Progesterone is a steroid hormone that is produced by the corpus luteum, or CL as veterinarians commonly say. Progesterone prepares the uterus for pregnancy and helps maintain the pregnancy by its effects on the uterine lining. Progesterone is also produced by the placenta later during pregnancy. Progesterone is important for successful conception and maintenance of pregnancy. However, the presence of progesterone at the wrong time can make getting cows pregnant more difficult. Thus, the correct amount of progesterone at the right times is important to ensure adequate reproductive performance in dairy cows. This is critical for successful use of timed A.I. programs.
High progesterone is beneficial at the time of the first GnRH injection of an ovsynch protocol, while low progesterone is beneficial at the time of the second GnRH injection and at breeding. Many modifications to timed A.I. protocols have been researched or recommended to attempt to increase the percentage of cows with optimal progesterone levels at the right time. For example, if the first shot of an ovsynch protocol is given randomly to normal, cycling cows, a significant percentage will not ovulate, will not form a CL and hence will have a low concentration of progesterone during the follicular growth phase, resulting in subfertility and higher risk for pregnancy loss and twinning. Pre-synchronization protocols were developed to reduce the percentage of cows that are in a low progesterone phase at the first GnRH injection. However, even pre-synchronization protocols will not necessarily synchronize cows that are not cycling and ovulating normally. Thus, more elaborate pre-synchronization protocols have been developed, such as double ovsynch for example, in an attempt to induce ovulation and create a functional CL that will be producing progesterone at the time of the first ovsynch GnRH injection. Though not originally de-signed for this purpose, CIDRs can be used to provide progesterone in ovsynch protocols where no CL is present. A CIDR essentially replaces the functional CL, and its use can achieve similar conception rates to cows with a CL.
In normal, fertile cows, blood progesterone levels decline rapidly near the end of the estrus cycle. This occurs because the CL regresses due to prostaglandin release. In ovsynch, we inject prostaglandin to regress the CL. However, a young CL, as found when a cow has ovulated following GnRH seven days prior, does not necessarily fully respond to prostaglandin. Also, for unknown rea-sons, young CLs respond better if there is another, mature CL present at the same time. When the CL does not fully regress, progesterone levels stay too high, which can drastically reduce fertility. A variety of strategies have been used to solve this problem, including an additional prostaglandin injection 24 hours later and an increased dose of prostaglandin. Studies mostly have shown that an increased dose does not work as well as an additional dose to regress the CL, so an additional dose of prostaglandin has become a very common practice in ovsynch protocols today. An additional prostaglandin dose was first shown to be effective in a double ovsynch protocol, and it was believed that it would be more effective there than in a simple ovsynch protocol because cows in the double ovsynch protocol are much more synchronized and thus a greater proportion of them will have young CLs. However, a recent study by Holper, et. al, in the January Journal of Dairy Science showed an increase in the percentage pregnant per insemination of over 6 points in a standard ovsynch protocol. An additional dose was particularly effective in cows that did not have a CL at the time of the first GnRH injection in an ovsynch protocol. Cows with no CL that received two doses of prostaglandin had 47.9% pregnancies per A.I., while cows with a double dose had 32.7%.
In this study, cows that were given CIDRs that had more vaginal discharge had higher rates of pregnancy per A.I., and the overall accuracy of detection of a functional CL by transrectal ultra-sound was 87.2%. The optimum cutoff was a 20 mm diameter of the CL. Increased vaginal discharge might be due to higher vaginal blood flow that may result in increased hormonal absorption. Accuracy of diagnosis of a CL capable of producing progesterone is important if one wants to use CIDRs to increase fertility in cows without mature CLs.
The estrus cycle of a cow is an intricate dance of hormones and other factors. Understanding the role of progesterone and other hormones is critically important to designing timed A.I. protocols for herds. The best source to answer questions regarding the optimal reproductive protocol for your herd is your veterinarian. Do not be shy about asking.
Bennett is one of four dairy veterinarians at Northern Valley Dairy Production Medicine Center in Plainview, Minnesota. He also consults on dairy farms in other states. He and his wife, Pam, have four children. Jim can be reached at [email protected] with comments or questions.