Genetic parameters were estimated for weight at the beginning of breeding season (PEM), considered as an indicative of mature weight of Nelore cows. Data file comprised 7,902 records from 1,556 cows from to a selection experiment conducted at the Estação Experimental de Zootecnia de Sertãozinho, SP, Brazil. PEM were analyzed either as the last weight available for each cow in the data file (PEM_U) or as repeated records, including all weights (PEM_R). The analyses were also performed excluding the records of cows culled before reaching 4 years of age, and for both the last (PEM_U2) and the repeated (PEM_R2) records. Variance components were estimated by restricted maximum likelihood, fitting single and multiple trait animal models. The multiple trait models included the selection weights adjusted for 378 (males only) and 550 (females only) days of age. Heritability estimates obtained from the single trait analyses were 0.30±0.05, 0.37±0.06, 0.35±0.04 and 0.42±0.05, for PEM_U, PEM_U2, PEM_R and PEM_R2, respectively. Corresponding values for the multiple trait analyses were 0.34, 0.42, 0.56, and 0.57. Repeatability estimates for PEM_R and PEM_R2 were 0.58±0.01 and 0.69±0.01 for the single, and 0.61 and 0.72 for the multiple trait analyses, respectively. Estimates of genetic changes were significantly positive and equal to 0.40±0.08 and 0.35±0.07 percent of the mean per year, for the two selected lines in the experiment. The results obtained in the present study indicated the multiple trait repeated records models as the most appropriate for analyzing the PEM as an indicative of mature weight. The PEM could be included in a selection program aiming at monitoring a desirable mature weight.
beef cattle; growth; genetic correlation; heritability; repeatability
