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. / Doctor of Philosophy / We need to improve reproductive efficiency in beef cows if we are to combat the challenges of producing more food while using less resources due to limited land availability and concerns with greenhouse gas emissions from agriculture. In cow-calf production systems, this means producing one healthy calf per cow per year. Cattlemen can implement a variety of assisted reproductive technologies to achieve this goal. Achieving maximal reproductive efficiency will require using technologies that are incorporated into herd management, individual animal care, and in vitro embryo production.
Fixed time artificial insemination (FTAI) allows cattlemen to maximize the number of cows becoming pregnant and calving earlier in the season to increase efficiency. Unfortunately, use of FTAI is uncommon in cow-calf production systems because of labor and economic restraints. In order to improve economic feasibility of FTAI, bull-related costs need to be reduced, which can be done through increasing the number of cows serviced per bull (decreasing the bull:cow ratio). This study retrospectively examined correlations between the bull:cow ratio and pregnancy rate on first return to estrus after FTAI. There was little to no correlations between bull:cow ratio and pregnancy rates, and if they were significant, there was much variation in the data. With this we concluded that a reduced bull:cow ratio does not affect pregnancy rate on first return to estrus, allowing producers to increase the number of cows serviced by a single bull and reduce bull related costs.
Supplementing calcium salts of soybean oil (CSSO) that are rich in omega-6 fatty acids can enhance beef cow fertility, but it is unclear why this happens. Using non-pregnant cows as a model, we studied the effects of either saturated fat or CSSO on reproductive parameters such as ovarian structures, hormone concentrations, and uterine gene expression. There were no changes in any of these parameters between treatments, but there were changes in the concentrations of certain plasma and follicular fluid fatty acids. There was also reduced activity of lipid metabolism enzymes. We were unable to pinpoint how CSSO supplementation improves reproduction, but the altered fatty acid content of tissues and altered enzyme activity likely plays a key role, thus ultimately impacting fatty acid utilization and growth of the embryo.
In-vitro embryo production can increase the number of offspring produced from a single female and accelerate the incorporation of animals with high genetic merit into herds. To obtain optimal pregnancy rates with in vitro embryos, we should develop non-invasive, objective methods for identifying the most viable embryos. This study examined if size parameters of activated and fertilized oocytes are indicative of successful development. We discovered that oocytes with large diameters and those with thin zona pellucida were most likely to develop to the blastocyst stage, and that the area of the cell was positively correlated with blastocyst total cell number. 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. This suggests that digital measurements of fertilized oocytes may have potential as objective selection criteria.
Addressing issues of reproductive inefficiency in beef cows requires a comprehensive approach, as there is not one ideal solution. Management techniques can alleviate the cost of FTAI by reducing the number of bulls used without affecting pregnancy rates. Supplementing CSSO can alter tissue fatty acids to enhance fertility. Finally, the efficiency of in vitro embryo production can be improved by selecting better embryos for transfer without compromising the embryo. Combinations of all these techniques can create more reproductively efficient animals.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/98841 |
| Date | 12 June 2020 |
| Creators | Timlin, Claire |
| Contributors | Animal and Poultry Sciences, Mercadante, Vitor R. G., Lee, Kiho, Rhoads, Michelle, Eyestone, Willard H., Ealy, Alan D. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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