Weed competition

Primary symptoms

Maize is very sensitive to weed competition during the critical period between the V3 and the V8 stages.

Weeds damage the crop primarily by competing for light, water, and nutrients.

Confirm the problem by checking the tables below.


Causes of high weed competitionAdditional evidence required
Poor manual weed control. Ask the farmer about the timing, frequency, and methods of weeding.
Weeding too late. Look for large dead weeds lying in the field and stunted maize. Ask the farmer about when he controlled weeds.
Ineffective herbicide application. Ask the farmer about conditions during application.
Planting delayed after land preparation. Ask the farmer for dates of field operations.
Problem weed species not controlled by current method. Identify weed species. The perennials are poorly controlled manually. Most herbicides only control certain types of weeds (see Table 9 below).
Alleopathic weed. Identify weed species (see Table 8 below).
If the land has been used for continuous maize cropping for many years, the load of weed seeds will be very high. Ask the farmer about the history of the field.

The importance of weed competition in maize depends on four factors: the crop growth stage, the amount of weeds present, the degree of water and nutrient stress, and the weed species. Weeds damage the crop primarily by competing for light, water, and nutrients. Maize is very sensitive to this competition during the critical period between the V3 and the V8 stages.

Before the V3 stage, weeds are usually important only if they are larger than the maize or if the crop is suffering from water stress. Maize needs a period between the V3 and V8 stages when few weeds are present. From the V8 stage to maturity, the crop usually reduces the sunlight reaching the weeds enough to provide good weed control. In the later part of the cycle, weeds are important mainly if water or nutrient stress is a problem, or if a very aggressive weed overtops the maize and shades it, or if the weed has some allelopathic effect. In addition, some weeds make harvesting difficult, and thus increase production costs.

Some weed species cause more damage than others. This can be because the weeds actually produce toxic substances which damage the crop (allelopathy) or because the weeds are very effective competitors for water or nutrients. Some weed species which are reported to be allelopathic are listed in Table 8 below.

Are weeds a problem?

Note: These observations should be made before the maize reaches the 8-leaf stage. If the farmer controls weeds, you should make the observations just before the farmer cultivates or applies herbicide, and note the crop growth stage. If you visit the field after flowering, it will be difficult to estimate the importance of weeds on yield.

Evidence: observations.

Examine a 5-m length between the rows at 10 random locations in the field.

  1. Are many weeds larger than the crop? Is the crop shaded by the weeds? If so, the weeds are a problem.

  2. Is the crop suffering from drought stress? Estimate the percent of sunlight which falls on weeds rather than the crop or bare ground. That percentage is close to the percentage of the available water which is being used by the weeds rather than by the crop.

  3. Note the growth stage of the crop. Between the V3 and V8 stages, weed density should be low to avoid yield reduction.

  4. Do maize plants in weedy spots in the field look different from those growing in clean spots? This can indicate severe competition for nutrients, water, and light, and/or allelopathic effects.

  5. Compare different areas of the field with the photographs below. Estimate the amount of yield loss. This comparison should be made around the V8 stage.

  6. What are the main weed types present? Narrow or broad leaves? Annual or perennial? This information is needed to identify a method for addressing weed control problems and to know if toxins produced by the weeds might be important.

Possible solutions

  • Recommend improving the method or timeliness of weed control.
  • Reduce problems of drought or nutrient stress.
  • Increase population density of crop and/or N application rate to get more shading of weeds.
  • Move to another site or rotate with another crop that will allow better control of problem weeds.

The following photos demonstrate the impact of weed growth on maize yields. Maize at V8 under good, fair, poor, and no weed control. If there were no subsequent weeding, respective yields would be approximately 100, 75, 25, and 8% of potential. In the no-weeding treatment, even if weeds were controlled from this stage on, the crop would already have suffered irreversible damage (note the reduced plant size and early leaf senescence).

Good weed control. Yield = 100%

Good weed control. Yield = 100%.

Fair weed control. Yield = 75%

 Fair weed control. Yield = 75%.

 Poor weed control. Yield = 25%

 Poor weed control. Yield = 25%.  

No weed control.Yield = 8%

No weed control. Yield = 8%.

Table 8. Some common weeds reported to have allelopathic properties.

Scientific nameCommon name
Abutilon theophrasti Velvetleaf
Agropyron repens Quackgrass
Amaranthus sp. Pigweed
Ambrosia sp. Ragweed
Avena fatua Wild oat
Brassica sp. Mustard
Chenopodium album Common lambsquarters
Cynodon dactylon Bermuda grass
Cyperus esculentus Yellow nutsedge
Cyperus rotundus Purple nutsedge
Digitaria sanguinalis Crabgrass
Echinochloa crusgalli Barnyardgrass
Helianthus annuus Sunflower
Imperata cylindrica Alang-alang, Speargrass
Poa sp. Bluegrass
Portulaca oleracera Common purslane
Rottboelia exaltata Rottboelia
Setaria faberi Giant foxtail
Sorghum halepense Johnsongrass

Sources: S.O. Duke, 1985. Weed Physiology, Vol. 1: Reproduction and Ecophysiology. eRe Press. E.L. Rice, 1984. Allelopathy. Academic Press, N.Y.

Table 9. Selectivity of some important herbicides used in maize-based cropping systems.

ChemicalSpecies controlledSpecies not controlled *
2,4-D Many annual broad leaf weeds. High rates can be used on Cyperus sp. Many annual and perennial grasses.
Paraquat Most annual grasses and broad leaf weeds. Perennial weeds, also Parthenium hysterophorus.
Dicamba Many annual broad leaf weeds. Most perennial weeds.
Glyphosate Most annual plants (including maize!) and many perennial weeds, including Cyperus, Imperata, and Sorghum halapense. Species with underground storage tubers may require additional treatments. Weeds should be growing when the chemical is applied.
Atrazine Most annual broad leaf weeds, some annual grasses. Most perennial weeds, many annual grasses.
Alachor Many annual grasses and broad leaf weeds. Most perennial weeds.
Metolachlor Most annual grasses, some broad leaf weeds. Most perennial weeds, many broad leaf weeds.
Pendimethalin Many annual grasses and broad leaf weeds, including Rottboelia. Most perennial weeds.
* When the chemical is applied at the recommended rate.