Water is key to pesticide
When considering the factors that can hurt pesticide performance, farmers often overlook problems with the main ingredient in their spray tanks, according to Fred Whitford, coordinator of pesticide programs for Purdue University. “You’re spraying 99% water,” he reminded farmers at the recent West Ohio Agronomy Day held in Fort Loramie.
A farmer can choose the ideal product and use the most sophisticated application equipment guided by the most precise guidance system, but none of that will matter if the water used to mix up the pesticide has quality problems, he stressed.
Factors such as pH and dissolved minerals can influence product effectiveness by reducing solubility of the active ingredients, or decreasing absorption by the pests or weeds. To figure out what quality factors are important for a particular pesticide, read the product label, but the information may be hard to find, Whitford explained. “This stuff is buried in these labels.”
No pH level is perfect for every pesticide, so it’s important to figure out the parameters for the product you’re using. For instance, Whitford explained, most herbicides perform best in a solution that is slightly acidic, with a pH of 4 or 5. However, sulfonylurea products typically work best when pH is slightly alkaline, from 7 to 8.
In some cases, when pH is outside the recommended range, the pesticide won’t dissolve into solution. The pH of the spray solution can also affect the electrical charge of pesticide molecules, which can hurt the product’s effectiveness by reducing the molecules’ penetration into weed leaves.
Another concern is that pH can influence how long a pesticide molecule lasts before it starts to break down. For instance, if a weakly acidic pesticide is mixed with weakly acidic water, it’s likely to remain stable. But that same product mixed with water that’s slightly alkaline might lose half its effectiveness in a matter of minutes.
“By the time you fill up your tank and take a smoke, half of it’s gone,” Whitford warned. This is a particular concern for farmers who use treated municipal water, he added. Treated water often has pH levels higher than 8, in the alkaline range.
Keep in mind that different formulations of the same herbicide may have different water-quality requirements. For instance, some companies add an acidifier to their glyphosate products, but others don’t. As a result, the products might perform differently if the pH of the solutions is not adjusted accordingly.
Another concern, particularly for farmers using well water, is water “hardness,” or dissolved minerals such as calcium, magnesium and iron. These mineral molecules are positively charged. Some pesticide molecules are negatively charged, so the minerals will bind with them, Whitford explained. If this happens, the mineral-bound pesticides might be less effective because the weeds or pests take them up more slowly or not at all. In other cases, the pesticide precipitates out of solution.
For instance, if a glyphosate solution looks cloudy, the herbicide is not dissolved in the solution, Whitford pointed out. A farmer might put the right amount of herbicide in the spray tank, but if it’s not completely dissolved, it won’t perform as it should. “You are not putting out that rate — it’s a lot less,” he stressed.
Using pond water is an option for some farmers, but quality can vary greatly and isn’t necessarily the same from year to year. Ideally, farmers would catch rainwater before it hits the ground to use in spray solutions. Some producers have installed systems to catch and store rainwater, Whitford said, but most don’t consider that to be practical.
To get optimum performance from a pesticide, evaluate water quality before mixing up spray solutions, Whitford recommended. In most cases, it’s not necessary to spend a lot on lab testing or electronic meters. Instead, go to an aquarium store and pick up test kits with sensitive paper strips, he suggested. The test trips are easy to use, inexpensive and accurate enough for analyzing pesticide spray water.
Refer to product labels to adjust for water quality. In some cases, the label might call for different rates for different water pH levels. Others might recommend water conditioners to change the pH of the spray mix, Whitford noted. Research has shown that it doesn’t matter if the pH is corrected before or after the pesticide is added, but he prefers to clean up the water before adding the pesticide.
Ammonium sulfate, often used in spray mixes, reduces problems with water hardness. The negatively charged sulfate molecules help by binding with the positively charged mineral molecules. However, the mix will still test high for hardness, Whitford said. The minerals are still in the water, but they are no longer free to bind with negatively charged pesticide molecules.
Keck writes from Raymond.
This article published in the February, 2012 edition of OHIO FARMER.