Herbicide Hangover

February 11, 2012 04:28 AM
Herbicide Hangover

By Claire Benjamin

Don’t get burned by carryover this spring

The extreme weather events of 2011 left some dark clouds threatening the upcoming crop before the first seed is even planted. University of Illinois Extension weed specialist Aaron Hager says the extreme wet and dry conditions of this past season could contribute to herbicide carryover issues in 2012.

"Wet soil conditions slowed spring planting and delayed applications of soil-residual herbicides in many areas during 2011," Hager says. "Those long wet periods were followed by particularly dry conditions across large geographic areas as the season progressed into July and August."

Hager says the combination of late spraying and dry soil conditions contributed to less-than-ideal performance of some foliar-applied herbicides. Weeds growing under hot and dry conditions were "hardened off" and difficult to control with postemergence herbicides.

"Poor weed control is only one outcome of a dry growing season. Herbicide degradation and dissipation can also be reduced when soil moisture is limited," Hager adds. Because a large portion of the herbicide is degraded in the summer and early fall following application, very dry conditions during this period increase the potential for herbicide residues to remain high enough to injure rotational crops.

Factors that favor herbicide carryover include the herbicide applied, when the application was made, soil pH and soil moisture. Higher herbicide rates and uneven distribution might also result in carryover problems.

Read the label. Most herbicide labels list the amount of time required between application and planting of a rotational crop. Hager says late-season herbicide applications with soil-residual activity can result in crop injury if the time interval is too short.

Soil pH affects herbicide stability and persistence. A pH of 7.0 or greater might also slow herbicide dissipation.

"Even in conditions of adequate soil moisture, degradation of some triazine and sulfonylurea herbicides under high-pH soil conditions can be reduced enough to result in carryover," Hager says. "Soil moisture is often the most critical factor governing the efficacy of many soil-residual herbicides."

Many herbicides are degraded in soil by soil microorganisms. When soil moisture is limited, microorganism populations can be greatly reduced. Dry soil also allows colloids to absorb herbicide, reducing plant uptake and degradation by soil microbes, he says.

If carryover is a concern, a chemical analysis or bioassay can determine if residues are high enough to cause injury. "Soil chemical analyses can be a bit expensive," Hager says. "Bioassays, using the rotational crop of choice, will not quantify the amount of herbicide residue remaining in the soil but can give an indication if the rotational crop might be injured by remaining herbicide residues."

Tips to Avoid Carryover

  • Select the appropriate herbicide rate based on soil type.
  • Calibrate the sprayer and apply herbicide accurately and uniformly.
  • If incorporating, make sure it is done thoroughly and uniformly.
  • Consider applying reduced rates of a persistent herbicide in combination with a less persistent herbicide.
  • Select herbicides by rotation plans.
  • Follow the label’s rotation restrictions.
  • Apply herbicide as early as possible and delay planting the following crop if carryover is suspected.
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