Revisiting Total Weaning Weight as a Selection Criterion

Data from a single prolific Merino flock (N=10705 joining records) recorded over 10 years were used to estimate genetic parameters for annual total weight weaned (TWW) of ewes which weaned lambs (N=8615), treated as a ewe trait, and the accompanying reproductive traits. TWW was the sum of individual weights of weaned lambs to birth ewe. An alternative trait definition included ewes which lambed and lost, which received zero trait values (TWW0, N=9509). Both TWW and TWW0 were lowly heritable (range h²: 0.06-0.11). Most of the phenotypic variation in these traits resulted from variation in the number of lambs weaned. Trait definition significantly influenced both the observable variation in the ewe weaning weight traits (eg. TWW vs TWW0) and correlations with reproductive traits. Because total weight weaned traits combine direct and maternal effects, and multiple non-genetic sources of variation, prediction of response to selection for total weight weaned and its components depends on the trait definition used and accompanying population characteristics and genetic parameters. We conclude that selection on an index which combines breeding values for reproductive performance, and both direct and maternal contributions to weaning weight traits, should be considered to improve ewe productivity in a more predictable manner under dual purpose breeding goals. This index is provided by Sheep Genetics, which also appropriately analyses individual animal reproductive and weight data while accounting for systematic effects and multiple records.