Are you a little foggy on the definition and purpose of a “bioassay?” You’ll not only want to learn what it is, but will want to do one, particularly if you had some droughty conditions on your farm last year.

If you were short on moisture in the 2005 crop year, one of your concerns this spring should be the potential for your crops to get stung by the herbicides that may still be lying in wait in your fields. Corn doesn’t typically grow well in the presence of soybean herbicides, and soybeans really don’t like your weed control program for corn either. Yes, glyphosate may be an exception where you plant both Roundup-Ready corn and soybeans, but if there are some other weed management chemicals present, be prepared for seedling damage if you were dry in 2005.

Herbicide carryover can devastate your revenue, about as much as a large carryover of corn or soybeans. But the impact of herbicide carryover is something you can control. Weed scientist Kevin Bradley at the University of Missouri says, “Depending on where you are in the state, carryover could be an issue.” His colleague, Aaron Hager at the University of Illinois agrees, and adds, “Reduced weed control was one obvious outcome of the dry growing season, but herbicide degradation and dissipation also can be reduced when soil moisture is limited. Reduced herbicide dissipation in soils may result in herbicide residues high enough to injure susceptible rotational crops.” Specifically, the crops you are planning to plant in a few days.

Hager says, “Dry soils can enhance herbicide adsorption to soil colloids, rendering the herbicide unavailable for plant uptake and degradation by soil microbial populations. Some herbicide rotational intervals are increased if a specified amount of precipitation is not received by a certain date.”

Bradley says the lack of rainfall is one issue, but the herbicide is another. “The type of herbicide applied to the previous crop is one of the most important factors that will determine the likelihood of injury.” Consider the potential damage on soybeans from the atrazine you used on the corn in that same field last year. “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. If a producer made a late atrazine application last fall, there may be concern about planting soybeans in that field this year.”

If you are planting corn in last year’s beans, your glyphosate is a non-issue because of its degradation. But if you used other herbicides, Bradley warns, “Carryover is possible with protox-inhibiting herbicides such as Reflex or Flexstar.” And Hager says, “Harmony GT XP (thifensulfuron) and Classic (chlorimuron) are both in the sulfonylurea herbicide family, but Classic is inherently more persistent in the soil. Rotational crop-planting intervals range from 0 to 45 days after application of Harmony GT XP, while the range is 0 to 30 months for Classic.”

So how do you know if you are going to suffer from herbicide carryover? Hager says look at the labels of the chemicals you used last year. That will give you a good idea. For safety, Bradley suggests doing a bioassay. “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.” And Bradley says, “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, and don’t even fool around with it.”

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

Summary:

Not every Cornbelt farmer will be affected by the potential for herbicide carryover in 2006. However, it is a real problem for row crop farmers in the plains, and parts of Missouri, Iowa, and Illinois. If your soils were dry, and herbicides were applied late, it is better to be safe than sorry when you are counting on your crop to emerge with vigor this year.

Stu Ellis

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