The Effects of Biweekly Administration of Recombinant Bovine Somatotropin During the First Trimester on Fetal and Placental Development in Beef Heifers

A total of 97 Angus-based crossbred beef heifers were exposed to the 7-day CO-Synch + CIDR estrus synchronization control protocol and then artificially inseminated. At fixed-time artificial insemination (TAI; d 0) heifers were randomly assigned to receive one of two treatments: single subcutaneous injection with 500 mg of bST in the neck at TAI and then biweekly until day 57 of the experiment (BST; Figure 1); or an untreated control (CONT). Blood samples were collected on days -0, 22, 50, and 64 relative to TAI for analysis of plasma concentrations of IGF-1. Heifer body weight (BW) was recorded on d -9, -3, 0, 15, 22, 29, 43, 50, 57, 64, and 77. Pregnancy was determined via transrectal ultrasonography on d 29 and d 64 after TAI. On day 85 after TAI heifers were harvested and a subset of pregnant heifers (n = 7 for BST, n = 5 for CONT) were retained for assessment of fetal and placental characteristics. At time of harvest complete gravid reproductive tracts and liver tissue from the dams were collected for analysis. Specific fetal measurements assessed were: brain weight, crown-to-nose length, crown-to-rump length, heart girth, fetal BW, eviscerated BW, liver weight, and umbilical cord diameter; whereas extraembryonic measurements were: fetal fluid volume, fetal membrane weight, placentome weights, and placentome number. In addition, maternal tract parameters of gravid uterine weight, empty uterine weight, ovarian weight, and corpus luteum weight were recorded. Mean change in BW (158 lb) and ADG (2.09 lb/d ± 0.59) 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 plasma 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 kg), 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 = 0.03) volume of fetal fluid compared to CONT heifers (429.6 ± 27.14 g).