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

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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. Heritability estimates were .05 ± .12 for preweaning RGR, .25 ± .14 for postweaning RGR and .64 ± .18 for postnatal RGR. Preweaning RGR was negatively correlated with birth weight (rg = −1.04 ± 1.63) and positively correlated with postweaning average daily gain (ADG) (rg = 1.23 ± 1.59) and 452-day weight (rg = .28 ± .70). Either postweaning or postnatal RGR would seemingly be an effective criterion for changing the shape of the growth curve. Respective genetic correlations were negative with birth weight (−.62 ± .36; −.61 ± .32) and positive with postweaning ADG (.78 ± .15; .74 ± .17) and 452-day weight (.54 ± .29; .34 ± .23). Positive genetic correlations with percentage of fat trim (1.24 ± .36; .59 ± .21), fat thickness (.85 ± .34; .32 ± .25) and marbling score (1.04 ± .41; .60 ± .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 favorably correlated with daily metabolizable energy (ME) intake (.70 ± .26; .27 ± .26) and ME per unit of live weight gain (−.93 ± .87; −1.21 ± .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. Copyright © 1976. American Society of Animal Science . Copyright 1976 by American Society of Animal Science