Direct and correlated responses to selection for yearling weight on reproductive performance of Nelore cows.

Data from a selection experiment for growth carried out in Brazil were analyzed in order to evaluate the direct responses on yearling weight (YW) and the correlated responses on the size and reproduction traits of cows. The experiment was started in 1976, and in 1980 three lines of Nelore cattle were established: selection (NeS), traditional (NeT), both selected for higher YW, and control (NeC), selected for mean YW. The NeT was an open line that eventually received bulls from other herds. Yearling weight records for animals born from 1978 to 1998 and yearling hip height (H550) offemales born from 1985 to 1998 were analyzed by fitting an animal model in order to obtain the genetic trends. The means for weight, height, and body condition score at the start of the breeding season, days to calving, and calving success of cows born from 1993 to 1996 (pertaining to the third to fourth generations of selection) were compared between the selected (NeS and NeT) and control lines. The genetic trends obtained after 16 yr for YW were 1.7 +/- 0.2, 2.3 +/- 0.2, and -0.1 +/- 0.1 kg/yr for males and 1.9 +/- 0.2, 2.4 +/- 0.2, and -0.1 +/- 0.1 kg/yr for females, for the NeS, NeT, and NeC lines, respectively. Corresponding values for H550 were 0.25 +/- 0.03, 0.24 +/- 0.04 and -0.04 +/- 0.03 cm/yr for females. Heifers and cows from NeS and NeT were 19% and 15% heavier and 4% taller at the start of the breeding season than those from NeC. No significant differences between selected (NeS and NeT) and control females were detected for body condition scores and for reproductive performance. The results indicate that selection for body weight promoted high and consistent weight and height responses both at the yearling and later ages, without compromising the reproductive performance of the cows with respect to days to calving and calving success.