Effect of recombinant bovine somatotropin on superovulatory response and recipient pregnancy rates in a commercial embryo transfer program

Recombinant bovine somatotropin (rbST) has been shown to increase follicular growth in cattle and some studies have demonstrated an increase in superovulatory response for rbST-treated cows. Pregnancy rates have also been shown to increase when rbST was administered around the time of insemination or prior to embryo transfer. The application of rbST for the purpose of increasing superovulatory responses of donor cows and increasing pregnancy rates of recipient heifers was tested in a commercial embryo transfer program. In Experiment 1, embryo donor cows (n=56) underwent three cycles of control superovulation (two before and one after weaning) and subsequently underwent up to four additional superovulations while being treated with either rbST (500mg sustained-release rbST; Posilac, Monsanto, St. Louis, MO; n=28) or excipient (control; n=28) once every 14 days. In Experiment 2, lactating embryo donor cows (n=37) underwent a control superovulation and then underwent a superovulation while lactating and being treated with either rbST (n=16) or excipient (n=21). In Experiment 3, embryo recipient heifers that were being implanted with either in vitro or in vivo produced embryos were treated with either rbST (n=146) or excipient (n=143) at the time of embryo transfer. Treatment of non-lactating (Experiment 1) or lactating (Experiment 2) donor cows with rbST during repeated superovulation did not affect the number of corpora lutea, the sum of transferable embryos, degenerate embryos, and unfertilized oocytes, or the number of transferable embryos. Treatment of recipient heifers with rbST (Experiment 3) did not affect pregnancy rates for either in vitro or in vivo produced embryos. We conclude that superovulatory response and pregnancy rates (respectively) are similar to control for rbST-treated cows undergoing repeated superovulations and rbST-treated recipient heifers treated at the time of embryo transfer.