Estimation of genetic parameters of yearling scrotal circumference and semen characteristics in line 1 hereford bulls
Kealey, Courtney Gail.
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Objectives of this research were to estimate heritabilities of scrotal circumference and semen traits, and genetic correlations among these traits and birth weight. Line 1 Hereford bulls (n = 841), born in 1963 or from 1967 to 2000, were selected for use by USDA-ARS at Miles City, Montana or for sale. Semen was collected by electro-ejaculation when the bulls were approximately one year of age (mean = 446d) and all samples were evaluated by one person. Traits analyzed were scrotal circumference, color, volume, concentration, swirl, motility, and percents normal, live, abnormal heads, abnormal mid-pieces, proximal distal droplets, bent tails, coiled tails, distal proximal droplets, and primary and secondary abnormalities. Data were analyzed using MTDF-REML. Models included fixed effects for contemporary group, age of dam, age of bull at evaluation, inbreeding of the bull and his dam, and random animal, maternal, permanent maternal environmental, and residual effects. Heritability estimates for scrotal circumference, color, volume, concentration, swirl, motility, and percents normal, live, abnormal mid-pieces, proximal distal droplets, coiled tails, and primary and secondary abnormalities were 0.57, 0.15, 0.09, 0.16, 0.21, 0.22, 0.23, 0.34, 0.17, 0.34, 0.30 0.34, and 0.29, respectively. Estimates of genetic correlations between birth weight and scrotal circumference, color, volume, concentration, swirl, motility, and percents normal, live, abnormal mid-pieces, proximal distal droplets, coiled tails, and primary and secondary abnormalities were 0.36, 0.60, 0.07, 0.58, 0.44, 0.21, 0.20, 0.34, -0.03, -0.52, -0.20, -0.25, and 0.05, respectively. The moderate estimates of heritability for many of the traits indicate potential for favorable selection response. Positive genetic correlations between birth weight and majority of the traits suggest selection to reduce birth weight may compromise semen traits. However, for most traits the expected correlated responses are small. Desirable genetic correlations among scrotal circumference and semen traits suggest selection for one trait would not compromise the other semen traits. Expected correlated responses in semen traits to selection for increased scrotal circumference appear favorable.