Introducing 305-AA: The new national standardized yield measurement enabling fair comparisons across individual cows and environments.

Multiplicative adjustment factors to standardize milk, fat, and protein lactation yields for age, parity, season of calving, geographical region, milking frequency, and previous days open were last updated in 1994. Since then, the national animal model has updated age-parity adjustments within each 5-year period, but those were not publicized or summarized until now. New multiplicative adjustment factors were estimated using 101.5 million milk, 100.5 million fat, and 81.2 million protein lactation records from 1960 to 2022 in a multitrait model. The pedigree file included 91.3 million animals of all dairy breeds and crossbreds. Along with breeding values for those, the animal model included 392 unknown parent groups, 39.9 million permanent environmental effects, 1.3 million herd-by-sire interactions, and regressions on pedigree inbreeding and heterosis. New age-parity factors were estimated within each 5-year time period-breed combination, and new season factors were estimated within each time period-climate region combination. Instead of mature equivalent, the new age factors standardize records to second-parity cows calving at 36 mo, which was already the policy in genetic evaluations since 2005, to make averages of standardized records much closer to observed herd averages. Seasonal effects were estimated within 5 new regions defined by the average climate zone scores for each state. Within each region, the seasonal differences in lactation yields are smaller in recent decades, suggesting that improved housing and management is mitigating the effect of the environment. The final adjustment factor is the product of 2 new multiplicative factors for age-parity and season-region and the 2 original factors for previous days open and milking frequency, all combined to produce a single multiplier for adjusting actual yields to get standardized yields. These new factors were designed for application to lactation records in the national database and have been distributed to cooperators who also use standardized records to encourage widespread adoption of a uniform approach to improve management comparisons of yields within herds.