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Extension > Agriculture > Livestock > Horse > Pasture management > Grass pasture weed control

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Grass pasture weed control

Krishona Martinson, PhD, University of Minnesota

Pasture weed control is a challenge for most horse owners. Considerable grass pasture acreage in the Midwest is infested with broadleaf and grassy weeds. Weeds are generally less palatable, less nutritious, less dependable and lower yielding than recommended species. The first step to good weed control is correct identification of the weed and an understanding of its lifecycle. There are three types of weeds: annuals, biennials and perennials. Annuals complete their life cycle in one growing season. Biennials complete their life cycle in two growing seasons. Perennials normally live for three or more years.

For annuals, mechanical weed control, such as mowing, is an effective method of control if done prior to flowering and seed production. Using an herbicide to control annual weeds is most effective when applied in the spring to actively growing, small weeds. For biennials, mowing is not an option in the first year of growth because the rosettes are too close to the ground. Mowing is an effective control method in the second year as long as the mowing is complete before seed production. Chemical control of biennials is most effective when applied during the first year's growth. If treatment is delayed until the second year, an early season application of an herbicide before the flowers bloom is recommended.

Management of perennial weeds requires integrating mechanical (i.e. mowing), chemical (i.e. herbicides) and cultural (i.e. over-seeding) weed control methods. Mowing alone may take several growing seasons to effectively control perennial weeds and may never result in complete control. Herbicides alone, or a single herbicide application, likely will not eradicate perennial weeds. Fall herbicide applications (i.e. August 15 to September 15) provide the most effective perennial weed control. Application of herbicides in spring, or mowing during the summer, can be effective in controlling growth until fall. Apply herbicides selectively, carefully, and only if necessary. Always read and follow the herbicide label and comply with all grazing restrictions. Herbicides labeled for use in pastures are not harmful to horses when applied at the recommended usage rates and when all directions are carefully followed.

The key to management of any weed is to prevent its establishment with good pasture management. Recommended pasture management includes avoiding overgrazing, testing soil pH and fertility every three years, fertilizing if needed, and resting the pasture for 30 days after each grazing event. Mowing, dragging and chemical weed control (if needed) should be done during the rest period. A well-managed pasture will out-compete most weed species and will reduce weed infestations. Once weeds are established, effective control requires a combination of mechanical, chemical, and cultural methods.

For a mixed pasture of legumes and grasses, no herbicides are available that will selectively control broadleaf weeds while not injuring or killing the legumes. Good pasture management should be used to control weeds in a mixed pasture.

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