Variation Within and Between Groups of Genetically Identical Nuclear Transfer Brangus Bulls and Steers for Growth Measures Taken Over Time

Abstract

A study was conducted to determine the variability within and among groups of beef cattle produced through nuclear transfer (NT) for postweaning growth of various body measurements. Four groups of male, Brangus, second-generation NT calves (four to six per group) were used in this project. At weaning (6 to 7 mo) two randomly selected animals in each group were surgically castrated and subsequently maintained under simulated feedlot conditions. The remaining bulls were maintained under standard 140-d postweaning gain test conditions. Beginning at weaning, BW, linear measurements of hip height (HH), body length (BL), rump length (RL), rump width (RW), shoulder width (SW), round mass (RNMS), metacarpal length and circumference (MCL and MCC), metatarsal length and circumference (MTL and MTC), and ultrasonic measurements ofbackfat thickness (BFT), Longissimus muscle area (LMA), Longissimus muscle depth (LMD), body wall thickness (BWT), rump fat thickness (RFT), and rump muscle depth (RMD) were obtained on each animal at 28-d intervals, until 16 to 17 mo of age. Both linear and quadratic models were used to determine the changes in these traits over time; however, the linear models appeared to best describe growth in both sexes. Linear regression coefficients were obtained for each trait by animal, and these were analyzed partitioning sex, genotype, and genotype x sex as sources of variation. The interaction effect of genotype by sex was not significant (P>0.10) for any of the measures and was dropped from the model. Genotype was significant (P<0.01) for measures of BW, HH, BL, RW, SW, RNMS, MCC, BFT, and BWT, whereas sex was significant (P>0.01)for BW, BL, MCC, BFT, and RFT. Over all traits, the percentage of variation attributable to genotype ranged from 19.8 to 96.2%. When traits lacking significance of the genotype effect were omitted, genotype accounted for 86.7 to 96.2% of the total variation.