ABSTRACT. The objective of this study was to develop a bio-economic model for the estimation of economic values of economically important traits in a full-cycle beef cattle production system. The bio-economic model calculated economic values by simulating the impact of changes in the profit of the system consisting of a 1% increase in each trait of the breeding objective, while the other traits were kept constant. The bio-economic model was effective in estimating the sources of revenues and expenses of the production system. The estimated economic values were, in the order of importance for the full-cycle system, R$ 3.69 for male slaughter weight, R$ 3.63 for male weaning weight, R$ 3.58 for weaning rate, R$ 3.40 for female slaughter weight, R$ 2.30 for female weaning weight, and R$ 0.13 for mature cow weight. The results showed that all traits evaluated in the full-cycle system had positive economic impact, indicating that selection would increase profitability maximizing the expected response for the traits of the breeding goal.

Os programas de avaliação genética de bovinos de corte, no Brasil, estão incluindo novas características, principalmente as ligadas à eficiência reprodutiva. Dessa forma, faz-se necessário estudar a efetividade da seleção e quantificar ganho genético para estas características nos rebanhos selecionados. Objetivou-se, neste estudo, estimar parâmetros genéticos e fenotípicos e tendências genéticas para características reprodutivas utilizadas em programas de avaliação genética de bovinos de corte da raça Nelore. As características estudadas foram o perímetro escrotal aos 365 e 450 dias de idade (PE365 e PE450), idade ao primeiro parto (IPP), período de gestação considerada como característica da vaca (PGvaca) e do bezerro (PGbez). Os componentes de (co) variância foram obtidos pelo Método de Máxima Verossimilhança Restrita em análises uni e bi-características, sob o modelo animal. Para o perímetro escrotal (PE365 e PE450), foi observado ganho genético positivo e favorável. Para IPP, PGvaca e PGbez as tendências foram no sentido favorável da seleção, porém sem ganho genético significativo. A seleção para maior perímetro escrotal pode reduzir a IPP e melhorar a eficiência reprodutiva das fêmeas. A seleção para características reprodutivas (PE365, PE450, IPP e PG) pode melhorar a eficiência reprodutiva e produtiva do rebanho, se forem usadas como critérios de seleção para bovinos da raça Nelore.

Genetic breeding programs of beef cattle in Brazil are including new features, mainly related to reproductive efficiency.Thus, it is necessary to study the effectiveness of selection and quantify genetic gain for these traits in herds. This study estimated genetic and phenotypic parameters and genetic trends for reproductive traits used in breeding programs for Nelore beef cattle. The traits studied were the scrotal circumference (SC) at 365 and 450 days of age (SC365 and SC450), age at first calving (AFC) and gestation length, as a cow trait (GLcow) and a calf trait (GLcalf). The (co)variance components were obtained with the Restricted Maximum Likelihood Methodology in a single and double-trait analysis of the animal model. For scrotal circumference (SC365 and SC450), positive and favorable genetic gains were observed. For AFC, GLcow and GLcalf, the trends were favorable for selection, but without significant genetic gain. Selection for large SC may reduce AFC and improve female reproductive efficiency. The selection for reproductive traits (SC365, SC450, AFC and GL) may improve reproductive and productive efficiency of Nelore cattle, if used as a selection criterion.