@conference {40616,
	title = {Accounting for scale effects in genetic evaluation of dairy cattle.},
	booktitle = {Proceedings of the New Zealand Society of Animal Production},
	volume = {54},
	year = {1994},
	month = {Jan},
	pages = {276-280},
	publisher = {New Zealand Society of Animal Production},
	organization = {New Zealand Society of Animal Production},
	abstract = {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 {\textpm} sd) High (H), 172 {\textpm} 28; Medium (M), 152 {\textpm} 26; and Low  
(L), 139 {\textpm} 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. },
	issn = {1176-5283},
	author = {Lopez-Villalobos N and Garrick DJ and Harris BL and Blair HT}
}
