GENETICS AND BREEDING Considerations for the Specification and Statistical Estimation of Differences Among Animals in the Partial Efficiencies of Feed Energy Utilization 1

The partial efficiencies of feed net en­ ergy use for lactation, maintenance weight change, and pregnancy are impor~ tant components of production system ef­ ficiency. If heritable variation in these efficiencies can be identified the efficien­ cies may become important 'selection cri­ teria in livestock. Estimates of the partial efficiencies of interest have been obtained using multiple regression procedures and data from nutrition experiments. Such ex­ pe~ents utilize variation among anunals, not necessarily within animals. V~ation within animals is necessary to estimate partial efficiencies for individu­ ~s. ~ithin-animal multiple regression YIelds mterpretable and meaningful esti­ mates of the partial efficiencies of energy use only when 1) the model is correctly and completely specified, 2) values of the independent variables are uncorrelated, 3;J1d ~) autocorrelation among observa­ tions IS accOlUlted for correctly. To obtain independent variation in weight, weight composition, weight change, pregnancy and milk yield may not be feasible wi~ animal~, especially in field data Analyses of reSIduals from prediction equations based on among animal estimates of the partial efficiencies may provide some in­ formation as to the amount of genetic va~ation available, but they are difficult to mterpret Alternatives, such as random coefficients models, may help in address­ ing the problems.