by Charles Geddes
Description and identification
Wild oat is the second most abundant mid-season weed in spring wheat in western Canada, and arguably the most problematic weed in the Canadian Prairies overall. This weed is common throughout the Prairie Provinces. Wild oat is found in about 50% of spring wheat fields following in-crop weed management, where it occurs at densities averaging 8 plants per square metre, but ranging up to 450 plants per square metre.
Like green foxtail, wild oat is an annual grass species that reproduces by seed. The seedlings are identified most easily by a counter-clockwise twist in the leaves when viewed from above, a large membranous ligule (somewhat irregularly toothed), absence of auricles, and hairs on the leaf margins. The leaf blades are rough on both sides, the sheath is split, and the seed head is a loose, open, and drooping panicle with long twisted awns. In early growth stages, the seed often remains attached to the root system when the seedling is plucked from the soil, making seedling identification rather easy.
Wild oat prefers cool temperate climates, moist soil conditions, and is most abundant in zero-tillage systems. Emergence of wild oat tends to coincide with seeding and emergence of most spring-seeded crops in the Canadian Prairies.
However, emergence of wild oat also can occur throughout the growing season. Wild oat remains one of the most difficult-to-manage weeds due to prolonged seedbank persistence (about 4 to 5 years) facilitated by seed dormancy, irregular germination throughout the growing season, and herbicide resistance. Several studies in Saskatchewan have shown wild oat-induced yield loss in spring wheat ranging from 10% to 60% depending on crop cultivar, plant density, agronomic management and environmental conditions.
Heavy reliance on the highly efficacious selective ACCase- and ALS-inhibitor in-crop herbicides have resulting in selection pressure for herbicide resistance in wild oat. Herbicide resistance in wild oat has become a major problem in the Canadian Prairies, where different populations have been found with resistance to ACCase-inhibitors (group 1), ALS-inhibitors (group 2), lipid synthesis-inhibitors (group 8), protoporphyrinogen oxidase (PPO)-inhibitors (group 14), very-long chain fatty acid (VLCFA) biosynthesis-inhibitors (group 15), and arylaminopropionic acids (group 25).
Recently, a population with five-way resistance to groups 1, 2, 8, 14 and 15 was identified in Manitoba. Two-way resistance to in-crop selective herbicide groups 1 and 2 is becoming more common, and blanket resistance to these two herbicide modes of action can drastically limit options for post-emergence management using herbicides in cereal crops, like spring wheat.
Integrating non-chemical weed management practices with non-selective herbicides applied pre-plant and post-emergence, with less-frequent use of in-crop selective herbicides and a diverse crop rotation is the an optimal strategy to mitigate selection pressure for herbicide resistance in wild oat. The most consistent non-chemical tool for management of wild oat is increased crop seeding rate. Tall cereal cultivars, crop rotation, and breaking up crop life cycles with either two years of a winter cereal or perennial forage are among some of the most effective tools for management of wild oat populations. Placement of fertilizer in a band rather than broadcasting can also promote a competitive crop and reduce the response of wild oat to fertilization. Historically, delayed seeding following a stimulation of wild oat germination via shallow tillage and a subsequent management pass, was used for management of wild oat, however this approach can promote a flush of later-emerging weeds like green foxtail.
A proactive approach is important for management of wild oat because once wild oat seedlings are present in cereal crops, few effective management options remain, aside from using an in-crop selective herbicide. Continual and sustained use of these herbicides, however, should be limited to mitigate selection pressure for herbicide resistance. Other potential management tools include clipping wild oat seed heads above the crop canopy, mowing dense wild oat patches prior to seed production, or some form of harvest weed seed control. Seed shatter of wild oat can create difficulties for management using harvest weed seed control because the plant tends to lose about 60% to 70% of its seed prior to crop maturity and harvest.