The optimum timing and method of herbicide application will depend on the herbicide, crop, and target weeds. Guidelines and directions for the proper application of each herbicide is provided in the herbicide label. Failure to follow label directions, warnings, and precautions can result in crop injury, poor weed control, off- target plant injury, environmental contamination, human hazards, and/or civil penalties.
Herbicides can have foliar and/ or soil activity depending on the chemical. A number of factors can affect the activity or performance of soil-applied and foliar-applied herbicides.
Soil-Applied Herbicides
Soil-applied herbicides persist in the soil for a period of time and can be absorbed from the soil by plant roots and shoots. Herbicides applied to the soil and mechanically incorporated before planting the crop are commonly referred to as preplant incorporated treatments.
Herbicides applied to the soil surface before the crop and weed emergence are commonly called preemergence applications. Herbicide properties, soil properties, environmental conditions, weed susceptibility, and application rate will affect the performance of soil-applied herbicides. Most herbicides need to
be in soil solution to be absorbed by germinating seedlings. Adequate precipitation or mechanical
incorporation after application is required to get the herbicide into solution or “activate” the herbicide. The amount of precipitation required to activate the herbicide will depend on the individual herbicide and soil properties. Herbicides are generally more active in coarse textured and low organic matter soils than in finer textured and higher organic matter soils. Consequently, herbicide rates may need to be adjusted for soil- applied herbicides depending on soil properties. Soil pH also can affect the activity and persistence of certain herbicides. For example, atrazine, and several of the
sulfonylurea herbicides like Finesse and Amber, are more active and more persistent at high than at low soil pH. Consult the herbicide label for the recommended application rates, crop rotation guidelines, and use restrictions as it pertains to soil properties.
Foliar-Applied Herbicides
Foliar-applied or postemergence herbicides are applied to the foliage of emerged plants for control. Some postemergence herbicides may have both foliar and soil residual activity.The performance of postemergence herbicides will depend on species susceptibility, weed size, environmental conditions, spray adjuvants, and application techniques. Annual weeds and seedling perennial weeds generally are most susceptible to postemergence herbicides when the weeds are small and actively growing. The suseptiblity and
Weed Management
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benefits from controlling annual weeds decreases as they mature. Established perennial weeds usually are most susceptible when treated at the early bloom stage of growth or to active growth in the fall. Application at these times generally results in the greatest translocation to the roots and the best long-term control.
Weather conditions can greatly impact the performance of postemergence herbicides. Postemergence herbicides are most effective when applied to actively growing weeds with favorable growing conditions. Weeds that are under stress due to extreme temperature or moisture conditions tend to be less susceptible to most herbicides. On the other hand, crops under stress may actually be more susceptible to injury than crops growing with good conditions due to a diminished ability to metabolize the herbicide.
Spray adjuvants are chemicals added to the spray to modify the properties of the spray solution for improved handling or pesticide activity. Adjuvants are often recommended with certain postemergence herbicides to optimize their performance. There are many types of adjuvants that each have a different function. Consult the herbicide label for the recommended type and rate of adjuvant to maximize performance. Inappropriate adjuvant use could result in crop injury, poor weed control, and application problems. Spray volume and droplet size may impact spray deposition,
canopy penetration, coverage, and herbicide performance. Consult each herbicide label for the recommended spray application guidelines to minimize spray drift and optimize herbicide performance.
Weed Management Decisions
Knowledge of field history and scouting fields to identify emerging weed problems throughout the growing season are important for making effective weed control plans and decisions. Herbicides are just one component of an effective weed management program. Consult herbicide labels and other weed control guides, such as the latest edition of Chemical Weed Control for Field Crops, Pastures, Rangeland, and Noncropland (SRP 1045) to assist in making effective herbicide choices. The guide is available at www.ksre.ksu.edu/library by typing chemical weed control in the search box.
Weed Management
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Study Questions
These study questions are to help you learn the material on pages 70 through 82.
1. Weeds reduce crop yields by competing for:
a. moisture b. light
c. soil nutrients d. all of the above
2. Important features used to identify grassy weeds include:
a. leaf shape
b. auricles and ligules c. leaf arrangement d. all of the above
3. An example of a dicot plant includes a:
a. broadleaf b. sedge c. soybean d. both a and c
4. Field bindweed, dandelion and johnsongrass are: a. summer annuals b. perennials c. winter annuals d. biennials
5. Winter annual grasses include: a. downy brome
b. cheat
c. jointed goatgrass d. all of the above
6. In cases where available herbicides will not control the weeds, which of the following management practices are helpful?
a. crop rotation b. tillage
c. a and b above d. none of the above
7. Glyphosate is an example of a: a. non selective herbicide b. EPSP inhibitor
c. selective herbicide when use with Roundup Ready crops d. all of the above
8. A weed species that has developed resistance to herbicides in Kansas includes:
a. kochia b. cheat
c. palmar amaranth d. all of the above
9. Summer crops such as corn, sorghum, and soybeans tend to increase:
a. summer annual weeds b. fall annual weeds c. winter annual weeds d. perennials
10. The risk of herbicide resistance can be decreased by using: a. crop rotations
b. herbicide rotations
c. mixtures of herbicides that have a different mode of action
d. all of the above
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11. Perennial weeds can reproduce from:
a. stolons
b. seed production c. tubers
d. all of the above
12. Examples of biennial broadleaf weeds are:
a. common mullein b. musk thistle c. western salsify d. all of the above
Weed Management
Weeds common in rangeland and pastures in Kansas include annual species such as lanceleaf ragweed and common broomweed. Biennial plants that take two years to complete their life cycle, include musk thistle and common mullein. Troublesome perennial weed species include sericea lespedeza, Baldwin ironweed, woolly verbena, and goldenrods. Grasses such as prairie threeawn, longspine sandbur, Johnsongrass, Kentucky bluegrass, and old world bluestems may also be considered weeds if they are growing where they are not welcome. Shrubs such as buckbrush, smooth sumac, and roughleaf dogwood, as well as trees such as common honeylocust, osage orange, and eastern redcedar, may become problems on range and pastureland.
There are several options for control and management of brush and weed species:
• management • mechanical • biological
• prescribed burning • chemical
Proper grazing management can reduce invasion of many problem weed and brush species. Stocking rate, the number of animals per unit area for a given period of time, is the most important factor influencing livestock and plant response to grazing. Overstocking can reduce the vigor of the most palatable species and allow invasion of plants that reproduce by seed. Maintaining a competitive cover can keep unwanted species at manageable levels. Grazing systems, such as rotational grazing,
that incorporate periodic rest, can help keep a competitive cover. Improper grazing distribution that results in concentrated grazing usually disturbs the soil and allows unwanted species to invade. Unfenced pond dams are a typical disturbed site where noxious weeds and other unwanted species occur. Other grazing management principles include proper season of use and kind of animal. Cattle are primarily grass eaters, but will consume significant amounts of forbs and browse during the year. Sheep prefer palatable forbs, and goats will eat woody plants if available.
Mechanical control includes hand clippers, mowers, tree cutters, and bulldozers. Mowing of annual and biennial species before seed production can be an effective control method. Non-sprouting species, such as eastern redcedar, can be controlled by cutting below the lowest green branch. Repeated mowing or top removal will be necessary to control most perennial species. Timing for top removal should correspond with the low point in the food reserve cycle that generally occurs at floral bud initiation.
Release of the musk thistle head weevil (Rhinocyllus conicus) and rosette weevil (Trichosirocalus horridus) has provided reduced seed production and is an example of biological control. Attempts are being made to introduce a leaf feeding beetle (Diorhabda elongata) for biological control of saltcedar. Leafy spurge and spotted knapweed are also targets for using biological control. Because of grazing preference, sheep and