Abstract
The use of direct seeding for dryland production of winter wheat has the potential to reduce soil erosion, reduce crop production expenses associated with tillage operations, and improve soil quality while maintaining or improving crop yields. In the driest parts of the PNW, a season-long fallow period is a necessary part of most dryland crop rotations, whether conven-tionally grown or direct seeded. In direct seed systems that require fallowing as part of the overall crop rotation, growers necessarily rely on herbicides rather than tillage to control weeds. The optimum timing of glyphosate applications, and/or the combined use of glyphosate plus herbicides that provide residual soil activity can reduce the overall cost of fallow while maximizing vegetation control. Since tillage is eliminated in chemical fallow systems, it is important to give special consideration to weed management beginning during wheat harvest, and continuing throughout the early chemical fallow period. Early season attention to management of crop residue and control of annual weeds when they are small in size will improve the likelihood of successful season-long chemical fallow. Effective chemical fallow begins during wheat harvest and must be managed throughout the entire fallow period until planting of the next crop. The chemical fallow period can be divided into three periods for the purposes of discussion and management planning. These periods include the harvest and early post-harvest period in the fall, spring and early summer fallow period, and late summer fallow period.
Introduction
In the inland Pacific Northwest (PNW), direct seeding for dryland winter wheat production continues to attract the attention of growers. The use of direct seeding for dryland production of winter wheat has the potential to reduce soil erosion, reduce crop production expenses associated with tillage operations, and improve soil quality while maintaining or improving crop yields. In the driest parts of the PNW, a season-long fallow period is a necessary part of most dryland crop rotations, whether conven-tionally grown or direct seeded. Fallow periods conserve scarce soil moisture necessary for crop production in the subsequent crop year. Control of volunteer cereals, grass weeds, and broadleaf weeds is critical for successful moisture conservation during these fallow periods. In direct seed systems that require fallowing as part of the overall crop rotation, growers necessarily rely on herbicides rather than tillage to control weeds. The elimination of tillage and the reliance on herbicides during the fallow period is termed chemical fallowing. Effective chemical fallowing typically relies on multiple applications of glyphosate (Roundup®) to maintain season-long control of weeds such as volunteer cereals, downy brome (Bromus tectorum), jointed goatgrass (Aegilops cylindrica), rattail fescue (Vulpia myuros), Russian thistle (Salsola iberica), and numerous other weed species. The optimum timing of glyphosate applications, and/or the combined use of glyphosate plus herbicides that provide residual soil activity can reduce the overall cost of fallow while maximizing vegetation control.
Effective chemical fallow begins during wheat harvest and must be managed throughout the entire fallow period until planting of the next crop. The chemical fallow period can be divided into three periods for the purposes of discussion and management planning. These periods include the harvest and early post-harvest period in the fall, spring and early summer fallow period, and late summer fallow period.
Harvest and early post-harvest period
During wheat harvesting, the use of a chaff spreader behind the combine will minimize chaff rows and the subsequent concentration of volunteer wheat and weed growth that occurs within chaff rows. Vegetation control with herbicides is hindered when heavy crop residues and dense vegetation growth prevents adequate herbicide spray coverage. If allowed to grow for an extended period, uncontrolled vegetation within chaff rows can become matted, which prevents adequate herbicide spray coverage. Matted vegetation also can complicate direct seeding operations at the end of the fallow period by hindering uniform seed placement.
If sufficient fall precipitation occurs to allow germination of volunteer cereals and/or weeds, a herbicide application at this time will control vegetation while it is still relatively small, has not become matted, and before it becomes hardened-off by over-wintering conditions. Control of vegetation that has emerged in the fall will facilitate improved vegetation control the following spring. Research conducted near Moro, Oregon, a 13-inch annual precipitation location, illustrates that split applications of glyphosate in the fall and spring provides better control of volunteer wheat and grass weeds than does a single spring application of glyphosate (Table 1).
In areas where emerging Russian thistle plants present a problem after wheat harvest, control in the fall is often necessary to prevent excessive weed growth. Herbicides containing glyphosate can be effective on small Russian thistle plants. Treatment with paraquat plus diuron (Surefire®) can also provide good effectiveness against post-harvest infestations of Russian thistle.
For late fall, post-harvest herbicide applications for vegetation control in areas with a Russian thistle problem, research has shown that a late fall timing of sulfentrazone (Spartan®) can provide extended control of Russian thistle well into the following spring and summer (Table 2). Sulfentrazone is a preemergence, soil-active herbicide that requires about a half inch of precipitation within 3 to 4 weeks of application to become active in the soil. In low rainfall areas of the inland PNW, spring application of sulfentrazone has not proven to be as reliable for control of Russian thistle as fall application (Table 2). This is due to the reduced probability of sufficient precipitation events in late spring to move the applied sulfentrazone into the soil for activation. In higher rainfall areas, an early spring application of sulfentrazone can provide season-long Russian thistle control if adequate precipitation for soil activation is received after herbicide application. Sulfentrazone needs to be combined with a foliar active herbicide such as glyphosate to control emerged weeds.
Spring and early summer fallow period
This is the ideal time for growth of volunteer cereals and cool-season weeds such as downy brome, jointed goatgrass, feral rye, and various broadleaf weeds such as mustards, tarweed, and kochia. An application of glyphosate should be timed to prevent excessive vegetative growth, and before flowering of weeds. Consider the vegetation density and stage of growth when deciding on the appropriate glyphosate rate. PNW research has shown that the specific glyphosate formulation is less important in determining weed control effectiveness than is the actual glyphosate application rate, prevailing environmental conditions at time of application, and the use of necessary surfactants. Some glyphosate formulations require additional surfactant to be effective, others do not. Also, ammonium sulfate (AMS) has been shown to improve effectiveness of glyphosate, particularly with low glyphosate application rates. However, the beneficial effects of AMS in the spray tank solution are lessened as higher rates of glyphosate are used (Table 3).
Late summer fallow period
Control of weeds during the late summer period can be difficult. Weeds that have escaped control by this time of the season are metabolically inactive due to water and heat stress, thereby making them tolerant to control with herbicides. Dusty conditions can also complicate herbicide effectiveness. For these reasons, it is important to control weeds early in the fallow period when they are young and actively growing. Late germinating summer annuals including Russian thistle, kochia (Kochia scoparia), prickly lettuce (Lactuca serriola), and horseweed (Conyza canadensis) can become a problem during the late summer. Light infestations may not be of particular concern, but a heavy infestation at this time may need to be mowed, or eliminated with tillage to prevent production of weed seeds. There are no consistently effective chemical controls for these late season weed escapes.
Since tillage is eliminated in chemical fallow systems, it is important to give special consideration to weed management beginning during wheat harvest, and continuing throughout the early chemical fallow period. Early season attention to management of crop residue and control of annual weeds when they are small in size will improve the likelihood of successful season-long chemical fallow.
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