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The objective of this study was to evaluate the efficiency of oestrus/ovulation synchronization using PGF(2alpha) and GnRH at intervals according to the sizes of ovarian follicles at the time of initial treatment. Oestrus synchronization was performed in 65 cows from 3 commercial dairy farms. Synthetic analogue of PGF(2alpha) cloprostenol was administered in cows bearing corpus luteum between days 50 to 100 post partum. The cows were divided into 3 groups according to diameters of ovarian follicles in ultrasonographical image: Group 1 - small follicles (3-7 mm, n = 10). Group 2 -,small and medium follicles (3-7 and 8-14 min, n = 35) and Group 3 - small, medium and large follicles (3-7 rum, 8-14 mm and 15-23 mm, n = 20). GnRH was administered 72 hours, (Group 1), 48 h (Group 2) and 24 h (Group 3) after PGF(2alpha) Ultrasonographical examinations of ovarian structures were performed on days 0 (day of PGF(2alpha) administration) 2, 4, 6, and 12. Blood samples for progesterone determination by RIA were obtained in 50 cows (10 cows in Group 1, 25 cows in Group 2 and 15 cows in Group 3) on days 0 and 12. Detection of oestrus Was performed twice daily and cows in oestrus were inseminated. Term of oestrus and ovulation, progesterone concentration and conception rate were evaluated. oestrus was detected and insemination was performed in 8 of 10 (80%), 33 of 35 (94%) and 12 of 20 (60%) cows in experimental Groups 1, 2 and 3, respectively, within 5 days after PGF(2alpha) administration. Ovulation Occurred in all inseminated cows in Groups I and 2 within 2 days after insemination. In spite of oestrus Occurring and insemination being performed only in 12 cows in Group 3 Ovulation occurred in 15 cows in this Group Within 5 days after PGF(2alpha). Corpora lutea were found in all ovulated cows (except 1 cow in Group 2) on day 12, Conception rates in inseminated cows were 50%, 64% and 42% in Groups 1, 2, and 3, respectively. The highest efficiency of oestrus synchronization was found in Group 2 in which oestrus, occurred in 29 of 35 (83%) observed cows oil day 3 after PGF(2alpha) and conception rate was 66% in these cows.

Oestrus Synchronization by PGF2α and GnRH in Intervals according to Stage of Follicular Development at Time of Initial Treatment in Cows