Site-Specific Weed Management
Using soil electrical conductivity maps to modify soil applied herbicide rates.
Most farmers know that there can be tremendous variability in the type of soils within a field. Although pre-emergence herbicide activity is highly dependent on soil type, most are applied at the same rate across the whole field leading to over application on some soils and not applying enough in other areas. This increases not only costs, but the possibility of contaminating ground and surface water as well. Utilizing the tools of precision agriculture, farmers may be able to vary the application of pre-emergence herbicides based on soil variability, applying less on lighter soils (coarse texture soils) and more on heavier soils (fine texture soils) within a field while maintaining good weed control. This would not only reduce the cost of herbicides for the farmer, but would reduce the possibility of water contamination due to the over application of herbicides to areas that do not need them.
One of the obstacles to implement variable herbicide application is gathering
the extensive amount of information necessary to accurately determine where
to apply different rates of herbicide. What is needed is an inexpensive and
efficient way to thoroughly map soil variability and relate it to factors that
affect herbicide performance. Veris Technologies has designed a trailer unit
that can create intensive field maps of bulk soil electrical conductivity (EC)
(See Farahani, HJ. 2002. Soil Electrical Conductivity Mapping of Agricultural
Fields.
http://www.colostate.edu/Depts/SoilCrop/extension/Newsletters/2002/Sensors/SoilEC.htm).
The EC of a soil is a measure of how easily an electrical current passes through
the soil and is dependent on water content, soil texture, soil organic matter,
salinity and exchangeable calcium and magnesium. Under non-saline conditions,
soil EC has been used to estimate many factors, including herbicide behavior.
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| Figure 1: EC maps of three fields in eastern Colorado. |
We have been conducting research to determine the relationship between soil EC and herbicide behavior in three different fields in eastern Colorado. We have maps of the EC zones within these fields (Figure 1) and have taken soil samples representing three different zones within the field and determined both the binding and biological activity of three pre-emergence herbicides, metolachlor, EPTC and metribuzin. Our results show that there is a good relationship between soil EC and herbicide binding, particularly for metolachlor and EPTC. These herbicides bind less tightly to soils taken from low EC zones compared to soils taken from high EC zones. This behavior is not unexpected because in these fields there is a strong relationship between EC and soil organic matter: the higher the EC the greater the organic matter. Since the binding of herbicides is highly dependent on soil organic matter, then one would expect to see more binding in the high EC zones compared to the low EC zones. The biological activity of these herbicides also varied depending on the EC zone. In soil taken from low EC zones, it took approximately 40% less metolachlor to give the same level of activity compared to soil taken from the highest EC zone.
We took the data that we generated from these relationships between soil EC and herbicide binding and created new zones within the three fields that predicted soil binding of the herbibides. We then went back to each of these fields and took new soil samples from areas of the field to determine how accurately we could predict herbicide binding. We found that we accurately predicted herbicide binding and biological activity about 80% of the time. This is very encouraging.
However, were the differences we measured in these fields enough to justify the cost of gathering the information? On the Dual Magnum label there are instructions on how to vary rates based on soil texture and organic matter. Using the recommended herbicide application rates for different soil types, we determined the difference in rate for the low and high EC zones in all three fields. In two of the fields (Wiggins 1 and 2), the differences between the low and high EC zones varied by less than 10%. It is unlikely that herbicides could be applied accurately enough to do this. In the third field (Yuma site), differences between the low and high EC zones were large enough that the application rate of the herbicide varied between 1.2 and 1.5 lb/a. Using this information, we constructed a theoretical application map (Figure 2). In this particular case the low application zones accounted for approximately 25% of the field. The savings in herbicide was approximately 1 gallon of formulated product, which translated into a savings on herbicide cost of approximately $1/acre.
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Figure 2: Application Map for metolachlor. |
How would a farmer use this type of approach? The first thing to do would be to make a soil EC map of a field to determine the pattern of variability. Then soil samples would be analyzed to determine the relationship between soil EC and soil organic matter and texture. Once this relationship is known, a new map can be drawn of the field delineating the field into different herbicide application zones based on the predicted variation in herbicide availability. We are currently conducting further research to determine if this approach is practical and can accurately predict herbicide efficacy and behavior.
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