Minnesota Horse Owners Warned of Toxic Plant Issue

Due to last year's drought and recent dry conditions in parts of Minnesota, the state's Extension service is again warning horse owners that hoary alyssum could become a problem for their animals. Hoary alyssum is a perennial weed common throughout Minnesota, the surrounding states, and Canada. It is particularly adapted to dry conditions and sandy or gravelly soils. Hoary alyssum tends to increase in pastures and hay fields following drought or winterkill, regardless of the soil type.

Hoary alyssum is toxic to horses when the fresh plant is grazed in a pasture, and when the dried plant is eaten in hay. Most hoary alyssum toxicity occurs when horses accidentally ingest it in hay. Hay containing greater 20% or more hoary alyssum should not be fed to horses.

To minimize hoary alyssum and other weeds in pastures:

  • Do not overstock or overgraze;
  • Have a sacrifice paddock and use it if necessary;
  • Rotationally graze;
  • Remember that each pasture paddocks need rest periods;
  • Soil test every three years and fertilize accordingly
  • Mow and drag after each rotation;
  • Control weeds (chemically, culturally, and mechanically).

If herbicides must be used in a grass pasture, the most effective and economical approach would be to apply 2,4-D (a member of the chlorophenoxy family of herbicides) in the spring and/or fall to suppress hoary alyssum. Retreatment will likely be necessary in following years.

In a pure alfalfa hay field, herbicides applied to dormant stands or following first cutting can suppress hoary alyssum, but are expensive and can injure alfalfa. No herbicide options exist after first cutting. There are no herbicide options for controlling hoary alyssum in a grass/legume (alfalfa or clover) mixed pasture or hay field that do not have significant injury potential to the legume as well.

When using herbicides, always read and follow the label carefully. Most herbicides have grazing and harvest restrictions that must be followed.

More information on hoary alyssum and other toxic weeds from the Univeristy of Minnesota.  

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