Manipulation of Progesterone Before Timed AI to Increase Fertility in Lactating Dairy Cows

A series of experiments were conducted to optimize progesterone concentrations during the Ovsynch protocol and improve fertility in lactating dairy cows during first TAI (Chapter I and II) and during resynchronization (Chapter IV). Overall, results showed that medium range (0.5 to 4 ng/mL) P4 at G1, high (> 1 ng/mL) at the PGF, and low (< 0.4 ng/mL) at G2 were associated with greater P/AI at first service as well as during resynchronization. The use of different presynchronization schemes before initiation of the Ovsynch protocol had differential effects on the percentage of cows initiating the Ovsynch protocol on D7 of the estrous cycle, which was then reflected in the final P/AI of each synchronization program. The use of a Double-Ovsynch protocol at first service decreased the percentage of cows with low ( 4.0ng/mL) P4 at G1, and decreases the percentage of cows with low (< 1.0 ng/mL) at the PGF treatment (Chapter I). During resynchronization, presynchronization with a GnRH treatment decreased the percentage of cows initiating the Ovsynch protocol with low (< 0.5 ng/mL) P4 concentrations (Chapter III). We were able to temporarily decreasing progesterone concentrations before initiation of the Ovsynch protocol without inducing complete luteal regression, which allowed for a greater ovulatory response to G1, and cows ovulating to G1 had greater P/AI than cows not ovulating to G1 (Chapter II). Addition of a second PGF treatment at 24 h was effective to reduce the percentage of cows with complete luteal regression. The results obtained with the addition of a second PGF treatment resulted in exceptional improvements in P/AI during resynchronization (Chapter III and IV).