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October 16, 2002

Fall herbicides wage cold war on winter annual weeds

WEST LAFAYETTE, Ind. – A nip in the air signals the coming winter. Right behind are winter annual weeds.

It's decision time for farmers concerned that their fields will blossom with unwanted vegetation at the first hint of spring. They might want to consider applying herbicides this fall while temperatures are warm enough for the products to be effective, said Glenn Nice and Bill Johnson, Purdue University Cooperative Extension Service weed specialists.

With many producers harvesting crops later than usual this year because of planting delays in April and May, the fall spraying calendar could be tighter, Nice said.

"Some herbicide labels have a cutoff date and a cutoff soil temperature for spraying," Nice said. "Last year we had a very late winter, and a lot of people were able to apply fall herbicides up to the end of November. This year may be a little different. Weather forecasters are calling for a colder winter, and it might come quicker, so our application window could be a lot smaller than it normally is."

Most herbicide labels recommend applying the product while the soil is cool but not frozen, Johnson said. Farmers might want herbicide applications to coincide with fall fertilization, he said.

"As a general rule, I'd say think about putting on your fall-applied herbicides about the time you put on fall-applied nitrogen," Johnson said. "You want your soil temperatures below 50 degrees, you want a good seedbed and you want some vegetation up and growing.

"But you don't want to do it too late, particularly if you use glyphosate-type products that don't work very well under cool weather conditions. If you get into some really cool weather conditions, you need to use more of the Contact-type products, the Gramoxone-based products."

Not all fields will benefit from fall herbicide applications, and not every product is right for every field, Johnson said. Fields with a history of winter annual weed problems – especially those in long-term no-till production and conventional tillage acres in creek and river bottoms – often respond better to fall herbicides, he said.

Some herbicides work only in certain crop rotations. Farmers should carefully read labels for planting restrictions, Johnson said.

"There are products you can choose that tie you into a specific cropping sequence and there are products that don't tie you into a specific cropping sequence," he said. "If you're determined to plant corn in a field, then you can pick products like Princep that allow you to go into corn. If you're unsure about crop rotation, you can use the Sencor/Python mixtures or the glyphosate or 2,4-D products, which wouldn't tie you into that crop rotation."

Johnson also recommends purchasing only those herbicides that come with a manufacturer's guarantee. Often if weather conditions or other unforeseen circumstances prevent a farmer from applying herbicides in a timely manner, or the product fails to perform as advertised, the manufacturer will provide a burndown treatment, Johnson said.

Both Purdue weed specialists said favorable fall weather could portend a weed explosion next spring, although predicting which species might top the list is impossible to say. This past spring Indiana farmland was covered in cressleaf groundsel, a bright yellow plant commonly known as golden ragwort and butterweed.

Whatever herbicide a farmer might select, the product needs to keep fields clean through the spring and, possibly, into the summer.

"One thing you have to consider is you want a fall-applied herbicide to stick around long enough to control your winter annuals and, maybe, even give you some help with the start of the summer annuals," Nice said.

"You don't want it to stick around too long, though. In the case of a wet, warm winter, you're going to get increased microbial degradation and hydrolysis. However, in a cold, dry winter, you're going to get that fall-applied herbicide sticking around for quite a while, and it'll probably serve you very well for the spring."

Additional fall herbicide recommendations and application rates can be found in Purdue Extension publication WS-28-W, "Fall-Applied Herbicide Applications in Indiana," written by a host of specialists in Purdue's Department of Botany and Plant Pathology, and Extension educators. The publication can be downloaded online.

Writer: Steve Leer, (765) 494-8415, sleer@purdue.edu

Sources: Glenn Nice, (765) 496-2121, gnice@purdue.edu

Bill Johnson, (765) 494-4614, wgjohnso@purdue.edu

Ag Communications: (765) 494-2722; Beth Forbes, bforbes@aes.purdue.edu; http://www.agriculture.purdue.edu/AgComm/public/agnews/

Purdue News Service: (765) 494-2096; purduenews@purdue.edu


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