Effect of Time of Artificial Insemination and Supplemental Estradiol on Reproduction of Lactating Dairy Cows

Abstract

Our objectives were to compare reproductive responses of dairy cows receiving timed artificial insemination (AI) either at 48 or 72h after induction of luteolysis and supplemented or not with estradiol cypionate (ECP). Holstein cows (971) had their estrous cycles presynchronized with injections of PGF2α at 37 and 51 d in milk (DIM) and then received an injection of GnRH at 64 DIM and an injection of PGF2α at 71 DIM. Cows were then assigned to a 2×2 factorial randomized block experiment; cows in the CoSynch 48h (CoS48) received a final injection of GnRH concurrent with timed AI 48h after PGF2α, whereas cows in the CoSynch 72h (CoS72) received GnRH and timed AI 72h after PGF2α. Half of the cows in each CoSynch protocol received an injection of 1mg of ECP 24h after PGF2α. Therefore, the 4 treatments were as follows: CoS48-NECP (n=240), CoS72-NECP (n=246), CoS48-ECP (n=245), and CoS72-ECP (n=240). Blood was sampled at 7 d before and at the first GnRH of the CoSynch from all cows for analysis of progesterone concentration in plasma. Cows were classified as anovular when progesterone was less than 1.0 ng/mL in both samples. Blood was also sampled during proestrus from a subset of 123 cows to measure concentrations of estradiol and at 7 d after timed AI to measure concentrations of progesterone. Ovaries from the same subset of 123 cows were examined by ultrasonography to determine ovulatory follicle diameter and incidence of ovulation. Pregnancy was diagnosed at 40 and 68 d after AI. Prevalence of cyclic cows was 72.4% and was similar among treatments. Concentrations of estradiol increased after ECP treatment and at 72h of proestrus with CoS72. Pregnancy at 40 and 68 d after AI and pregnancy loss were not affected by timing of AI or supplemental ECP. Delaying timed AI to 72h and supplementation with ECP increased the proportion of cows displaying estrus at AI, and cows detected in estrus had increased pregnancy per AI associated with improved ovulation and increased postovulatory progesterone concentration. These results indicate that extending the proestrus by delaying timed AI from 48 to 72h plus supplemental ECP, despite increased expression of estrus at timed AI, did not improve reproductive performance of lactating dairy cows at first AI.