Genetic Analysis of Relative Growth Rate in Crossbreed and Straightbred Hereford, Angus and Shorthorn Steers

Abstract

Relationships among preweaning (birth to 200 days), postweaning (200 to 452 days) and postnatal (birth to 452 days) relative growth rate (RGR) and a number of growth, carcass and feed traits were examined for 477 individually fed steers from 61 progeny groups. The h2 estimates were 0.05 for preweaning RGR, 0.25 for postweaning RGR and 0.64 for postnatal RGR. There were genetic correlations of preweaning RGR with birth weight (-1.04 plus or minus 1.63), postweaning av. daily gain (1.23 plus or minus 1.59) and body weight at 452 days (0.28 plus or minus 0.70). Either postweaning or postnatal RGR would seem to be an effective criterion for changing the shape of the growth curve. The genetic correlations of postweaning and postnatal RGR were negative with birth weight (-0.62 plus or minus 0.36 and -0.61 plus or minus 0.32) and positive with postweaning av. daily gain (0.78 plus or minus 0.15 and 0.74 plus or minus 0.17) and 452-day weight (0.54 plus or minus 0.29 and 0.34 plus or minus 0.23). Positive genetic correlations of RGR with percentage of fat trim (1.24 plus or minus 0.36 and 0.59 plus or minus 0.21), fat thickness (0.85 plus or minus 0.34 and 0.32 plus or minus 0.25) and marbling score (1.04 plus or minus 0.41 and 0.60 plus or minus 0.29) suggest that degree of maturity at 452 days would be increased by selection for either postweaning or postnatal RGR. Postweaning and postnatal RGR were also favourably correlated with daily metabolisable energy (ME) intake (0.70 plus or minus 0.26 and 0.27 plus or minus 0.26) and ME/unit of live weight gain (-0.93 plus or minus 0.87 and -1.21 plus or minus 0.50).The apparent positive effect of 452-day degree of maturity and, hence, the likely minor effect on mature weight suggest that selection for either postweaning or postnatal RGR may be more appropriate for maternal stocks than for terminal sire breeds.