Weed Management

Originally written February 1, 2006 | Last updated August 21, 2014

Successful weed management requires an understanding of weed biology

Successful weed management requires an understanding of weed biology and of weed management strategies.

Ineffective weed control can have serious consequences

Reduced crop yields weeds compete for light, moisture, and nutrients

Reduced crop quality weed seeds and foreign material can reduce the market grade of crops weed seeds and foreign material may be moist and cause storage problems

Weed Biology

Weed and Crop Life Cycles

Weed Seeds

Weed Interference

Weed Management Strategies




Methods of Weed Control

Cultural weed control

Mechanical weed control

Biological weed control

Chemical weed control: Some chemicals have carryover problems: Atrazine (AAtrex), Lasso

Herbicide Application Methods




Keys to Successful Weed Management with Herbicides

Weed ID is Key

Herbicide selection

Herbicide application

Hybrid/Variety Interactions with Herbicides

Most effective weed control = Combination of cultivation and chemicals

Timing of Weed Control

 Knezevic et al. 2003 (Nebraska)

Species Shifts

What will happen to RR technology?

Volunteer corn in corn and soybeans fields will affect yields if density levels are high and left uncontrolled.  In soybeans fields there are herbicides effective in controlling glyphosate or non-glyphosate resistant corn plants.  In corn it becomes more problematic if the volunteer plants from the previous crop are glyphosate resistant and growing in a glyphosate resistant field.

Cultivation is the only practical means available to remove these volunteer plants.  The yield impact can approach 13% under extremely high density levels.  Studies from Iowa State show a 1.3% yield loss with a volunteer plant every ten feet of row.  In many fields there are clumps and individual plants scattered about.  University studies show 0.5% yield loss with 500 volunteer plants per acre up to 1.5% loss at 2000 plants per acre.  If there are clumps in the field, then yield losses range from 1.2% per 500 clumps per acre to 2.4% per 1000 clumps per acre.  An acre is 208.7 feet by 208.7 feet.


  • Most farmers cultivate row crops at least once, many cultivate twice
  • Research shows cultivation may increase yields as much as 10-20% on soils that tend to crust
  • Cultivation breaks up crust and improves rainfall infiltration
  • Cultivation dries out upper 2-3 in of soil more rapidly, but it also breaks up capillary pathways that extend into the soil
  • Cultivation stirs the soil, which can increase microbial oxidation and release of nutrients
  • Breaking a crust to increase water infiltration can reduce soil erosion


Banding herbicides at planting is an excellent practice

  • Offers protection in a wet spring
  • Banding saves dollars
  • Usually use ½ to 1/3 as much chemical in comparison with broadcasting
  • Banding with the planter controls weeds in the row, cultivation later controls weeds between rows


  • Uniform weed control throughout a field, but more chemical applied/a means more expense
  • Many herbicides (except "burn down" or contact herbicides) lose effectiveness in a dry spring
  • Herbicide management is very important
  • On continuous corn, many farmers rotate herbicides from year to year
  • Heavy rates of some herbicides may damage corn
  • Hybrids may differ in tolerance
  • Environmental conditions important

Weed Control and Tillage Systems

  • Reducing or eliminating tillage can alter weed populations
  • Perennial weeds are often more prevalent in no-till systems than in moldboard and chisel systems
  • Higher rates of chemicals are often required as tillage is reduced because residue ties up chemicals
  • Uniform incorporation/distribution of herbicides is usually more difficult as amount of surface residue increases

Weed Control and Crop Rotations

Continuous corn: Weed control in reduced till fields can be as dependable as in conventional fields since the same herbicides and mechanical cultivation can be used

Corn-soybean rotations: If soybean was the previous crop in a reduced tillage system, much of the surface residue is buried with the first tillage operation. Therefore, uniform incorporation of herbicides is not much of a problem when following soybeans

No till: Weed control depends entirely on herbicides. A higher level of herbicide management is required

  • Herbicides requiring incorporation are not used
  • Both preemergence and postemergence herbicides are required

Herbicide Resistance 

List of herbicide resistance weeds

http://www.weedscience.org/in.asp (from Robert Gunn)

  1. Herbicide Resistant Crops

  2. Herbicide Resistant Weeds

  3. Management of Resistance

Weeds cause harvest problems

  • Lower stems of some weeds are very thick and woody at maturity (velvetleaf, sunflower)  can be tough on a combine
  • If there are many weeds, and the weeds are tough, may have to wait for a killing frost to combine soybeans
  • May have to operate the combine at a reduced ground speed, and at the same time increase combine rpm's
  • Too much weed trash may plug the sieves, and soybeans may be lost out of the back end of a combine

Problem weeds in WI

annual grasses (barnyard grass, crabgrass, downy brome, fall panicum, foxtails, longspine sandbur, prairie cupgrass, shattercane, wild proso millet, witchgrass, wooly cupgrass)

large-seeded broadleaf weeds (cocklebur, common sunflower, devil's claw, eastern black nightshade, puncture vine, smartweed, velvetleaf, Venice mallow)

perennial grasses (johnsongrass, quackgrass, wirestem muhly)

annual broadleaves  (common ragweed, lambsquarter, Pennsylvania smartweed, redroot pigweed, waterhemp)

perennial broadleaves (field bindweed, hedge bindweed, swamp smartweed, woolyleaf bursage)

small-seeded broadleaf weeds (kochia, lambsquarters)

How to calibrate a sprayer

Calibrating a boom sprayer is not as difficult as it sounds. Although there are many methods to use, the method described below for broadcast applications is simple and requires few calculations.

To calibrate your sprayer you need a measuring tape, a watch capable of indicating seconds, and a measuring jar graduated in ounces. A pocket calculator also will be handy. What follows is a calibration process:

  1. Fill at least half the tank with water.
  2. Run the sprayer, inspect it for leaks, and make sure all vital parts function properly.
  3. Measure the distance in inches between the nozzles. Then measure an appropriate distance in the field based on the nozzle spacing (204, 136 and 102 ft for nozzle spacings of 20, 30 and 40 inches of nozzle spacing, respectively).
  4. Drive through the measured distance in the field at your normal spraying speed, and record the travel time in seconds. Repeat this procedure and average the two measurements.
  5. With the sprayer parked, run the sprayer at the same pressure level and catch the output from each nozzle in a measuring jar for the travel time required in Step 4.
  6. Calculate the average nozzle output by adding the individual outputs and then dividing by the number of nozzles tested. If an individual sample collected is more than 10 percent higher or lower than the average nozzle output rate, check for clogs and clean the tip, or replace the nozzle.
  7. Repeat steps 5 and 6 until the variation in discharge rate for all nozzles is within 10 percent of the average.
  8. Then, the final average output in ounces is equal to the application rate in gallons per acre (Average output in ounces = Application rate in GPA).
  9. Compare the actual application rate with the recommended or intended rate. If the actual rate is more than 5 percent higher or lower than the recommended or intended rate, you must make adjustments.
  10. You can start the adjustments by changing the pressure. Lowering the spray pressure will reduce the spray delivered; higher pressure means more spray is delivered. Don't vary from the pressure range recommended for the nozzles used.
  11. You also can correct the application error by changing the actual travel speed. Slower speeds mean more spray is delivered; faster speeds mean less spray is delivered.
  12. If these changes don't bring the application rate to the desired rate, then you may have to select a new set of nozzles with smaller or larger orifices.
  13. Recalibrate the sprayer (repeat steps 5 through 12) after any adjustment.

Further Reading

To purchase hard copies of these publications, go to The UWEX Learning Store

Pest Management in Wisconsin Field Crops  UWEX Bulletin A3646

Scouting Corn--A Guide for Wisconsin Corn Production UWEX Bulletin A3547

Protecting Wisconsin's Resources through Integrated Weed Management UWEX Bulletin A3690

Herbicide persistance and carryover  UWEX Bulletin A3819

Avoiding Herbicide Resistance in Weeds  UWEX Bulletin A3615

Corn and Soybean Herbicide Chart

Herbicide Mode of Action Key for Injury Symptoms

IPM Quick Guide: Troublesome Weeds of Wisconsin

Wisconsin Integrated Crop Management Manual: Intro | Alfalfa | Corn | Soybean | Wheat | Nutrients | Weeds

Pest Management Fast Facts

Protecting Resources with Integrated Weed Management

Reduced Herbicide Rates in Corn  UWEX Bulletin A3563

Strategies to reduce risk of glyphosate resistance

Wisconsin 2-pass Weed Control Program

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