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Department of Dairy Science, University of Wisconsin, Madison 53706
Corresponding author: P. M. Fricke; e-mail: pmfricke{at}wisc.edu.
Nonlactating Holstein dairy heifers (n = 352) 13 mo of age were managed using a 42-d artificial insemination (AI) breeding period in which they received AI after removed tail chalk evaluated once daily. At AI breeding period onset (d 0), heifers were randomly assigned to receive synchronization of ovulation (100 µg of GnRH, d 0; 25 mg of PGF2
, d 6; 100 µg of GnRH, d 8) and timed AI (TAI; d 8) and AI after removed tail chalk for the entire AI breeding period (GPG; n = 175), or AI after removed tail chalk for the entire AI breeding period (TC; n = 177). As expected, 17.7% (31/175) of GPG heifers received AI after removed tail chalk before scheduled TAI. Pregnancy rate per artificial insemination (PR/AI) at ~30 d after first AI tended to be greater for TC (46.5%) than for GPG (38.3%) heifers. No treatment x inseminator interaction was detected; however, overall PR/AI was low for heifers in both treatments due to variation among the 3 inseminators (24.8, 30.0, and 58.0%). Pregnancy loss from ~30 to ~75 d after first AI was 10% and did not differ between treatments. Based on survival analysis, days to first AI was greater for TC than for GPG heifers, whereas days to pregnancy across the 42-d AI breeding period did not differ between treatments. Overall, 81.2% of GPG heifers receiving TAI synchronized luteal regression and ovulated within 48 h after the second GnRH injection. We conclude that this synchronization protocol can yield acceptable fertility in dairy heifers if AI to estrus is conducted between treatment with GnRH and PGF2 and AI efficiency is optimized.
Key Words: dairy heifer • synchronization of ovulation • timed artificial insemination
Abbreviation key: PR/AI = pregnancy rate per artificial insemination, P4 = progesterone, TAI = timed artificial insemination, WHI = wither height index
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