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Effect of Environmental Corrections on Repeatability of Weaning Weight in Angus^1,2,

Colorado State University, Fort Collins

Abstract

Weaning weights of 4,722 commercial Angus calves from 1,501 dams and 9 yr. were analyzed to compare the accuracy of environmental correction factors computed from herd data (Method A) as opposed to factors recommended by the beef cattle industry (Method B).

In each method weights were adjusted for calf age using a standard birth weight and the calf's own average daily gain. Adjustment for cow age was by multiplicative factors in Method B and by additive least-squares constants in Method A. The magnitude of the correction factors was similar in each case. Sex was corrected multiplicatively in each method. Industry correction terms assumed greater differences between bulls and steers than did factors utilized in Method A.

Estimated repeatability of weaning weight, computed as the intraclass correlation among calves of a cow, was O.257±O.O16 and 0.251 ± 0.016 from data corrected by Methods A and B, respectively. The correlation between the two adjusted records of an individual was 0.80. It appeared that more of the initial variability was accounted for in Method A analyses than in those of Method B. Results indicated, however, that under commercial conditions the additional expense of computing correction terms specific for the herd was not justified.

Possible reasons for the low magnitude of repeatability were presented and discussed. Included were the inclusion in the model of the cow age x year interaction, non-adjacency of records, a possible breed effect and an effect resulting from behavioral characteristics of Angus cows and calves.

¹ Approved for publication as scientific Paper 1400 of the Colorado Experiment Station. This study was conducted in cooperation with Western Regional Research Project W-1, Improvement of beef Cattle Through the Application of Breeding Methods.

² Appreciation is expressed to J. E. Rouse, One Bar Eleven Ranch, Saratoga, Wyoming, who made available the data for this study.