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ABSTRACT. Recently, transvaginal ultrasound-guided follicle aspiration technology has been found to be of great value for in vitro fertilization (IVF) programs, even though the oocyte recovery rate and cleavage rate of transferrable embryos were low. In this study, we investigated the effect of the removal of the dominant follicle at different stages of the estrous cycle on the ovarian response of donor cows. Four experiments (EXPs) were devised. In EXP 1, 3 cows received 20 mg FSH on Day 1, ovulation occurred on Day 0, and on Day 3 follicles were aspirated. In EXP 2, the dominant follicle of the first wave was removed on Day 6 from 3 cows which received 20 mg FSH on Day 7 and on Day 9 follicles were aspirated. In EXP 3, 2 pregnant cows received 20 mg FSH on 70 d of pregnancy and 48 hr later follicles were aspirated a total of 5 times at 5-day intervals. In EXP 4, after ovulation on Day 0, 9 cows received 20 mg FSH on Days 8 to 14 of the estrous cycle and 48 hr after the last injection, follicles were aspirated once. The respective mean ± SD numbers of aspirated follicles and recovered oocytes were higher (p<0.01) in EXP 1 (13.4 ± 1.7 and 8.7 ± 2.3), EXP 2 (12.1 ± 1.4 and 7.7 ± 1.7) and EXP 3 (10.7 ± 2.1 and 7.0 ± 2.2) than in EXP 4 (5.8 ± 2.3 and 3.1 ± 1.6). The oocyte recovery rates were higher (p<0.05) in EXP 1, EXP 2 and EXP 3 than in EXP 4. Similarly, the respective numbers of viable oocytes and cleavage rates were higher in EXP 1, EXP 2 and EXP 3 (6.0 ± 1.3, 5.0 ± 1.1 and 4.6 ± 1.5 viable oocytes (p<0.01); 66, 73 and 65% cleavage rates (p<0.05)) than in EXP 4 (2.4 ± 1.1; 46%). The numbers of morulae and blastocysts were higher (p<0.05) in EXP 1, EXP 2 and EXP 3 than in EXP 4. In conclusion 1) removal of the dominant follicles from lactating and pregnant cows enabled viable oocytes to be recovered constantly and repeatedly by aspiration at different reproductive stages, and that viable blastocysts can be produced after IVF. 2) The presence or absence of a dominant follicle significantly affects the ovarian responses to FSH treatment. 3) This ultrasound-guided procedure proved to be an effective, repeatable and safe method for viable oocyte recovery from valuable pregnant donors. -KEY WORDS: cattle, dominant follicle removal, follicle aspiration, oocyte, ultrasound.

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