Reproductive performance of purebred and crossbred beef cattle in the tropics of Mexico

Abstract

AbstractRecords of beef cows on a ranch in south-east Mexico gave 1377 ages at first calving (AFC) and 3932 calving intervals (CI) for the comparison by least-squares analysis of the genetic groups: commercial zebu (CZ), Brahman, other zebu breeds and their zebu crosses (zebu), F, crosses born to zebu dams by Bos taunts or Santa Gertrudis sires, and 3/4 bred zebu and 3/4 Bos taurus. Subsamples of 1090 ages at first calving and 3154 intervals were analysed considering the breed groups: CZ, Brahman, Guzerat, Gyr and Indo-brazil, and the F1 crosses resulting from matings between these zebu breeds, Santa Gertrudis and Charolais as sire breeds and the CZ as the dam breed. Three populations were analysed for each model, the original ranch population and two populations reduced by retrospective culling. The original population had a mean AFC and CI of 1226 and 489 days respectively reduced by the first and second culling by 24 and 37 days and by 62 and 83 days respectively. Repeatability of CI increased from 0·06 to 0·11 and 0·14 respectively.Genetic group significantly influenced AFC. In the original population F1 heifers had a significantly younger AFC than Brahman and zebu heifers but not significantly younger than the other genetic groups. Heifers with 0·25, 0·75 and 1·00 zebu genes did not have significantly different AFCs. After the retrospective culling only the 3/4 bred zebus were significantly superior to the worst group, the zebus. The effect of breed group was only significant in the original population but generally the same rank order was maintained after the retrospective culling. F, Charolais × CZ heifers had the youngest AFC, a superiority of 67 days over CZ and 92 days over Brahman. Purebred zebus and their F, crosses with CZ did not calve earlier than CZ dams. Genetic group had a highly significant effect on CI. Females with 0·50 and 0·75 Bos taurus genes had shorter CIs than females with 0·75 and 1·00 zebu genes. The superiority was maintained after culling. Breed group had a significant effect in each population. F1 Charolais × CZ dams had the shortest CI. CZ had equivalent or shorter CI than all zebu breeds and their F1 crosses with CZ. Heterosis effects for AFC and CI in the zebu crosses were generally advantageous but small, while the heritabilities for the zebu population were negative for AFC and 0·05, 0·22 and 0·06 for CI in the original and two culled populations respectively.