GENETIC PARAMETER-ESTIMATION OF POSTWEANING GAIN, FEED-INTAKE, AND FEED-EFFICIENCY FOR HEREFORD AND ANGUS BULLS FED 2 DIFFERENT DIETS

Postweaning gain performance and individual feed intake on 271 Herefor d and 263 Angus bulls were recorded during three 168-d test periods fr om 1984 to 1986. Each breed was composed of two lines and within each breed bulls were fed either a high-energy (HD) or a medium-energy (MD) diet. Energy intake was partitioned into energy for maintenance and g rowth based on predicted individual animal requirements. Estimates of heritability were obtained using Restricted Maximum Likelihood with an individual animal model including fixed effects of year, diet, and co variates of initial weight and backfat change by breed and with line e ffects for overall data. Bulls fed the HD grew faster and had higher m etabolizable energy intake per day (MEI), residual feed consumption (R FC), and gross and net feed efficiency (FE and NFE) (P < .001) than th ose fed the MD. Estimates of heritability for Hereford and Angus bulls , respectively, were .46 and .16 for 200-d weaning weight (WWT), .16 a nd .43 for average daily gain (ADG), .19 and .31 for intake per day (M EI), .43 and .45 for yearling weight (YWT), .07 and .23 for RFC, .08 a nd .35 for FE, and .14 and .28 for NFE. Genetic and phenotypic correla tions between MEI and ADG, MEI and YWT, ADG and YWT, ADG and FE, YWT a nd FE, and FE and NFE were moderately to highly positive for both bree ds. Negative genetic and phenotypic correlations between NFE and ADG s how partial correlations of FE with ADG after accounting for energy re quirement for maintenance. Residual feed consumption was negatively as sociated with YWT, FE, and NFE, indicating a possible genetic improvem ent.