137 DIFFERENCES IN EMBRYO PRODUCTION BETWEEN LACTATING AND NON-LACTATING HOLSTEIN DONOR COWS

Embryo transfer is a common practice in reproductive management of large dairy herds. Data from 2 dairies in the San Joaquin Valley in California (5000 lactating Holstein producing 27 500 pounds of milk in 365 days) was used to evaluate the differences in embryo production between lactating (105.3 ± 24.6 pounds of milk/day, 161 ± 85 days in milk and 3 ± 1 lactations) and nonlactating donor cows, from July 2006 to June 2007. Cows were observed daily for estrous signs and superovulation treatment was initiated 10 d later. Cows received FSH twice a day during 4 days in decreasing doses. On the fifth treatment of FSH, cows received an injection of PGF2a and 48 h later an injection of GnRH. Cows were inseminated 12 and 24 h after the GnRH injection. Uterine flush was performed 7 d after the first AI only in cows that had 2 or more CL. The total number of ova/embryos, viable embryos (transferred fresh or frozen), degenerated, and unfertilized structures were recorded (classified according to the guidelines of IETS). A Bernoulli regression model was fitted using the R software (www.r-project.org) for the variable superovulation response and the covariates were cow condition (lactating or nonlactating), season, and interactions. From 719 superovulated cows, 668 were flushed (92.9%): 86.8% (309/356) of the lactating cows and 98.9% (359/363) of the nonlactating cows (P < 0.01). There was evidence of season effect (P = 0.09). Cows tended to respond less to superovulation treatment during summer and fall. A negative binomial regression model was fitted to the response variables total number of ova/embryo and viable embryos and the covariates were cow condition, season, and interactions. Lactating cows produced fewer (6.77) ova/embryos than nonlactating cows (14.14; P < 0.01). The adjusted means were 5.93 v. 12.53 for fall, 6.30 v. 13.32 for winter, 8.24 v. 17.40 for spring and 6.32 v. 13.34 for summer, for lactating and nonlactating cows, respectively. Cows produced more ova/embryos during spring (P < 0.01). Less viable embryos were recovered from lactating cows (3.62) than nonlactating cows (5.44; P < 0.01). The adjusted means were 2.52 v. 3.98 for fall, 3.61 v. 5.69 for winter, 4.51 v. 7.12 for spring and 3.37 v. 5.31 for summer, for lactating and non lactating cows, respectively. The production of viable embryos in fall was different from winter and spring (P < 0.01). Lactating cows responded less to superovulation treatment and produced less viable embryos than nonlactating cows, probably because of the increased metabolism due to milk production. Warmer seasons were associated with lower viable embryo production. It is important to have a group of nonlactating donors to increase embryo production in large ET programs.