Source: North Carolina State University Cooperative Extension
Herbicide resistance is a major problem for conventional row-crop growers. In March 2014, the International Survey of Herbicide Resistant Weeds determined there were 432 different weed biotypes resistant to 22 different herbicide groups. Weed management is getting more complicated and farmers that are not on the continual lookout for herbicide resistance on their land may find themselves in a real pickle.
Once the problem arrives, the flexibility of the current weed management system may be greatly reduced. This can be costly and yield reducing. Avoiding herbicide resistance is the preferred route for this problem. However, if herbicide resistance has already arrived, recognizing and properly managing herbicide resistance can save some tremendous weed headaches.
Here are some general considerations:
1. Don’t wait for herbicide resistance to occur. Minimize the chances of herbicide resistance development by changing the herbicide modes of action (MOA) or tank-mixing different MOA herbicides in the weed management program. Simply buying a different herbicide product is not the same as using a different MOA. Some herbicide MOAs have hundreds of products with different trade and chemical names. Visit this website managed by the Herbicide Resistance Action Committee (HRAC) to see the various classifications of herbicides and consider rotating modes of action.
2. Recognize the presence of herbicide-resistant weeds vs. poor weed control due to herbicide application problems. Detection of an herbicide-resistant weed population early can allow for easier management while decreasing the chances that the herbicide-resistant biotype spreads to other production areas. Detection of a pest problem, including herbicide-resistant weeds, starts with regular scouting. Scout fields a week after herbicide applications to keep records of weed escape problems.
However, there are many herbicide application challenges that may result in poor weed control such as: not following the label directions; improper equipment calibration; improper solution mixing (antagonism of tank-mix herbicides or adjuvants); equipment failure (clogged nozzle resulting in non-uniform spraying); physical barriers (dirt clods reducing soil applied herbicide uniformity or dust on weeds binding up the chemical); soil factors (pre-emergence herbicides often require higher rates on higher clay content soils); environmental factors (rain at time of application or drought-stressed weeds not absorbing the herbicide); or improper application timing (spraying weeds that are too big).
Once it has been determined that poor herbicide performance was not due to some of the fore-mentioned factors, there are some clues that may indicate the presence of herbicide-resistant weed biotypes such as: small patches of uncontrolled weeds that get bigger the next season; patches of live weeds next to dead weeds of the same species that emerged at the same time; dense patches of escaped weeds with lower levels of weed escapes radiating out from the patch; a field history of continuously using the same herbicide mode of action; and confirmation of herbicide-resistant weeds in neighboring fields. If a suspect patch of weeds is spotted in a field, getting off the tractor and cutting those weeds before they produce seed might avoid a world of trouble.
3. If herbicide resistance is suspected get expert help immediately. Contact the Cooperative Extension Office, Crop Advisor, or other pest management consultant to develop a new weed management program. The key to effective herbicide-resistant weed management is an integrated approach using multiple tactics including: crop rotation; use of cover cropping; and tank-mixing. Use of pre-plant and pre-emergence herbicides with differing MOAs from the post-emergence herbicides typically used in weed management programs is another recommended tactic.
4. Prevent spreading the herbicide-resistant weed seed around by carefully cleaning equipment before moving it to other fields. This includes using a power washer or compressed air to clean off the tires and undercarriage of field equipment, vehicles and boots. Hundreds of herbicide-resistant weed seeds can be found in a small dirt clod stuck to the underside of a pick-up.
Read up on more details about herbicide-resistant weed management by taking this free on-line continuing education course by the Weed Science Society of America (course can be completed in less than 3 hours).
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