J. Anim Sci.
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J. Anim Sci. 1984. 59:1488-1500.
© 1984 American Society of Animal Science
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Selection and Concurrent Inbreeding in Simulated Beef Herds: Model Development1

M. E. Davis2,3, and J. S. Brinks3

Colorado State University, Fort Collins 80523

Abstract

The Monte Carlo method was used to simulate selection and inbreeding in a population of beef cattle consisting of 16 closed lines each with one sire and 10 dams. Production of progeny over a 25-yr period that included overlapping generations was simulated. Two traits with initial narrow sense heritabilities of .28 and .52, respectively, were studied. Genotypic values were determined two loci at a time for the 64 independent loci in an individual's genome on the basis of five modes of gene action, including dominance, epistasis and additive gene action. Phenotypic values were generated by the addition of normally distributed, random environmental factors to the genotypic values. Index values were calculated as the simple average of the phenotypes for traits 1 and 2. Selection was entirely within lines, with index values the sole selection criterion. Genetic gains were made despite moderate levels of inbreeding. The genotypic mean for trait 1 increased 5.8% from the base population to the last 5-yr period, while the genotypic mean for trait 2 increased 6.6%. Genetic gains were greatest in the section of the genome in which both traits were controlled by the additive mode of gene action. Substantial genetic progress also was observed in the sections in which both traits were under the influence of the dominance mode. Changes in gene frequency closely paralleled changes in sectional genotypic values with the greatest changes in gene frequency occurring in the sections in which both traits were controlled by the additive or dominance modes. The mean inbreeding coefficient for the entire genome in yr 21 to 25 was .352. Moderately high levels of inbreeding were attained under all modes of gene action. In general, heritabilities, correlations and variances obtained from analysis of the 25 yr of simulation data corresponded well with parameters reported in the literature for weight and gain traits in beef cattle.

Footnotes

1 Published with the approval of the Director, Colorado State Univ. Exp. Sta., as Scientific Ser. Paper No. 2872.

2 Present address: Anim. Sci. Dept., The Ohio State Univ., Columbus 43210.

3 Dept. of Anim. Sci.






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Copyright © 1984 by the American Society of Animal Science.