Farmers carefully select and plant pasture grasses to provide high nutritional content for grazing animals. Most weeds have little nutritive value, and some weeds are poisonous to animals if ingested. These weeds should be eliminated from pastures, while maintaining the nutritious forage grasses. If herbicides must be used, they must be safe for livestock. Chemical herbicides must be applied according to stringent guidelines, and proper stewardship must be maintained. Organic farmers use a holistic approach to pasture management that may include organic fertilizers that promote a dense growth of desirable grasses and organic herbicides, which kill weeds without leaving chemical residues to taint surrounding feed and forage grasses.
2,4-D Amine 4L
2,4-D is an herbicide that is used in more than 1,000 weed killer products in the United States. It is available as liquid, dust or granules and in different formulations for different applications. It may be highly concentrated, or ready-to-use. Although ubiquitous, 2,4-D is an extremely hazardous chemical. The National Pesticide Information Center advises to precisely follow directions for application and cleanup, and use all suggested protective equipment. The University of Tennessee Extension guidelines state that you can use 2,4-D Amine 4L on established pasture grasses to control broadleaf weeds. It also kills clover, alfalfa and other desirable legume crops in the pasture. Apply 2,4-D Amine 4L when the weeds are less than 4 inches tall and actively growing.
Grazon P + D
Grazon P + D, or Grazon Picloram + 2,4-D, is a restricted-use pesticide, or RUP. You must be licensed to purchase or apply it. You can applied to newly seeded pasture when the seedlings have begun their secondary root systems and tillers begin to appear, which is about the four leaf stage for most grasses. The University of Tennessee Extension cautions that Grazon P + D also kills pasture legumes, established or new. Reseeding of legumes may be possible one year after application. Determine the degree of soil residuals before attempting to replant.
Surmount
Surmount, another RUP, is a blend of Picloram and Fluroxypyr. It is effective against tough woody brush and brambles, such as blackberry canes, horsenettles and thistles. It also gives some residual control of broadleaf perennials. The University of Tennessee Extension recommends applying when the brush has fully leafed out in the summer. For blackberries, apply after fruit drop when there is plenty of moisture. Follow directions for broadcasting rates and use a non-ionic surfactant as directed for spray applications.
Vinegar
Strong vinegar at 20 percent acidity is the effective strength for use as an organic herbicide. Twenty percent acidity is strong enough to burn skin, and it is a breathing hazard. ProgressiveForage.com cautions that this strength of vinegar is not approved by the Environmental Protection Agency due to handling dangers, although permits to use it can be obtained through a lengthy and costly process. Vinegar breaks down completely in the environment and leaves no harmful residue in soil or water.
Plant Oils
Natural plant oil-based herbicides are safer to handle than caustic vinegar, and they are also effective organic weedkillers. Clove oil and cinnamon oil are commonly used organic herbicides. Diluted and sprayed on weeds, they break down the waxy leaf coating and cell membranes on contact, causing the plant to dehydrate and die. Natural plant oils break down quickly and completely in the environment, leaving no harmful residues in soil or water.
(Ref 3, organic herbicides section)
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