Using the same sprayer to treat multiple crops requires extra precaution. In my weed control project at Stoneville, we use a MudMaster to spray rice, cotton, and corn.

Early last week, we sprayed several rice studies with a tank-mixture of Storm and Permit. We needed to treat a cotton field with Ignite a few days later but noticed the spray tank was full of water that was not clear. Although the tank and boom should have been cleaned, we had been so busy that we were not sure this was the case. So, we cleaned the sprayer again just to be safe.

This can happen in any spray operation in the Delta during the weed control season, which has seemed long this year.

Whether due to rains in April, the flood, or the wind, it appears everyone (me included) has been in a rush. Tom Eubank and I have looked at several problem fields this year that we ultimately attributed to tank contamination.

In one case, a large acreage of soybeans was exhibiting dicamba injury symptoms. The grower used a bulk tank as his source for spray water. Lab tests of the water from the bulk tank revealed a high concentration of dicamba. During burndown, dicamba had back-flowed into the bulk tank from an attached mixing tank. The dicamba was still present in the bulk tank when the soybeans were treated with glyphosate over two months after burndown.

This past week, Tom and I were called to a cotton field that had shown injury symptoms of a growth regulator herbicide (2,4-D, dicamba, Facet) since emergence. Following a rain in late May, the symptoms had worsened. The field had been treated only with herbicides labeled for cotton, but the injury was obvious. The logical explanation was that a growth regulator herbicide had accidentally been added to the spray tank during an at-planting herbicide application.

Both of these were problems with contaminated spray tanks, but they could just as easily have been caused by improper tank cleaning.

Since herbicide-resistant weeds have become the top priority for weed control in the Delta over the past few years, it seems that most growers and commercial applicators are placing more emphasis on the quality of their herbicide applications.

There are several important components to a successful herbicide application — correct weed identification, matching the herbicide to the targeted weed species, proper spray nozzle selection, and accurate sprayer calibration. However, with the mix of crops common in the Delta and the types of herbicides required to manage herbicide-resistant weeds, sprayer cleanout after the herbicide application is just as critical as the considerations prior to and during the herbicide treatment.

Unfortunately, not all herbicides are created equally when it comes to sprayer cleanout. I am no chemist, so I will not attempt to discuss the properties that cause different herbicides to get stuck in various places in a sprayer.

Some herbicides, especially granular or dry flowable formulations, may settle out of solution and form residue in the bottom of the spray tank or get lodged in strainers. Others may adhere to the walls of the tank and/or hoses (especially in poly tanks or when hoses are worn).

Regardless of the herbicide used, there are three similar goals when cleaning a sprayer. These include diluting the active ingredient below damaging concentrations, deactivating the herbicide, or removing the herbicide from the spray system.

For example, most glyphosate products can be purged from a sprayer by flushing an adequate amount of clean water through the system. Some commercial tank cleaners or chlorine bleach are effective for reacting with sulfonylurea herbicides (Classic, Permit, Resolve) and converting them into their inactive forms. Industrial strength ammonia can be useful for dislodging herbicide residue in tanks and hoses.

Most herbicide labels contain information on sprayer cleanout following application. Some are more detailed than others. Several detailed Extension publications are available that outline sprayer cleanout.

Although some cleaning requirements may vary depending on the type of herbicide that was last sprayed, some general steps in sprayer cleanout apply to most herbicide chemistries.

Sprayers should be cleaned immediately following herbicide application to avoid drying of herbicide residues.

The spray system should be drained completely and clean water flushed through the tank, hoses, and nozzles for several minutes. While flushing the inside of the spray system, thoroughly rinse the exterior.

Drain the remaining rinse water from the tank and refill the tank with clean water, adding industrial strength ammonia or commercial tank cleaner per specific herbicide label instructions. Agitate this mixture and flush through the spray system, again for several minutes.

Remove all strainers and nozzles and rinse these in a bucket containing the ammonia or tank cleaning solution.

Drain the tank and flush the system for a third time using clean water.

Most new sprayers have enormous spray tanks and countless hoses and spray nozzles. These sprayers require a lot of water and time to thoroughly clean. But sprayer cleanout and maintenance are critical components to a herbicide application, so they should not be neglected.

When weeds are growing rapidly and there are a lot of acres left to spray, it is easy to say, “That’s clean enough, let’s go mix the next load.” But, with many of the herbicides required to control today’s problem weed species, the margin for error in sprayer cleanout is small. Taking time to thoroughly clean the sprayer may save money in the long run.

Jbond@drec.msstate.edu