The U.S. genetic base will be updated with the CDCB evaluations on April 1, 2025. The CDCB merit index values, along with the 49 individual selection traits and composites produced by CDCB, will be expressed on an updated genetic base relative to dairy cows born in 2020. In the U.S., this update occurs every five years to best align selection tools with the current dairy herd.
2025 Base Change
The routine five-year genetic base change will be implemented for the U.S. genetic evaluations produced by the Council on Dairy Cattle Breeding (CDCB) in April 2025. The genetic base population will move from cows born in 2015 to cows born in 2020.
Preliminary estimations of the changes in Predicted Transmitting Ability (PTA) values for all breeds are available at https://uscdcb.com/basechange. The final base change values, genetic versus environmental impact, and additional resources will be shared in late March.
Lifetime Net Merit Dollars (NM$) Revision
The Lifetime Net Merit (NM$) index – which ranks dairy animals based on their combined genetic merit for economically important traits – will be revised on April 1, 2025, along with Cheese Merit $, Fluid Merit $, and Grazing Merit $. The 2025 update revises some methods to estimate trait values and many income and cost variables, such as milk prices, feed requirements, and reproductive options. The 2025 revision does not include genetic evaluations for any new traits.
Updates to Breed Base Representation (BBR) reference population
In April 2025, all animals will receive a new Breed Base Representation (BBR) estimation based on updated business rules for selecting purebred bulls in the BBR reference population. We are now able to apply more refined business rules to better identify purebred animals for the BBR reference population, due to the increasing availability of genotyped animals, along with advancements in data quality, methodologies, and technology (such as more complete pedigrees and the introduction of constructed IDs).
The BBR reference population will now be selected from the genotyped, progeny-tested bull population (minimum 10 daughters) who are enrolled (excluding bulls with status codes C and N) with complete pedigree and classified as purebred within each breed of evaluation.
- Purebred bulls must have a pedigree-based heterosis value ≤ 1% (rounded to the nearest integer.
- A complete pedigree is defined as a pedigree with all ancestors identified for four generations. Animals with constructed IDs in their four-generation pedigree are disqualified.
- Exceptions:
1. All breeds (except Jersey): Bulls born before 2000 with more than 1,000 daughters in Holstein or more than 200 daughters in other breeds are exempt from the pedigree completeness rule. This accommodates older but influential bulls with partially complete pedigrees. Per an agreement with AJCA, this rule does not apply to the Jersey breed.
2. Jersey breed: All animals with “JX” at the beginning of their name are automatically disqualified from the reference population.
3. Brown Swiss breed: Intergenomics bulls with MACE proofs are treated equivalently to enrolled progeny-tested bulls.
To assess the impact of these updates, CDCB staff conducted a test run using a BBR reference population based on data available in August 2024. This test was applied to most animals included in the December 2024 evaluation, revealing that the percentage of animals receiving a new BBR value varies across breeds—from a maximum of 25% in Ayrshire to a minimum of 1% in Holstein.
Because the new methodology improves the detection of animals with non-purebred ancestors, the update will generally lead to a decrease in BBR values across breeds and the consequent exclusion of some animals from the evaluation reference population and/or purebred evaluations. Animals with the most significant changes in BBR values are typically those that were previously part of the BBR reference population but are no longer included, as well as those with weak connections to the current BBR reference population, primarily foreign animals and older to very old animals. Changes in BBR will impact PTAs, particularly when animals are removed from the reference population or when their BBR drops below the 90% threshold required for calculation using single-breed SNP effects.
Normalization of Type Trait Reliability Calculations in Non-Holstein Breeds
The Council on Dairy Cattle Breeding (CDCB), in collaboration with AGIL, is implementing an update in April to the way type trait reliabilities are calculated on all non-Holstein breeds. Historically, while PTAs for type evaluations have been derived from a multiple-trait model, reliabilities were still calculated as in a single trait model. With the growing volume of appraisal data, AGIL has recommended aligning the process so that both PTAs and reliabilities follow the multiple-trait methodology.
As a consequence of this change, traditional PTAs will remain unchanged, but reliabilities for most traits will increase. Traits with limited data will now benefit from genetic correlations with other type traits, leading to improved reliability estimates. Genomic PTAs will see a more noticeable impact, as reliability adjustments affect SNP solutions and weighting factors used in final calculations. The impact will be variable across traits, as it is linked to the amount of change in the reliability. Final Score (PTAT) will be the least affected trait across all breeds, as it’s considered an uncorrelated trait.
UPDATE REGARDING CALVING TRAITS
The genetic base change occurring with the April 2025 evaluations will not be applied to calving traits – Daughter Calving Ease, Sire Calving Ease, Daughter Stillbirth and Sire Stillbirth. Calving traits are an independent trait group in which the genetic evaluations are reported on an observed (phenotypic) scale. This means that both genetic and phenotypic bases need to be updated during a base change. When these base updates were applied for the April evaluation, unanticipated results were observed. Due to the timing of this discovery, CDCB decided to maintain calving traits in their current base in April. This means that both genetic and phenotypic bases used to calculate these traits in December 2024 will again be applied during this run. New phenotypes received in the National Cooperator Database since the December evaluation will be included. CDCB expects to update the bases for calving traits in August after further investigation can be conducted. Maintaining the calving traits’ bases in the April evaluation will not impact any other traits. More information will be shared as it becomes available.
Preliminary estimations of the changes in Predicted Transmitting Ability (PTA) values for all breeds are available at https://uscdcb.com/
Source: CDCB
