Issue No. 8, Article 2/May 25, 2012
Beware of the Dust
Dry soil conditions across many areas of Illinois have accelerated both planting of crops and spraying with post-emergence herbicides. It is unusual for such a high percentage of corn acres to already have been sprayed with post-emergence herbicides at this point in the season, and one potentially adverse consequence of very dry soil is the often voluminous dust propelled into the air by application equipment.
While planting an on-farm research location last week, we noticed a large cloud of dust in the distance. Closer observation revealed that the cloud was being generated by application of a postemergence corn herbicide. The composition and size of the weeds in that field suggested treatment was justified, but reduced weed control could be an outcome of applications made during very dry, dusty conditions.
Dust generated during the application of postemergence herbicides can reduce herbicide phytotoxicity.
Airborne dust has been shown to reduce the activity of some foliar-applied herbicides, including glyphosate. Greenhouse research conducted at North Dakota State University (J. Zhou, A. Tao, and C.G. Messersmith, 2006, "Soil dust reduces glyphosate efficacy," Weed Science, Vol. 54, pp. 1132-1136) demonstrated that control of nightshade species with glyphosate was reduced when dust was present on plant leaf surfaces. The reduced phytotoxicity occurred whether dust was present on the leaf surfaces before glyphosate application or deposited there within 15 minutes. If dust was deposited more than 15 minutes after glyphosate application, or if glyphosate was applied 30 minutes before dust was deposited, reduced phytotoxicity was not reported. Dust generated from a silty clay soil tended to reduce glyphosate phytotoxicity more than dust generated from a loamy sand soil.
Glyphosate readily adsorbs to soil colloids, whether they are first encountered on the soil surface, suspended in the air (i.e., as dust) above the soil surface, or on the leaf surface of target weeds. Glyphosate adsorbed onto soil colloids is less able to be absorbed into plant leaves, which can result in reduced phytotoxicity.
Remedies for reduced herbicide phytotoxicity caused by dust are few. Spray booms mounted at the front of the sprayer can discharge the spray solution before it encounters dust generated from the tires, but dust deposited on leaf surfaces shortly after application has been shown to reduce herbicide performance. Increasing carrier volume and some spray additives have been shown to reduce, but not eliminate, the deleterious effects of dust. It is advisable to scout fields that are treated with postemergence herbicides under very dusty conditions to determine the level of weed control.--Aaron Hager