Resynchronization of ovulation and timed insemination in lactating dairy cows I: use of the Ovsynch and Heatsynch protocols after non-pregnancy diagnosis by ultrasonography

Abstract The objective was to compare pregnancy rates to resynchronization and timed AI (TAI) protocols in lactating dairy cows that received GnRH at 23d and were diagnosed not pregnant at 30d after the pre-enrollment AI. Nonpregnant cows (624) at ultrasonography on day 30 (study day 0) were classified as diestrus (74.8%), metestrus (5.6%) and without a CL (19.5%). Cows in diestrus were assigned either to the GnRH group (PGF 2α on day 0, GnRH on day 2 and TAI 16h later, n = 238) or the estradiol cypionate (ECP) group (PGF 2α on day 0, ECP on day 1, and TAI 36h later, n = 229). Cows in metestrus were assigned to the Modified Heatsynch Group (GnRH on day 0, PGF 2α on day 7, ECP on day 8 and TAI on day 9, n = 35). Cows without a CL ( n = 122) were classified either as proestrus (10.6%), ovarian cysts (7.5%) or anestrus (1.4%), and assigned to factorial treatments (i.e., use of GnRH versus CIDR) to either the GnRH group (GnRH on day 0, PGF 2α on day 7, GnRH on day 9 and TAI 16h later, n = 28), the CIDR group (CIDR insert from days 0 to 7, PGF 2α on day 7, GnRH on day 9 and TAI 16h later, n = 34), the GnRH + CIDR group (GnRH on day 0, CIDR insert from days 0 to 7, PGF 2α on day 7, GnRH on day 9 and TAI 16h later, n = 32), and the control group (PGF 2α on day 7, GnRH on day 9 and TAI 16h later, n = 28). For cows without a CL, plasma P 4 concentrations were determined on days 0, 7, 10 and 17 and ovarian structures determined on days 0, 7 and 17. Pregnancy rates were evaluated at 30, 55 and 90d after the resynchronized AI. For cows in diestrus, there were no differences in pregnancy rates on days 30, 55 and 90 for cows in the GnRH (27.5, 26.5 and 24.2%) or ECP (29.1, 25.5 and 24.1%) groups. In addition, there were no differences in pregnancy losses between days 30 and 55 and 55 and 90 between the GnRH (7.0 and 8.6%) and ECP (9.8 and 5.4%) groups. For cows without a CL, GnRH on day 0 increased the proportion of cows with a CL on days 7 and 17 and plasma P 4 concentration on day 17 in cows with ovarian cysts but not for cows in proestrus. The CIDR insert increased pregnancy rate in cows with ovarian cysts but reduced pregnancy rate for cows in proestrus.