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Tuesday, Sep. 16, 2014

What weeds should you watch for?

Posted Tuesday, December 6, 2011, at 10:44 AM

Good weed control starts by knowing what the predominant weed species problems for each field. Knowing which are the worst weeds will aid in choosing the best product or mix of products to control them. Most major weed and herbicide resistance problems in corn vary by location across the corn belt, within a state, and even within different fields. The following six weeds have been ranked as being the most difficult to control.

Waterhemp was ranked as number one in difficulty to control because of its predominance and because it is ideally suited to adapt to today's farming practices. It does well in limited tillage situations, it has an extended period of emergence, one female plant can produce one million seeds and it can be easily transported on machinery. Nationwide, waterhemp has evolved resistance to ALS-inhibitor herbicides, PPO-inhibitor herbicides, glyphosate, HPPD inhibitors, photosystem II inhibitors, and now to 2,4-D in Nebraska. Weed height at postemergence application is an important factor for effective weed control.

Giant ragweed was ranked as the second-hardest weed to control for corn. Its emergence extends into June or even late July in Illinois. It is difficult to control with just one application. Pockets of glyphosate resistance have developed. Giant ragweed has become difficult to control with postemergence chemicals with the exception of 2,4-D and other auxinic herbicides. Illinois has identified some pockets of ALS-resistance giant ragweed. Giant ragweed control is better when the weeds are small and if you use glyphosate, do not skimp on the rate and apply sooner than later.

Marestail's (also known as horseweed) biggest issue is it's resistance to glyphosate. Marestail tends to be more of a problem in no-till fields. Controlling marestail prior to planting is critical. Fall or spring burndown can control marestail but it is unrealistic to expect residual control to last from fall through the first postemergence application. A little tillage in the spring can take care of marestail. It needs to be controlled while it is still a rosette and before it bolts and sends up a flower stalk. At that stage it is more difficult to control.

Foxtail is very common and widespread across all the major corn growing states. At present there are no herbicide resistant issues with the foxtails. Early season competition with corn can significantly lower yields. Post-emergence applications need to be timely and recommended rates will provide better residual activity than reduced rates.

Velvetleaf was ranked fifth because of its prevalence, competitiveness and the longevity of seed in the soil. Research from Nebraska has demonstrated lower levels of control with glyphosate when velvetleaf's photosynthetic rate is slower in the early morning or late day

Morningglory is difficult to control with glyphosate effectively. For glyphosate to be effective it needs to be tank mixed with a product that offers good morningglory control. Timing of treatment is extremely important with morningglory.

The pros and cons of all weed controls should be evaluated. Producers need to think about implementing a production system for the next five years not just next year. There is no silver bullet on the near horizon that will solve weed management issues. Producers need to consider strategies including rotating crops, herbicide modes of action, tankmix combinations and tillage practices from season to season.



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As our local University of Missouri Agronomy Extension Specialist, Crook has been writing a column for the print edition Agriculture page for the past three years and we will now be sharing it on our web version. Crook has a bachelor and masters degree in agronomy from University of Missouri and received his doctorate in Agronomy from Kansas State University. He was in soybean variety development research for 22 years for various seed companies and has been Saline County's agronomy specialist for 10 years.
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