Modifications of the G6G timed-AI protocol improved pregnancy per AI and reduced pregnancy loss in lactating dairy cows

In dairy cows, subjected to a G6G protocol, objectives were to determine effects of (1) extending the interval from prostaglandin F2 alpha (PGF2 alpha) to gonadotropin-releasing hormone (GnRH) during presynchronization; and (2) adding a second PGF2 alpha treatment before artificial insemination (AI), on ovarian response, plasma progesterone (P4) concentrations and pregnancy per AI (P/AI). In a 2x2 factorial design, lactating cows were randomly assigned to one of four timed AI (TAI) protocols: (1) G6G (n=149), one injection of PGF2 alpha, GnRH 2 days later and a 7-day Ovsynch (GnRH, 7 days, PGF2 alpha, 56 h, GnRH, 16 h, TAI) was initiated 6 days later; (2) G6GP (n=144), an additional PGF2 alpha treatment (24 h after the first) during Ovsynch of the G6G protocol; (3) MG6G, one injection of PGF2 alpha, GnRH 4 days later before initiation of the G6G protocol; and (4) MG6GP, an additional PGF2 alpha treatment (24 h after the first) during Ovsynch of the MG6G protocol. Blood samples were collected (subset of 200 cows) at first GnRH and PGF2 alpha of the Ovsynch, and at TAI to measure P4. Ultrasound examinations were performed in a subset of 406 cows to evaluate ovarian response at various times of Ovsynch, and in all cattle to determine pregnancy status at 32 and 60 days after TAI. Extending the interval by 2 days between PGF2 alpha and GnRH during presynchronization increased (P < 0.01) ovulatory response to first GnRH of Ovsynch, circulating P4 during Ovsynch, and P/AI at 32 and 60 days after TAI. Adding a second PGF2 alpha treatment before AI increased the proportion of cows with luteal regression (P = 0.04), improved P/AI at 60 days after TAI (P = 0.05), and reduced pregnancy loss between 30 and 60 days after TAI (P = 0.04). In summary, extending the interval from PGF2 alpha to GnRH during presynchronization increased response to first GnRH of Ovsynch and P4 concentrations during Ovsynch, whereas adding a second PGF2 alpha treatment before AI enhanced luteal regression. Both modifications of the G6G protocol improved fertility in lactating dairy cows.