Editor’s note: The following was written by Aaron Hager, University of Illinois associate professor and Extension weed science specialist, for the university’s crop management Bulletin website May 4.
Dry soil conditions across many areas have contributed to accelerated crop planting. It is a bit unusual at this point in the season that such a high percentage of corn and soybean acres already have been planted.
Soon, post-emergence herbicide applications will begin. However, one potentially adverse consequence of very dry soil is the often large amount of dust propelled into the air by either application equipment or high winds.
Airborne dust has been shown to reduce the activity of some foliar-applied herbicides, including glyphosate. Greenhouse research conducted by Zhou et al. at North Dakota State University demonstrated that control of nightshade species with glyphosate was reduced when dust was present on plant leaf surfaces.
The reduced phytotoxicity occurred regardless of whether dust was present on the leaf surfaces before glyphosate was applied or if dust was deposited within 15 minutes after glyphosate application. 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, regardless of whether these colloids are first encountered on the soil surface, suspended in the air (i.e., as dust), 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 in which post-emergence herbicides were applied under very dusty conditions to determine the level of weed control.