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Chapter 5 - Weed Control, No. 12, Summer 1988

Weed Control Considerations for Conservation Tillage

Roger Veseth

Effective weed control is essential for successful crop production under conservation tillage, as in any farming systems. Alex Ogg, research leader for the USDA-Agricultural Research Service Weed Science Research Unit at Washington State University in Pullman, points out that lack of adequate weed control is one of the most frequently cited reasons for failure of conservation tillage systems. He stresses that only those farmers who fully understand the factors affecting crop production and weed control in conservation tillage are likely to be successful. Ogg is one of over 100 scientists involved in the STEEP (Solutions To Environmental and Economic Problems) conservation farming research effort underway in Idaho, Oregon and Washington.

Integrated Management Strategy

Ogg explains that the weed control strategy with the greatest potential for success is the one that fully integrates the many management options producers have available. It is important to have a long range strategy to help predict and avoid potential weed problems in the future. He lists the following weed control options and considerations in an approximate chronological order.

Crop Rotation

Where feasible, consider crop rotations of 3 or more years in length, preferably 2 or more years out of winter cereals. Include a variety of spring cereal and fall- or spring-seeded noncereal crops in the rotation where possible. Longer rotations with spring crops and noncereal crops are particularly helpful.d in controlling numerous weed and disease problems associated with winter cereals.

Rotations with spring crops are particularly important in helping to control winter annual grass weeds such as downy brome and jointed goatgrass. Spring crops allow for control of these fall-germinating weeds in the fall and again in the spring before seeding. Herbicides, tillage or a combination of both can be used either in the fall, spring or both, depending on weather conditions annually. Fall or spring-seeded noncereal crops, with grass weed herbicide options, provide additional potential for improved control of winter annual grass weeds. If spring cereals are to follow winter cereals, weeds and volunteer should be controlled beginning in the fall, if possible, to reduce the severity of Rhizoctonia root rot in the spring cereal.

Preceding Crop Weed Control

When planning to use some type of conservation tillage system for seeding the next crop, Ogg stresses the importance of achieving good weed control in the preceding crop. Reducing weed seed production, and thus the future weed population, before beginning conservation tillage will greatly improve the chances of a successful crop under conservation tillage. This is especially beneficial for control of weeds such as downy brome which has a 'short seed-life in the soil.

Spread Combine Residue Uniformly

The next important factor in weed control under conservation tillage is uniform distribution of straw and chaff from the combine during harvest of the preceding crop. Ogg explains that weed seeds, which pass through the combine during harvest of the previous crop, are usually distributed with the chaff. If the chaff is concentrated in a narrow row behind the combine, then a major portion of the weed seeds will also be concentrated there. He points out several problems associated with concentrated straw and chaff rows.

1. Soil-active herbicides applied after harvest can be tied= up in the residue. Foliar-active herbicides can also be intercepted by the residue and not contact small emerging weeds.

2. The shaded, cool, moist environment under the straw and chaff row results in a longer period of germination and emergence of weeds, volunteer and the crop. than in the rest of the field. This makes a single application of foliar-active herbicides less effective because of the wide range plant stages and continued emergence after the herbicide application.

3. High populations of weeds and volunteer in the straw and chaff row could require higher herbicide application rates to provide effective control.

4. Drill penetration for good seed-soil contact is often inadequate in heavy straw and chaff rows, resulting in crop stands which are thin and not uniform. This gives less crop competition against weeds, which is an important part of effective, season-long weed control.

5. Concentrated straw and chaff also provide a favorable environment for several soilborne diseases of cereals, including Pythium and Rhizoctonia. Poor crop stands and low plant vigor as a result of disease reduce competition against weeds in those areas as well as lower crop yield potential.

6. There is a potential for immobilization of plant nutrients during microbial decomposition of the concentrated residue. This could reduce the available nutrient supply for the crop, particularly early in the season. Nutrient deficiencies would result in reduced crop vigor and competitive ability.

7. Concentrated residue provides a food source and cover for rodents. Rodent damage reduces crop stands and consequently reduces competition against weeds.

Fallow Weed Control... Early and Throughout

When a summer fallow year follows harvest, the next weed control consideration in the conservation tillage system is when to initiate the chemical or chemical-tillage fallow portion of the system. Ogg points out that, in the Northwest, herbicide applications on fallow maybe made in the fall, late winter or spring, depending on weather conditions and weed species present.

Ogg points out that soil-active herbicides are generally most effective when applied in the fall. Foliar-active herbicides can be applied in the fall or in the spring. Application time is determined primarily by the amount of weed growth in the fall. If weeds emerge in significant numbers after the fall application, another application or tillage operation maybe required in the spring. Soil-active and foliar-active herbicide are often used in combination depending on weed growth and the species present.

If rains occur early in the fall and there is significant emergence of fall-germinating weeds and volunteer in October and early November, the herbicides are usually applied in the fall. On the other hand, if little or no precipitation occurs until late fall and weeds do not emerge before December, herbicide application can be delayed until late winter to very early spring.

Frequently, herbicides can be used without fall tillage.However in regions where runoff on frozen soil is a common problem, fall chisel plowing with or without herbicides may be used to increase water infiltration and to reduce weed growth.

Downy brome is a particularly difficult weed to control. If the herbicide application is delayed until spring, herbicides can still control the weed, but a dense sod may have already formed and soil water storage may be reduced. A dense sod of weeds and volunteer grain can interfere with seeding the following crop. Extended periods of stormy weather in the spring may delay herbicide application and allow weeds to grow excessively large.

Preplant Herbicide Application

Herbicides, tillage or a combination of both can be used to control weeds and volunteer before planting. Downy brome can be killed before seeding with a non-selective foliar-active herbicide, or by rod-weeding or other tillage operation. Ogg points out that control with herbicides or a herbicide/tillage combination is generally more effective. Where only tillage is used, some of the plants will become reestablished if the soil remains moist. Compared to a herbicide application, tillage can also result in increased soil water losses from evaporation.

Ogg also stresses that preliminary research results indicate that when herbicides are applied to control weeds and volunteer before planting, they should be applied at least 2 weeks before seeding, if possible, This has been shown to reduce the incidence of Rhizoctonia root rot in the following crop.

Quality Seed of Adapted Varieties

A basic consideration for any farming system is using clean, high quality certified seed of varieties most adapted to the area. Drill box surveys in the Northwest have shown that the use of weed-contaminated crop seed is a significant contributor to weed problems in the region. Ogg believes that the rapid, extensive increase of jointed goatgrass infestations in recent years can be partially attributed to using contaminated crop seed.

High quality seed is important for rapid emergence and early competition against weeds. Using varieties adapted to the area will also improve the competitive ability of the crop.

Seeding Rates

Crop density is an important component of the crop's ability to compete with weeds. One management option which helps ensure adequate crop density is the adjustment in seeding rates. Because conservation tillage often presents a less favorable microenvironment for seedling establishment, higher than normal seeding rates are often used to help ensure a competitive crop.

Seeding rate increases of 10 to 15 percent are commonly used under conservation tillage to compensate for potential stand losses. However, the need for increased seeding rates must be evaluated for the particular situation. This would depend on the previous crop, available water, tillage and planting equipment used, final seedbed condition, the tillering ability of the cereal variety and other factors. Significant increases in seeding rate should not be considered without adjustments in row spacing and other arrangements to prevent excessive in-row competition for the crop.

Seeding Date

Ogg points out that earlier seeding of spring crops can often improve the crop's yield potential and ability to compete with weeds. However, selection of the seeding date must also be based on soil temperature, soil water content, disease potential, the potential for soil compaction under wet soil conditions and other considerations.

If weeds can be controlled, earlier seeding of winter wheat and other fall-seeded crops can also be advantageous for yield potential and weed control. But, other management options must be considered for control of early seeding-related diseases. For example, when early seeding of winter wheat is considered, a crop rotation with 2 years out of winter cereals should be considered for control of Cephalosporium stripe if the disease is a potential problem. In addition, herbicide options need to be considered for control of winter-annual grass weeds which may emerge after an early seeding date.

If a field has a history of dense populations of downy brome, some producers will delay fall seeding until fall rains stimulate weed germination. However, late seeding also reduces crop yield potential and increases the potential for damage from Pythium root rot under cool wet conditions after seeding.

Fertilizer Placement

Banding of fertilizer for increased availability and early access by the crop roots is commonly beneficial for both crop yield potential and weed control. Ogg explains that although research has not shown that fertilizer banding will significantly reduce fertilizer availability to the weeds, it can improve crop yields under certain conditions and provide more vigorous crop competition against weeds. Broadcast applications of nitrogen fertilizer have been shown to increase populations of some grass weeds, such as wild oats.

The importance of fertilizer use and placement depends on the amount and distribution of plant nutrients in the soil (based on soil tests), soil temperature and moisture, the crop and many other factors. Crop response to banded fertilizer is generally highest where there are low nutrient levels in the soil and under cool, wet soil conditions, particularly with spring crops.

Post-Plant Herbicides

After the crop is planted, producers still have several weed control options. Non-selective, foliar-applied herbicide can be applied before emergence of the crop. This option is quite risky because of the possibility of weather conditions preventing application before the crop emerges. Some pre-emergence herbicides for wheat require thorough soil incorporation, which limits their use in conservation tillage.

A variety of selective herbicides, both foliar-active and soil-active, are available for in-crop application. Ogg stresses that herbicides must be applied at the recommended rates and at the proper stages of weed and crop development to achieve optimum weed control while minimizing crop injury. Check the herbicide label!

Control Plant Diseases and Insects

Maintaining a healthy, vigorous crop, relatively free of disease and insect pests, increases the crop's ability to compete with weeds. Considerations include in-season measures, and probably more importantly, long-term pest management strategies such as crop rotation.


Producers have several management options that must be integrated to improve weed control in conservation tillage systems. Ogg points out that an effective weed control strategy must begin with an awareness of the weed control impacts and opportunities of management options. This would include crop rotation selection, practices for production and harvest of the preceding crop and the establishment and management of the current crop under conservation tillage. In addition to herbicide use, the more competitive the crop is, the more effective the weed control program will be.

The following is a summary schedule of management considerations affecting weed control in conservation tillage:

1. Weed control in the preceding crop

2. Combine residue distribution

3. Weed control in fallow

4. Preplant herbicide application

5. Clean, quality seed of adapted varieties

6. Seeding rate

7. Seeding date

8. Fertilizer placement

9. Post-plant herbicides

10. Control of diseases and insects


Contact us: Hans Kok, (208)885-5971 | Accessibility | Copyright | Policies | WebStats | STEEP Acknowledgement
Hans Kok, WSU/UI Extension Conservation Tillage Specialist, UI Ag Science 231, PO Box 442339, Moscow, ID 83844 USA
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