Weeds can become a serious problem in pastures because they compete with desirable pasture species and can reduce the longevity and nutritional value of a pasture stand. However, eliminating weeds creates an open space, which eventually will be filled with additional weeds if the pasture is not properly managed. The best weed control is achieved by maintaining a dense healthy stand of grasses though proper fertilization, cutting, and rotation.
In order to decide upon a means of weed control, it is important to know which species exist in your pastures. Collections of full color photos of weeds and/or detailed illustrations are found in these web sites and books:
If you choose to control the weeds by means of a chemical herbicide, 2,4-D, Banvel, or a combination of the two will control most of the broadleaf weeds. The link below titled, Relative Effectiveness of Pasture Herbicides provides a numerical rating of the effectiveness of these products. Banvel and 2,4-D must be absorbed by the plant leaves and translocated to the roots. The herbicides should be used when the weeds are actively growing and healthy. Hot, dry weather causes the weeds to become dormant and they will not be effectively controlled. Winter annual weeds should be treated in spring, before they produce seeds.
Perennial weeds such as thistle, dogbane, multi-flora rose, and brambles are best controlled in late summer when the plants are translocating nutrients to the roots. Banvel and/or 2,4-D can be used to control broadleaf perennial weeds. Round-up can also be used to remove difficult perennials such a multi-flora rose and brambles. It is non-selective and will kill grasses and broadleaf plants. It should be used only as a spot treatment.
Ally is another herbicide that effectively controls weeds. It is characterized by high specificity, providing excellent control of a relatively small number of weeds. It is very good at removing multi-flora rose. There is no grazing restriction for Ally. It can be used anytime from early spring to late summer. Apply Ally to grasses that have been established for at least 6 months. Fescue should be established for at least 24 months. Grasses should be actively growing at the time of application. Timothy should be at least 6 inches tall or discoloration and stunting may occur. Ryegrasses are less tolerant to Ally than other grasses. Ally persists in soil and should generally not be used if you intend to over-seed any pastures within 1 to 2 years. Follow label crop rotation restrictions carefully.
Stinger is a highly weed-specific grass pasture herbicide. It is effective on a small number of pasture weeds. In particular it has very good activity on thistle and marestail, but it not effective on the majority of weeds present in most pastures. Roundup can also be used to remove difficult perennials such as multi-flora rose and brambles. It is non-selective and will kill grasses and broadleaf plants. It should be used only as a spot treatment. Most fertilizer companies can custom-apply herbicides as well as fertilizers. Farmers may also be willing to apply herbicides to pastures. Remember that these herbicides will remove the clover in your pastures as well as the weeds.
Caution: Use pesticides only when necessary and at the recommended dosages and timing to keep residues within the limit set by the law. Before using any pesticide, read the label and follow all directions and safety precautions listed. Where trade names are used, no discrimination or endorsement of products is implied.
The following tables from the 2005 Penn State Agronomy Guide contain more complete information about available pasture herbicides: