Sex Effects on Breed of Sire Differences for Birth, Weaning, and Yearling Weights

Weights of males and females can be considered to be correlated traits with different averages and variances. This study attempted to determine whether defining traits as expressed in males or in females would change estimates of breed of sire differences needed to calculate across-breed factors for adjustment of within-breed EPD to across-breed EPD. Records from the US Meat Animal Research Center of progeny of Hereford, Angus, and MARC III composite dams mated to 12 sire breeds that had been used to calculate breed of sire adjustments in 1996 were used. Breeds of sire were Hereford, Angus, Shorthorn, Brahman, Simmental, Limousin, Charolais, Maine-Anjou, Gelbvieh, Pinzgauer, Tarentaise, and Salers. Female and male records for birth (BWT), weaning (WWT), and yearling (YWT) weights were considered to be separate although correlated traits. Heritability estimates for expression as females and males were as follows: .44 and .47 for BWT, .25 and .19 for WWT, and .55 and .49 for YWT. Corresponding genetic correlations between expression in males and females were .85, 1.00, and .92. Phenotypic standard deviations were slightly larger and coefficients of variation slightly smaller for males than for females; the largest differences were for YWT. Breeds ranked similarly for female and male weights; the major exception was Brahman for BWT. Averages of breed of sire contrasts for expression in females and males were almost identical to contrasts from analyses of combined male and female records. Largest differences between averaged and combined breed of sire contrasts were approximately 1 kg for BWT and WWT and approximately 2 kg for YWT. The results show that considering male and female weights as separate traits is not needed in calculation of across-breed adjustment factors from US Meat Animal Research Center records