COLUMBIA, Mo. -- While recent rains have eased some concerns, producers still need to weigh the risks of herbicide carryover injury when considering planting options for corn and soybeans this spring, a University of Missouri weed scientist said.

"Depending on where you are in the state, carryover could be an issue," said MU weed scientist Kevin Bradley. "It's not going to be a big deal in areas that received timely rain last fall, but areas impacted by drought conditions may still have residual herbicides active in the soil profile."

Herbicides with soil activity typically are degraded in the soil by microscopic organisms, which break down the chemicals to inactive products, he said. A lack of water minimizes microbe activity, leaving more herbicides in the soil profile that could be active into the next cropping system.

"Before planting, producers should review their weed management program, determine if issues might exist and plan accordingly," Bradley said.

Drought conditions have increased the probability of herbicide injury, but rainfall is just one factor that can contribute to carryover, Bradley explained. "The type of herbicide applied to the previous crop is one of the most important factors that will determine the likelihood of injury."

In fields where corn was the previous crop, herbicide carryover may be a concern if atrazine or prepackaged mixtures containing atrazine were applied. Herbicide application rate and timing may influence the likelihood of injury to the 2006 soybean crop.

"In general, the higher the rate and the later the application, the greater the risk of injury," Bradley said. "If a producer made a late atrazine application last fall, there may be concern about planting soybeans in that field this year."

Carryover injury to corn is much less likely today because most producers plant glyphosate-resistant soybean varieties. Glyphosate is a non-residual herbicide.

"It's almost not an issue except for those who planted conventional soybeans," Bradley said. "I haven't heard of problems, but carryover is possible with protox-inhibiting herbicides such as Reflex or Flexstar."

If a producer suspects a carryover issue in a field, Bradley said a bioassay can help gauge its severity.

"Collect soil from the field you suspect and from one you don't, plant soybeans in it, and see what happens," he said. "A little experiment like this can help avoid a 1,000-acre mistake."

If there is carryover herbicide present, the injury will appear as yellow-white blotches on the interior of the first true leaf.

"If you see this, plant corn in that field," Bradley said. "I wouldn't even fool around with it."

Step-by-step instructions on how to set up a herbicide bioassay experiment can be found online.

For more information on crop rotation restrictions, reference the 2006 Missouri Pest Management Guide: Corn, Grain Sorghum, Soybean, Winter Wheat.

SOURCE: University of Missouri Extension.