Design and analysis of open nucleus breeding systems for cattle in Senegal

Data collected during 26 yr of the Gobra cattle selection program to improve beef production in Senegal has been analyzed to determine genetic parameters and evaluate the selection program. Estimates of (co)variance components and genetic parameters for birth, 6-mo, 12-mo and 18-mo weights for Gobra cattle were obtained using single trait animal models. Data were analyzed by restricted maximum likelihood. Maternal effects were important for all traits. Estimates of direct and maternal heritabilities were 0.07 $\pm$ 0.03 and 0.04 $\pm$ 0.02, 0.20 $\pm$ 0.05 and 0.21 $\pm$ 0.05, 0.24 $\pm$ 0.07 and 0.21 $\pm$ 0.06, and 0.14 $\pm$ 0.06 and 0.16 $\pm$ 0.06 for birth, 6-mo, 12-mo and 18-mo weights. Correlations between direct and maternal genetic effects were negative for all traits and large for weaning and yearling weights with estimates of ${-}0.61 \pm 0.33$ and ${-}0.51 \pm 0.31$, respectively. Linear trends for additive direct and maternal breeding values were not significant for any traits except maternal breeding value for 12-mo weight. New breeding plans within the framework of open nucleus breeding systems for selection for growth and milk yield have been designed and analyzed using simulation to predict genetic gain and economic returns. The open nucleus breeding systems showed greater rates of genetic gain and less variation in selection response than the closed nucleus systems. In the open nucleus systems, greater rates of genetic gain were obtained when migration of pre-nucleus females to the nucleus was below 40%. The open nucleus system with screening of pre-nucleus cows for milk (Model 1), was most profitable when nucleus size was small (200 cows). Model 2 in which nucleus female replacements from the pre-nucleus were selected on weight and milk yield did not seem to have an advantage over a closed nucleus system. The open nucleus system with selection of nucleus female replacements from the pre-nucleus based on milk yield only (Model 3) gave the best economic return among the schemes tested