Individual performance and production per unit area of grazing steers of different potential growth rates

Abstract

AbstractThe purpose of this work was to investigate the relationship between individual animal performance and production per unit area of land of steers of different potential growth rate under different grazing stocking rates (SR). The analysis involved 160 steers divided equally between two breeds, over 2 years at four SRfor each breed and two replicates for each SR. Thus there werefive steers per replicate. The breeds were purebred Aberdeen Angus (Angus) and 4 Limousin (Limousin). After weaning steers were grazed for 251 days on a pasture dominated by Festuca arundinacea. Given that equal SR does not equate to equal treament for breeds characterized to be of different size and rate of growth at a given age, the analysis was undertaken on the basis of three categories of metabolic body size per ha (S). The variables analysed were individual live-weight gain (final minus initial weight, kg), relative individual live-weight gain (logefinal weight - loge initial weight) and production (kg/ha). A mixed model was used to examine effects of breed, S, year and the interactions between these variables. In general significant (P < 0·05) differences were found for all factors of classification and interactions. Independent linear regressions were calculated for each breed between absolute live-weight gain, relative live-weight gain, and production on S. It was concluded that there was a genotype × environment interaction, with the Angus breed tending to show a higher production per unit area than the Limousin, especially at high levels of S when scarcity of forage was apparent. Nevertheless Limousin steers had higher absolute individual performance than the Angus. The higher production of the smaller breed may be explained by a higher SR at a given S level and also by its higher relative individual live-weight gain in relation to that of the larger breed. The data were coded to use Mott's model to study the relationship between absolute performance (kg) and production (kg/ha). However this model ignores the existing interaction involving breeds.