J. Anim Sci.
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J. Anim Sci. 1972. 35:13-19.
© 1972 American Society of Animal Science
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Adjusting Yearling Weight Ratio for Selection on Weaning Weight1

Alan Emsley2, G. E. Dickerson3, L. V. Cundiff4 and J. A. Gosey2

U. S. Department of Agriculture and University of Nebraska, Lincoln 68503

Abstract

Herd average yearling weights (Ys) are biased upward by culling smaller calves at weaning, to the extent that means for selected weaning weights (Ws) and postweaning gains (Ps) exceed those for unselected animals (Wu and Pu). Hence, ratios of individual yearling weights to the selected herd mean, Yi/Ys=(Wi+Pi)/(Ws+Ps), underestimate ratios based on unselected herd means: Yi/Yu=(Wi+Pi)/(Wu+Pu). To correct for such bias,
Figure 1
was obtained by substituting
Figure 2
in Yi/Ys where
Figure 3
(method 2) or
Figure 4
(Ws–Wu) (method 3) or
Figure 5
(method 4). The ratio (Yi^/Yu also was obtained from (Yi/Ys) [1+bYR·WR(Ws–Wu)/Wu] (method 5); PR, YR and WR symbolize ratios to Pu, Yu and Wu, respectively.

Pooled within-year regressions (b) were calculated separately in four sets of unselected records for Hereford selection lines (both sexes in two management periods of 5 and 3 years, respectively). Predicted and actual yearling weight ratios were computed by each method for five percentages of culling (bottom 13, 25, 50, 73 and 87.5) and for three record lengths (365, 452 and 550 days). Negative bias of unadjusted ratios, Yi/Ys (method 1), increased sharply and linearly with proportion culled, averaging –1.2% for each 10% culled; bias was greater for the shorter postweaning records and for the slower gains of heifers. Bias after simply substituting (Wu+Ps) for Ys (method 2) ranged from zero to maxima between 1.7% and –3.14%, increasing with proportion culled or under full feeding and becoming more negative for longer records under delayed full feeding. All three regression methods reduced bias to essentially zero regardless of selection intensity or record length, but appropriate regressions differed with sex and feeding regime. At less than 50% culling, method 2 reduced bias to negligible levels.

Footnotes

1 Published as Paper Number 3233, Journal Series, Nebraska Agricultural Experiment Station. Research reported was conducted under Project No. 46-2. Contribution from North Central Regional Project NC-1, Improvement of Beef Cattle through Breeding Methods.

2 Department of Animal Science, University of Nebraska.

3 U.S. Meat Animal Research Center, Animal Science Research Division, A.R.S., U.S.D.A., Lincoln.

4 Beef Cattle Research Branch, Animal Science Research Division, A.R.S., U.S.D.A., Lincoln.






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Copyright © 1972 by the American Society of Animal Science.