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Heat stress has negative effects on pregnancy rates of lactating dairy cattle. There are genetic differences in tolerance to heat stress, and it has been shown that Bos indicus cattle are more resistant to high temperatures than Bos taurus. In the present work, effect of sire and sire breed on conception rate of Holstein cows during the Brazilian summer (January and February, 2003 and 2004) was determined. Lactating Holstein cows (n = 1113), producing an average of 25.2 kg milk/day and at 247 ± 136 days postpartum, were AI approximately 12 h after estrus detection (n = 433) or fixed-time AI (FTAI) after hormonal induction of ovulation (n = 680), using semen from 4 Gir (Bos indicus dairy bull, 657 AI) and 3 Holstein sires (Bos taurus, 456 AI). Pregnancy diagnosis was performed by ultrasonography or rectal palpation at 28–45 and 70–84 days after AI. Cows diagnosed pregnant at the first examination and open at the time of the second were considered as having lost their embryo. The results were analyzed by logistic regression using PROC GENMOD: the model considered the effect of sire, sire breed, inseminators, milk production 7 days before AI, period of lactation, and AI type (FTAI vs. AI after estrus detection). There was no significant effect of sire breed on early pregnancy rate (EPR: Gir = 78/657, 11.9%; and Holstein = 30/456, 6.6%, P = 0.34) or early pregnancy loss (EPL: Gir = 25/78, 32%; and Holstein = 10/30, 33.3%, P = 0.73). However, there was a sire effect on pregnancy loss (43.7, 35.3, 10, and 40% for Gir bulls 1, 2, 3, and 4, respectively) and (50, 18.2, and 38.5% for Holstein bulls 5, 6, and 7, respectively), with bull 3 having the lowest rate of pregnancy loss (P = 0.03). There was no effect of the AI type on EPR (FTAI = 65/680, 9.5%; and AI after estrus = 43/433, 9.9%; P = 0.8). Surprisingly, there were higher pregnancy losses in cows AI after estrus detection (17/43, 39.5%) as compared to FTAI cows (18/65, 27.7%; P = 0.03). The variables days at postpartum, milk production 7 days before AI, and inseminator did not significantly influence EPR or EPL. It is concluded that use of Bos indicus sires did not improve EPR of lactating Holstein cows during summer as compared to use of Bos taurus sires. However, some bulls, either indicus or taurus, were more effective in decreasing EPL and selection of bulls by this criterion may result in higher parturition rates in lactating Holstein cows.This work was supported by USDA IFAFS Grant # 2001-52101-11318.

22 INFLUENCE OF SIRE AND SIRE BREED (GIR VS. HOLSTEIN) ON EARLY PREGNANCY AND EARLY EMBRYONIC LOSS RATES IN HOLSTEIN COWS DURING SUMMER HEAT STRESS