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Corn+Soybean Digest

Hostile Herbicides?

Corn growers need to watch carryover and hybrid-herbicide combinations

Are you cutting yield potential with your corn weed control program?

More than 15 years ago, seed company agronomists noticed in field tests that certain herbicides were causing noticeable yield reductions when used in combination with some hybrids. Today's herbicides and hybrids offer growers an even more complex set of interactions.

Pioneer Hi-Bred International and Golden Harvest conduct research at numerous locations looking at the effect of herbicides on their hybrids. "Our research agronomists noticed yield differences in our hybrids depending on the herbicide applied as far back as 1983," says Rick Smelser, Golden Harvest research agronomist in Estherville, IA.

In an effort to get the best performance possible out of each hybrid, the company developed herbicide ratings.

"The primary criterion is yield damage potential," says Smelser. "That doesn't mean yield reductions will occur every year, but that the potential is there based on dozens of replications over a period of years."

Paul Gaspar, Pioneer research agronomist in Mankato, MN, stresses that more than 30 herbicide-hybrid replications are needed before the company will issue a herbicide rating for a hybrid. "Many factors, including weather, soil conditions and application timing, factor into our herbicide ratings," he says.

Herbicide carryover is another reason to look at mixing modes of action, says John Oolman of Agri-Growth, Inc., an independent research company near Hollandale, MN.

In a three-year study, his company looked at the carryover effects from postemergent applications of ALS herbicides in a corn-soybean rotation. Results show that overuse of that herbicide family can have a detrimental impact on both crops.

According to Oolman, the 50-acre study measured yield in the same location through three years of corn-soybean rotations. In 1997, '98 and '99, strips were planted to corn or soybeans each year, then were rotated to the other crop the following year.

"Work on other research projects gave us an insight into the carryover potential of some ALS herbicides," says Oolman.

Herbicides were grouped into ALS and non-ALS modes of action. Amino acid synthesis inhibitors, imidazolinones and sulfonylureas are all ALS herbicides. The remaining modes of action, including growth regulators, lipid synthesis inhibitors, seedling growth inhibitors, photosynthesis inhibitors, membrane disruptors and pigment inhibitors, were grouped under non-ALS herbicides.

Three-year individual and averaged corn and soybean yields under non-ALS chemistry consistently topped average crop yields by more than 15%. However, adding one ALS chemical application out of the three years brought yields down to average, while adding two ALS chemical applications out of three years produced yields nearly 4% below average. The biggest drop in yield came with three years of back-to-back ALS applications, resulting in yields averaging more than 8% below average (see charts).

University research has shown that ALS inhibitors generally provide good weed control. "ALS herbicides are widely used," says Mike Owen, Iowa State University agronomist. "It's often hard to find herbicides that provide good weed control without additional negative characteristics."

Growers know they should avoid applying the same herbicide mode of action year after year to help prevent resistant weeds from developing. "The addition of new chemicals, tankmixes and heavy advertising by chemical companies often confuses the herbicide mode of action for growers," says Owen.

Smelser says Golden Harvest urges growers to choose the best hybrids for their conditions and then pick the chemical that provides the best weed control for the cost. "If this hybrid-herbicide combination is rated as possible yield loss, we offer advice on how to minimize its impact," he says.

For the 2000 growing season, Golden Harvest went a step further and provided specific guidelines to reduce potential yield impact. "These combinations are rated as `application management required' and Golden Harvest provides specific guidelines such as application timing, tankmix additives and spray applications for each herbicide to help the grower avoid possible yield loss," says Smelser.

Pre-emergent and postemergent herbicides used to control grass in corn have a large potential for damage, he points out. "The biological systems of grass and corn are so similar it's tough to control the grass without having some herbicide effect on the corn," he says.

According to Gaspar, Pioneer provides management suggestions to help customers reduce the risk of crop response to herbicide applications. "Pioneer works very hard to educate our sales reps on herbicide families," he notes. "In turn, they are able to answer the growers' questions and help them make profitable decisions on hybrids and herbicides."

"The top-flight growers know and understand mixing modes of action to prevent herbicide resistance and yield loss," says Agri-Growth's Oolman. "Not all ALS herbicides have the same impact on crop injury or carryover. Producers need to read the product labels and check with representatives in their local areas for more information."

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