The Effects of Biweekly Administration of Recombinant Bovine Somatotropin during the First Trimester on Fetal Development in Gestating Beef Heifers

To evaluate the effects of recombinant bovine somatotropin (bST; sometribove zinc, Posilac, Elanco Animal Health, Indianapolis, IN) on fetal and placental development during the first trimester of gestation, ninety-seven Angus heifers were exposed to a 7-d CO-Synch + CIDR ovulation control protocol and then to receive either 500 mg of bST (BST) biweekly, or to receive no bST (CONT) immediately prior to fixed-timed artificial insemination (TAI; d 0). On d 0, 15, 29, 43, and 57 BST heifers received a single subcutaneous injection in the neck. Heifer BW was assessed on d -9, -3, 0, 15, 22, 29, 43, 50, 57, 64, and 77. Blood samples were collected on d 0, 22, 50, and 64 for analysis of concentrations of plasma IGF-1. Pregnancy was diagnosed via transrectal ultrasonography on d 29 and d 64 after TAI. A subset of pregnant heifers (n = 7 for BST, n = 5 for CONT) were retained for assessment of fetal and placental characteristics. Heifers were harvested and complete gravid reproductive tracts and liver tissue were collected for analysis. Mean change in BW (71.9 kg) and ADG (0.95 kg ±0.27) of the heifers from TAI to d 77 did not differ between treatments (P > 0.05). Likewise, no differences were detected between treatments with regards to carcass quality grade, carcass yield, or carcass weight. Mean concentrations of IGF-1 were greater (P < 0.001) in BST (346.50 ± 27.7 ng/mL) treated than CONT (134.70 ± 32.8 ng/mL) heifers. Mean placental weight (66.46 g), fetal membrane weight (0.256 kg), uterine weight (1.42 g), as well as ovarian and corpus luteum weights (15.1 g and 4.8 g, respectively) did not differ (P > 0.05) between treatments. Similarly, fetal crown to rump length, fetal weight, heart girth, and liver weight did not differ between treatments (P > 0.05). However, extraembryonic samples collected from heifers receiving bST (521.6 ± 22.9 g) resulted in greater (P = 0.027) quantities of fetal fluid compared to CONT heifers (429.6 ± 27.14 g). There was also a tendency for BST heifer reproductive tracts to have fewer placentomes (P = 0.084) and greater umbilical diameter (P = 0.091) than CONT heifers. In conclusion, administration of bST during the first trimester of gestation increased concentrations of IGF-1 which resulted in an increase in extraembryonic fluid, a decreased quantity of placentomes, and greater umbilical diameter, but failed to alter fetal development.