GENETIC AND STATISTICAL PROPERTIES OF RESIDUAL FEED-INTAKE

Residual feed intake is defined as the difference between actual feed intake and that predicted on the basis of requirements for production and maintenance of body weight. Formulas were developed to obtain gene tic parameters of residual feed intake from knowledge of the genetic a nd phenotypic parameters of the component traits. Genetic parameters o f residual feed intake were determined for a range of heritabilities ( h2 = .1, .3, or .5) for component traits of feed intake and production , and genetic (r(g) = .1, .5, or .9) and environmental ( r(e) = .1, .5 , or .9) correlations between them. Resulting heritability of residual feed intake ranged from .03 to .84 and the genetic correlation betwee n residual feed intake and production ranged from -.90 to .87. Heritab ility of residual feed intake depends considerably on the environmenta l correlation between feed intake and production. Residual feed intake based on phenotypic regression of feed intake on production usually c ontains a genetic component due to production. Residual feed intake ba sed on genotypic regression of feed intake on production is geneticall y independent of production and its use is equivalent to use of a sele ction index restricted to hold production constant. Multiple-trait sel ection on residual feed intake, based on either phenotypic or genetic regressions, and production is equivalent to multiple-trait selection on feed intake and production. Residual energy intake in dairy cattle was examined as an example. Heritability of residual energy intake bas ed on genotypic regression was close to zero and indicated that measur ement of feed intake provides little additional genetic information ov er and above that provided by milk production and body weight. The pri nciples outlined in this study have broader application than just to r esidual feed intake and apply to any trait that is defined as a linear function of other traits.