Effect of mature size and body condition of Nelore females on the reproductive performance.

The size and body condition of cows can affect their reproductive efficiency. However, few studies reported genetic correlations between these traits in beef cattle. Thus, we estimated the genetic parameters and correlations between weight (MW), height (MH) and body condition score (BCS) of cows and reproductive traits (SC: scrotal circumference, AFC: age at first calving, GL: gestation length, DC: days to calving, and CI: calving interval) in Nelore animals. In addition, it has also obtained direct and correlated responses aiming at determining whether changes in cow size and body condition may affect the herd reproductive performance. A series of two-trait Bayesian analyzes were performed including MW, MH or BCS with each of the reproductive traits. The heritability estimated for MW, MH and BCS were 0.46 ± 0.02, 0.35 ± 0.01 and 0.17 ± 0.02, indicating an involvement of additive gene action mainly in cows' size determination. For the reproductive traits of females, heritability ranged from 0.05 ± 0.00 for CI to 0.18 ± 0.01 for GL, and was 0.37 ± 0.01 for SC. Low repeatability were estimates for GL (0.19 ± 0.00), DC (0.19 ± 0.00) and CI (0.05 ± 0.00). The MW was positive correlated with AFC (0.23 ± 0.08), CI (0.25 ± 0.15) and, with lower magnitude, of GL (0.14 ± 0.03). Null genetic correlations were obtained between MW with SC (0.03 ± 0.03) and DC (-0.01 ± 0.04). The MH showed positive and low genetic association with all female reproductive traits, but negative with SC (-0.08 ± 0.03). Negative and favorable genetic correlations were estimated between BCS and females reproductive traits, but unfavorable with SC (-0.13 ± 0.06). In summary, the negative impact of increase the size of Nelore females on their reproductive performance is expected to be small in tropical regions. The use of sires with high genetic breeding values for SC should result in slight reduction of BCS of cows, but no effective genetic response in female size is expected. Selection based on BCS should promote little improvement in reproductive efficiency of cows.