Records of 1021 Nelore cows were analyzed in order to evaluate the effect of selection for growth on their stayability in the herd up to five years, considering that they were selected (P5|S). The coded variable (0=culled and 1=not culled) was analyzed with a threshold model. The Nelore cows were born from 1981 to 1997 within the selected (NeS and NeT) and control (NeC) lines of the Experimental Station of Sertãozinho (SP-Brazil). In NeS and NeT the selected (replacement) animals were those with high selection differentials, within line and year, and in NeC those with null selection differentials for yearling weight. After 3.5 generations of selection, the NeS and NeT selection weight was 22% higher compared to NeC. The observed P5|S mean was 68%. Line and selection weight effects were not significant for P5|S. The generation effect was significant, showing a decreasing trend of P5|S along the years in a similar fashion for all the three lines. This tendency could be explained mainly to environmental effects, like increasing replacement rates of heifers, and not to selection for weight. The additive genetic variance, estimated by animal model, sire model and sire and maternal grandsire model were 0.06, 0.16, e, 0.17, proportionally to sigma²e constrained to one, and the heritabilities were 0.06±0.06, 0.15±0.13 and 0.16±0.13, respectively. The annual average breeding values of sires showed variation, without a regular trend along the years, similarly for the three lines. The selection for yearling weight within-herd, after 3.5 generations, did not cause any change in cow stayability up to five years of age. The threshold sire model was able to separate the genetic from the environmental trend.
beef cattle; selection experiment; threshold model; body weight; stayability of cows
