But opinions on their effectiveness are mixed A growing number of northern Corn Belt farmers and chemical dealers are applying corn herbicides in fall and seeing good results. But most university weed specialists are cool on the practice.

Fall-applied weed killers generally are labeled for the northern portions of Illinois and Nebraska and for Iowa, Minnesota, Wisconsin and North and South Dakota. They're not to be applied until the sustained 4" soil temperature is 55ΓΈ or less.

Dual II Magnum has been the most popular fall-applied product, while Axiom, Frontier, Harness and Surpass are also in the race. Fall application, which often can be combined with fertilizer, relieves some of the spring rush.

"We've been fall-applying Dual and Dual II Magnum since 1994, usually with liquid fertilizer as the carrier," reports Doug Larson, Ty-Walk Liquid Sales, Minooka, IL. "In our side-by-side comparisons, the fall herbicide applications have performed as well as, and sometimes better than, the spring applications.

"Fall applications have done better when we have had gentle fall rains to incorporate the herbicide, and especially when followed by dry springs where spring applications did not get well-incorporated," says Larson.

Larson explains that fall application is especially suited for farmers who plant corn as early in spring as soil conditions allow. They aren't delayed by waiting for a grass herbicide application.

Ty-Walk treats 6,000-7,000 acres with fall-applied Dual II Magnum.

At Charles City, IA, Denny Shollenbarger of the Floyd County Ag Center has fall-applied both Axiom and Dual II Magnum. "We have compared fall and spring applications, and the fall applications have performed just as well," he says.

Shollenbarger points out that fall application works best on flat, black ground where runoff and leaching aren't problems.

Fall application of Frontier has caught on in northwestern Iowa and southwestern Minnesota, says Chad Estebo, Farmers Co-op Society, Little Rock, IA.

"We've been fall-applying Frontier, often in combination with dry fertilizer, and have had excellent weed control," Estebo reports. "In addition, the farmers save money by applying both herbicide and fertilizer in one trip, and by buying those products in fall."

The co-op has compared fall- and spring-applied Frontier and has seen comparable weed control. Estebo says fall application gives more time for moisture to incorporate the herbicide. During the dry spring of 2000, spring-applied herbicides didn't activate as well.

(Note: BASF is replacing Frontier with Outlook, which has additional qualities.)

Ronn Akey of the Windom (MN) Co-op reports that Surpass has been very effective for fall application in that area. "We apply about one-third of our grass herbicide in fall," says Akey. "It has held as well as the spring-applied."

Although some on-farm results show advantages for fall-applied herbicides, it's been a different story with university research.

"Iowa State University has not recommended fall application of pre-emergence herbicides due to concerns about performance and possible environmental risks," says weed scientist Bob Hartzler. "Herbicides applied in fall are subject to the environment for a longer time than spring applications. That increases the risk of control failures from early degradation."

In Iowa State research, Dual II was applied to no-till and minimum-till corn in both fall and spring. Foxtail control in the no-till plots was 82% with fall application; 98% with spring application. For minimum-till, foxtail control was 78% with fall application; 92% with spring application.

"Planting dates for the research plots were later than for most growers in the areas of the study," Hartzler points out. "And weed pressure was heavy. Thus, many growers have had more consistent results than shown in our studies."

University of Wisconsin research also indicates that fall applications don't control weeds as well as spring applications. "Another concern is the need for higher rates in fall in an attempt to equal the performance of spring application," says weed scientist Chris Boerboom.

Boerboom points out that weed pressure may be heavier and springs wetter in Wisconsin than in the western Corn Belt. That could explain why fall-applied herbicides may perform better farther west.

Bill Simmons, University of Illinois weed scientist, applied four conventional and two encapsulated herbicides in five timings. The conventional products were Dual, Dual Magnum, Axiom and Harness/Surpass. The encapsulated herbicides were Degree and TopNotch. The timings were fall, 60 days before planting, 30 days before planting, preplant incorporated and pre-emergence.

The study was done at three locations for two years.

When performance was averaged across all herbicides and locations, foxtail control was 82% with fall application, about 90% at 60 days before planting, about 92% at 30 days before planting, and about 95% with both preplant incorporated and pre-emergence applications.

All the herbicides produced equivalent foxtail control when applied preplant incorporated and pre-emergence. However, the Dual formulations, Axiom and Degree performed better than Surpass or TopNotch when fall-applied.