8 Reasons Why Unwanted Herbicide Residues Might Happen

8 Reasons Why Unwanted Herbicide Residues Might Happen

There are several reasons why unwanted herbicide residues continue to find their way onto the wrong crops. This was one of main topics discussed in a report published by Purdue Extension titled “Removing Herbicide Residues from Agricultural Application Equipment.”


Herbicides Are Active At Low Concentrations. Some herbicide products still call for applying pints or quarts of herbicide per acre, but many herbicides now have application rates measured in ounces per acre. Residues from products with these lower application rates can affect crops even when trace amounts are left in the application equipment.

Post-emergence Herbicide More Common. By definition, post-emergence herbicides are applied over an existing crop. So if there are any unwanted herbicide residues in the equipment, future applications can damage or kill crops that you didn’t intend.

Dry Formulations Are Popular. Herbicides formulated as dry products have become quite popular, because they are easy to handle, ship, and store. Most dry formulations create suspensions of the herbicide (similar to mixing clay in water), so they do not dissolve completely into the spray mixture. So unlike most liquid formulations, dry formulations require constant agitation to evenly suspend the product.

When applicators fail to allow enough mixing time, larger particles of the dry product can get trapped in a series of screens. The particles may remain in the sprayer system until applicators run enough water through the screens or another product solubilizes them into smaller pieces — which can then pass through the screens and out the nozzles.

Be aware that the low use rates, longer breakdown time, and mixing requirements of dry formulations pose challenges when preventing crop damage.

Adjuvant Use Is Growing. Adjuvants, such as surfactants and crop oil concentrates, are products that make foliar-applied herbicides work better. Many herbicide products are formulated with adjuvants to improve their effectiveness.

But adjuvants also may dislodge old herbicide residues that are embedded in tank walls or hoses, or they may help break down particles in screens. When they do, the adjuvants may cause an old, unwanted herbicide residue to be part of the spray liquid.

Tank-mixes of Herbicides Are Common. Glyphosate-tolerant crops simplified weed control by allowing one active ingredient to control a broad spectrum of weed species. As glyphosate-resistant weed populations have grown, it has become common to use multiple herbicides to control resistant weeds. It is important, of course, to learn which herbicides can be tank-mixed to control these resistant weeds. This practice also makes cleaning application equipment between sprays more important than ever.

Products Can Be Incompatible. Problems with product hiding or building up in application equipment can be traced back to how different products get along. If you do not mix agricultural chemicals in the correct order, they can settle out of solution or form a paste. If such things happen, there are going to be problems. It is important to pay attention to compatibility issues, especially when field mixing. If you’re receiving premixed “hot loads” from a mixing facility, always tell the people at the facility if you observe potential compatibility issues.

In recent years, pre-emergence herbicide use has increased as a resistance management strategy. Using pre-emergence herbicides is reminiscent of the weed-and-feed days of the past when applicators used UAN as a carrier for preplant and pre-emergence herbicides. The differences are that the herbicide formulations today are different from those in past, and there is an increased interest in adding sulfur to these mixtures.

Incompatibility problems can arise when you use new herbicide formulations, mix products in the improper sequence, and add plant nutrients beyond just nitrogen, phosphorus, and potassium. These incompatible mixtures often leave difficult-to-remove residues inside spray equipment.

Application Timing Is More Crucial Than Ever. Optimum performance of post-emergence herbicides often depends on the size of the weeds at the time of the application. As weeds get larger, the herbicide’s effectiveness is greatly reduced, which is why herbicide labels provide maximum weed size restrictions. When weather delays early-season herbicide applications, controlling weeds becomes more challenging. With little time to spare, applicators may gamble and only partially clean sprayers in order to save time and cover more acres.

Plumbing Is Complicated and Interconnected. Modern sprayers have many places where herbicide residues remain trapped, even after flushing hundreds or thousands of gallons of water through the system. Simply not opening one valve might leave quarts or gallons of herbicide in a hose. When the applicator opens the valve to that hose during the next application, the remaining residue will then be mixed with the new product and pushed out the boom.

We know that cutting corners to save time can increase the chance for herbicide injury to crops. It’s a false priority to think that herbicide application timing is more important than stopping to thoroughly clean possible herbicide residues.

Read more at Purdue.edu.

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