8 Resynchronization of ovulation and timed insemination in lactating dairy cows using the Ovsynch and Heatsynch protocols initiated 7 days before pregnancy diagnosis on day 30 by ultrasonography

Abstract

Rapid re-synchronization of ovulation and insemination in cows found nonpregnant to a previous service is important to maintain high reproductive efficiency. The objective was to compare pregnancy rate (PR) and pregnancy losses (PL) in cows subjected to Ovsynch or Heatsynch protocols 7 days before pregnancy diagnosis by ultrasonography (U/S) on Day 30. Initiation of both protocols included administration of GnRH on Day 23, which was considered the optimal time according to the distribution of interestrus intervals in cows previously inseminated. The study was conducted in a large dairy herd located in north central Florida from March to May, 2003. Of 593 cows treated with GnRH on Day 23 (�1) after insemination and found nonpregnant at U/S on Day 30 (�1; experimental Day 0), 75% (445) had a CL (diestrus) and were sequentially assigned to the two experimental groups. Cows in the Ovsynch Group (n�=�225) received 25�mg (i.m.) PGF2� (Lutalyse, Pharmacia, Kalamazoo, MI, USA) on Day 0, 100�_g (i.m.) GnRH (Cystorelin, Merial Limited, Iselin, NJ, USA) on Day 2, and were timed inseminated 16�h later. Cows in Heatsynch Group (n�=�220) received 25�mg (i.m.) PGF2� on Day 0, and 1�mg (i.m.) estradiol cypionate (ECP, Pharmacia, Kalamazoo, MI, USA) on Day 1, and were timed-inseminated 36�h later. Pregnancy at Day 30 was determined by U/S and at Day 55 by rectal examination of the genital tract. Pregnancy rate and PL were evaluated using multiple logistic regression. Pregnancy rate at Day 30 for Ovsynch (64/225, 28.4%) and Heatsynch (63/220, 28.6%) and PR at Day 55 for Ovsynch (58/225, 25.8%) and Heatsynch (54/220, 24.5%) were not different. In addition, PL between Days 30 and 55 for Ovsynch (6/64, 9.4%) and Heatsynch (9/63, 14.3%) were not different. There were no effects of parity, inseminator and days in milk on PR. However, PR at day 30 was higher in lots with cooling systems (46/141, 32.6%) and lots of first calf heifers (41/135, 30.4%) compared to lots of multiparous cows late in lactation without cooling systems (32/148, 21.6%). In conclusion, Heatsynch and Ovsynch initiated 7 days before pregnancy diagnosis on Day 30 had comparable PR of 25% at Day 55 in lactating dairy cows during the period of March-May.