Hoary Alyssum: A Plant You Don’t Want to Find in Your Hay

Recently several horses in Wellington, Florida presented with swollen legs, fevers, and laminitis. The culprit appears to be hoary alyssum, a member of the mustard family (Brassicaceae). It was found in bales of timothy/alfalfa hay from Michigan.

Hoary Alyssum flowers

Toxic to horses

Hoary alyssum is toxic only to horses, not other livestock. Signs of toxicity are seen 12-24 hours after a horse ingests the plant. Common reactions include swelling (stocking up on all four legs), a fever of 103 or higher, unwillingness to move, and potentially laminitis.

The specific toxin is unknown, but the plant retains its toxicity for up to nine months in hay.¹

Currently there is no antidote to hoary alyssum toxicity in horses.

An invasive species

As an invasive plant native to Europe and Asia, the USDA states that hoary alyssum “should not be planted or propagated under any circumstances in North America.”

Speculation is that it got introduced to North America as a contaminant in clover and alfalfa seeds. The seeds can be further dispersed on mowers, vehicles, or other machinery. It is possible that wind disperses the seeds as well.

This plant tends to be most abundant in disturbed, dry areas. In places like dry prairies where vegetation is sparse, it will displace native species.

Michigan alfalfa hay has contained hoary alyssum before

Because it is such a pernicious weed, and easily spread, an infestation becomes very challenging for hay growers.

The situation this year in Wellington isn’t a first-time thing, unfortunately. In 2007, Kentucky Equine Research reported that hoary alyssum had been found in alfalfa hay baled in Michigan as well.²

Habitat

This is a plant that likes cold winters and hot, dry summers. In the 1960s, it began to appear more frequently in North American hay fields and pastures associated with drought and overgrazing, as well as those with coarse limestone and soil of poor fertility — conditions where hoary alyssum thrives.

The plant is considered a noxious species in California, Colorado, Idaho, Montana, Oregon, Washington, Wyoming, and Michigan, as well as in the Canadian provinces of Alberta, British Columbia, and Saskatchewan. It is not found in the coastal states from Texas to North Carolina.

Researchers at Montana State University have found that, in pastures, this toxic weed decreases forage value because the woody stems of mature plants are low in crude protein and digestible carbohydrates.³

Further research has shown that the plant does poorly where alfalfa is vigorous, because alfalfa’s shading canopy deprives it of light. Where alfalfa growth is poor, hoary alyssum thrives.

Management and prevention

According to the same Montana State study, hoary alyssum growing in Minnesota plots that were clear of other vegetation produced an average of 2,407 seeds per plot, while those growing in plots with other competing vegetation produced an average of only 104 seeds — a 96% reduction.

The plant likes poor soil, needs plenty of light, thrives in dry conditions, and does not do well with competition from other vegetation.

With this information, researchers have provided key insights on managing and preventing hoary alyssum growth:

• Mowing alone will not control it, and could worsen infestation by cutting down the shading canopy of neighboring vegetation.
• Irrigation can be useful to support forage grasses, increasing their competitiveness with hoary alyssum.
• Over-utilization of pastures and ranges weakens perennial grasses and creates an ideal scenario for weed invasion.
• Herbicides are frequently mentioned as a tool for managing this specific plant, particularly after the growing season.
• Pastures and areas that have been cleared, disturbed by construction, or had introduction of new soil or fill should be monitored for three years or more.
• Hand-pulling weeds is more manageable when the plants are small and have not spread widely.
• A vigorous stand of alfalfa, grown in healthy soil and either irrigated or receiving plenty of rainfall, can help reduce hoary alyssum in alfalfa hay.

Hoary alyssum can be reduced in alfalfa hay when alfalfa is grown in healthy soil with plenty of rainfall.

References

1- https://www.nwcb.wa.gov/images/weeds/Berteroa-incana-2003.pdf

2- https://ker.com/equinews/forage-alternatives/

3- https://www.montana.edu/extension/invasiveplants/documents/mt_noxious_weeds/hoary%20alyssum_EB0194.pdf