Intensidades de seleção e resposta direta e correlacionadas em 10 anos de progênies de bovinos das raças nelore e guzerá selecionadas para peso pós-desmame

The selection applied on postweaning weights and the direct and correlated responses were evaluated on l0 year’s progeny of Nelore and Guzera zebu breeds. Beginning 1980, two selection herds, one Nelore (NeS) and the other Guzera (GuS) each one composed of 120 cows and 6 sires and a Nelore Control herd (NeC) with 60 cows and 4 sires were established, from the herds of the Estacao Experimental de Zootecnia de Sertaozinho, State of Sao Paulo, Brazil. Three males and 20 females with highest selection differentials for yearling weights (P378) and 18 months weights (P550), respectively, were selected annually from each heard NeS and GuS. The males P378 weights were obtained on postweaning feeding tests conducted in feedlot lasting 168 days, shortly after weaning at tire average age of 210 days. The females P550 were obtained based on their performances on pasture conditions. In the NeC herd, 2 males and 10 females with close to zero selection differentials for those traits were selected annualy. Concerning the progenies born from 1981 to 1990, the midparent effective selection differentials were, on the average 31.5; 25.7 and 4.2kg for tire NeS, GuS and NeC herds, respectively. The corresponding selection intensities were 1.18; 0.92 and 0.17 standard deviation unities. The generation interval was 5.0 years, in average, and the generation coefficient reached 2.3 for the 1990 progeny for the three herds. The total genetic change (average) for P378 was 22.5 and 17.2kg, for NeS and GuS, respectively. Corresponding values for P550 (females) were 9.4 and 4.6kg. The animal response for P378 selection was estimated to be 2.4 and 2.2kg/year for NeS and GuS, respectively. There were significant correlated responses for birth and weaning weights and for daily gain on feedlot test for NeS and GuS.