sixteen and nine second-calving cows also received cloprostenol + relaxin and cloprostenol + phosphate-buffered saline, respectively. Radioimmunoassay of daily plasma samples indicated an abrupt decrease in progesterone with time (p less than 0.001), from 7.5 +/- 0.50 to 1.0 +/- 0.30 ng/ml (mean +/- SE) within 48 h for all groups. The mean rate of progesterone decrease (ng/ml in 24 h) was accelerated (p less than 0.01) in relaxin-treated heifers (5.3 +/- 0.36), in contrast to dexamethasone- and cloprostenol-treated control heifers (2.8 +/- 0.40). Relaxin combined with cloprostenol or dexamethasone shortened the calving period in these heifers by reducing the interval between treatment and calving (33 vs. 56 h; p less than 0.01). The incidence and duration of retained placenta were reduced by 22 vs. 75% and 14 vs. 34 h for relaxin combined with cloprostenol or dexamethasone as compared with cloprostenol- or dexamethasone-treated controls, respectively (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)