Objetivos de seleção e valores econômicos para o melhoramento genético das raças Hereford e Braford no Brasil

Abstract

The objective of this study was to characterize the different production systems occurring in southern Brazil by developing bio-economic models to derive economic values (EV) for traits that affect the profitability of these systems. Three bio-economic models were developed and used to estimate EV, being the first model for the cow-calf operations, in which only weaned calves, heifers and cull cows are marketed. The second bio-economic model developed describes the full cycle of production system characterized by the sale of steers with 24 months, surplus heifers with 30 months and cull cows. The third bioeconomic model describes the system of rearing and finishing, based on crop - livestock integration, where part of the property area is cultivated with soybean (Glicyne max spp.) during the summer, providing cultivated pasture for beef cattle during the winter and early spring. In this production system weaned calves are bought and raised till slaughter with 24 months. The EV for the selection objectives were defined as the expected increase in annual herd profit, resulting from a unit increase in a trait keeping all the others constant. The evaluated trait that had the highest economic value in the cow-claf operation was the survival rate (SR) (R$ 17.23), followed by the birth rate (BR), weaning weight (WW) and mature cow weight (MCW), with respective values of R$ 6.42, R$ 1.96 and R$ 0.55. Despite having greater economic importance, due to extreme influence of environmental factors and very low heritability, the SR can hardly be used as a direct criterion for genetic selection and should be improved through health and feed management or by indicator traits. Among the traits amenable to genetic improvement, the higher relative importance was for the BR (68.03%), followed by WW (23.99%) and MCW (12.62%). In the full cycle system, the trait with the highest EV was also the SR (R$ 41.4), followed by BR, carcass weight (CW), WW, average daily gain (ADG) and MCW, with EV equal to R$ 10.22, R$ 4.36 R$ 1.19, R$ 0.80, and R$ 0.37, respectively. When ordered by the relative importance of each trait, BR showed higher importance (59.0%), followed by CW (16.0%), ADG (13.0%), WW (6.0%) and MCW (5.0%). In the rearing and finishing system, the SR was again the trait with higher EV (R$ 21.48), followed by CW, ADG and WW, with EV of R$ 6.55, R$ 1.24 and R$ -2.37, respectively. The trait of greater relative importance was the CW (41.99%), followed by ADG (36.18%) and WW (27.91%). The traits EV varies according to production systems, being necessary to estimate these values using specific bioeconomic models that consider the characteristics of each system. The selection of animals that are more adapted and/or resistant to productive conditions can raise the SR and consequently increase the profitability of the systems, due to high EV of this trait. Selection for reproductive traits has greater relative importance than growth traits in cow-calf and full cycle systems, and should be prioritized in genetic evaluation programs.