Crop–Weed Competition


Competition from weeds is the most important of all biological factors that reduce agricultural crop yield. This occurs primarily because weeds use resources that would otherwise be available to the crop. The magnitude of yield loss is affected by numerous agronomic and environmental factors, most importantly, weed density and time of emergence relative to the crop. Practices that (1) reduce the density of weeds, (2) maximise occupation of space or uptake of resources by the crop or (3) establish an early‐season size advantage of the crop over the weeds will minimise the competitive effects of weeds on crops. Longer term management of crop–weed competition can be achieved through crop rotations, specifically crop sequences that reduce the weed seed bank, and therefore seedling density, and prevent proliferation of perennial weeds.

Key Concepts

  • Plant growth requires sunlight, water and nutrients, which in turn are converted into biomass that captures additional resources.
  • In crop or weed monocultures, increasing density increases total production to a maximum that is determined by the resource status of the site, generally with a corresponding decrease in per plant mass.
  • In crop–weed mixtures, increasing weed density results in large initial reductions in crop yield, again reaching an asymptote of maximum yield loss that varies among crop species, environments and weed cohorts.
  • Farmers manage crop–weed competition by (1) reducing weed density (i.e. ‘weeding’), (2) establishing an early‐season competitive advantage to the crop and (3) maximising resource capture by the crop using competitive species, competitive cultivars, high sowing densities, optimal spatial arrangement, intercropping complimentary species or transplanting.

Keywords: agroecology; crop yield loss; inter‐specific; interference; weed management

Figure 1. Annual weeds interfere with pumpkin harvest and reduce fruit yield. The abundant seed rain ensures an abundant weed population in future years. Photo: E. Gallandt.
Figure 2. The effects of plant density on biomass in single‐species populations (monoculture). At very low densities, there is no competition and total biomass increases linearly with density (a). As density increases, competition becomes important and the increase in biomass is less than proportional to density (curved part of graph a). At high densities, further increases in density do not lead to any increase in biomass (Law of Constant Final Yield). The effects of density on mean individual plant biomass are best illustrated with a log–log plot (b). The proportional increase in total biomass with density at low densities on graph (a) corresponds to the flat part of graph (b). The flat part of graph (a) corresponds to the linear decrease (slope = −1) on graph (b).
Figure 3. The right‐rectangular hyperbolic model relating crop yield loss to weed density (Cousens, ). Crop yield loss = i*N/1+((i*N)/A) where N is the weed density; i and A fit parameters with i representing the slope as N approaches zero and A the slope as N approaches infinity.
Figure 4. Results from an experiment designed to determine the ‘critical period’ for weed control. Data from two complementary experiments are plotted on the same axes. Results from one experiment show cabbage yield when the crop is maintained weed‐free for 0, 1, 2, 4, 6 and 14 weeks. Results from the second experiment show cabbage yield when the crop is unweeded for 0, 2, 4, 6 and 14 weeks. Weeds present for only a short period of time have no effect on yield, and the end of the critical period marks the point at which the crop has a sufficient size advantage to be unaffected by subsequently emerging weeds. (Data from Miller and Hopen, © Allen Press Inc.)
Figure 5. Effect of sequence of seed sowing, and resulting sequence of seedling emergence, over 10 consecutive days on space, estimated as proportional to biomass, occupied by individual plants. Panels show the randomly selected positions in which seeds were sown, and the area occupied on days 4, 8, 12, 16 and 20. Seedlings from seeds sown on days 7 through 10 occupy very little space which have been pre‐empted by earlier‐emerging individuals. (Adapted with permission from Harper (). © Academic Press Inc.)


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Further Reading

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Liebman M, Mohler CL and Staver CP (2001) Ecological Management of Agricultural Weeds. Cambridge, UK: Cambridge University Press, 532 pp.

Radosevich SR, Holt JS and Ghersa CM (1997) Weed Ecology, Implications for Management, 2nd edn. New York: John Wiley & Sons, Inc, 589 pp.

Tilman D (1990) Mechanisms of plant competition for nutrients: the elements of a predictive theory of competition. In: Grace JB and Tilman D, (eds). Perspectives on Plant Competition, pp. 117–141. San Diego, CA: Academic Press.

Zimdahl RL (1980) IPPC #31‐A‐80, (ed). Weed‐Crop Competition, a Review. Corvallis, OR: International Plant Protection Center, 196 pp.

Zimdahl RL (2004) Weed‐Crop Competition: A Review, 2nd edn. Ames, IA: Blackwell Publishing 32 pp.

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Gallandt, Eric R, and Weiner, Jacob(May 2015) Crop–Weed Competition. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0020477.pub2]