Genetic and environmental effects on the fertility of beef cattle in a tropical environment.

Calving data recorded over a 28-year period were analysed for the effects of breed, filial generation, lactational status, cow liveweight at mating and year of mating. The herd comprised six distinct breeding lines ('breeds') and analysis was performed both within and across breeds. The least fertile breeds were the high grade (314-718) Africander (A) and high grade (718-purebred) Brahman (B) cows which had average calving rates of 56% and 55%, respectively. Crossing these with a Hereford- Shorthorn line, which had an average calving rate of 6O%, resulted in 19% heterosis (in absolute calving percentage units) in the F1 Africander-cross (AX) and 16% heterosis in the F1 Brahman-cross (BX). In subsequent generations, 13% residual heterosis remained in the AX, while in the BX only 1% remained. Reciprocal crossing of the AX and BX lines generated 6% heterosis in the Fl AXBX, and all of this was maintained in subsequent generations. In the F2 et seq. generations of the AXBX, the line of animals carrying the Bos indicus Y chromosome was similarly fertile (P> 0.05) to the line carrying the Bos taurus Y chromosome. Lactating mature (3 5-year-old) cows were on average 15% less fertile than non-lactating cows. This lactation effect was larger in younger cows. There was a curvilinear response in fertility to increasing liveweight at mating which was most pronounced in 2-year-old heifers. Breed appeared to react differently to the effects of environmental stress where the AX and HS cows were limited more by liveweight, and the BX and AXBX were more limited by lactation effects per se. Lactating cows that also lactated in the previous year were 6% more fertile than lactating cows that were dry the previous year, suggesting that calving records are moderately repeatable and therefore that permanent differences in fertility could be established by phenotypic selection.