Important Updates on Early Onset Muscle Weakness Syndrome from Holstein Association USA - Cowsmo

Important Updates on Early Onset Muscle Weakness Syndrome from Holstein Association USA

Holstein Association USA has continued to monitor information related to Early Onset Muscle Weakness Syndrome (MW) over the past several months. The development of the haplotype calls for Muscle Weakness (HMW) has been a top priority for Holstein Association USA, along with colleagues at the Council on Dairy Cattle Breeding (CDCB), USDA-AGIL and the National Association of Animal Breeders (NAAB).

During the Fall 2023 Holstein Association USA Board meeting in Chicago, Illinois, the Holstein Association USA Board of Directors approved staff’s recommendations for further action regarding MW.

Holstein Association USA will make the haplotype results available 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 HMW will be reported as 0 (free), 1 (carrier), 2 (homozygous affected, 3 (probable carrier), or 4 (probable homozygous) on evaluations. CDCB predicts a number of Holstein genotypes in the database will receive a status of 3 or 4 for HMW, due to the inability to confirm the presence of the mutated haplotype through available pedigree information.

Next Steps

Holstein Association USA staff will analyze the HMW haplotypes released in December and collect comments from Holstein breeders and scientific researchers. Following a satisfactory review outcome, Holstein Association USA plans to add MW to the list of officially recognized undesirable genetic conditions in February 2024.

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

The best tool available for discovering and understanding new genetic conditions is farmer reporting. Therefore, breeders performing gene tests on animals are asked to forward those results to Holstein Association USA, at [email protected]. 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 [email protected], or contact the Genetic Services department at 800.952.5200.

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