Bos taurus × Bos indicus crosses are widespread in tropical and subtropical regions, nonetheless, quantitative information about the influence of B. indicus genetics on the reproductive performance of beef cattle is lacking. Herein, we determined the association between level of B. indicus genetics and reproduction from a 31-yr dataset comprising sequential breeding seasons of the University of Florida multibreed herd (n = 6,503 Angus × Brahman cows). The proportion of B. indicus genetics in this herd is evenly distributed by each 1/32nd or approximately 3-percentage points. From 1989 to 2020, the estrous cycle of cows was synchronized for artificial insemination (AI) based on detected estrus or timed-AI (TAI) using programs based on gonadotropin-releasing hormone and prostaglandin, and progestin/progesterone. All cows were exposed to natural service after AI and approximately 90-d breeding seasons, considering the day of AI as day 0. The proportion of B. indicus genetics of cows was associated negatively with pregnancy per AI, ranging from 51.6% for cows with 0%-19% of B. indicus genetics to 37.4% for cows with 81%-100% of B. indicus genetics. Similar association was found for estrous response at the end of the synchronization protocol, ranging from 66.3% to 38.4%, respectively. This reduced estrous response helped to explain the pregnancy results, once the pregnancy to AI of cows showing estrus was 2.3-fold greater than for those not showing estrus and submitted to TAI. Despite reduced pregnancy per AI, the increase in the proportion of B. indicus genetics of cows was not associated with a reduction in the proportion of pregnant cows at the end of the breeding season. Nevertheless, the interval from entering the breeding season to pregnancy was lengthened as the proportion of B. indicus genetics of cows increased. The median days to pregnancy was extended by 25 when the proportion of B. indicus genetics surpassed 78% compared with less than 20%. Thus, the increase in the proportion of B. indicus genetics of cows was related to a reduction in pregnancy per AI and lengthening the interval to attain pregnancy during the breeding season, but not with the final proportion of pregnant cows. As a result, reproductive management strategies directed specifically to cows with a greater proportion of B. indicus genetics are needed to improve the rate of pregnancy in beef herds.
Keywords: Bos indicus; artificial insemination; estrus; pregnancy; synchronization.
Cow–calf operations in the tropics and sub-tropics have benefited from the environmental adaptation provided by Bos indicus genetics. However, reproductive performance has been a cause of concern, although poorly quantified. This study characterized how much the B. indicus genetics in crossbred cows influence herd reproduction. We analyzed data from cows with known proportions of Angus and Brahman genetics, from the same crossbred herd, for 31 sequential breeding seasons. The increase in the proportion of B. indicus genetics reduced estrous response and pregnancy per artificial insemination after estrous synchronization, but not the proportion of pregnant cows at the end of the breeding season. Interval from the beginning of the breeding season to pregnancy was extended by 25 d when the proportion of B. indicus genetics surpassed 78%. In conclusion, reproductive management strategies directed specifically to cows with a greater proportion of B. indicus genetics are needed to improve the rate of pregnancy in beef herds.
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