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Identifying and controlling weeds

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Quick facts

  • Weeds can lower the nutritional value of pastures and some may harm your horse’s health.

  • Hoary alyssum is the most common poisonous plant to horses in Minnesota.

  • To best control weeds, don’t overgraze pastures, mow after each grazing and use herbicides as needed.

  • Always follow application and grazing directions when using an herbicide.

Weed control is key to managing grass pastures. Compared to grasses weeds are generally:

  • Less palatable

  • Less nutritious

  • Lower yielding

  • Less dependable as a forage supply

Many pasture weeds are on the Minnesota noxious weed list. Noxious weeds are weeds that can cause harm to humans, animals or the environment. The Minnesota noxious weed law requires control of primary noxious weeds on all private and public land in the state. Five of the ten primary Minnesota noxious weeds are common pasture weeds.

  • Bull thistle

  • Canada thistle

  • Musk thistle

  • Perennial sowthistle

  • Plumeless thistle

Common poisonous plants in Minnesota include:

  • Bracken fern

  • Buttercup

  • Common cocklebur

  • Common lambsquarter

  • Hoary alyssum

  • Horsetail

  • Nightshades

  • Poison hemlock

  • Redroot pigweed

  • White snakeroot

Most poisonings occur in the early spring, late fall or during a drought when the pasture forage is in short supply.

From a control standpoint, grouping weeds into categories based on life span is most practical. Annual, biennial and perennial are the main life spans of weeds.

Annuals

An annual sprouts from seed, grows, matures and dies in less than one year or when killed by frost. Chemical control of annuals works best when applied in the spring to actively growing, young weeds. Mechanical control, such as mowing, is very effective against annuals.

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Biennials

Biennials require two years to complete their life cycles. They form a rosette (group of leaves at ground level) and store food in their roots the first year and flower the second year. Chemical or mechanical control are most effective during the first year's growth. If you delay treatment until the second year, apply herbicide early in the season before bloom.

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Perennials

Perennials live over two years and grow back from the same roots each year. Make sure to use a systemic herbicide in the fall to best treat perennials. You can apply herbicides in the spring or mow often during the summer to control growth until fall. Mowing alone may take a few growing seasons to effectively control weeds.

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Other problem plants

The following plants may also cause harm to grass pastures and horses.

  • Trees

  • Shrubs

  • Wildflowers

  • Weedy grasses

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Herbicides

When using herbicides ALWAYS:

  • Carefully read and follow labels when using herbicides.

  • Follow grazing directions after using an herbicide.

  • Make sure the product is labeled for use in a pasture.   

Keep horses out of the sprayed area until the grazing restriction found on the herbicide label has passed.  An additional 7 to 10 days of restricted grazing is recommended if poisonous plants are present. Herbicides may make toxic weeds more palatable to horses.

Herbicides alone won’t result in a weed-free pasture.

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Reducing weeds in grass pastures

  1. Properly manage grazing.

    • Overgrazing tends to pull out grass roots, which gives weeds space to grow.

  2. Protect new seedings from grazing until they become well established.

  3. Allow established pastures to recover (e.g. re-grow) after grazing. This will reduce weeds and increase pasture yield and nutrition value.

  4. If possible, mow after each grazing period to control many pasture weeds and promote new pasture growth.

    • Don’t mow the pasture closer than four inches above the soil.

  5. Remove horses from pastures during hot and dry spells.

  6. Reseed very weedy pastures if pasture forages are thin.

  7. Remember that thick, well-managed pastures will choke out weeds.

Author: Krishona Martinson, equine Extension specialist

Reviewed in 2023

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