seal  Purdue News

June 13, 2003

Controlling weeds a tall order for no-till farmers, specialist says

WEST LAFAYETTE, Ind. – When tall weeds overcome a crop field, a popular method for removing them before planting is tillage. But what does a no-till farmer do if unwanted vegetation is knee-high?

That's the quandary many southern Indiana farmers find themselves in during a spring that's produced one rainy day after another, said Bill Johnson, a Purdue University Extension weed specialist. Johnson assured producers there's still time and options available for both conquering weeds and planting corn and soybeans in a soil-conserving way.

"In southern Indiana they've experienced a recent rash of very frequent storms," Johnson said. "The frequency of the storms has not allowed the fields to dry out. As a result, farmers have not been able to get spray equipment over their fields to control the winter annual vegetation, and now we have summer annual weeds coming up in these fields, as well.

"This is a dilemma. In southern Indiana we have a lot of no-till acres because the soils tend to be erosive. When you get into the early part of June there's a lot of pressure to get the crops planted as soon as possible, because each day planting is delayed it can result in rather significant yield loss."

Through Sunday (6/8) just 66 percent of corn and 40 percent of soybean acres were planted in southern Indiana counties, according to the Purdue-based Indiana Agricultural Statistics Service. By comparison, corn planting was 99 percent complete in northern and central Indiana counties, with soybean planting 90 percent done in the north and 87 percent finished in the central region.

Wet weather patterns and cool temperatures have been a boon to weeds. Many southern Indiana fields are overrun with combinations of marestail, henbit, chickweed, giant ragweed, pigweed, smartweed and prickly lettuce, Johnson said. Weeds are so large in some fields that certain herbicides can't control them fast enough to spare farmers huge crop losses, he said.

"When you have air temperatures in the 70s, which is where we're at right now, it can take five to 15 days for these weeds to completely melt to the ground if you use a translocated product, such as 2,4-D or glyphosate-based," Johnson said. "With tillage equipment you can get in there and knock weeds down very quickly. However, that's not the goal with no-till fields."

Translocated herbicides take longer to act because a weed's leaves and roots must first absorb the herbicides and be "translocated" to growing parts of the plant before the weed begins to die. Another class of herbicides kills plant tissue from the outside. These "contact" herbicides cause more rapid tissue damage and weed "burndown."

"Typically, contact herbicides don't work as well on large weeds," Johnson said. "They do a good job burning off some of the leaves at the top of the plant, but you have to be careful about how you use them because there are certain big weeds that these contact-type products will miss."

Instead, Johnson recommended farmers use a combination of contact herbicides with other weed control products.

"What we would like to see people do in these no-till areas is use a contact-type product, such as Gramoxone Max," he said. "If you mix that with a photosynthetic inhibitor – atrazine in corn, Sencor in beans – you can greatly increase not only the speed of activity, but also the ability of that product to control large weeds. That will give you activity within about three days.

"At that point you should have the vegetation burned to the ground and you can make the decision on what to do next, as far as being able to plant or giving it a few more days and possibly hitting it a second time with just the contact material."

If a no-till crop has emerged and weeds are a problem, farmers should not use Gramoxone, Johnson said.

"Gramoxone will kill emerged crops," he said. "About the only thing those producers can do is use postemergence herbicides labeled for that crop. The biggest concern in corn is that once grass weeds are more than 4 inches tall postemergence herbicides lose their effectiveness very quickly."

Writer: Steve Leer, (765) 494-8415,

Source: Bill Johnson, (765) 494-4656,

Ag Communications: (765) 494-2722; Beth Forbes,;

Related Web site:
Purdue University Weed Science Page

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