Effect of altering follicular dynamics on fertility of lactating dairy cows receiving artificial insemination after detected estrus.

Fertility of lactating dairy cows is reduced following a naturally occurring estrus compared with fertility programs that controlled antral age of the ovulatory follicle. The aim of this study was to determine the effect of exogenous GnRH on d 5 to 8 of the estrous cycle before 1st AI on fertility of dairy cows after a subsequent estrus. We hypothesized that GnRH during the 1st follicular wave would increase the percentage of cows that ovulated a third follicular wave; and that cows ovulating a third follicular wave would have reduced antral age of the ovulatory follicle resulting in an earlier time to conceptus attachment. Lactating Holstein cows (n = 1,208) were fitted with accelerometer-based activity monitors. Cows detected in estrus during the voluntary waiting period (VWP-estrus; 45 - 67 DIM) were blocked by parity and randomly assigned to one of 2 treatments: no treatment (Control) or GnRH administration 5 to 8 d following onset of VWP-estrus. Cows that returned to estrus following treatment within the breeding period were inseminated 8 to 20 h after onset of estrus (AI-estrus; 68 to 94 DIM). Ovaries were examined on d 5 - 8, 12 or 13, 16 or 17, 20 or 21, and 24 or 25 after VWP-estrus to identify the follicular wave origin of the ovulatory follicle. Ovulation was determined 2 to 8 d post AI-estrus with ultrasound. Day of "conceptus attachment" was defined as an increase of ≥12.5% PSPB from an individual baseline in addition to 2 consecutive d of ≥12.5% increases. Treatment with GnRH during the 1st follicular wave altered follicular dynamics of the remainder of the estrous cycle. It caused more cows to ovulate a 1st-wave dominant follicle compared with the Control group (81 vs. 0%). It also caused more cows to develop a third follicular wave compared with the Control group (68 vs. 12%) and increased the percent of cows that ovulated a shorter antral age follicle compared with Controls. Cows that ovulated a follicle with an antral age of 7 - 9 d had a greater incidence of (53 vs. 37%) of conceptus attachment compared with cows with a greater antral age of 12 to 14 d. These cows also had a lower proportion with late conceptus attachment (11 vs. 28 percent at ≥ d 22). Ovulation of a 1st wave dominant follicle with GnRH shifted the distribution of antral ages of ovulatory follicles at time of AI-estrus. More Treated cows fell within an antral age of 7 to 9 d. More Control cows fell in the less favorable 12 to 14 d antral age group. However, extended time to estrus in cows that developed a 3rd ovulatory wave did not allow for enough cows with reduced antral age, thus, treatment did not improve the incidence or timing of conceptus attachment or reduce pregnancy losses. These outcomes support estimated antral age of the ovulatory follicle as an important fertility-associated phenotype in lactating dairy cows inseminated after detected estrus.