Updating Your Selection Plan to Keep Pace with the April 2020 Base Change

Marjorie Faust, PhD Data Driven Genetics

  • Marjorie Faust

    The April 2020 genetic evaluations in the U.S. incorporated a scheduled genetic base change to account for genetic change that had taken place across the cow populations of all dairy breeds.

  • For many traits, genetic change was desirable, although each breed included at least one trait with notable undesirable genetic change.
  • Good management practice specifies updating selection values dairies use for traits and indexes ahead of selecting sires and dams and purchasing semen based on the April 2020 genetic evaluation results. Neces
    sary details can be found in the article.


Sire selection programs vary from dairy to dairy and across the U.S. to best achieve the goals of dairies and the specific market conditions they face.  An approach used by many is to select only those bulls meeting or exceeding a minimum index level (such as Net Merit $) or specified values for a combination of several traits such as Milk / Protein, Type, and Udder Composite.  For programs that rely on specified figures for traits and indexes, refreshes and updates are necessary from time to time to keep pace with genetic progress and scheduled industry-wide genetic base changes.

The April 2020 Sire Summary incorporates a scheduled genetic base change, and as a result, good management practice specifies updating the dairy’s selection values for traits and indexes ahead of selecting sires and dams and purchasing semen.  This article includes data and discussion for updating selection values and ensuring that the sires and dams selected will be the most current and relevant for the dairy.

Genetic Evaluations and Genetic Bases

It is evident that genetic evaluations do not predict the total genetic contribution to individual traits; instead evaluations report the advantage of using one animal as a parent as compared to using others.  In the U.S., we elected to designate a group of animals as the ‘standard’ or base for these comparisons.  Understandably, as genetic progress occurs, PTAs (Predicted Transmitting Abilities) of top animals can become quite large when the base remains static and unchanged across time.  To avoid reporting PTAs that are excessively large or small due to genetic change across years and decades, the genetic base and population that underpins it is updated every 5 years.  Beginning with the April 2020 Sire Summary, the genetic base for U.S. evaluations is defined as cows born in 2015.  For each trait, the genetic average of this group is set as zero and PTAs for individual sires and dams are shown as deviations from this base group.

Genetic Change and Genetic Base Updates

The portion of change in PTAs arising from the April 2020 genetic base update reflects 5 years of genetic change in the dairy population.  For most traits, differences between the old and new bases of 2010- and 2015-born cows reflect desirable genetic change.  However, there are a few notable exceptions where genetic change measured by comparing 2010- and 2015-born cows has been undesirable.  These exceptions which demonstrated undesirable genetic change include: female fertility traits for Ayrshires, Brown Swiss, Guernseys, Jerseys, and Milking Shorthorns; Cow Livability for Ayrshires, Brown Swiss, and Milking Shorthorns; Somatic Cells Score (SCS) for Milking Shorthorns; and Milk Fever for Holsteins.  Notable examples of favorable genetic change over this 5-year period are: Lifetime Cheese Merit $ for Ayrshires, Jerseys, and Holsteins; Productive Life (PL) for Guernseys, Holsteins, and Jerseys; Protein for Jerseys and Holsteins; and Service Sire Calving Difficulty and Cow Livability for Holsteins.



Genetic Base and Selection Values Used by Dairies

As a result of implementing the new genetic base in 2020, many trait PTA and index values exhibit notable differences from respective December 2019 values even when no new data for individual animals have been added.  Results in Table 1 illustrate for select traits the genetic base differences that have been implemented.  This means that April 2020 individual animal results are lower (higher when values in Table 1 are negative) than December 2019 counterparts by these figures (final values determined during the April 2020 computations).  Accounting for such base related differences when selecting sires and dams is important in helping ensure that herd genetic programs remain relevant and effective.

Table 1. Difference in Predicted Transmitting Abilities of cows born in 2015 from those born in 2010 and implemented in the April 2020 genetic base change.*


Trait / Index



Brown Swiss  






Milking Shorthorn
Milk, lb. 182 214 150 492 524 36
Fat, lb. 6 8 6 24 25 2
Protein, lb. 6 8 4 18 20 2
Somatic Cell Score, score -0.01 0.00 0.00 -0.08 0.00 0.02
Productive Life, months 0.12 0.24 0.90 1.86 1.54 0.63
Daughter Pregnancy Rate, % -0.47 -0.62 -0.12 0.24 -0.99 -0.53
Calving Difficulty, % 0.0 -0.6
Final Score 0.5 0.4 0.4 0.76 0.7 0.1
Foot Angle 0.1 0.1 0.2 0.50 0.1 0.0
Udder Depth 0.3 0.2 0.3 0.84 0.9 0.2
Feet & Leg Composite 0.49
Udder Composite 0.85
Lifetime Net Merit, dollars 121 60 77 231 191 45
Lifetime Cheese Merit, dollars 123 63 77 239 196 45
Lifetime Fluid Merit, dollars 117 56 78 219 179 45
*Values that are underlined indicate undesirable genetic change. See all calculated values here.




To account for base related changes, dairies with genetic programs that rely on high and low threshold values for traits and indexes will want to subtract values in Table 1 from values used by the herd in December 2019 for genetic program decisions.  For example in December 2019, a dairy may have required Holstein sires to have PTA Productive Life of 3.5 months or more for use in the herd.  In April 2020, this same dairy may refresh their threshold to require PTA PL of 1.5 or 1.6 months for Holstein sires (3.5 mo. – 1.86 mo.).  For Jersey sires, a December 2019 threshold that required a PTA Daughter Pregnancy Rate of -3.5 percent for Jersey sires, now may require Jersey sires to be over -2.5% (-3.5% – [-0.99%]) for DPR in April 2020.  For DPR for Jerseys, it is important to keep in mind that the higher April 2020 threshold does not imply that the same bulls are genetically superior to their status in December 2019.  On the contrary, the genetic merit of these sires remains the same after accounting for the 2020 base change.  Instead, the undesirable genetic change in DPR (-0.99%) for Jerseys highlights the unfortunate fact that the genetic decline in cow fertility traits for Jerseys continued during the 5-year period represented by the current base change.


Using the base change related values in Table 1, dairies can update their selection standards to today’s 2020 genetic base and ensure that herd genetic decision making remains current and relevant.


Norman, H. D. and co-workers  2020.  U.S. Council on Dairy Cattle Breeding.  https://www.uscdcb.com/cdcb-changes-to-evaluation-system-april-2020/

Norman, H. D. and co-workers  2020.  U.S. Council on Dairy Cattle Breeding.  https://www.uscdcb.com/wp-content/uploads/2020/02/Norman-et-al-Genetic-Base-Change-April-2020-FINAL_new.pdf

Editor’s note: Marj Faust has extensive experience in dairy genetics,  both in academia and the AI industry. Based near Madison, Wis., she may be contacted at: +1 608.219.1834 and


Be the first to comment

Leave a Reply

Your email address will not be published.