Ovulation failure and double ovulation rates were examined in 1917 inseminations performed in high-yielding dairy cows under standard commercial conditions. The ovulation rate was determined 11 days post-insemination by ultrasound detection of at least one corpus luteum in the ovaries. Analyzing the double ovulation and pregnancy rates, the study population consisted only of ovulated cows (n = 1792). Data were analyzed using logistic regression methods. A failure to ovulate was recorded in 125/1917 (6.5%) services: 82/663 (12.4%) during the warm and 43/1254 (3.4%) during the cool period. Based on the odds ratios, the risk of ovulation failure was 3.9 times higher for inseminations performed during the warm period. No significant effects of estrous synchronization, milk production and days in milk at AI, and service and lactation number on ovulation failure were found. Double ovulation was recorded in 277/1792 (15.5%) services: 146 (52.7%) unilateral double ovulations (42.5% left versus 57.5% right); 115 (41.5%) bilateral double ovulations; and 16 (5.8%) triple ovulations. Double ovulation was recorded in 72 (12.4%) and 205 (16.9%) AI during the warm and the cool period, respectively. The percentages of double ovulation for first, second and third or more lactations were 6.7, 16.6 and 25%, respectively. Double ovulation rates for early (less than 90 days), mid- (90-150 days) and late (more than 150 days) lactation periods were 13, 20.7 and 14.2%, respectively. Reaching estrus during the warm period decreased the likelihood of double ovulation by a factor of 0.86; the risk of double ovulation was lower in cows with higher milk production (a I kg increase in milk yield led to a 0.97-fold reduced risk of double ovulation); cows in their second and in their third or more lactations showed a likelihood of double ovulation (using the first lactation as reference) increased by factors of 3.4 and 5.6, respectively; and reaching estrus during the early and late lactation period was related to a decreased probability of double ovulation (using the mid-lactation period as reference) by factors of 0.56 and 0.84, respectively. No significant effects of synchronization and service number on the double ovulation rate were found. Pregnancy was recorded in 914/1792 (51%) services: rates of 53.5% (811/1515) were recorded for single ovulations; 37.2% (103/277) for double ovulations: 28.8% (42/146) for unilateral double ovulations; 45.2% (52/ 115) for bilateral double ovulations; and 56.3% (9/16) for triple ovulations. The likelihood of pregnancy diminished in cows: inseminated during the warm period (by a factor of 0.5); inseminated by one particular bull (by a factor of 0.33); with higher milk production (a 1 kg increase in milk yield decreased the probability of pregnancy by a factor of 0.98); or undergoing unilateral (by a factor of 0.31) and bilateral (by a factor of 0.64) double ovulation. Logistic regression analysis indicated no significant effects of synchronization, days in milk, lactation number and service number on pregnancy rate. Collectively, our results indicate that cows showing estrus in conditions of heat stress had a high risk of ovulation failure. The effect of milk production on double ovulation was negative, whereas lactation number was positively correlated with this factor; the highest incidence of double ovulation occurring during the mid-lactation period. (C) 2004 Elsevier Inc. All rights reserved.
