Effective Weed Control
The most effective weed control strategy is to maintain thick, healthy forage stands. Weeds are plants of opportunity and will utilize any weakness to establish themselves in a pasture or hay field. These weaknesses may include soil fertility issues, overgrazing, scalping during mowing, and soil acidity issues. If these issues are not addressed, weed control will be unsatisfactory at best.
The first step in effective weed control is to evaluate the pasture or hay field and determine what conditions may have led to existing weed problems. Soil testing to determine the current nutrient and pH status is the place to begin. After soil fertility issues have been corrected, overgrazing problems should be eliminated with a change in stocking rate, a change in rotation schedule, or an addition of grazing land. For hay fields, scalping-during-mowing and mowing-too-low issues should be corrected by raising the mower to the correct height. Weak forage stands may also require some type of renovation.
Weed Identification and Herbicide Selection
After these issues have been corrected, the weed or weeds of concern must be accurately identified so that the appropriate herbicide may be selected. Proper weed identification is critical, since herbicides vary in their effectiveness in controlling specific weeds, and some may not provide any control against the targeted weed. Choosing the wrong herbicide due to a misidentified weed can easily result in large amounts of money spent with no little or effect on the weed problem. Local Extension Agents can help with weed identification and can also help determine which herbicide will be most effective based on research data and herbicide label information. They can also point out possible issues with using a given herbicide, such as wind drift to nearby crops, volatilization problems, spray water pH and water quality issues, and requirements to apply the herbicide during a specific growth stage of the forage crop or the weed to be controlled.
The herbicide selected must be labeled for both the weed to be controlled and the pasture or forage crop grown. It is against Federal law to use a herbicide on a crop or forage not listed on the label.1 The label on the herbicide provides information on weeds controlled, forages the herbicide may be applied to, mixing procedures, application rates, and proper safety apparel required during mixing and application. More information on herbicide handling and safety may be found in the Clemson Cooperative Extension Pesticide Information Program’s fact sheet PIP-16, “Handle Pesticides Safely”.2 Selecting the wrong herbicide, or over-applying the correct one, is not only illegal, but can also destroy the desirable forage. The herbicide label is the law and is there to provide directions on proper and safe application and use. Read the label!
Local Extension Agents utilize the South Carolina Pest Management Handbook to determine the most effective herbicide for the weed to be controlled and the forage grown.3 This handbook is available online and provides information including efficacy of herbicides on various weeds, recommended application rates, mixing and handling instructions, and sprayer calibration information. Proper sprayer calibration is critical to ensure correct application rates.
The Weed Control Timing Chart
Once the existing pasture/forage issues have been corrected, the sprayer has been calibrated, and the herbicide has been selected based on weed identification and forage grown, the grower must determine what time of year to apply the herbicide. The most effective herbicide for the control of a given weed may be almost completely ineffective if applied at the wrong time of year. There is an optimum time frame to apply herbicide to each weed. In most cases, this time is early in the weed’s life cycle, but in some cases, herbicides should be applied during a different growth stage. In other cases, only a pre-emergent herbicide (herbicide applied before the weed emerges to prevent weed seed germination) may provide effective control. Remembering when to apply herbicide for many different weeds can be a challenge. The herbicide label provides definitive information concerning the crop stage, time of day, and weather conditions when the product may be applied, but a simple chart to help initially plan a weed control season would be of great benefit.
The Pasture/Hay Field Herbicide Application Timing Chart was developed to help remove confusion and guesswork when determining the recommended time of year to control various weeds in pastures and forages. The chart is arranged with a list of common weeds in rows on the left and months of the year in columns on the top of the chart. The row of each weed listed has a colored bar in the row under the time frame when the weed may be most effectively controlled. Green bars indicate application time ranges for post-emergent herbicides; goldenrod bars indicate application time ranges for pre-emergent herbicides.
Growers with a known weed problem may use this chart to plan their herbicide purchases and application timing. The chart may also be used to determine if a weed issue recently discovered may be best addressed immediately, or if a herbicide application should be delayed until later in the season or the following year to be most effective.
If there are several different weeds to be controlled, the chart may also help the grower decide if a single application of one herbicide will be appropriate, or if there will need to be two different applications due to the timing required, or different herbicides needed to control the different weeds. If the recommended control times for several weeds the grower needs to control overlap, an application time may be selected that will offer good control for all of those weeds (provided the herbicide selected is effective in controlling them).
Your local Extension Agent is the best source for information related to weed identification, herbicide selection, and determination of application rates. The Agent may also be able to offer information or cautions based on the pesticide label – for instance, should care be exercised or another herbicide used if a soybean crop or fruit orchard is planted nearby. Consult the list of Clemson Extension county offices for contact information for your local office.
This chart is provided as a planning tool. It is not provided as a substitute for herbicide label information, which is the law, nor is it to be used in absence of the directions on the herbicide label. It is simply provided to help the grower plan approximate weed control timing as a portion of an effective weed control strategy, which includes following all herbicide label directions and observing all warnings and cautions, including setbacks from water bodies, roads, property lines, and wells; warnings concerning volatilization and wind drift; requirements for surfactants, nozzle types and sizes, spray pressures, and water volumes applied; and worker protection apparel required.
- Office of Enforcement and Compliance Assurance. Federal insecticide, fungicide, and rodenticide act (FIFRA) inspection manual. Washington DC, DC: U.S. Environmental Protection Agency; 2019. p. 121–121.
- Bellinger RG. Handle pesticides safely. PIP-16; Clemson (SC): Clemson Cooperative Extension Pesticide Safety Education Program.
- South Carolina pest management handbook. Clemson (SC): Clemson University Cooperative Extension Service, c2019 [accessed 2019 August]. http://www.clemson.edu/extension/agronomy/pest%20management%20handbook.html.
Smith WB, Beer B, Croft J, van Vlake L. Pasture / hayfield weed control timing chart. Land-Grant Press by Clemson Extension. 2019; LGP 1018: http://lgpress.clemson.edu/publication/pasture-hayfield-weed-control-timing-chart.