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

¹ Montana State University, Bozeman 59717

The partial efficiencies of feed net energy use for lactation, maintenance, weight change, and pregnancy are important components of production system efficiency. If heritable variation in these efficiencies can be identified, the efficiencies may become important selection criteria in livestock. Estimates of the partial efficiencies of interest have been obtained using multiple regression procedures and data from nutrition experiments. Such experiments utilize variation among animals, not necessarily within animals. Variation within animals is necessary to estimate partial efficiencies for individuals. Within-animal multiple regression yields interpretable and meaningful estimates 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, and 3) autocorrelation among observations is accounted for correctly. To obtain independent variation in weight, weight composition, weight change, pregnancy, and milk yield may not be feasible within animals, especially in field data. Analyses of residuals from prediction equations based on among animal estimates of the partial efficiencies may provide some information as to the amount of genetic variation available, but they are difficult to interpret. Alternatives, such as random coefficients models, may help in addressing the problems.

Key Words: efficiency • metabolizable energy • genetic parameters

Submitted on October 31, 1989
Accepted on May 9, 1990