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1 Department of Animal and Range Sciences, North Dakota State University, Fargo 58105
2 Department of Dairy Science, University of Wisconsin, Madison 53706
Corresponding author: P. M. Fricke; e-mail: pmfricke{at}wisc.edu.
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 µg of GnRH (d –10), 25 mg of PGF2
(d –3), and 50 µg 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 PGF2 (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 (P4) profiles determined for a subset of cows (n = 109), P4 concentrations decreased for Presynch cows after the first 2 PGF2 injections, and Presynch cows had greater P4 concentrations at the PGF2 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.
Key Words: Ovsynch • Presynch • TAI • GnRH • PGF2
Abbreviation key: Ovsynch = synchronization regimen using sequential injections of GnRH and PGF2 to control ovulation for timed insemination, Presynch = postpartum regimen using 2 injections of PGF2 to prepare cows for Ovsynch, PR/AI = pregnancy rate per artificial insemination, P4 = progesterone, TAI = timed artificial insemination
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