How do you control large weeds?
Heavy rains across Iowa have made timely post-emergence weed control difficult in the 2014 growing season, even in fields treated with preemergence herbicides. This has led to questions as to what can be done to ensure control of weeds larger than specified on herbicide labels. Bob Hartzler, Iowa State University Extension weed management specialist, has been answering those questions this summer.
“Keep in mind that if there was a way to improve the consistency of a herbicide’s performance on large weeds, the manufacturer probably would have included that practice on the label,” he says. In the following example, Hartzler focuses on managing large waterhemp in soybean fields, but many of the principles are the same with other weeds, he notes.
The occurrence of waterhemp that’s resistant to Group 2 (ALS inhibitors) and Group 9 (glyphosate) herbicides greatly complicates waterhemp management. The remaining effective post-
emergence options for waterhemp are the Group 14 herbicides: aciflourfen, fomesafen and lactofen in soybeans. There are some options in corn that simplify things for that crop, but with soybeans you are limited.
These three soybean herbicides (acifluorfen, fomesafen, lactofen) are very similar in their performance on waterhemp when applied at equivalent rates. They are contact herbicides that require application to small waterhemp (maximum of six leaves) for consistent control. While application to weeds beyond the size restriction is not a violation of the label, the manufacturer is not responsible for performance.
• Excessive rain has made timely weed control difficult.
• Mixing multiple herbicides to boost performance is risky.
• Best bet is to improve the coverage of weeds.
Mixing to improve control?
One approach to enhance performance is to add multiple herbicides (herbicide cocktails, witches’ brews, etc.) to the spray tank in the hope an additive or synergistic response will improve weed control performance. Unfortunately, products registered for use in soybeans (such as Cadet, 2,4-DB, any Group 2 herbicide) that can be added to a Group 14 herbicide have minimal activity on waterhemp, thus little or no benefit exists in using these mixes in terms of improving waterhemp control.
The addition of these products may, however, help control other weeds on which they have good activity, such as velvetleaf. But remember, says Hartzler, a combination of multiple herbicides can increase the likelihood of significant crop injury compared to the products individually.
Best bet is better coverage
The tactic with the greatest likelihood of improving control of large weeds is to adjust the sprayer to improve coverage of the target weed or weeds, he advises. Steps to consider include:
• higher spray volumes
• slower tractor speeds
• nozzles that produce a smaller range of droplet sizes
• lower boom height
Another consideration is the impact that late applications may have on the selection of new herbicide-resistant weed biotypes. Large weeds that survive these applications have essentially been treated with a sub-lethal herbicide dose, the same process as when below-labeled rates are used. This has been shown to contribute to resistance evolution within weed populations.
“Although many people have had success at killing weeds larger than specified on the label, the variability in herbicide performance increases rapidly with increasing weed size,” notes Hartzler. “You need to weigh the cost of the application and potential for crop injury against the likelihood of successfully controlling the weeds. It’s never easy to concede to weed control failures, but in certain situations, it is necessary to recognize that an effective chemical control option is not available.”
Source: Iowa State University
This article published in the August, 2014 edition of WALLACES FARMER.
All rights reserved. Copyright Farm Progress Cos. 2014.