Interactions of herbicides with crop residue

Introduction

Conservation tillage is widespread in Australia because of its many advantages including reducing  soil erosion, saving fuel and labour costs, conserving soil moisture, improving soil tilth, lowering surface runoff of herbicides and fertilisers, sowing crops in a more timely fashion and improving yield reliability and profit.   Conservation tillage is heavily dependent on herbicides for weed management, but the heavy crop residues in this type of farming system may have a negative impact on the activity of herbicides used.  The herbicides can be intercepted by the crop residue and not reach the soil surface without timely rainfall.  The herbicide on the residue may also dissipate due to volatilization, photodegradation and/or microbial activity.   We conducted a study to determine how much herbicide is intercepted and retained by maize, wheat, and sorghum residue.  We compared the behaviour of three herbicides, atrazine, metolachlor and pyroxasulfone.  Pyroxasulfone is a new herbicide in the Australian market for weed control in wheat.  Pyroxasulfone is a Group K herbicide with the same mechanism of action as the acetanilides, but it is used at much lower rates and appears to require less moisture for activation.  In this study we examined the relative binding of these herbicides to different types of crop residue, the relationship between residue cover and herbicide interception, the early loss of herbicide from a dry residue and the relationship between rainfall amounts and wash-off of the herbicide.

Binding to the crop residue

In the first study we determined how tightly these herbicides bind to different crop residues.  We used a batch-slurry technique where 1 gram of the crop residue was exposed to 10 mL of water containing the herbicides for 24 h under continuous shaking.  We found major differences in the ability of the herbicides to bind to the different crop residues, as well as differences within the type of residue (Table 1).  The order of binding was metolachlor >atrazine> pyroxasulfone.  All of the herbicides bound the most to the sorghum stem residue and the least to the corn cob residue. (Table 1).  These data suggest that, while all of the herbicides bind to crop residue, pyroxasulfone will bind the least and metolachlor the most.

Table 1: Binding (%) of pyroxasulfone, metolachlor and atrazine to different crop residues.

In a study in which we determine the relationship between per cent residue ground cover and herbicide interception, we found, not surprisingly, a linear relationship.  The greater the ground cover, the more herbicide that was intercepted (Figure 1).

Figure 1: Relationship between per cent ground cover and herbicide interception

We then conducted a study looking at the relationship between rainfall amounts and herbicide wash-off from sorghum crop residue (Table 1).  In this study the herbicides were applied to the residue with a moving head sprayer and then the treated residue sat in a greenhouse for 6 days before applying the simulated rainfall.  We found that the majority of the herbicides washed off with 5 mm of rainfall, but there was increased wash off with increased rainfall (Table 2).  In this same study we found that approximately 30% of the metolachlor was lost from the residue during a 6 day period between herbicide application and rainfall but there was no loss of either the atrazine or pyroxasulfone.  This loss was most likely due to volatilization of the herbicide from the residue surface.

When we looked at the effects of sequential rainfall of 12 mm applied at 1, 4, and 7 days after treatment to wheat residue treated with pyroxasulfone, approximately 80% of the residue was washed off with the first 12 mm, an additional 10% with the second and another 4% with the third rainfall event.  Approximately 6% of the intercepted herbicide remained on the wheat residue after the 3^rd rainfall event.

Table 2: The effect of different rainfall amounts and wash-off of atrazine, pyroxasulfone, and metolachlor from sorghum residue 6 days after application.

^aApproximately 30% of the initial application of metolachlor was lost during the 6 days of incubation on the sorghum residue after application.

^bThis is the percent of residue remaining on the sorghum residue at 6 days after application.

Conclusion

Soil-applied herbicides are intercepted by crop residues on the soil surface in a linear fashion, as residue amounts increase, more herbicide is intercepted.  There are differences among herbicides in terms of how much they bind to the residue, how well they are washed off the residue with rainfall and how much dissipates between the time the herbicide is applied and rainfall occurs.  In this study, metolachlor bound the most tightly to the crop residues and pyroxasulfone the least.  20mm of rainfall was enough to wash 75 to 88% of the herbicides from sorghum residue, but there was less metolachlor washed off compared to pyroxasulfone and atrazine.  In addition, approximately 30% of the metolachlor dissipated over the 6 day period between herbicide application and the rainfall event.  These results suggest that although crop residue does intercept herbicides, the herbicides will wash off the residue with rainfall.

Contact details

Dale Shaner
USDA-ARS
Phone: 01-970-556-4826
Email: daleshaner@aol.com