Selection for yearling weight has been applied in two (120-cow, 6-sire) purebred herds from Nelore (NeS) and Guzera (GuS) breeds of Zebu cattle since 1980. A Nelore control (NeC) herd (60-cow, 4-sire) has been used to evaluate the genetic change. Selection criteria were the bull's weight at 378 d (W378) and heifer's weight at 550 d (W550). Average age of parents were 4.79; 4.72 and 4.75 yr for NeS, NeC and GuS, respectively. For calves born in 1985,the midparent cumulative selection differentials in standard units were 1.863 in NeS; 1.335 in GuS and .258 in NeC. Data recorded on 629 progeny were used to evaluate the selection response. The yearly deviations NeS-NeC in W378 were 14.0; 18.3; 17.3 and 14.7 kg for males born from 1981 to 1984, respectively. Corresponding values for GuS-NeC were 0.0; 17.1; 6.7 and 10.4 kg. In the females,the early deviations NeS-NeC in the W550 were 17.8; -.7 and 10.5 kg for the years 1981 to 1983, respectively. Corresponding values for GuS-NeC were -2.4; -5.9 and 5.6 kg. These results indicate that selection for yearling weight can be an efficient tool for improving the growth ability of Zebu cattle.

I. U Packer, A. G Razook, J. BF Trovo, L. M Bonilha, L. A Figueiredo, J. Nascimento, L. Pacola, J. G Candido, B. ES Campos, W. B Machado

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume IX. Breeding programs for dairy and beef cattle, water buffalo, sheep, and goats., , 419–423, 1986
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