Use of applied reproductive technologies (FTAI, FTET) to improve the reproductive efficiency in dairy cattle
Chebel, Ricardo C
Background: Reproductive inefficiency of dairy cattle, characterized by reduced estrous expression and detection rates, reduced pregnancy per artificial insemination (number of cows pregnant divided by number of cows inseminated), reduced pregnancy rates (number of cows pregnant divided by the number of cows eligible to become pregnant during a time interval), and increased pregnancy losses, has a large financial impact on dairy operations across the world. Although the most important component of reduced reproductive efficiency in dairy cattle is unquestionably poor management and diseases that result from it, the genetic selection and the resulting increased milk yield have caused physiological changes in lactating dairy cows that also affect fertility. The most important of these changes is the increased feed intake and the consequent increased mesenteric and liver blood flow to supply the nutrients necessary for milk yield. This causes significant decreases in concentrations of progesterone and estradiol that affect estrous expression, follicular growth, oocyte quality, and embryo development and survival. This review will discuss reproductive technologies used in large dairy herds to mitigate the effects of these physiological changes on reproductive performance. Review: The use of ovulation/estrous synchronization protocols (OSP), pre and post-ovulation hormonal treatments, and embryo transfer (ET) in the reproductive management of lactating dairy cows was reviewed. Several OSP have been developed in the past 20 years. To achieve acceptable pregnancy per artificial insemination (P/AI) OSP should result in synchronized recruitment of a new follicular wave, growth of follicles under P4 concentration > 2 ng/mL, synchronized luteolysis, and synchronized ovulation at the end of the protocol. When embryo recipient cows are submitted to OSP, these protocols must aim to tightly synchronize luteolysis and ovulation at the end of the protocol. The use of ET in lactating dairy cows in the U.S. has been limited to herds of registered animals, to mitigate the negative effects of exposure to heat stress, to improve genetics of expanding herds, and in a few herds to salvage repeat-breeders. Lactating dairy cows are sensitive to heat stress because of the high metabolic rate resulting from the increased feed intake necessary to supply nutrients for milk production. Several studies have demonstrated that lactating dairy cows exposed to heat stress that receive ET have improved reproductive performance compared with cows receiving AI. Finally, the use of hormonal treatments to increase P4 concentration during early diestrus was reviewed because several studies have demonstrated a strong association among P4 concentration and embryo development and pregnancy establishment. The effects of hormonal treatments during ovulation synchronization protocols, after AI or at ET on P4 concentration and reproductive outcomes are controversial and likely dependent on management, milk yield, and diet of the lactating dairy cows. Conclusion: The use of reproductive technologies in lactating dairy cows, particularly AI, is extremely well disseminated and has resulted in significant improvements in milk yield in the past 50 years. Recent developments in the understanding of reproductive physiology of lactating dairy cows have resulted in ovulation synchronization protocols that optimize fertility after AI or ET.
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