Understanding Early Onset Muscle Weakness Syndrome in Holstein Calves

Black and white holstein calf with coat on

In 2023, a genetic defect now called Early Onset Muscle Weakness Syndrome (MW) was identified within the Holstein breed. This genetic defect – formerly known as calf recumbency – is an important concern and high priority for you as a producer and GENEX, as your genetics supplier. Below, you will find questions and answers explaining MW and our approach to mitigating its impact. 

As of February 2024, the Holstein Association USA officially recognized MW as a genetic condition, and direct gene test results for MW are now labeled on Official Holstein Pedigrees as well as other products where officially recognized genetic conditions are displayed.

What is Early Onset Muscle Weakness Syndrome (MW) in Holstein calves?

MW is a genetic defect that affects Holstein calves’ ability to stand. Calves affected by MW are unable to stand at birth or lose the ability to stand shortly after birth.

What is the frequency of MW in the Holstein population?

As of January 2024, the frequency of the allele in the Holstein breed population is 5%. Although this may seem significant, we can reduce the frequency of MW with good breeding practices. In particular, influential bloodlines known to carry this genetic defect trace back to Roylane Socra Robust and his son Seagull-Bay Supersire (both are MW carriers).

A gene test is available for MW. Results from Holstein Association USA can be found here. Breeders can also share results with the association by emailing them to [email protected] to be added to the database. 

Newborn Holstein calf laying on straw
MW is characterized by the calf’s inability to stand at birth or the loss of the ability to stand shortly after birth.

How can I mitigate the impact of MW on my herd?

The GENEX MAP+™ mating program and GENEX DairySuite™ precision genetics program have been updated to prevent matings between known MW carriers. These genetic programs use an industry file from the Council on Dairy Cattle Breeding (CDCB) and Holstein Association USA which provide gene test results for MW.

Where can I access MW test results on GENEX bulls?

The most recent test results on GENEX active sires is available to download by clicking the button below.

How are the test results labeled?

The nomenclature used for gene test results and the haplotype results vary and are described below. These labels are used across our GENEX platforms including our online catalog and the GENEX Dairy Bull Search App.

Holstein Association USA has established an official nomenclature for the gene test results as follows:

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

The CDCB haplotype results will be included in genomic evaluations as:

  • 0 = Noncarrier
  • 1 = Carrier
  • 2 = Homozygous
  • 3 = Probable Carrier
  • 4 = Probable Homozygous

Note that these results deviate from the historic understanding of genetics where a homozygous designation is lethal. At this point, data has confirmed that not all animals that are homozygous for the MW defect are affected the same way.

If you have further questions, regarding the MW (or calf recumbency) genetic defect impacting Holstein calves, please reach out to your local GENEX representative.

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