Weeds can reduce the quantity and the stand life of desirable forage plants in pastures and hayfields. These unwanted plants are often more aggressive than existing or desired forage species and compete for light, water, and nutrients. Weeds can also diminish the quality and palatability of the forage available for livestock grazing, and certain weed species are potentially poisonous to grazing animals. The aesthetic value of a pasture is also impacted by weeds. Therefore, it may be desirable to initiate weed management strategies that reduce the impact of weeds on forage production. However, not all weedy plants are detrimental to pastures or hayfields. In fact, some weedy plants provide nutritional value to grazing animals; thus, prudent management decisions are often required to determine when or if weed control should be initiated in a pasture or hayfield.
In general, weedy plants are usually not high yielding and are considered to be low in quality. However, many weeds are eaten along with the desired forage grasses and legumes. In fact, the dry matter digestibility of several weed species during their early vegetative stage of growth is generally high and comparable to seeded forage species. Crude protein levels are also adequate for consumption by cattle. But, like many cultivated forage grasses, digestibility and crude protein decline as weeds mature. Thus, the greatest benefits in digestibility and crude protein are obtained from weeds and desirable forage species that are grazed during their early growth stages. On the other hand, some weedy plants are unpalatable compared with the desirable forage species; thus, they are not normally consumed by animals. For example, weeds such as curly dock and tall ironweed are selectively grazed to a greater extent compared with more palatable species such as crabgrass.
One consideration before allowing livestock to graze fields heavily infested with weeds is the potential for exposure to poisonous plants. The potential for livestock poisonings depends on the availability and quantity of the poisonous plant, the stage of plant growth, the time of year, and the kind of animal. Most potentially poisonous plants (but not all plants) must be consumed in large enough quantities to cause animal death. Many plants have an undesirable taste, and animals do not consume them at levels that are toxic unless other forages are limited during periods of drought or long winter seasons. Several plants found in Kentucky that are potentially toxic to livestock are Buckeye's, Wild black Cherry, Yew, Perilla Mint, Poison Hemlock, Star-of-Bethlehem , and Nightshade to name of few.
The way a pasture or hayfield is managed can have a major impact on the presence of weedy plants. Production practices that result in overgrazing and low fertility levels favor emergence, propagation, and growth of weeds. The ideal approach is to incorporate practices that are more adaptable to the growth of the desirable forage species and less favorable for unwanted plants. Although there are exceptions, most weeds do not compete well with a dense stand of desirable forage species. Further, to minimize the effects of weedy plants, pastures and hayfields should be managed to favor the vigorous growth of the desired forage species. Effective pasture management programs include these practices:
• maintaining proper soil pH and fertility levels
• using controlled grazing practices
• mowing at proper timing and stage of maturity
• allowing new seedlings to become well established before use
• renovating pastures when needed. Herbicides can be another useful tool for weed management in pastures and hayfields.
They should be used where appropriate and when cost effective. A program that integrates several different control strategies is generally more successful than relying on only one method. Weeds present at the time of herbicide application may be controlled, but if the forage stand is not vigorous and actively growing, new weed seedlings will soon emerge and occupy the bare areas that remain. Thus, without proper use of mechanical control methods and good cultural practices, herbicide use will not be beneficial.
It is sometimes necessary to consider the use of a herbicide for control of problem weeds. Herbicide selection is based on the type of forage and weed species present, but the decision to use herbicide treatments will also depend on a variety of other factors. Some of these factors may include stage and severity of weed growth, the intended use of the forage, the time of year, environmental conditions such as temperature and rainfall, potential damage to nearby sensitive crops or plants, waiting period after treatment to use forage, and cost of treatment. Always consult the label before using a herbicide product. The type of forage grown and whether the desirable forage is a new seeding or an established stand can greatly limit the herbicide options available for use in grazed pastures and hay fields. Herbicides currently available for use in established grass pastures are listed in Table 2. In grass pastures inter seeded with clover or other forage legumes, selective herbicide options are not available for use as broadcast treatments. Lack of herbicide options in mixed stands is primarily due to the potential for legume species to be killed or severely injured. Another factor that limits some herbicide options is that the allowed residue levels have not been determined or established by the EPA for some forage species.
Another consideration or limitation when choosing a herbicide product is the waiting period after application before livestock are allowed to graze. Use of the area as a hayfield can lengthen the waiting period for some products. Also, the kind of animal present, whether beef or lactating dairy animals, can be a factor in determining the waiting period. Since horses are not always specifically mentioned on herbicide labels, the waiting period for beef animals should be applicable. Although some herbicide labels indicate a zero day waiting period for grazing, a general practice to follow is removing animals from the treated area for at least 7 to 14 days following application. The type of weeds to be controlled is also a major consideration when selecting a herbicide product. The control option can often depend on the life cycle of the plant (whether it is an annual, biennial, or herbaceous perennial) or on whether it is a woody plant such as multiflora rose. The age and size of the plant can also determine the herbicide rate and its potential effectiveness. Herbicide treatments are most often used for weeds such as musk thistle, multiflora rose, and other broadleaf-type plants in which herbicides are known to be effective. Herbicides that will selectively control broom sedge, purple-top, and other weedy-type grasses in grass pastures are not available.
For More information on the control of hay and pasture weeds contact the Pulaski County Extension Service. Information gathered from UK Publication AGR-172 " Weed Management in Grass Pastures, Hayfields, and Other Farmstead Sites J. D. Green, W. W. Witt, and J. R. Martin, Department of Plant and Soil Sciences.