Effect of milk production on the incidence of double ovulation in dairy cows

To determine the effect of parity and milk production on the incidence of d ouble ovulation, the synchronization of ovulation, using GnRH and prostagla ndin F-2 alpha followed by timed AI (Ovsynch(R)), was initiated at a random stage of the estrous cycle in lactating Holstein cows (n=237). Ovulatory r esponse at 48 h after the second GnRH injection and conception rate at 28 d post AI were determined by transrectal ultrasonography. Ovulation was sync hronized in 84% of cows receiving the Ovsynch(R) protocol. Of the synchroni zed cows, 14.1% exhibited a double ovulation and 47.6% conceived. Conceptio n rate tended to be greater (P=0.08) for cows exhibiting double (64.0%) rat her than single ovulation (45.2%). To determine the effect of milk producti on on the incidence of double ovulation, cows were classified into low (les s than or equal to 40 kg/d) or high (> 40 kg/d) milk production groups base d on the average milk production of 40.5 +/- 0.8 kg/d collected 2 d before AI. Although the incidence of double ovulation tended to increase linearly (P=0.09) with increasing parity, the incidence of double ovulation was near ly 3-fold greater (P<0.05) for cows in the high (20.2%) than the low (6.9%) milk production group. Furthermore, the increase in the incidence of doubl e ovulation with parity apparently occurred because, within a parity group, the proportion of cows with high milk production was greater for the older cows. Twinning rate of cows that calved (n=58) was 5.2%. In a secondary ob jective, cows were retrospectively classified as cystic or normal based on ultrasonographic ovarian morphology at the time of the second GnRH injectio n. Incidence of ovarian cysts was 11%, and the synchronization and concepti on rate of cows classified as cystic was 73.1 and 36.8%, respectively, whic h did not differ from that of normal cows. We conclude that milk production is the primary factor affecting the incidence of double ovulation in lacta ting dairy cows and may explain the effect of parity on twinning rate. In a ddition, Ovsynch(R) appears to be an effective method for establishing preg nancy in lactating dairy cows with ovarian cysts. (C) 1999 by Elsevier Scie nce Inc.