PARTURITION IN BEEF-COWS FOLLOWING ADMINISTRATION OF PORCINE RELAXIN AT 10 DAYS PREPARTUM

Administration of procine relaxin (pRLX) to heifers 5 d prepartum has been reported to expedite parturition. Thirty-eight mature crossbred beef cows were randomly assigned to one of three treatment groups. Control animals (C; n = 13) received an intramuscular (i.m.) injection of 2 ml corn oil and 2 ml i.m. phosphate-buffered saline (PBS) 24 h later; relaxin treated animals (RLX; n = 13) received 2 ml i.m. corn oil and 1.0 mg i.m. pRLX 24 h later; estradiol-relaxin treated animals (E-RLX; n = 12) received 20 mg i.m. estradiol benzoate (EB) and 1.0 mg i.m. pRLX 24 h later. Treatment with pRLX occurred at 272.6±0.14 d of gestation. The pRLX had been purified to homogeneity from porcine ovaries collected during late pregnancy and was determined to have ≥3000 U/mg by the mouse interpubic ligament bioassay. Peripheral blood samples were collected from all cows at 0, 4, 8 and 24 h, respective to corn oil or EB administration, and assayed for plasma estradiol-17β. At 24 h post administration of EB, plasma estradiol-17β concentrations were 48.0±10.5 pg/ml for C and RLX cows and 178.5±14.8 pg/ml for E-RLX cows. There were no treatment effects (P≥0.10) for elapsed time from treatment to parturition (304.2±22.4 h), gestation length (285.2±0.9 d), calving difficulty score (1.05±0.04), calf vigor score (1.05±0.04) or calf birth weight (38.0±0.88 kg). Additionally, there were no retained placental membranes in any cows. Administration of pRLX intramuscularly to beef cows at 10 d before expected parturition was not effective in inducing premature parturition. Furthermore, the effectiveness of pRLX in inducing parturition was not enhanced by pretreatment with estradiol benzoate. © 1990.