What is Causing the Differences in Progesterone and Estrogen Concentrations Between Lactating Dairy Cows and Heifers?
Wiltbank and co-workers at the University of Wisconsin-Madison found that feed dry matter intake differences of lactating dairy cows impacted on circulating concentrations of progesterone and estrogen . These hormones are primarily metabolized in the liver. When dry matter intake was increased so was blood flow through the liver. Thus, the greater the blood flow through the liver, the greater the steroid hormone metabolism, resulting in less estrogen and progesterone remaining in circulation.
The research suggests that lactation per se is not the root of the problem. The reduced hormone concentrations likely are associated with the large amounts of feed that lactating cows must consume to produce large quantities of milk.
How Can We Resolve this Problem?
Increasing circulating progesterone concentration during the growth of the ovulatory follicle is a key factor to increase fertility in lactating dairy cows. Cows with greater progesterone concentrations at the time of prostaglandin F2α (PGF) of Ovsynch have greater probability of pregnancy (Figure 2). Therefore, creating higher concentrations of progesterone during Ovsynch should compensate for the progesterone lost due to high metabolism by the liver of lactating dairy cows and enhance fertility.
Strategy to Increase Progesterone Concentrations at Time of PGF of Ovsynch
Multiple strategies previously have been tested to increase progesterone prior to PGF injection of Ovsynch. In studies that used a controlled progesterone releasing intravaginal device (or CIDR) to achieve greater concentrations of progesterone during Ovsynch, there was no significant increase in progesterone on the day of PGF injection of Ovsynch or in resulting conception rates of cycling cows . Concentrations of progesterone in cycling cows with a CIDR were similar to cows without a CIDR (2.7 compared with 2.8 ng/mL) .
Thus, use of a CIDR may not be the best strategy to increase progesterone concentrations and enhance fertility in lactating dairy cows. We therefore examined the possibility that GnRH inducement of a second (accessory) CL during Ovsynch could be used to more effectively enhance progesterone concentrations. In this case, cows would have two CL during Ovsynch, an older CL and a younger CL (accessory CL). The idea was that this accessory CL would increase the production of progesterone and compensate for the lost by liver metabolism.