Weeds may affect wheat production in many ways. Wheat yield may be reduced significantly when weeds compete with wheat plants for light, water and minerals. Weeds may also inhibit wheat growth through release of allelopathic chemicals that are toxic to wheat plants. Weeds or weed seeds contaminating harvested grain may reduce quality. In addition, weeds may interfere with harvesting or raise the moisture content of the harvested grain, leading to damage from heat and pests in storage.
In the relatively mild and wet climate of western Washington, the primary weeds in wheat are annual broadleaf weeds that germinate just before or as the wheat germinates. Wheat, whether seeded in the fall or spring, frequently becomes a solid mat of well established broadleaf annual weeds if weed control practices are lacking. However, some areas are also plagued by grasses: annual ryegrass, wild oats, and quackgrass. Perennial weeds may also be troublesome in either crop.
An integrated weed management system that combines cultural and chemical weed management practices is the most effective and economical way to manage weeds in wheat. Although several effective herbicides are available to control broadleaf weeds and grasses in wheat, herbicides should be viewed as an additional tool, not as a remedy. Often, when one control method, whether chemical or mechanical, is used continuously, a shift in the weed population toward a difÞcult-to-control species will occur. Herbicide resistant plants within a species can be selected from a susceptible species and can increase in number. Most commonly, tolerant species can replace sensitive ones that have been eliminated by herbicides. This problem can be avoided by integrating as many control measures as possible such as crop rotation, using mixtures of herbicides with different modes of action, and by rotating herbicides from one season to the next.
A properly prepared seedbed can signiÞcantly reduce weed infestation. If possible, germinate the first flush of weeds before beginning tillage operations using rainfall or irrigation. Plow as deeply as possible to break up soil compaction and reduce risk of herbicide carryover if wheat is planted after vegetable seed crops, ornamental bulbs, corn, and other crops. The final tillage should be just before planting wheat so that any germinated weeds do not have a competitive advantage.
Good field sanitation is essential for weed control. When possible select fields free from hard-to-control weeds. Clean planting, harvesting and tillage implements prior to entering a field to eliminate introducing new weeds. Keep field perimeters weed free because they serve as an initial reservoir for seed to infest the field.
Planting wheat seed contaminated with weeds is one of the most common ways to introduce weeds into wheat fields. Plant certiÞed wheat seed. Certified seed is slightly higher priced, but it is cheap compared with managing a weed problem that can result from seed contaminated with weeds.
Weed infestations can be reduced by rotating to crops with a different life cycle or ones in which different cultural and chemical practices are used. Crop rotation regularly changes the crop in each field, the soil preparation practices in that field, subsequent tillage, and weed-control techniques. All these factors affect weed populations. Rotating wheat with summer crops is a very logical weed control practice. Summer crops such as green peas, vegetable seed, and cucumber benefit when planted following wheat. Unlike broadleaf weeds, annual grasses in soils are largely depleted after two to three years of summer cropping.
Management practices which encourage a healthy and vigorous wheat crop will reduce losses from weeds. Some of these practices may include: seeding at the proper depth; seeding at the appropriate rate and time; selecting the correct amount, timing, and placement of fertilizers; using adapted cultivars; and controlling insects and diseases.
Weed management is not accomplished by using cultural practices exclusively. Some weeds are favored by the same management practices that favor wheat. Herbicides offer an additional tool to control weeds in conjunction with cultural practices, but are not intended as a replacement for proper management practices.
The success of a herbicide application is dependent upon weed species, the timeliness and thoroughness of application, conditions at the time of application, herbicide rate, and crop management after the application. If the decision is to use a herbicide, carefully read the label. Following the label will reduce the likehood of crop injury, reduce off-target movement of herbicide, and maximize weed control.