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.
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).
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.
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
| Trait | Units | Ayrshire | Brown Swiss | Guernsey | Holstein | Jersey | Milking Shorthorn |
| Milk | Pounds | 142 | 381 | 68 | 752 | 355 | 6 |
| Fat | Pounds | 3 | 9 | 0 | 44 | 16 | -7 |
| Protein | Pounds | 5 | 14 | 2 | 29 | 14 | -3 |
| Productive life | Months | 0.08 | 0.9 | 0.72 | 2.31 | 1.61 | 0.37 |
| Somatic cell score (SCS) | Log base 2 units | 0.02 | -0.04 | 0 | -0.1 | 0.02 | 0.02 |
| Daughter pregnancy rate | % | -0.99 | -0.61 | -0.45 | -0.21 | -0.39 | -0.53 |
| Heifer conception rate | % | -0.69 | 0.14 | -0.32 | 0.94 | 1.41 | -0.52 |
| Cow conception rate | % | -1.15 | -0.48 | -0.85 | 0.45 | 0.05 | -0.37 |
| Cow livability | % | -0.86 | 0.47 | -0.03 | 0.41 | 0.61 | -0.04 |
| Gestation length2 | Days | 0.15 | -0.08 | 0.16 | -0.65 | 0.28 | ̶ |
| Residual Feed Intake | Pounds | ̶ | ̶ | ̶ | -42.34 | ̶ | ̶ |
| Milk fever / Hypocalcemia | % | ̶ | 0 | ̶ | 0.07 | 0.1 | ̶ |
| Displaced abomasum | % | ̶ | 0 | ̶ | 0.35 | 0.21 | ̶ |
| Ketosis | % | ̶ | 0 | ̶ | 1.04 | -0.06 | ̶ |
| Mastitis | % | ̶ | -0.01 | ̶ | 0.7 | -1.05 | ̶ |
| Metritis | % | ̶ | 0 | ̶ | 1.02 | -0.02 | ̶ |
| Retained Placenta | % | ̶ | 0 | ̶ | 0.01 | -0.11 | ̶ |
| Early first calving | Days | -0.25 | 0.66 | 0.23 | 2.37 | 1.93 | -1.72 |
| Heifer Livability | % | ̶ | ̶ | ̶ | 0.46 | 0.18 | ̶ |
| Final Score | Points | 0.2 | 0.2 | 0.3 | * | 0.5 | 0.2 |
| Stature | Points | 0.5 | 0.4 | -0.1 | * | 0.5 | 0.2 |
| Strength | Points | 0 | 0.1 | 0.1 | * | 0.1 | 0 |
| Dairy form | Points | 0.2 | 0 | -0.1 | * | 0.4 | 0.2 |
| Front Teat Attachment | Points | 0.3 | 0.2 | 0.2 | * | 0.2 | 0.1 |
| Rear Legs – Side View | Points | -0.1 | 0 | -0.2 | * | 0 | -0.1 |
| Body depth | Points | 0.1 | 0 | 0 | * | 0 | 0.1 |
| Rump angle | Points | 0 | 0.2 | -0.5 | * | -0.3 | 0 |
| Rump width | Points | 0.2 | 0.1 | 0.2 | * | 0.3 | 0.2 |
| Fore udder attachment | Points | 0.5 | ̶0.5 | ̶0.7 | * | 0.9 | 0.2 |
| Rear udder height | Points | 0.4 | ̶0.4 | ̶0.5 | * | 0.5 | 0.2 |
| Rear udder width | Points | 0.2 | 0.2 | 0.2 | * | 0.1 | 0.1 |
| Udder depth | Points | 0.6 | 0.4 | 0.6 | * | 0.7 | 0.2 |
| Udder cleft | Points | 0.3 | 0.1 | 0.1 | * | 0.2 | 0.1 |
| Front teat placement | Points | 0.3 | 0.3 | 0.2 | * | 0.4 | 0.2 |
| Teat Length | Points | -0.2 | -0.4 | -0.2 | * | 0.1 | -0.1 |
| Rear Legs – Rear View | Points | ̶ | 0.1 | 0.2 | * | 0 | 0.1 |
| Mobility | Points | ̶ | 0.1 | ̶ | ̶ | 0 | 0.1 |
| Milking Speed | Points | ̶ | 0 | ̶ | ̶ | 0 | 0 |
| Rear teat placement – rear view | Points | ̶ | 0.1 | ̶ | ̶ | 0.3 | ̶ |
| Rear teat placement – side view | Points | ̶ | ̶ | ̶ | ̶ | -0.1 | ̶ |
| Lifetime Net Merit** | Dollars | 71 | 130 | -15 | 404 | 179 | -12 |
| Lifetime Cheese Merit** | Dollars | 65 | 117 | -17 | 375 | 166 | -4 |
| Lifetime Fluid Merit** | Dollars | 73 | 135 | -13 | 417 | 184 | -15 |
| Lifetime Grazing Merit** | Dollars | 47 | 104 | -39 | 386 | 151 | -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.
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 2024 | April 2025 | Difference | |
| Raw Genetic Value | 1,417 | 673 | 744 |
| Inbreeding Effect | -440 | -302 | -138 |
| Final PTA | 977 | 371 | 606 |
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.
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.
For those in the industry who require additional technical information such as:
– Performance Differences Attributed to Genetic and Environmental Change
– Standard Deviations
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.
