Weed Resistance Practices Advised

Weeds have long been a challenge to farmers attempting to efficiently produce food products and maintain profitability. At first the only options of controlling the moisture robbing pests were tillage and hand labor. Approximately 50-60 years ago the agriculture community was introduced to herbicides that would attack the weeds, mostly broadleaf, but not harm the crop. Since then, weed control practices have drastically changed. For most crops, farmers rely very little on tillage as the primary weed control practices. They have learned that less tillage also reduces weed germination in addition to protecting the soil from erosion and conserving soil moisture.

In place of tillage and hand labor, agriculture has come to rely on herbicides to reduce the impact of weeds. However, weed populations have a wide genetic diversity giving some members of the weed population the ability to develop resistance to a herbicide. Research has shown that the resistant type will increase with each use of the same herbicide(s).

Our weed experts in both public and private research tell us that weed resistance to herbicides cannot be prevented but can be delayed through implementation of specific strategies.

The first is to scout fields on a regular basis. By monitoring changes and presence of weed populations, a grower can take other actions to kill or remove escaped weeds, whether they are single plants or small patches. Dead plants, unaffected plants, and plants showing intermediate responses indicate herbicide resistance has probably occurred. Most individual weed plants have the ability to produce massive amounts of seed. So, a zero tolerance approach to weed management should be given top priority.

Another approach to combating herbicide resistance is to apply effective herbicides in tank-mix prepackage, or sequential mixtures that include multiple mechanisms of action. Two or more herbicides in mixture must have activity against potentially resistant weeds to be effective. Herbicides in most commercial mixtures do not target the same weed species. Effective tank-mixtures will reduce selection of herbicide-resistant biotypes more successfully than rotating herbicide modes of action. It is possible antagonism may occur with some mixtures, especially between contact and systemic herbicides.

Crop rotations, particularly those crops with different life cycles can be effective. Examples might be winter annual crops such as winter wheat, perennial crops (alfalfa), and summer annual crops such as spring wheat, corn, pulses, etc.

The use of high herbicide rates and effective adjuvants is strongly urged. In an effort to control cost of production it is very tempting to use the lower labeled rates but this allows plants with low-level resistance to survive, hybridize and produce progeny with elevated resistance. Hybrid plants express a higher level of resistance and require a higher herbicide rate to kill the plant. Again, dead weeds mean zero tolerance which results in zero seed production and effective resistance management.

Both public researchers and educators in addition to private industry tell us herbicide resistance is very real for western North Dakota. Those who closely monitor the development of resistance are documenting the westward march of resistant weeds.

For those of you who want more information, I suggest an on-line herbicide resistance education and training course at: http://wssa.net/lessonmodules/herbicide-resistant-weeds/