Ovarian dynamics and fertility in beef cows administered a 5-day or 6-day CO-Synch protocol for timed artificial insemination

Alexandria Crist
Graduate (MS)
Alvaro Garcia Guerra
Animal Sciences

Timed artificial insemination (TAI) treatment protocols allow synchronization of ovulation and the ability to inseminate all cows at a self-appointed time. One of the most extensively used TAI protocols, the 5-day CO-Synch, entails a short period of progesterone (P4) administration and multiple administrations of prostaglandin F2± (PGF). The objective of the present study was to evaluate the effect of increasing the period of progesterone (P4) device insertion from 5 to 6 days during a TAI protocol on ovarian dynamics, estrous expression, and fertility in beef cows. Suckled beef cows (n=867) at six locations in Ohio were randomly assigned to either a 5-day or 6-day CO-Synch protocol for TAI. Cows received an intravaginal P4 device (CIDR) and 100 µg of gonadorelin acetate (GnRH, Parnell) either on d-9 (6-day) or d-8 (5-day). On d-3 CIDRs were removed, an estrus detection patch applied (Estrotect, Rockway Inc.), and either 500 µg (6-day) or 1000 µg (5-day) of PGF (Estroplan, Parnell) was administered. On d0 all cows received 100 µg of GnRH, the estrus detection patch was evaluated, and AI was performed using frozen-thawed semen. Pregnancy was determined by ultrasonography 34 days after FTAI. Ovarian dynamics were evaluated, using transrectal ultrasonography, in a subset of animals (n=20) between d-3 and d7 to determine preovulatory follicle size, time of ovulation, and corpus luteum size. Blood samples for P4 quantitation were collected between d-3 and d0. Data were analyzed using generalized linear mixed models (SAS 9.4). Cows in the 6-day group tended (P=0.09) to have a larger maximum follicle diameter (16.2 ±0.4mm) than cows in the 5-day group (15.0±0.6mm). In addition, CL volume on d7 was greater (P=0.01) in 6-day (4302±495mm3) than 5-day (2406±455mm3) cows. There were, however, no differences (P=0.15) in the interval between CIDR removal and ovulation. Cows assigned to the 6-day group had greater (Plt;0.05) estrus expression [56.1% (215/385) vs 41.5% (161/388)] and pregnancy rate [54.6% (236/438) vs 47.4% (207/435)] than cows in the 5-day group. In conclusion, an extended period of exogenous P4 during an ovulation synchronization protocol results in a larger preovulatory follicle size, increased estrous expression, thus increasing fertility to timed artificial insemination.