Introducing 305-AA: The New Standardized Yield Measurement

Written by CDCB

June 21, 2024

Introducing 305-AA: The New Standardized Yield Measurement

Beginning in June 2024, the Council on Dairy Cattle Breeding’s (CDCB) publication of 305-ME (Mature Equivalent) will be discontinued and replaced by 305-AA (Average Age) as the standardized yield estimate for U.S. dairy cows. This change incorporates current knowledge about the key variables that impact milk yield and improves the accuracy of the estimate.

In August 2024, the new 305-AA will be implemented in the calculation of genetic evaluations by CDCB. As part of this transition, the 305-AA yield estimates were added to CDCB in mid-June.

Dairy producers rely on a variety of measurements to compare animals of different ages, geographical locations and breeds. Whether using Net Merit $ to measure an animal’s genetic merit, Breed Age Average to compare type conformation, or the new 305-AA for milk yield, each tool is designed to help producers make informed decisions. As dairy herds evolve and as we learn more about cow performance, it is important we update these tools to remain relevant and accurate.
What’s the backstory?
Since 1994, the U.S. dairy industry has compared cows in different lactations, geographical regions and breeds using 305-ME (Mature Equivalent) adjustments. This production estimation standardizes milk, fat and protein yields to a 305-day lactation at a mature equivalent status regardless of a cow’s current age.
In 1995, a Journal of Dairy Science® paper authored by Norman et al outlined how genetic selection changes maturity patterns. Thirty years later, in 2023, the variables impacting the equation in the 305-ME estimations were reevaluated by USDA’s Animal Genomics and Improvement Laboratory (AGIL) and the Council on Dairy Cattle Breeding (CDCB). With CDCB’s stewardship of the National Cooperator Database, the dataset utilized in this review included 101.5 million milk records, 100.5 million fat records, and 81.2 million protein records!
The result of the 2023 AGIL-CDCB study is the introduction of 305-AA – an age-adjusted yield estimation standardized to a 305-day lactation record at 36 months of age – and the discontinuation of 305-ME.
How will the 305-AA differ from the 305-ME?
Before diving into the changes, let’s note the two variables that remain unchanged.

  • The new 305-AA still standardizes lactations to a 2x milking frequency.
  • The methodology for accounting for previous days open is unchanged.

Here are the primary changes:

  • Age: Average Age adjustments standardize to 36 months. Mature Equivalent standardized to a range of 61-86 months depending on the breed.
  • Climate: 305-AA includes five climate region adjustments accounting for cows residing in the hottest to coldest part of the U.S. The previous 305-ME region adjustments incorporated three geographical regions moving east to west across the U.S.

Breed adjustments: Previously, breed adjustments were calculated along with the season-region adjustments but are now estimated along with the age-parity adjustments. Because of the increase in data over the past 30 years, smaller breeds are now more represented in age-parity adjustments where the prior 305-ME adjustments were heavily influenced by Holstein.