06/30/2026
🌱 𝐇𝐨𝐚𝐫𝐲 𝐀𝐥𝐲𝐬𝐬𝐮𝐦: 𝐀 𝐓𝐨𝐱𝐢𝐜 𝐖𝐞𝐞𝐝 𝐄𝐯𝐞𝐫𝐲 𝐇𝐨𝐫𝐬𝐞 𝐎𝐰𝐧𝐞𝐫 𝐒𝐡𝐨𝐮𝐥𝐝 𝐊𝐧𝐨𝐰
Time to kick off a new educational series on common plants and w**ds found in horse pastures and hay!
Some are simply a nuisance, while others can have serious consequences for your horse's health. First up is one of the most common toxic w**ds horse owners should recognize – Hoary Alyssum.
🌿 𝐖𝐡𝐚𝐭 𝐢𝐬 𝐡𝐨𝐚𝐫𝐲 𝐚𝐥𝐲𝐬𝐬𝐮𝐦?
Hoary alyssum (Berteroa incana) is an herbaceous forb that belongs to the broadleaf mustard family. It grows up to two feet tall and has small white flowers, each with four deeply notched petals.
📍 𝐖𝐡𝐞𝐫𝐞 𝐢𝐬 𝐢𝐭 𝐟𝐨𝐮𝐧𝐝?
Originally native to Eurasia, spanning Eastern and Central Europe as well as Western Asia, hoary alyssum is now widespread throughout North America. It is primarily found in the Northern and Western United States as well as southern and central Canada.
It was accidentally introduced as a contaminant in clover and alfalfa seed in the late 19th and early 20th centuries and was first documented in Wallbridge, Ontario, in 1893. Since then, its rapid spread has been driven by the movement of contaminated hay, gravel, soil, and farm equipment.
🌱 𝐖𝐡𝐞𝐫𝐞 𝐝𝐨𝐞𝐬 𝐢𝐭 𝐭𝐡𝐫𝐢𝐯𝐞?
Hoary alyssum is most abundant in disturbed areas. These disturbances may result from overgrazing, animal traffic, tilling, plowing, mowing natural borders, or removing topsoil. Additionally, hoary alyssum is common in sandy soils and can take advantage of poor, drought-stressed conditions where it outcompetes desirable forage species.
⚠️ 𝐖𝐡𝐞𝐧 𝐝𝐨𝐞𝐬 𝐢𝐭 𝐩𝐨𝐬𝐞 𝐚𝐧 𝐢𝐬𝐬𝐮𝐞 𝐭𝐨 𝐡𝐨𝐫𝐬𝐞𝐬?
Both fresh and dried hoary alyssum are toxic to horses, meaning it can be dangerous whether it is growing in pasture or baled in hay.
Typically, hay or pasture containing 30% or more hoary alyssum by weight can be dangerous for horses. However, some horses develop signs of toxicity with as little as 10 to 20% of their diet containing the w**d.
Due to its bitter taste, horses often avoid hoary alyssum on pasture unless other forage is limited. The greater risk occurs when it is incorporated into hay, where horses cannot easily sort it out.
Notably, while hoary alyssum is toxic to horses, ponies, and donkeys, other livestock such as cattle, sheep, and goats can safely consume the plant.
🩺 𝐖𝐡𝐚𝐭 𝐜𝐥𝐢𝐧𝐢𝐜𝐚𝐥 𝐬𝐢𝐠𝐧𝐬 𝐚𝐫𝐞 𝐨𝐛𝐬𝐞𝐫𝐯𝐞𝐝?
Currently, the toxins in hoary alyssum have yet to be identified, but they disrupt the circulatory and inflammatory systems within the horse.
If a horse consumes a toxic amount of hoary alyssum, clinical signs typically appear within 12 to 24 hours. The main signs include swelling of the limbs, lameness, laminitis, and colic.
💉 𝐇𝐨𝐰 𝐢𝐬 𝐢𝐭 𝐭𝐫𝐞𝐚𝐭𝐞𝐝?
The first steps are to immediately remove the source containing hoary alyssum and contact your veterinarian to assess the severity of the condition. Treatment often includes IV fluids, electrolyte therapy, and management of laminitis if it develops.
With proper supportive care, many horses show improvement within 2 to 4 days, although it can take months for some horses to return to full performance.
🚜 𝐇𝐨𝐰 𝐝𝐨 𝐲𝐨𝐮 𝐠𝐞𝐭 𝐫𝐢𝐝 𝐨𝐟 𝐢𝐭?
Hoary alyssum is primarily an annual, meaning it completes its life cycle in a single growing season and reproduces entirely by seed. Preventing seed production is therefore one of the most effective ways to reduce future infestations.
Because it has a taproot, hand-pulling or mechanical removal is the most effective control method for small infestations. If the w**d is widespread, mowing before seedheads develop can help reduce future spread, although it will not eliminate existing plants. Avoid mowing once seeds have formed, as this can spread the seeds even farther.
When infestations are severe, cultural practices such as prescribed burning and reseeding with desirable forage species may be beneficial. As a last resort, the herbicide 2,4-D can provide effective control when applied before the plants flower.
✅ 𝐂𝐨𝐧𝐜𝐥𝐮𝐬𝐢𝐨𝐧
The best way to protect your horse is to recognize toxic plants before they ever make it into the pasture or hay bale. Regular pasture inspections, purchasing quality hay, and learning to identify common w**ds can go a long way toward preventing a potentially serious poisoning event.
Have you ever encountered hoary alyssum in your pasture or hay?
What w**ds or toxic plants would you like me to cover next?
Cheers,
Dr. DeBoer
