A modified presynchronization protocol improves fertility to timed artificial insemination in lactating dairy cows

To compare 2 hormonal protocols for submission of lactating dairy cows for timed artificial insemination (TAI), nonpregnant lactating Holstein cows (n = 269) >60 d in milk were randomly assigned to each of 2 treatments to receive TAI ( TAI = d 0). Cows assigned to the first treatment (Ovsynch, n = 134) received 50 mug of GnRH (d-10), 25 mg of PGF(2alpha) (d-3), and 50 mug of GnRH ( d-1) beginning at a random stage of the estrous cycle. Cows assigned to the second treatment (Presynch, n = 135) received Ovsynch but with the addition of 2 PGF(2alpha) ( 25 mg) injections administered 14 d apart beginning 28 d (d-38 and -24) before initiation of Ovsynch. All cows received TAI 16 to 18 h after the second GnRH injection. Ovulatory response after each GnRH injection for a subset of cows ( n = 109) and pregnancy status 42 d after TAI for all cows were assessed using transrectal ultrasonography. Based on serum progesterone (P-4) profiles determined for a subset of cows (n = 109), P-4 concentrations decreased for Presynch cows after the first 2 PGF(2alpha) injections, and Presynch cows had greater P-4 concentrations at the PGF(2alpha) injection on d -3 compared with Ovsynch cows. Although the proportion of cows ovulating after the first and second GnRH injections did not differ statistically between treatments (41.1 and 69.6% vs. 35.9 and 81.1% for Ovsynch vs. Presynch, respectively), pregnancy rate per artificial insemination (PR/AI) at 42 d post TAI was greater for Presynch than for Ovsynch cows (49.6 vs. 37.3%). Parity, DIM, and body condition score (BCS) at TAI did not affect PR/AI to TAI. These data support use of this presynchronization protocol to increase PR/AI of lactating dairy cows receiving TAI compared with Ovsynch.