|Kealey, Courtney - MONTANA STATE UNIVERSITY|
|Tess, Michael - MONTANA STATE UNIVERSITY|
|Bellows, Robert - RETIRED ARS|
Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 26, 2005
Publication Date: February 1, 2006
Citation: Kealey, C.G., Macneil, M.D., Tess, M.W., Geary, T.W., Bellows, R.A. 2006. Genetic parameter estimates for scrotal circumference and semen characteristics of yearling line 1 hereford bulls. Journal of Animal Science 84:283-290. Interpretive Summary: Economic success of cow-calf production depends on percent calf crop, which in turn depends on fertility of parents and survival of progeny. Sire fertility should be established before the breeding season due to its impact on profitability. Semen evaluation is a generally accepted way to estimate potential fertility of bulls. Previous research suggests improvement of semen traits may be warranted. Objectives of this study were to estimate genetic parameters for birth weight, scrotal circumference, ejaculate characteristics, and morphology of spermatozoa and to predict correlated responses in ejaculate characteristics and morphology of spermatozoa when selection pressure is applied to improve fitness by either reducing birth weight or increasing scrotal circumference. Data were from Line 1 Hereford bulls that were either retained for breeding or distributed to industry over the period from 1963 to 2000 by the USDA-ARS Fort Keogh Livestock and Range Research Laboratory, Miles City, MT. Given current selection practices, these data suggest correlated responses affecting semen characteristics that result from selection for reduced birth weight or increased scrotal circumference may be unimportant. However, if breeders are to place greater emphasis on indicators of fitness as selection criteria, it may be prudent to consider semen characteristics in selection of seedstock bulls in order to further enhance economic efficiency of cow-calf production. Since many of the semen characteristics appear to be moderately heritable, identification of molecular markers for them may be a reasonable approach.
Technical Abstract: Objectives of this research were to estimate heritability for scrotal circumference (SC) and semen traits, and genetic correlations among these traits and birth weight (BW). Phenotypes were recorded for Line 1 Hereford bulls (n = 841), born in 1963 or from 1967 to 2000 that had been either selected for use at Fort Keogh or for sale. Semen was collected by electro-ejaculation when the bulls were approximately 14.7 mo old. Phenotypes were BW, SC and volume of the ejaculate and subjective scores indicative of ejaculate color and swirl, sperm concentration and motility; and percentages of sperm classified as being normal and live, or having abnormal heads, abnormal mid-pieces, proximal cytoplasmic droplets, bent tails, coiled tails, or distal cytoplasmic droplets. Percentages of primary and secondary abnormalities were calculated. Data were analyzed using MTDFREML. Models included fixed effects for contemporary group, age of dam, age of bull at evaluation, inbreeding coefficients of the bull and his dam, and random animal, maternal, permanent maternal environmental and residual effects. Heritability estimates for BW, SC, semen color, volume, concentration, swirl, motility, and percents normal, live, abnormal mid-pieces, proximal cytoplasmic droplets, coiled tails, and primary and secondary abnormalities were 0.46, 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 the genetic correlations of SC with color, volume, swirl, motility, and percents primary and secondary abnormalities were 0.73, 0.20, 0.40, 0.34, 0.36 and -0.45, respectively. Estimates of the genetic correlations of BW with SC, color, volume, concentration, swirl, motility, and percent normal, percent live, and percents primary and secondary abnormalities were 0.36, 0.60, 0.07, 0.58, 0.44, 0.21, 0.20, 0.34, 0.16 and -0.16, respectively. Selection pressure applied to increase SC would be expected to improve all the phenotypes evaluated except percent primary abnormalities. Predicted correlated responses in semen characteristics per genetic SD of selection applied to SC were 0.16 to 0.93 genetic SD. Selection pressure applied to reduce BW would be antagonistic to improving all the phenotypes evaluated except percent primary abnormalities. Predicted correlated responses in SC and semen characteristics per genetic SD of selection applied to BW were 0.11 to 0.38 genetic SD.