Age and Herd Effects in New Zealand Dairy Cow Records

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Abstract
Age correction factors designed to take account of any interaction that may exist between herd environment and the effects of age on production, are compared with regular multiplicative factors. Comparison is made on the basis of herd-by-age interaction variance components estimated in a series of analyses, of records corrected by both sets of factors. The variance components are estimated by 3 different methods, using the completely random model, the mixed model, and an analysis of subclass means. In all cases, the interaction component when using herd-level factors was approximately equal to that of the actual records, whereas that for the multiplicatively-corrected records was larger. This suggests that multiplicative factors perhaps introduce interaction effects, or magnify those already present in uncorrected records, while herd-level factors may not.