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Relationships among Absolute Growth Rate, Relative Growth Rate and Feed Conversion during Postweaning Feedlot Performance Tests^1,2,

University of Arkansas³, Fayetteville 72701

Abstract

Knowledge of the relationships between absolute growth rate (AGR), relative growth rate (RGR) and feed conversion (FCONV) of bulls in postweaning feedlot performance tests can give cattle producers important information for selecting superior sires. Weight gain and FCONV data that were collected during 16 yr were analyzed from 393 Angus and 340 Hereford bulls by 26 and 27 sires, respectively, that were individually fed in 140-d tests. Sire variance and covariance components were used to obtain heritability (h²) estimates for AGR, RGR and FCONV and the genetic correlations (r_g) and phenotypic correlations (r_p) among these traits. Respective mean AGR, RGR and FCONV were 1.27 ± .14 kg/d, .4378 ± .0395%/d and 7.32 ± .58 kg/kg for the Angus and 1.28 ± .12 kg/d, .4552 ± .0388%/d and 6.56 ± .46 kg/kg for the Hereford bulls. Estimates of h² were similar for AGR and RGR in both Angus (.36 ± .11 and .22 ± .09) and Hereford (.33 ± .11 and .20 ± .09) bulls. The h² estimates for FCONV were .14 ± .07 for Angus and .13 ± .08 for Herefords. For the Angus and Hereford bulls, respectively, r_g were .86 ± .09 and .86 ± .13 between AGR and RGR, –.84 ± .38 and –.74 ± .49 between AGR and FCONV and –.84 ± .49 and –.61 ± .64 between RGR and FCONV. The r_p were .80 ± .03 and .68 ± .04 between AGR and RGR, –.58 ± ,05 and –.51 · .05 between AGR and FCONV and –.71 ± .04 and –.73 ± .04 between RGR and FCONV for the Angus and Hereford bulls, respectively. These results suggest that AGR is comparable to RGR as a selection trait for bulls with similar genetic composition developed in standardized postweaning feedlot performance tests because those bulls that rank high for AGR would also rank high for RGR and FCONV.

¹ Published with the approval of the Director of the Univ. of Arkansas Agric. Exp. Sta.

² A publication of Southern Region Beef Cattle Breeding Project, S-10.

³ Dept. of Anim. Sci.