Resumo:
Fixed-time artificial insemination is a tool of reproductive biotechnology that improves the
herd's genetics to obtain more productive animals. The objective of this work was to evaluate
the associations between the occurrence of estrus after synchronization and ovulation
protocols and follicular diameter at the time of fixed-time artificial insemination (TAI) on
ovarian responses and pregnancy rate after TAI in females Bos taurus taurus/ Bos taurus
indicus. Thus, possible practical implications of using this information to improve TAI
programs in lactating mixed-breed females were evaluated. In the first chapter, the
associations between follicular diameter at the time of TAI and the occurrence of estrus
between the end of the synchronization protocol and the moment of TAI were evaluated on
the ovarian response and the conception rate after TAI. It was observed that the increase in
follicular diameter at the time of TAI increases the conception rate after TAI and that the
occurrence of estrus after the synchronization protocol is associated with better ovarian
responses and a better conception rate after TAI. Therefore, strategies to improve the
occurrence of estrus and follicular diameter at the time of TAI are essential for the success
of ovulation synchronization programs for TAI of Bos taurus taurus/Bos taurus indicus
females, and such criteria can be used in the selection of females to receive high genetic
quality semen. The second chapter reports on four studies in which the associations between
the use of progesterone and estrogen and their effect on luteal characteristics in
resynchronization protocols using TAI were evaluated, as well as the determination of the
pregnancy rate in Bos taurus taurus/Bos taurus indicus cows during the resynchronization
protocol. The results indicate that it is possible to obtain higher conception rates after
ovulation resynchronization in crossbred cattle but without estradiol or with low doses.
