High
Yield Grass Seed Production and Water Quality Protection HandbookHigh
Yield Grass Seed Production and Water Quality Protection Handbook| Nutrient Management | Residue
Management | Weed Control | Herbicide
Drift Management |
| Disease Management | Management
of Insects, Slugs and Related Pests | Vole control
|
| Conservation Practices | References
and Credits | Home |
 Wild oats |
 Mayweed chamomile |
 Field bindweed |
 Canada thistle |
 Quackgrass |
Keys to the success of spring treatments
include biochemical selectivity between the crop and weed and rapid recovery by the corp after any injury. |
Weed control systems are designed to prevent or delay weed seed germination, interfere with the growth and development of seedlings, reduce or eliminate seed production by established plants and destroy vegetative survival structures. These goals are accomplished through cultural and chemical practices that interact with a wide variety of environmental factors.
While successful systems require site-specific management, general principles exist: Starting with a clean field helps prolong stand life and maintain high seed quality. This can be accomplished by practicing aggressive control of grass weeds during crop rotation cycles, staying out of grass seed crops for at least two years and employing best stand establishment and weed control methods in new grass seed plantings. Such practices will reduce other crop species and off-types of seeded species to less than 1 percent of the new stand, satisfying seed certification requirements.
Because many common grass seed weeds have a soil seed bank lifetime exceeding six years, crop rotation will not entirely eliminate them, but their population density in new stands can be reduced to levels that are nearly undetectable in the field and in seed test samples.
In established stands, a fall program of several herbicide treatments timed to specific stages of weed seed germination and seedling development can greatly reduce the establishment of volunteer crop seedlings and many annual and perennial weedy grasses, including annual bluegrass, roughstalk bluegrass, Italian ryegrass and California brome. Prowl, Dual, Axiom and Frontier all interfere with germination and early development and must be applied around the time of the first fall rains (typically early October). Each of these preemergence herbicides has specific strengths and weaknesses regarding resistance to volatilization and photo decomposition, amount of rainfall required for activation, potential for crop injury, duration of effectiveness, activity on already emerged seedlings and performance against individual weed species.
Postemergence treatments with various mixtures of Goal, Diuron, Sencor/Lexone (metribuzin) and Terbacil are generally required to control seedlings that survive preemergence treatment. These postemergence treatments are applied to seedlings in a wide range of growth stages and can be applied as early as October and as late as January. They work by destroying leaf tissue and often cause nearly as much damage to the established crop plants as they do to young weeds.
Adequate safety is achieved by limiting herbicide rates and minimizing direct foliar uptake. Direct foliar uptake is reduced when light to moderate rainfall soon after application washes the herbicide off the leaves and into the upper 1 inch of the soil profile, an area dominated by roots of young seedlings. Conversely, application during dry periods with foggy nights and sunny afternoons will maximize direct foliar uptake and crop injury.
Fall treatment programs that cause extensive crop damage are ineffective and often counterproductive against weeds such as annual bluegrass that possess extensive dormancy and light quality sensing mechanisms that allow seed to delay germination until the crop canopy is open or absent and the seed and soil are exposed to light.
Treatments causing crop injury in late winter and early spring will often reduce crop yield and may or may not succeed in reducing weed seed contamination of the seed crop.
Treatments that injure the crop in late spring almost invariably reduce seed yield.
Despite the general hazards of spring treatment, several useful programs have been developed to control specific weed species in particular crops. Keys to their success include biochemical selectivity between the crop and weed and rapid recovery by the crop after any injury. Herbicides successfully used in the spring include growth hormone types (2,4-D, MCPA, Banvel and Stinger), ALS inhibitors (Express), ACCase inhibitors (Poast and Fusilade on fine fescues), Horizon or Whip on ryegrasses, tall fescue and fine fescue, and Rely (with proper rates and timing on some crops).
Residue management systems can reduce weed pressure for some species: Germination is delayed for weeds such as annual bluegrass through reduction in sunlight and change in light quality at the soil surface under full-recovery management. Other species have little or no requirements for light: Volunteer perennial weeds germinate and emerge through chopped straw as soon as adequate moisture is present (between 0.5 and 1 inch of rain).
Prompt removal or processing of the straw and stubble after harvest can help promote crop regrowth. A good crop canopy is very effective at controlling weeds. If straw is baled, the remaining stubble should be promptly flail chopped to within 1 inch of the ground to encourage basal regrowth rather than survival of aerial tillers. If the full straw load is chopped, cutting height should also be within 1 inch of the ground and additional (second or third) passes with the flail should be made until the plant crowns are exposed to sunlight and the old rows can be seen.
The full-straw load alters performance of some herbicides. Preemergence herbicides "tied up" on top of the straw may not reach the germinating seeds soon enough in sufficiently high concentrations to stop them from germinating. However, the better moisture maintained at the soil surface under the full-straw layer may promote shallow root growth and increase the adsorption of herbicides throughout the fall. Improved weed control and decreased crop safety have sometimes been reported for Diuron applied in full-straw-load conditions. The decaying straw itself may be somewhat phytotoxic, providing direct weed control but potentially injuring the crop.
General observations of weed control in full-straw-load conditions include less annual bluegrass, more volunteer crop and more roughstalk bluegrass compared to residue removal systems. Moderately low populations of volunteer crop seedlings can fill in space vacated by dead or missing crop plants and occasionally increase seed yield, whereas high populations of volunteer crop seedlings act as competitive weeds and reduce seed yield 10 percent to 20 percent. Open field burning is an effective method of destroying weed seeds, but allowable burn acreage is limited by the state of Oregon.
Considerable movement of applied herbicide to groundwater and surface water is not likely when the chemical is applied under the proper conditions. Thus it is essential that chemicals be used under strict accordance to the label.
Growers can assess the groundwater contamination risk of a chemical by using Oregon State University's Oregon Water Quality Decision Aid (OWQDA). This tool provides information on the likelihood of a specific chemical when applied to a specific Oregon soil to move through the soil and contaminate groundwater. For example, based on OWQDA, the herbicide Diuron has a low groundwater vulnerability rating on Dayton soil. Thus the likelihood of this chemical moving to groundwater is small. Recent water quality field research supports this. It was shown that Diuron applied to perennial ryegrass seed crops on Dayton soil in western Oregon in the fall did not move to shallow groundwater, which indicates that Diuron residues persist in the soil. In rare cases, during the rainy season, a very small percentage of applied Diuron was removed by surface runoff.