Replacement of the first GnRH administration in the Ovsynch protocol by selecting cows according to the stage of follicular development

The aim of the study was to replace the first GnRH in the Ovsynch protocol by selecting cows bearing corpus luteum as well as follicles in a defined stage of development at PGF(2 alpha) administration. Additionally, various terms of GnRH administration after PGF2a were tested. Seventy five non-pregnant cows bearing corpus luteum were divided into groups according to the phase of follicular development on D 0 ( day of PGF2a administration) -growth ( GR, follicles 3.0 - 7.9 mm in diameter), early dominance ( ED, dominant follicle 8.0 - 14.9 mm) and late dominance ( LD, dominant follicle 15.0 - 23.0 mm). In addition, the cows were divided into groups according to the terms of GnRH administration ( 24, 48 or 72 h after PGF2a). In this way, groups GR 48 ( n = 5), GR 72 ( n = 6), ED 24 ( n = 10), ED 48 ( n = 12), ED 72 ( n = 12), LD 24 ( n = 10), LD 48 ( n = 10) and LD 72 ( n = 10) were established. Growth of ovulatory follicle, term of ovulation, insemination and conception rates as well as relation of the size of preovulatory follicle ( day of ovulation) to the size of following corpus luteum ( day 14) were evaluated. The highest level of synchronization of ovulation ( 100% on D 3) as well as conception rate ( 50%) was achieved in group ED 48. This protocol gives an opportunity of timing artificial insemination to 18 - 24 hours after GnRH administration, as ovulation occurs with a high probability within 24 - 48 hours after GnRH administration.