Age of dam and age at measurement adjustments and genetic parameters for scrotal circumference of Canadian Hereford bulls

The objective of the study was to estimate age of dam and age at measurement as adjustment factors for scrotal circumference in Canadian Hereford bulls (n = 9029) and to estimate genetic parameters for scrotal circumference, and birth and yearling weights. Quadratic effects of age at measurement and the interaction of age of dam with age at measurement were not important. Adjustment factors recommended for yearling bulls were 1.02, 0.33, 0.10, and 0.16 cm to adjust scrotal circumference of sons of 2-, 3-, 4-, and > 10-yr-old cows to a mature dam (5 to 9 yr of age) equivalent. The linear partial regression coefficient for age at measurement was 0.036 cm d^-1. Genetic parameters were estimated using a multiple trait animal model and REML. The heritability estimate for (age of dam and age at measurement) adjusted scrotal circumference was 0.40 ± 0.03, and heritability estimates were 0.43 ± 0.05, 0.21 ± 0.09, and 0.36 ± 0.03 for direct birth weight, maternal birth weight, and yearling weight, respectively. The genetic correlation of adjusted scrotal circumference with direct birth weight was low (0.15), and was moderate (0.38) with yearling weight, but was near zero (-0.01) with maternal birth weight. Environmental and phenotypic correlations of adjusted scrotal circumference were low with birth weight, and were high with yearling weight. These results indicate that there was a positive association between adjusted scrotal circumference and body weight. Genetic improvement of fertility through the use of adjusted scrotal circumference as an indicator trait would not be expected to be antagonistic to that for body weight.