Age and Herd Effects in New Zealand Dairy Cow Records

Abstract

summary Age-correction factors related to the level of herd production have recently been shown to differ little from multiplicative factors in reducing the herd-by-age interaction variance component in age-corrected records. The herds initially available for studying these variance components were from New York data, averaging eight cows per herd. Larger herds than these are more suitable for such a study and results are reported here of investigating the same variances among New Zealand herds, averaging 58 cows each. Variance components were estimated by three different methods, and in all analyses 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, whereas herd-level factors may not.