The information below has been supplied by dairy marketers and other industry organizations. It has not been edited, verified or endorsed by Hoard's Dairyman.

Holstein Association USA will declare Early Onset Muscle Weakness Syndrome (MW) as an officially recognized genetic condition beginning in February 2024. Holstein Association USA’s Genetic Advancement Committee recently endorsed a recommendation from staff to do so. The recommendation comes after their analysis of the MW haplotypes following the December 2023 genetic evaluations. Comments from Holstein breeders and scientific researchers further support this decision.

“This latest review provides more evidence and support on the recent action taken by the Holstein Association USA Board of Directors to add Early Onset Muscle Weakness to the list of officially recognized genetic conditions in February 2024,” says Dr. Tom Lawlor, Holstein Association USA’s Executive Director of Research and Development. “Soon, we’ll have MW in the rearview mirror as we move ahead with our genetic advancement of the Holstein breed.”

Starting in February 2024, direct gene test results for MW will be labeled on Official Holstein Pedigrees, and other products where officially recognized genetic conditions are displayed.

The following codes will be used to designate animals with official gene test results on file:

  • TE = tested free of MW
  • MW = Heterozygous Carrier
  • MW2 = Homozygous Carrier

Breeders can forward lab reports for MW test results to to have them added to the Holstein Association USA genetic conditions database.

MW haplotype results were made available for all genomic-tested animals in conjunction with the official genetic evaluation release on December 5, 2023. This information can be found in the same place as other haplotype information, including the free Family Tree Search, within Red Book Plus Online, on genomic reports, and within Enlight.

Results for MW haplotypes are reported as 0 (free), 1 (carrier), 2 (homozygous affected), 3 (suspected carrier), or 4 (suspected homozygous) on evaluations.

MW Resources

Additional resources and information on MW can be found on the Holstein Association USA website. A regularly updated list of bulls with a NAAB code that have a direct test result on file can be found at

The best tool available for discovering and understanding new genetic conditions is farmer reporting. Farmers who have calves born with abnormal features or health issues that are suspected to be caused by genetic problems are asked to complete the Abnormality Report Form available at and email it to, or contact the Genetic Services department at 800.952.5200.

For more background on the previous steps taken regarding MW, see Holstein Association USA’s December 2023 press release at Look for an article in the upcoming Winter 2024 issue of The Pulse for more detailed information.

Holstein Association USA, Inc., provides programs, products and services to dairy producers to enhance genetics and improve profitability — including animal identification and ear tags, genomic testing, mating programs, dairy records processing, classification, communication, consulting services, and Holstein semen.

The Association, headquartered in Brattleboro, VT., represents approximately 25,000 members throughout the United States. To learn more about Registered Holsteins® and the other exciting programs offered by the Holstein Association, visit, and follow us on Instagram, Facebook, and Twitter.