ESTIMATES OF BREED DIRECT, MATERNAL AND HETEROSIS EFFECTS FOR WEANING AND YEARLING WEIGHTS OF BEEF CATTLE IN THE HUMID TROPICS OF MEXICO

Abstract

The objective of this study was to estimate the breed-direct, breed-maternal and heterosis effects from data obtained for weaning and yearling weights of Brahman and its crosses with Charolais, Simmental, Brown Swiss breeds in a beef herd in Tabasco, Mexico. The climate of the region is tropical humid. Data were obtained on 1217 calves born from 1995 to 2007; among the 16 breed-group combinations one was purebred mating (Brahman), 3 two-breed static crosses, 7 three-breed static crosses and 5 backcrosses. Cows were fed in paddocks of Jaragua grass (Hyparrenia rufa) of poor condition and invaded with native grass of Paspalum and Axonopous genera. Calves remained with their dams until weaning. They were weighed at weaning and about one year of age. Calve weights from the record cards kept at the ranch were used to calculate adjusted weaning (W240) and yearling weights (W365). During the 13 year period of data collection, 711 cows and heifers were inseminated with semen of 23 Charolais (Ch), 50 Simmental (Si), 19 Brown Swiss (BS) and 15 Brahman (Br) unrelated sires. Also 32 Br bulls were used for natural mating. A multiple regression procedure was used to obtain estimates of the breed-direct, breed-maternal and direct heterosis effects for W240 and W365. The overall means and standard errors for W240 and W365 were 178.5+0.89 and 219.9+1.78 kg, respectively. The analysis of variance showed significant (P<0.01) effects of year of birth, season of birth, sex of the calf, calving number of the cow and year x season interaction on W240, and of year, sex and year x season interaction on W365. Overall, breed-direct, breed-maternal and direct heterosis effects were significant on W240, but only breed-direct effect on W365. Charolais (P=0.009) and BS (P= 0.019) breed direct effects and breed maternal Ch (P= 0.029) effects were significant for W240. The breed-direct and breed-maternal effects of Ch, BS and Si breeds were nonsignificant (P>0.20) for W365. Except for the Si x Br negative heterosis value for W240, all heterosis effects were positive but nonsignicant (P>0.12).