Strategies for Improving Reproductive Efficiency of Beef Cattle with Assisted Reproductive Technologies

Reproductive efficiency in beef cattle can be improved with reproductive technologies at the herd, individual cow, and embryonic levels. Decreasing the bull:cow ratio for natural service after fixed time artificial insemination (FTAI) can alleviate economic burden associated with FTAI. In experiment 1, the total number of cows exposed per bull was negatively correlated with pregnancy rate to natural service on first return to estrus after FTAI in fall herds. The number of open cows per bull in fall herds using one natural service sire was negatively correlated with pregnancy rate on first return to estrus. There was no correlation between number of cows exposed per bull and pregnancy rates in fall herds with multiple sires or in spring herds. However, bull:cow ratio accounted for only 5–11% of the variation in pregnancy rates, thus we conclude that a reduced bull:cow ratio did not affect natural service return to estrus pregnancy rate. Experiment 2 examined how supplementing calcium salts of soybean oil (CSSO) improves beef cow fertility. Non-pregnant cows received supplement with either saturated fat or omega-6 rich CSSO. There were no changes in dominant follicle diameter, corpus luteum volume, plasma progesterone, or endometrial gene expression (PTGES and AK1B1, PPARA, PPARA, PPARD) between treatments. Plasma and follicular fluid fatty acid compositions were altered between treatments. Experiment 3 examined if size parameters of zygotes have potential as a non-invasive, objective embryo selection method. The outer diameter, area of ooplasm, and thickness of zona pellucida (ZP) was digitally measured on individual artificially activated oocytes and in-vitro fertilized (IVF) zygotes. Larger outer diameter increased probability of development to the blastocyst stage by days 7 and 8 for activated oocytes and tended to by day 8 for IVF zygotes. Thinner ZP increased probability of development to blastocyst stage on days 7 and 8 for oocytes, and to day 8 for IVF zygotes. Area did not affect development but was positively correlated with blastomere number on day 8. An interaction between diameter and ZP thickness was observed in zygotes, but not activated oocytes, suggesting oocyte activation is not always a suitable replacement for in-vitro fertilization.