Photo from Andrea Haines

2025 Genetic Base Change

The routine five-year genetic base change was applied in April 2025.

What is a Genetic Base Change?

A genetic base is when the average PTAs (Predicted Transmitting Abilities) of cows born in a specific year are set to zero. In other words, the starting line from which we measure PTAs is moved because of genetic progress.

On April 1, 2025, the genetic base population moved from cows born in 2015 to cows born in 2020. Routine base adjustments, which happen every five years in the United States, allow new evaluations to be easily compared with previous evaluations.

Those involved in setting selection thresholds for mating programs should pay close attention. A bull or cow who previously reached standards may not do so after the April base change.

How is it calculated?

Average PTA (cows born in 2020)
– Average PTA (cows born in 2015)
Base Change

Changes greater than 0 mean the younger cows have a higher genetic merit than the older cows. April 2025 PTAs show the inverse effect (i.e. a positive milk value below resulted in a decrease in milk PTA).

Why is it updated every 5 years?

The genetic bases to which (most) dairy traits are expressed in the U.S. have been updated every five years since 1980. This adjustment schedule allows PTAs to reflect genetic improvement over time while still maintaining stability in evaluations.

2025 Genetic Base Change Values

The final genetic base change values implemented in April 2025 are below. These numbers are reported as the change in base populations (2015 vs 2020).

The genetic base change occurring with the April 2025 evaluations was not 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 reviewed before the April evaluation, unanticipated results were observed. Due to the timing of this discovery, CDCB maintained calving traits in their current base in April. This means that both genetic and phenotypic bases used to calculate these traits in December 2024 were again applied in April. 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 2025 after further investigation can be conducted. Maintaining the calving traits’ bases in the April evaluation will not impact any other traits.

Table 1: Value of the genetic change between cows born in 2020 and cows born in 2015 1

TraitUnitsAyrshireBrown SwissGuernseyHolsteinJerseyMilking Shorthorn
MilkPounds142381687523556
FatPounds3904416-7
ProteinPounds51422914-3
Productive lifeMonths0.080.90.722.311.610.37
Somatic cell score (SCS)Log base 2 units0.02-0.040-0.10.020.02
Daughter pregnancy rate%-0.99-0.61-0.45-0.21-0.39-0.53
Heifer conception rate%-0.690.14-0.320.941.41-0.52
Cow conception rate%-1.15-0.48-0.850.450.05-0.37
Cow livability%-0.860.47-0.030.410.61-0.04
Gestation length2Days0.15-0.080.16-0.650.28̶
Residual Feed IntakePounds̶̶̶-42.34̶̶
Milk fever / Hypocalcemia%̶0̶0.070.1̶
Displaced abomasum%̶0̶0.350.21̶
Ketosis%̶0̶1.04-0.06̶
Mastitis%̶-0.01̶0.7-1.05̶
Metritis%̶0̶1.02-0.02̶
Retained Placenta%̶0̶0.01-0.11̶
Early first calvingDays-0.250.660.232.371.93-1.72
Heifer Livability%̶̶̶0.460.18̶
Final ScorePoints0.20.20.3*0.50.2
StaturePoints0.50.4-0.1*0.50.2
StrengthPoints00.10.1*0.10
Dairy formPoints0.20-0.1*0.40.2
Front Teat AttachmentPoints0.30.20.2*0.20.1
Rear Legs – Side ViewPoints-0.10-0.2*0-0.1
Body depthPoints0.100*00.1
Rump anglePoints00.2-0.5*-0.30
Rump widthPoints0.20.10.2*0.30.2
Fore udder attachmentPoints0.5̶0.5̶0.7*0.90.2
Rear udder heightPoints0.4̶0.4̶0.5*0.50.2
Rear udder widthPoints0.20.20.2*0.10.1
Udder depthPoints0.60.40.6*0.70.2
Udder cleftPoints0.30.10.1*0.20.1
Front teat placementPoints0.30.30.2*0.40.2
Teat LengthPoints-0.2-0.4-0.2*0.1-0.1
Rear Legs – Rear ViewPoints̶0.10.2*00.1
MobilityPoints̶0.1̶̶00.1
Milking SpeedPoints̶0̶̶00
Rear teat placement – rear viewPoints̶0.1̶̶0.3̶
Rear teat placement – side viewPoints̶̶̶̶-0.1̶
Lifetime Net Merit**Dollars71130-15404179-12
Lifetime Cheese Merit**Dollars65117-17375166-4
Lifetime Fluid Merit**Dollars73135-13417184-15
Lifetime Grazing Merit**Dollars47104-39386151-30

– Trait not calculated and published for breed
* Trait calculated by Holstein Association USA
** Economic weights applied to Lifetime Merit Indices are also updated in April 2025.

Impact to PTAs

The genetic component is only half the story when considering how PTAs will be impacted by a genetic base change. After the genetic difference of cows born in 2020 compared to cows born in 2015 is determined, inbreeding and heterosis adjustments are applied. In the Holstein breed especially, the rate of inbreeding has increased in the five-year difference in base populations which causes greater effect on how these adjustments impact the PTAs.

For example, let’s consider Bull A, a purebred Holstein bull – so, no heterosis correction. In December 2024, his raw genetic value for milk was 1,417 pounds. After moving the genetic base to cows born in 2020, his raw genetic value in April becomes 673 pounds. A difference of 744 pounds – matching closely the estimations of the base change previously shared. However, when the inbreeding adjustment is applied in December and April, -440 and -302, respectively, his final milk values become 977 in December and 371 in April. A change of only 606 pounds, not 744. 

 December 2024April 2025Difference
Raw Genetic Value1,417673744
Inbreeding Effect-440-302-138
Final PTA977371606

The inbreeding adjustment in April 2025 is a result of how a base change affects multiple parameters of the evaluations at the same time. It does not demonstrate a lack of – or lessening – concern related to inbreeding.


What is happening with Net Merit $?

Preliminary calculations of the Lifetime Merit Indexes – Net Merit $, Cheese Merit $, Fluid Merit $, and Grazing Merit $ – indicate favorable genetic gains for all lifetime indexes. However, in April 2025, these indexes will also undergo an adjustment to the economic weights used in the formulas.

Where is additional technical information available?

For those in the industry who require additional technical information such as:
– Performance Differences Attributed to Genetic and Environmental Change
– Standard Deviations

What role does inbreeding play in genetic base changes?

After the April 2025 base change, observed changes in Predicted Transmitting Abilities (PTAs) for individual animals may not match the base change values as closely as they have in the past. This is not due to any errors in the evaluation. It reflects Expected Future Inbreeding (EFI) changes in the population that are larger than they’ve been in the past.

  1. The noted differences in PTAs included in this table do not yet account for inbreeding and heterosis adjustments. These adjustments will impact published PTAs.

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