Accounting for scale effects in genetic evaluation of dairy cattle.

N. Lopez-Villalobos, D.J. Garrick, B.L. Harris and H.T. Blair

Department of Animal Science, Massey University, Palmerston North, New Zealand.

Proceedings of the New Zealand Society of Animal Production 1994, 54: 276-280

Lactation records of dairy cows calving between 1986 and 1989 were obtained from the Livestock Improvement Corporation of the New Zealand Dairy Board. There were milkfat yields from 2,004,854 lactations in 83,805 contemporary groups. The data were divided into three equal-sized subsets based on average level of production (scale) in the contemporary group; these being (kg milkfat ± sd) High (H), 172 ± 28; Medium (M), 152 ± 26; and Low (L), 139 ± 25. The objectives of this study were to provide evidence of heterogenous variance and to identify the best of three methods to stabilise the variance of milkfat yields for use in the genetic evaluation of New Zealand dairy cattle based on a best linear unbiased prediction procedure using an across breeds animal model which will be implemented in July 1996.

The methods investigated for the accounting of scaling were adjustment based on: contemporary group standard deviation (SD); contemporary group means (MEAN); and logarithmic transformation (LOG) of milkfat yield. The overall correlation between contemporary group means and standard deviations was 0.44. This value was reduced to 0.31 in SD-transformed data, -0.27 (MEAN) and -0.24 (LOG).

Correlations between breeding values for sires estimated from the independent data sets using a mixed model but without adjustment for scale were 0.76, 0.73 and 0.78 in the L-M, L-H and M-H comparisons. These were lower than expected correlations of 0.85, 0.85 and 0.87, reflecting inaccuracies in sire evaluation when scaling is ignored. Calculated correlations were similar for SD and MEAN (0.78, 0.75 and 0.80; 0.78, 0.74 and 0.78), but LOG reduced the calculated correlations (0.73, 0.69 and 0.75).

Results confirm the problem of scaling on genetic evaluation of New Zealand dairy cattle and indicate other methods need to be studies to correct the scaling problem.

Keywords: NZSAPAB; animal model; BLUP; scaling effect.

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