Controlling weeds is probably one of the most important decisions to think about when managing horse pastures. Weeds are generally less palatable, less nutritious, and are less dependable as a forage supply to horses than the desirable pasture species they replace. Some pasture weeds are also poisonous to horses. Milkweed is a common example.
Annual, biennial and perennial weeds have different strategies for control. Annual weeds germinate from seed, grow, mature, and die in less than one year. Annual weeds tend to put out a lot of seeds! Chemical control of annual weeds works best when applied in the spring to actively growing, young weeds before they get to seed stage. Mechanical control, such as mowing, is also very effective against annual weeds. Biennial weeds 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. Control measures, chemical or mechanical, are most effective when applied during the first year’s growth before they can build up nutrients in their root. If treatment is delayed until the second year, early season application of a herbicide before bloom is important for seed control. Perennial weeds live more than two years, and grow back from the same roots year after year. Perennials move nutrients into their roots during fall to prepare for winter. Because of this, chemical control of perennials works best when applied in the fall to actively growing and well-developed foliage. As the nutrients move into the roots, the chemical will too. Application of herbicides in spring, or frequent moving during the summer is also effective in controlling growth until fall, weakening the plant. However, mowing alone may take several growing seasons to effectively control the perennial weeds.
When using herbicides, always read and follow labels carefully. Always follow grazing recommendations after herbicide application. Herbicides may make toxic weeds more palatable to horses. Horses should be excluded from the sprayed area for seven to ten days after treatment if poisonous plants are present. Also, most herbicides control either grasses or broadleaves (i.e. alfalfa and clover). If you have a mixed pasture (grasses with legumes like alfalfa and clovers), there are no herbicide options that will control unwanted weeds and leave BOTH legumes and grasses. Broadleaf weeds tend to be the primary problem in grass pastures. Because the herbicides recommended for broadleaf weed control in a grass pasture will kill legumes, they should not be used as broadcast treatments if legumes are present, but rather should be used as a spot treatment for problem areas.
Herbicides alone will not result in a weed-free pasture. High yielding, well-managed pastures will choke out weeds. Proper grazing management is a must. Overgrazing easily damages pastures. Overgrazing pastures tends to pull out roots of desirable plant species, giving weeds space to take hold.
Management Practices to Minimize Weeds in Pastures:
Adapted from University of Minnesota Extension, "Weed Control in Horse Pastures" (2009) http://www.extension.org/pages/15647/weed-control-in-horse-pastures#.VWTJ1M9Viko
Last updated May 29, 2015