Weed control in winter wheat is aided by the crop’s fall-growth habit, vigorous spring growth, and early maturity. In many instances, wild oat control is not required and broadleaf weed control can be achieved with relatively inexpensive products. This benefit not only is of value in the year winter wheat is grown, but is also an important tool for maximizing the effectiveness of other crop protection products in other crop years. For example, avoiding a graminicide during the winter wheat year can help avoid or manage the development of herbicide resistance.
Few crops grown on the Prairies are as competitive as winter wheat, thus allowing for more efficient use of crop inputs.
This is one of the main advantages of growing winter wheat. However, it is important to remain on top of your weed situation throughout the winter wheat growing season. This means starting your weed management in the crop prior to seeding right through to harvest of your winter wheat crop.
By incorporating winter wheat and these weed management practices into your rotation, you can be certain that not only your winter wheat, but also all subsequent crops in your rotation will reap the rewards of having lower weed pressure.
Weed Control in Previous Crops
Proper weed management in the crop prior to winter wheat is an important first step for crop establishment in the fall. This is particularly true of biennial and perennial weeds, which can be very competitive in late fall.
Pre-Seeding Weed Control
Winter wheat is no different than other crops in its need to have a competition-free establishment period. Controlling weeds prior to seeding is a particularly effective time for that second step in control of biennial, perennial, and winter annual weeds, especially downy brome. Glyphosate products provide the most effective control in this window for weed control.
Pre-seeding herbicide applications also control early emerging volunteer plants. Control of volunteer seedling is particularly important when seeding into wheat stubble. Pre-seed glyphosate one to two weeks prior to seeding will break the “green bridge,” preventing wheat streak mosaic virus from carrying over to the winter wheat crop. Some growers tank mix residual broadleaf chemistry with their glyphosate to extend the weed control into winter wheat emergence and establishment. Early emerging weeds are generally some of the most competitive weeds a crop has to face. Removing early weed competition helps develop a well-established winter wheat crop, which will have a better ability to survive the winter.
Fall In-Crop Weed Control
Controlling winter annual weeds is the next important step in successful winter wheat production. Common winter annual weeds include stinkweed, shepherd’s purse, and flixweed. Due to their habit of emerging in the fall and resuming growth early in the spring, winter annuals can be very competitive and difficult to control in the spring. This makes fall control a very practical and cost effective approach to controlling winter annuals if sufficient densities are present. The common practice to control these weeds involves a phenoxy herbicide, such as 2,4-D or MCPA, applied in the first half of October for central areas and mid to end of October in the southern Prairies. Research agronomists report that 2,4-D amine applied at a rate of 560 g a.i. when plant stage is at least three leaf and weather conditions are warm provides good results. Phenoxy injury can occur when other products are used, when the application takes place at the wrong growth stage or when frost preceded or followed the application. Herbicide applications should target actively growing plants that are able to metabolize the product quickly. According to Brian Beres of Agriculture and Agri-Food Canada, “Fall 2,4-D when used in the form and rate specified above is very effective in the control of winter annuals and I prefer it to any spring phenoxy applications as it’s very hard to apply in spring before the plant stage is too advanced.”
Spring In-Crop Weed Control
Considerations for spring in-crop weed control in winter wheat are generally very similar to any other cereal crop. In-crop applications in spring generally occur at times that coincide with pre-seeding glyphosate applications and seeding operations. Waiting until spring seeding is complete to spray winter wheat often results in poor weed control due to weeds becoming too large. The early growth habit of winter wheat also leads to increased crop canopy cover and potential crop injury due to the herbicide being applied beyond the safe application stage.
The lifecycle of downy brome is similar to those of winter wheat and fall rye and is therefore a troublesome weed for those crops. To make it worse, downy brome has the competitive advantage because it will start growing in the spring before the fall-seeded crops, using up the soil moisture at shallow depths.
An infestation of downy brome in winter wheat can reduce yields 30-40 per cent.
Downy Brome can be managed in winter wheat in a few ways:
Control in Surrounding Areas
Downy brome should be controlled when first observed on field borders or fence lines. Also, banding rather than broadcasting nitrogen, and establishing a competitive crop, will help control spreading.
Maximizing Seed Germination
Because downy brome seeds are viable for three to four years, stimulating their germination all at once through fall harrowing and increasing soil-to-seed contact, allows more plants to be controlled in late fall leaving fewer seeds to germinate in spring or the following fall.
Managing Crop Residue
Chaff spreading easily distributes downy brome seeds in the field, which will prevent large patches of downy brome, making it easier to control with tillage or herbicides.
Only planting winter wheat or fall rye once every three to four years helps keep downy brome at low infestation levels. Downy brome can also be controlled with herbicide application or tillage before a spring crop.
Using Certified Seed
Planting clean seed reduces the risk of pre-contaminated seed.
Using Optimum Dates
Early establishment of winter wheat will allow the crop to compete better with the downy brome. A downy brome infestation emerging before the crop will significantly reduce yields.
Grassy winter annual weeds such as Japanese brome and downy brome can be particularly difficult problems for winter wheat. Japanese brome can be controlled with a fall or spring application of pyroxsulam (Simplicity) herbicide. Initial studies indicate that flucarbazone (eg. Everest) and new chemistries flumioxazin and pyroxasulfone also have good promise for Japanese brome control, but these are not registered for use on winter wheat at this time.
Downy brome is a more difficult winter annual weed to control. Metribuzin (eg. Sencor) is registered for use on the Norstar winter wheat variety only after it has started tillering. Pyroxsulam is registered for control of downy brome in fall applications and suppression for spring applications. As with Japanese brome, new chemistries flumioxazin and pyroxasulfone also show good promise for downy brome control, but are not registered for use on winter wheat at this time.