Iowa State University
Dust, Tire Tracks and Glyphosate
by Bob Hartzler
October 03, 2007 - Complaints about poor performance of glyphosate (Roundup) in the tire tracks of sprayers have been fairly common since the introduction of Roundup Ready soybeans in 1996. Attempts to overcome this problem by placing larger or multiple nozzles in line with tires or changing the position of the boom have provided inconsistent results. Although the specific cause of the problems has never been clearly identified, stress induced by compaction of weeds by tires or interception of spray droplets by dust were considered to be the primary culprits. Research at North Dakota State University (Zhou et al. 2006) provides evidence that dust may be the source of many of these control problems.
The reduction in glyphosate activity in response to dust level is shown in Figure 1. In the absence of dust, glyphosate reduced nightshade fresh weight by 81%. Dust at 8 lbs/A (the level reported to be found on weed leaves after cultivation) reduced glyphosate activity by 27%.
Additional experiments were conducted to better define the interactions between dust and glyphosate activity. Simulated wind between application of dust and glyphosate application reduced the dust effect more on eastern black nightshade than on hairy nightshade, presumable because the hairs on hairy nightshade trapped dust and prevented it from being blown off. The reduction in glyphosate activity was directly related to the amount of clay in the soil used to generate the dust. Finally, glyphosate activity was reduced when dust was applied before glyphosate application or within 15 minutes after application. If the dust was applied more than 15 minutes after glyphosate there was no negative effect. Spray additives reduced the negative impact of dust, but did not eliminate it.
Chris Boerboom and colleagues (Boerboom et al. 2006) at the University of Wisconsin were able to replicate these results in the field, generating dust either by running a lawn mower along alleyways or using a leaf blower. Control plots were 'washed' with water prior to glyphosate application to remove dust. They reported that at one location lambsquarter with dust 'appeared to be nontreated' following glyphosate application.
Glyphosate binds tightly to soil colloids, and this is the reason why it has no soil activity. Glyphosate that comes in contact with dust kicked up by the sprayer or already present on the target will adsorb to the dust particle before it has a chance to penetrate into the leaf. The coorelation between the dust's clay content and glyphosate activity supports the idea that adsorption of glyphosate to soil colloids prevents absorption into the leaf. Most other postemergence herbicides do not bind as tightly to soil colloids as glyhosate, and this is probably why the tire track phenomenon is less common with other products. The results also indicate why attempts to overcome this problem by mounting booms on the front of tractors fail to eliminate the tire track problem since dust applied after application still can negatively impact glyphosate performance.
Zhou, J., B. Tao and C.G. Messersmith. 2006. Soil dust reduces glyphosate efficacy. Weed Sci. 54:1132-1136.
Boerboom. C., D. E. Stoltenberg, M. R. Jeschke, T. L. Trower , and J. M. Gaska 3 Factors affecting glyphosate control of common lambsquarters. Proceedings NorthCentral Weed Sci. Soc. 61:54.
Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University
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