Age and Herd Adjustment of First Lactation Milk Yield

A new procedure for age and herd adjustment for first lactation milk yield was developed and evaluated with Canadian Holstein Record of Performance records. Multiple regression prediction equations from 2 years' data considered the average yield and age of contemporaries in second and later lactations as well as effects of age at first calving within and between herds. Quadratic effects of individual and herd average ages were not significant (P ~ .05). Seasonal differences in effects of age at first calving within and between herds required separate equations for spring and fall. Milk yield for a separate 2 years was expressed as deviations from and as percentages of predicted yield. Variance of adjusted records was more homogeneous across herds for percentages than for deviations. Both measures of adjusted yield contained a 20 to 25% herd component of variance and a significant (P ~ .05) effect of age within herd in fall seasons. Procedures previously developed for milk yield in second and later lactations were satisfactory. Investigation of the usefulness of measures of condition or weight loss and of more effective adjustment techniques is recommended.