Summary. Crossbred beef heifers (N = 36) were assigned to one of three treatment groups: untreated controls (C; N = 15); Group R4, treated with pig relaxin (1·0 mg i.m.) 4 days pre partum (N = 11); or Group R7, treated with pig relaxin (1·0 mg i.m.) 7 days pre partum (N = 10). Bioactivity of the pig relaxin (UMC-R-P8) was determined by the mouse interpubic ligament assay to be ≥3000 U/mg, both before and after the experiment was conducted. Peripheral serum immunoreactive relaxin values were 7·5, 3·4, 2·5, and 1·5, ng/ml at 2, 4, 6 and 8 h after injection of relaxin, respectively. Gestation lengths were 282·9 ± 1·1, 285·5 ± 1·3 and 285·6 ± 1·5 days for Groups C, R4 and R7 (C vs R4 + R7; P ≃ 0·08). Calving difficulty score (1 to 4) tended to be greater (P ≃ 0·08) for Group R4 and R7 heifers (C vs R4 + R7; 1·3 ± 0·24 vs 1·75 ± 0·28 + 2·04 ± 0·32), but the incidences of dystocia and retained placentae were not influenced by treatment (P ≥ 0·10). The mean concentration and concentration profile of daily serum progesterone, oestradiol-17β, dihydroprostaglandin F-2α and relaxin were not affected by treatment from 6 days pre partum through 2 days post partum. Cervical diameter, cervical softness score, pelvic measurements, and vulva opening length during the periparturient period were not affected (P ≥ 0·10) by treatment, but all of these characteristics changed over time (P ⩽ 0·01), relative to calving. We conclude that i.m. administration of pig relaxin (≥3000 U) does not effectively alter periparturient characteristics of beef heifers. Discrepancies between these results and those reported for intracervical administration cannot be readily explained.
Keywords: relaxin; beef heifers; pre partum; dystocia