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Fertility of Lactating Dairy Cows Treated with Ovsynch after Presynchronization Injections of PGF₂ and GnRH

Department of Animal Science Michigan State University, East Lansing 48824

Corresponding author:
J. R. Pursley; e-mail:
pursley{at}msu.edu.

Synchronization of ovulation (Ovsynch) using GnRH and PGF₂ allows control of follicle growth, corpus luteum regression, and ovulation, but resulting pregnancy rates vary. This study examined whether presynchronization to allow initiation of Ovsynch during diestrus would improve pregnancy rates at timed artificial insemination (AI). Lactating dairy cows (n = 427), 69 to 75 d postpartum, were randomly assigned to two groups by parity. Control cows received Ovsynch (GnRH, d 0; PGF₂, d 7; GnRH, d 9; timed AI 16 h after second GnRH). Treated cows received presynchronization injections of PGF₂ and GnRH, 10 and 7 d, respectively, before starting Ovsynch. Pregnancy diagnoses were performed 36 d after AI. Progesterone (P₄) concentrations from a subset of cows (n = 84) were determined in serum samples collected on d 0, 3, and 7 of Ovsynch. Presynchronization increased the percentages of cows with 1 ng/ml serum P₄ compared with control cows at first injection of GnRH (d 0; 93 vs. 56%) and on d 3 (90.7 vs. 51.2%) during Ovsynch. On day of PGF₂, d 7 during Ovsynch, percentages of cows with 1 ng/ml serum P₄ were similar (95.3%, treated vs. 82.9%, control) but more treated cows had 2 ng/ml serum P₄ (95.3 vs. 63.4%). However, pregnancy to timed AI was similar between treated (41.5%) and control cows (38.3%). Cows with above-average milk production had greater pregnancy rate (45.8 vs. 33.8%) compared with lower producing cows. Although presynchrony increased the proportion of cows with luteal function at onset of Ovsynch, pregnancy rate to timed AI was not improved. Cows with above-average milk production had greater fertility at timed AI than herdmates with lower milk production.

Abbreviation key: Ovsynch = synchronization of ovulation (GnRH, d 0; PGF₂, d 7; GnRH, d 9; timed AI 16 h after second GnRH), PR = % pregnant to a timed AI

Key Words: Ovsynch • presynchronization • fertility

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