Practical Manipulation of the Estrous Cycle in Dairy Animals

Considerable advancements in reproductive biology and technology have occurred over the past 15 years that are useful tools to dairy producers to improve reproductive management of heifers and lactating dairy cows. These are even more important when we deal with increasing herd size and cope with seasonal periods of reduced fertility, such as in the south, due to heat stress. Utilization of these tools needs to be founded on a thorough understanding of the reproductive events that they control, and implemented in a manner that is compatible with: the cow, the management system, goals of the farm, and the veterinarian or staff responsible for the health care of the cattle. It is clear to all who have managed dairy cows that use of drugs that regulate reproductive events are no substitute for good management. Indeed, they will only work efficiently if management is good. Our current knowledge to successfully manipulate the estrous cycle, control the time of ovulation, and to enhance embryo survival in dairy cattle has provided the dairy industry with new and novel strategies of reproductive management. Optimization of reproductive management in lactating dairy cows is a challenge in that reproductive performance has declined with increasing milk production of the herd.4 Our comprehension of the various factors controlling ovarian follicle development, intensity of estrous behavior, corpus luteum development and regression, and time of ovulation has led to several tested strategies to improve reproductive management in dairy cattle. It is important to recognize that in the United States only two classes of drugs are approved currently for use in lactating dairy cows. They include Gonadotrophin Releasing Hormone and its analogues (GnRH) and Prostaglandin (PG) F2α and its analogues (PGF2α). Other compounds like various progestins and estrogens that have been utilized in dairy heifers, beef heifers and beef cows have not been approved for use in lactating dairy cows. An additional exciting aspect of applying current reproductive technology to control and improve reproductive management is that new and basic strategies can be investigated and applied to improve fertility by increasing conception rates or embryo survival. Several studies have documented that conception rates are increased after 60 days postpartum, and this is associated undoubtedly with improved uterine health and body condition, and increasing energy balance. The opportunity for producers to take advantage of this relationship has been hampered by inefficient detection of estrus leading to an inability to control precisely time of first service on a herd basis. Herd pregnancy rates are the product of estrus detection and conception rates. It is not practical to recommend producers to delay first service or to set the voluntary waiting period until 70 days if heat detection rates are only 50%. At this level of reproductive management, a producer would need to begin heat detection and inseminations at approximately 50 days postpartum to have a mean interval of first service of 70 days. Furthermore, the range of day to first service would be between 50 to 90 days postpartum. In contrast, if time of ovulation and first service can be controlled precisely, then a great increase in efficiency of reproductive management can be achieved with all inseminations made precisely at day 70 postpartum. This permits a programmed delay in the necessity to heat detect and inseminate cows until after 70 days postpartum, and first service is made when factors associated with optimal postpartum fertility are in place. Such strategies are reasonable for producers to implement and are cost effective. The objective of this presentation is to integrate the various reproductive strategies that are currently available for application in dairy animals.