The aim of this research was to compare two artificial insemination protocols (AIP): hormonal synchronization with fixed time artificial insemination (SC-FTAI) and the use of a table based on visual observation of estrus signs (VO) in order to identify cows in natural or spontaneous estrus being assigned to AI (NSE-IA). Two groups were formed: in the first group 109 cows were assigned to SC-FTAI, in which a commercial protocol is used; the second one included 108 randomly chosen cows, which were assigned to NSE-AI and in this group a modified table was used. Response variable was first service fertility rate (FSF), which was coded 1 for pregnant and 0 for empty. Predictor variables were AIP, postpartum anestrus, daily milk yield, body condition score at AI and calving number. Statistical analyses included association chi-square tests and logistic regression. Results showed an overall 41.94% FSF and a significant association was detected ( ) between FSF and daily milk yield; pregnancy rates were 42.20% and 41.67% for the SC-FTAI and NSE-IA groups, respectively ( ). The odds ratio for the effect of AIP was only 1.050, suggesting no differences in FSF between groups. The NSE-AI protocol can enhance both the technique of VO and reproductive efficiency. Further validation of the table is required. 1. Introduction The Gyr (Bos indicus) cattle breed from India has been used pure or in crossbreeding programs with dairy breeds such as Holstein and Brown Swiss, in order to combine milk yield ability and adaptation genes that cannot be found in each breed alone. This practice has been helpful in the process to form that is known as dual purpose cattle (DP). In Venezuela, the dairy cattle of Gyr have been imported from Brazil. In DP farms, using artificial insemination (AI), estrus visual observation (VO) is routinely performed during the milking hours. The traditional and more widespread technique for detecting cows in estrus is the quiet acceptance (standing) to be mounted (QAM) by a bull or a herdmate and it is the only sign used for identifying the cows in estrus to be assigned to an AI program or to control natural mating [1–3]. It is the standard practice that cows that are not seen with QAM from a teaser bull or any herdmate are not considered in estrus and, therefore, are not served. This practice discards numerous secondary estrus signs which can be used to identify such a physiological state [2]. Despite the great progress that has been achieved in recent decades in understanding the reproductive physiology of the cow and its relationship to sexual
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