Effects of age on milk yield: time trends and method of adjustment.

Multiplicative adjustment factors for age that were used in the US from 1974 through 1994 were developed from performance records of the 1960s. Rapid genetic improvement and improved management may have altered the relationship between lactation yield and calving age. The purpose of this study was 1) to examine changes in age effects on lactation milk yield over time; 2) to compare multiplicative, additive, or a combined method to adjust records for calving age; and 3) to examine effect of adjustment method on estimates of genetic trend. Data were complete lactation records (n = 765,413) of 383,011 Holsteins from Wisconsin DHI. An animal model was used with fixed effects for herd-year and month of calving by period by age-lactation class and with random effects for EBV, permanent environment, and error. Correction factors were applied additively, multiplicatively, and in combinations. The combination of additive and multiplicative adjustments that minimized heterogeneity of standard deviations within age class was considered to be optimal. The increase in yield that was associated with advancing age and lactation number was greater during recent years than during earlier years. Heterogeneity of standard deviations within age class was nearly minimum with additive adjustment for lactations 1 to 3. For lactations 4 and 5, heterogeneity was not sensitive to method of adjustment; however, multiplicative adjustment was near the optimum. The genetic trend from records with multiplicative adjustment was substantially greater than additive and optimal adjustments, especially during recent years.