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Northwest Iowa Dairy Outlooks

A local discussion of current science and issues concerning dairying in northwest iowa

Effective pesticide application requires attention to factors that influence product performance such as: product selection, label instructions, equipment calibration and application timing. However, one factor that doesn’t get much attention is the quality of the water used to spray the product.

For the most part, if the water runs clear, we don’t give much thought to its purity. But the research clearly shows the quality of water used for spraying can affect how pesticides’ perform.

Extension researchers have proven that spray water pH and hardness can reduce the effectiveness of herbicides, making it vitally important for crop producers to test water sources. Hard water, or water with pH values as low as 4 or as high as 9 have been shown to lower the efficacy of herbicides, including glyphosate. An ideal pH value would be 6-7.

Spray water at a high pH, can cause the molecules in some pesticides to break apart, turning the herbicide into a different compound altogether. Pesticides normally are formulated as weak acids or neutral to weakly alkaline products. As a general rule, herbicides, insecticides and fungicides perform best in slightly acidic water. When water pH falls outside of the preferred upper or lower boundaries, product performance can be reduced.

In addition, using water with suspended solids such as silt, clay and organic matter can lower the effectiveness of pesticides with high soil sorption coefficient and soil organic carbon sorption coefficients. Herbicides with high  Kd or Koc values bind tightly to soil as well as sediment and organic matter in water. So, the more sediment and organic matter in the water, the less herbicide remains available to be taken us by the plant.

Water hardness can affect some pesticides like magnets attracting opposite charges. Negatively charged pesticide molecules are attracted to the positively charged iron, calcium and magnesium cations in hard water. The binding of pesticides with these minerals creates molecules which cannot enter the target pest, or enter at a slower rate, or precipitate out of solution.

Growers can buy test kits for both pH and hardness, and there are pH adjuster treatments to neutralize spray water. Hard water issues can sometimes be corrected by adding ammonium sulfate.

Producers are warned against making complex mixtures of herbicides, insecticides, fungicides and foliar fertilizers because the characteristics of each chemical potentially could change the way the other chemicals perform, especially with questionable water.


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