Weed management and the critical period of crop–weed competition
How weeds cut yields, what the critical period of weed competition is and how it is measured, and how to combine control methods.
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Weeds cause the largest potential crop losses of any pest group. Global assessments put the loss they would cause, if left uncontrolled, at around a third of attainable yield in major crops, more than insects or diseases. Most of that is prevented by control, which is why weeding takes so much of a farm's labour, fuel and herbicide budget.
The critical period of crop–weed competition (CPWC) is the idea that makes weed control efficient: there is a window in a crop's life when weeds must be kept out, and outside it they do little harm. It is a standard exam topic, together with the indices used to judge weed control.
How weeds reduce yield
A weed is any plant growing where it is not wanted. Weeds harm crops in several ways:
- Competition for light, water and nutrients. Weeds often emerge quickly, grow fast and take up nitrogen early. Competition for light is decisive once canopies close: a weed that overtops the crop shades it for the rest of the season.
- Allelopathy: some weeds release chemicals that inhibit crop germination or growth.
- Hosting pests and diseases, and parasitism: Striga (witchweed) and Orobanche (broomrape) draw their water and nutrients directly from crop roots.
- Harvest and quality losses: contaminated grain, clogged machinery, poisonous seeds in fodder.
- Building the seedbank: one plant of a prolific weed can shed tens of thousands of seeds. The old farmers' saying "one year's seeding means seven years' weeding" reflects how long seeds survive in soil.
The damage depends on density (more weeds, more loss, but with diminishing effect at high densities), species (tall, fast or climbing weeds compete hardest), and above all on time of emergence relative to the crop. Weeds that emerge with or before the crop are far more damaging than those emerging a few weeks later, which the established crop suppresses.
The critical period
The critical period is the span of crop growth during which weeds must be controlled to keep yield loss below an acceptable level. It is defined from two kinds of field experiment run together.
- Weedy treatments: the crop is left weedy until a given time (for example 2, 4, 6 weeks after emergence), then kept weed-free. The longer weeds stay, the greater the loss. This curve tells you when control must begin.
- Weed-free treatments: the crop is kept weed-free until a given time, after which weeds are allowed to grow. The longer the crop is kept clean, the smaller the loss from later weeds. This curve tells you when control can stop.
A line is drawn at the acceptable yield loss, commonly 5 per cent (some studies use 2.5 or 10 per cent). The critical period runs from the point where the weedy curve crosses that line to the point where the weed-free curve does.
What the two ends mean:
- Before the start, early weeds and the young crop are both small, and resources are plentiful. Weeds can be tolerated briefly without loss.
- After the end, the crop canopy is large enough to suppress late weeds. They may still set seed, which matters for next season, but they hardly reduce this crop's yield.
As a rule of thumb the critical period often falls in roughly the first third to half of a crop's life: in many field crops, between about two and six to eight weeks after emergence. It is not a fixed property of a crop. It is shorter and later when the crop is vigorous and well fertilised, sown at higher density or in narrower rows, and when weeds emerge later; it is longer and earlier with heavy weed pressure, poor crop establishment, or drought. Slow-starting crops with open canopies, such as onion, are sensitive for much of the season.
Measuring weed competition and control
Field experiments describe weed control with a few standard indices.
Here X is the yield from the weed-free plot and Y the yield of the treatment. It is the yield lost to weeds in that treatment.
Here Wc is the weed dry weight (or density) in the unweeded control and Wt in the treatment.
Comparing two weed control treatments
In a maize trial, the weed-free plot yields 6.0 t/ha and the unweeded plot 3.9 t/ha, with 320 g/m2 of weed dry matter. Treatment A (one hoeing at three weeks) yields 5.4 t/ha with 96 g/m2 of weeds. Treatment B (two hoeings) yields 5.8 t/ha with 32 g/m2.
- Loss in the unweeded plot: WI = (6.0 − 3.9) ⁄ 6.0 × 100 = 35 per cent.
- Treatment A: WI = (6.0 − 5.4) ⁄ 6.0 × 100 = 10 per cent; WCE = (320 − 96) ⁄ 320 × 100 = 70 per cent.
- Treatment B: WI = (6.0 − 5.8) ⁄ 6.0 × 100 = 3.3 per cent; WCE = (320 − 32) ⁄ 320 × 100 = 90 per cent.
Treatment B keeps loss under a 5 per cent threshold; A does not. Whether the second hoeing pays depends on its cost against the extra 0.4 t/ha.
The methods of weed control
No single method is enough for long. Integrated weed management combines several, chosen to keep weeds below damaging levels during the critical period while shrinking the seedbank over time.
Preventive
Clean seed, clean machinery, composting manure well enough to kill weed seeds, keeping field margins and irrigation channels from seeding, and quarantine rules against new invasive species.
Cultural
These make the crop the stronger competitor.
- Crop rotation breaks the cycle of weeds adapted to one crop.
- Competitive crops and varieties: early vigour, tall or leafy types.
- Higher seed rates, narrower rows and good fertiliser placement to close the canopy sooner, which ends the critical period earlier.
- Timing of sowing, the stale seedbed (letting a flush of weeds emerge and killing it before sowing), and cover crops and mulches that suppress emergence.
- Flooding in transplanted rice, which suppresses most upland weeds.
Mechanical and physical
Hand weeding and hoeing, inter-row cultivation, harrowing, mowing, and in some systems flame weeding or soil solarisation (heating moist soil under clear plastic). Mechanical control is effective and leaves no residues but costs labour or fuel, depends on dry weather, and can increase erosion and moisture loss.
Biological
Using natural enemies, mainly insects and pathogens, against weeds, most successfully against invasive species in rangelands and waterways (classic examples are prickly pear cactus and water hyacinth controlled by imported insects). Grazing animals and allelopathic cover crops are also used.
Chemical
Herbicides are grouped by mode of action, using the global HRAC/WSSA classification (for example inhibitors of amino-acid synthesis, of photosynthesis, of cell division, synthetic auxins). They are also described by timing (pre-plant, pre-emergence, post-emergence) and selectivity (selective herbicides kill weeds but spare a given crop; non-selective ones kill all green plants). Registrations and permitted uses differ by country and change over time; always follow the current local label.
Relying on one mode of action has produced herbicide-resistant weeds in hundreds of species worldwide. Rotating and mixing modes of action, and combining herbicides with cultural and mechanical methods, slows resistance.
Key points
- Uncontrolled weeds would cost around a third of attainable yield in major crops, more than any other pest group.
- Damage depends on weed density, species and especially emergence time relative to the crop.
- The critical period is found from paired weedy and weed-free experiments and an acceptable-loss line (often 5%).
- It typically lies in the first third to half of crop life, and shifts with crop vigour, density, fertility and weed pressure.
- Weed index = (X − Y) ⁄ X × 100; weed control efficiency = (Wc − Wt) ⁄ Wc × 100.
- Integrated weed management combines preventive, cultural, mechanical, biological and chemical methods; rotating herbicide modes of action delays resistance.
What the exams ask
- School-leaving and entrance papers: definitions of a weed, harmful effects, examples of parasitic weeds, and methods of control.
- Degree and research-fellowship entrances: the critical period and how it is determined, weed index and weed control efficiency calculations, herbicide classification and selectivity, allelopathy.
- Professional and licensing examinations for agronomists: designing a weed management plan for a crop, timing control to the critical period, and managing herbicide resistance.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Do I need to weed after the critical period?
For this crop's yield, usually not. But weeds that set seed late in the season enlarge the seedbank and make next season harder, and some interfere with harvest. Late, cheap control of seeding weeds often pays over several seasons.
Why does the critical period change from year to year?
Because it depends on how fast the crop and weeds grow, which depends on weather, sowing date, fertility and which weeds emerge when. That is why local trials give better guidance than a fixed number of days.
Is hand weeding better than herbicides?
Each has a place. Hand and mechanical weeding leave no residues and suit small fields and labour-rich farms; herbicides save labour on large areas but need careful choice and rotation. Integrated programmes use both with cultural methods.
Sources
- Zimdahl, R. L. Fundamentals of Weed Science, 5th edition. Academic Press, 2018.
- Zimdahl, R. L. Weed–Crop Competition: A Review, 2nd edition. Blackwell, 2004.
- Knezevic, S. Z., Evans, S. P., Blankenship, E. E., Van Acker, R. C. and Lindquist, J. L. "Critical period for weed control: the concept and data analysis." Weed Science 50, 773–786 (2002).
- Oerke, E.-C. "Crop losses to pests." Journal of Agricultural Science 144, 31–43 (2006).
- Heap, I. The International Herbicide-Resistant Weed Database. Online, continuously updated.
- FAO. Procedures for Weed Risk Assessment. Food and Agriculture Organization of the United Nations, 2011.
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