Beginning in February, Holstein Association USA will officially recognize early onset muscle weakness syndrome as a genetic condition in the breed. That means that direct gene test results for the trait, abbreviated MW, will be listed on official Holstein pedigrees and other places genetic information is outlined.
The condition was first identified less than a year ago when farmers reported multiple calves having difficulty standing up. It typically presents in the first two months of life, when calves may be able to stand initially and then go down, be unable to stand but then appear to recover, or remain unable to stand.
Research done at Penn State University uncovered that the condition was related to a mutation of chromosome 16 that causes problems forming calcium channels. It is recessive, so an animal would need two copies of the mutation to express the condition. However, it also seems to exhibit incomplete penetrance, meaning not all animals with two copies would show symptoms. Like other conditions, early onset muscle weakness can be concentrated in certain cow families and affect some farms more than others.
Because of the variability of the condition, genetic testing has been critical to understanding its prevalence in any herd. The Council on Dairy Cattle Breeding evaluated the results of those tests and introduced a haplotype call for the trait with the December 2023 tri-annual evaluations. This made MW haplotype results available for all genomic-tested animals. However, the results of the haplotype — noncarrier, carrier, homozygous, probable carrier, or probable homozygous — have not been displayed on Holstein pedigrees because the breed association had not declared it an officially recognized genetic condition.
That will soon change based on an endorsement from Holstein Association USA’s genetic advancement committee, which analyzed the results of the December haplotype call.
Animals will be designated as:
- TE: tested free of muscle weakness
- MW: heterozygous carrier
- MW2: homozygous carrier
Avoiding homozygous matings is a dairy farmer’s best tool to limit the condition’s spread in their herd. Breeders can search a list of bulls that have an NAAB code and a direct MW test result on file here.