Dust could affect glyphosate’s activity because glyphosate is tightly bound by soil, which is the reason why glyphosate lacks residual activity. On a leaf, the glyphosate could bind to the dust and not be available to be absorbed into the leaf.
Dust Interferes with Glyphosate Activity
Chris Boerboom, Extension Weed Scientist
At several meetings last winter, I shared a couple of our experiences with dust affecting the performance of glyphosate. In one case, county agents Greg Andrews and Lee Milligan demonstrated serious antagonism between dust and glyphosate such that common lambsquarters was not controlled. It is logical that dust could affect glyphosate’s activity because glyphosate is tightly bound by soil, which is the reason why glyphosate lacks residual activity. On a leaf, the glyphosate could bind to the dust and not be available to be absorbed into the leaf.
With the arrival of the postemergence spray season, I thought it would be good to review recent research on the effect of dust on glyphosate because it may explain some cases of inconsistent weed control. Weed scientists at North Dakota State University tested if dust from soil affected the control of eastern black and hairy nightshade by glyphosate. In these experiments, a reduced rate of glyphosate was used to determine if dust was interfering with control. The dust was from dried, ground field soil and was filtered on to 2- to 3-inch tall nightshade seedlings in a specially designed chamber. Seven variables were tested and are summarized below.
Effect of dust rate: Rates of silty clay dust equal to 0 to 9 lb/a were applied to the leaves of nightshade seedlings and then the seedlings were sprayed with glyphosate. At the highest rate of 9 lb/a, the control of the nightshade was reduced by 40%. This rate of dust is apparently similar to the dust raised when cultivating. This first test demonstrated that dust can affect glyphosate’s activity and needs to be considered as a source of variation in control.
Effect of wind: In this test, dust was applied to the nightshade and then the seedlings were placed in a wind for 2 minutes before being sprayed with glyphosate. Wind speeds up to 9 mph were tested. With eastern black nightshade, control was similar to non-dusted plants when placed in a 9 mph wind. Control was reduced with lower wind speeds to blow the dust off the leaves. Hairy nightshade was not controlled as well after the wind treatments, presumably because the hairs trapped the dust on the leaves. The researchers caution that dust may not be blown off the leaves as easily under field conditions.
Effect of soil type: Dust made from loamy sand, silty clay loam, and silty clay soils were compared. All of the soils had the same pH and had organic matter between 4 and 5%. Dust from all the soil types reduced glyphosate activity, but the interference increased slightly as the clay content increased. While the loamy sand dust had the least interference and the silty clay had the most interference, the potential appears to exist with all soil types.
Effect of soil pH: Soil dust with a pH ranging from 5 to 9 did not substantially affect the dust’s interference of glyphosate.
Effect of dust timing: Dust applied before the application of glyphosate was compared to dust applied after the glyphosate application. When glyphosate was applied 30 minutes or longer before the dust was applied, the nightshade control was not affected. However, if the dust was applied 5 minutes after the glyphosate, the control was reduced by the same amount when dust was applied before the glyphosate. Dust applied 15 minutes after the glyphosate still partially reduced control. These results suggest that dust created by a spray rig and lands on leaves shortly after the application can reduce glyphosate’s control.
Effect of spray volume: Glyphosate was sprayed in 10, 20 and 30 gpa of water. Without dust, control increased as the spray volume was decreased, which is the typical response when spraying glyphosate. However, control partially improved when dusted seedlings were sprayed with 20 and 30 gpa compared with the 10 gpa spray volume. The researchers did not speculate why a higher spray volume might be beneficial when spraying dusty plants.
Effect of adjuvants: The adjuvants tested were AMS, nonionic surfactant, organosilicone surfactant, crop oil concentrate, and methylated seed oil (the glyphosate formulation was not preloaded with surfactant). The addition of crop oil concentrate or methylated seed oil did not reduce the interference caused by the dust. However, the addition of AMS, nonionic surfactant, and organosilicone surfactant partially improved the control of dusted plants. Still the control suffered compared with the non-dusted plants.
The results of this research do not provide any easy solutions to the dust problem. Whether the source of dust is a spray rig, a nearby road, or droughty conditions, dust may be a problem. Even with front-mounted booms on a sprayer, control may still suffer because of dust settling on leaves after the sprayer (and kicked up dust) passes over the treated weeds. This research also helps to understand the problem that can occur with tire tracks. Even with a front-mounted boom and extra nozzles behind the rear wheels, dust can still land on these weeds in the wheel tracks and interfere with the glyphosate. (Damage caused by running over the weeds may also be part of the problem in tire tracks.)
Source: Zhou, J., B. Tao, and C. G. Messersmith. 2006. Soil dust reduces glyphosate efficacy. Weed Science 54:1132-1136.