Primary symptoms

Wheat is very sensitive to early weed competition. Weeds damage the crop by competing for light, water and nutrients. Be careful with the use of herbicides.

Early weed control is important for good yields.

Summary

About weed competition

Weeds compete with the crop for light, nutrients, water and root space. Some weeds can damage the crop by producing toxic substances or acting as hosts for diseases. Annual weeds compete most effectively with wheat during the seedling stages and early tillering. So this is the critical period for weed control. Once the crop is covering 50-70% of the soil surface at jointing, it will dominate most newly germinating weeds.

Many selective herbicides are highly effective against weeds in the crop but they may cause some damage to the crop, as may manual or mechanical weed control methods. The likely yield loss from chemical or mechanical damage should be assessed against the yield loss from the weeds. Pre-emergence herbicides cause little or no damage so often result in better yields than later sprays.

Weeds can be a problem during harvesting; weed seeds can contaminate the grain and the green matter from late maturing weeds can contaminate the straw. Herbicides and desiccants can be used at this late stage, but only in particularly weedy situations and then with great care.

Conventional tillage if shallow and done hurriedly during seedbed preparation can exacerbate a weed problem. It can break up and spread the propagating root systems of some perennial weeds and bring buried seeds of annuals to the soil surface where they can germinate. Consider whether the option of minimum or zero tillage (direct drilling) may better suit your requirements in some years. If there is a well-established problem of perennial weeds, deep tillage to undercut and expose the deep propagating roots may be the first step towards control.

Is the crop weedy?

• Check the crop within 2-3 weeks of crop emergence.
• Are many of the weeds as large or larger than the crop plants? Any weeds that are similar in size to the crop at this stage will soon dominate crop plants if not removed. They will intercept the light and take up water and nutrients.
• Look generally at the crop. Does the crop cover much more or less ground surface than the weeds? Remember that the percentage of ground covered by weeds is the percentage of resources not available to the crop.
• What are the dominant weeds in the crop? Try to identify them so that you can check on their vigour and methods of control.
• Are wheat plants in the weedy areas smaller or less developed than in clean areas of the field? If so, weeds are already dominating resources.

Causes of weed infestation

• Your wheat seed was contaminated with weed seeds. Always examine your seed before sowing. Machines used for harvesting and tillage are a source of weed seeds and should be cleaned before leaving each field.
• Weed removal by hand was not effective. Check on its timing and frequency.
• Herbicide application was ineffective. Check what herbicide was used, its concentration and whether there was rain or heavy dew during application. Were the manufacturer's recommendations on the label followed?
• Monoculture and repeated use of herbicides with the same mode of action led to herbicide resistant weeds as reported in Malik et al. (1998) in South Asia.
• Perennial weeds propagating from broken roots or underground stem sections are difficult to control manually. Was a herbicide used? Is it effective against perennial weeds?
• Planting was delayed too long after seedbed preparation. Check the field sheets for dates of land preparation and planting. Did rain occur at this time encouraging weeds to germinate? Use minimum tillage (see the section "Notes on two tillage systems") if a fast turn around between crops is essential.
• Each crop species has its own set of cultural practices that create niches for certain weeds. If the land has been used continuously for wheat, a bank of weed seeds may have built up in the soil. As some weed seeds can lie dormant for many years, the problem will continue annually for some time despite control methods.

What you can do about weeds

• Seed quality: Make sure that your wheat seed for sowing is not contaminated with weed seeds. Buy good quality seed if possible. If you are using your own seed and your seed samples commonly have weed problems, consider preparing a small weed-free area on your farm solely for producing seed for planting next season. This will save work in the long term.
• Crop vigor: A crop that emerges quickly and is vigorous and dense will dominate most weeds. Assist the crop by preparing a good seed bed.
• Timing of weeding: The weeding operation should be done closer to crop seedling emergence and/or the method should be modified. Remove weeds when they are most susceptible to damage, when they are small.
• Weed seed: If a few weeds escape earlier control, remove them prior to flowering to avoid seeding, but do not damage the crop. Weed seeds mature extremely quickly.
• Herbicides: In the case of herbicide use, identify the weed before choosing the herbicide. Follow the directions on the label very closely. Never exceed the recommended dose. A strong mix may damage the crop. Apply the herbicide uniformly with a calibrated sprayer. Do not use the same herbicide year after year.
• Herbicides: Check on alternative herbicides that can be used to control difficult weeds. Most post-emergence herbicides that are absorbed by leaves work better when the weeds are growing actively. Spraying early morning after dew has lifted may be better than late afternoon. Do not spray if it is raining or about to rain.
• Pre-emergence herbicides: Leave a few days between application and cultivation and sowing to avoid damage to emerging seedlings.
• Seed rate: If perennial weeds cannot be controlled, increase the seeding density of the crop to achieve ground cover more quickly after sowing and out-compete the weeds. Consider deep cultivation to control them.
• Crop rotation: Rotate with a crop that will dominate the weeds or leave fallow and cultivate. Rotation changes the conditions to which the weeds are specially adapted.
• Crop residues: If the preceding crop (e.g. rice or maize) has left residues that are free of disease, and the land is not weedy, consider sowing the wheat crop through these residues with minimum tillage implements. The residues will help to suppress annual weeds and turn around time between crops will be short.
• Crop cycle: If you have an herbicide resistant weed, try a shorter duration crop variety or species that allows you to leave a 1-2 months fallow between crops. During that break irrigate, then control weeds mechanically or with a non-selective herbicide. Consider using bed planting as mechanical weed control is possible in the channels between beds. Use competitive varieties.
• Late weeding: Late removal of weeds by hand is possible but hard.