By Claire Benjamin, University of Illinois
Producers contended with precipitation "extremes" in the 2011 growing season that could lead to herbicide carryover, said Aaron Hager, University of Illinois Extension weed specialist.
"Wet soil conditions slowed spring planting and delayed applications of soil-residual herbicides in many areas, whereas particularly dry conditions were encountered across large geographic areas as the season progressed into July and August," he said.
Dry soil conditions undoubtedly contributed to less-than-ideal performance of some foliar-applied herbicides, he said. Weeds growing under hot and dry conditions were frequently "hardened off" and difficult to control with post-emergence herbicides.
"Poor weed control is one obvious outcome of a dry growing season, but herbicide degradation and dissipation also can be reduced when soil moisture is limited," Hager added.
Reduced herbicide dissipation in soils may result in herbicide residues high enough to cause injury to rotational crops. Hager said several factors should be considered when determining the potential for herbicide carryover, including the herbicide applied, when the application was made, soil pH, and soil moisture.
The labels of most soil-residual and many foliar-applied herbicides indicate the required amount of time between application and planting a rotational crop. Late-season applications of herbicides that possess soil-residual activity can result in rotational crop injury if the rotational interval is not achieved, he said.
In addition, soil pH affects the stability and persistence of some herbicides. Soil pH of 7.0 or greater may slow the dissipation of certain herbicides by reducing the degradation process known as hydrolysis.
"Even under 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," he said. "Soil moisture is often the most critical factor governing the efficacy and persistence of many other soil-residual herbicides."
Many herbicides are degraded in soil by the activity of soil microorganisms. When soil moisture is limited, these microorganism populations can be greatly reduced. Additionally, dry soils can enhance herbicide adsorption to soil colloids, reducing the availability of the herbicide for plant uptake and degradation by soil microbial populations, he said.
If herbicide carryover is a concern, a soil chemical analysis or bioassay can be performed to determine if herbicide residues are sufficiently high to cause injury to rotational crops.
"Soil chemical analyses are performed by commercial laboratories and can be a bit expensive," he said. "Bioassays, often conducted 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."
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