Tank and plumbing clean-out is a regular part of sprayer operation to remove herbicide residues before spraying canola. Pre-seed burnoff programs often include three or four active ingredients, including Group 2 and Group 14 products ahead of cereal crops. You want to make sure these Group 2 and 14 products are completely cleaned from the sprayer tank, plumbing and nozzles before spraying canola. (Seeing problems with plants? This article will help with diagnosis.)
Common canola-crop herbicides and surfactants left in a tank for any extended period of time are very good at removing these residue deposits from tank walls and sprayer plumbing, causing them to mix in with the spray solution. If wind or rain delays spraying, the concentration of scrubbed-free herbicide residue in the tank and boom ends can continue to increase.
This underlines the importance of a complete clean out between products, and of an understanding of what risks each product and each tank mix presents.
Tips to avoid spray tank contamination
Spray immediately after filling and spray until the tank is empty. Do not leave herbicide solution sitting in the tank for an extended period and do not park the sprayer without at least initiating the clean-out process. All herbicides must be crop-safe at twice the label rate to be registered, so going over previously sprayed crop to empty the tank should not damage the crop. Reduce the rate to be certain, and if applying a second pass, keep in mind what area has already received the higher load of herbicide for the season. Second, while still out in the field add 10x the sump’s remnant of clean water, circulate, and spray it out in the field as well. Flush boom ends. Repeat.
Look for solid herbicide residue. Some herbicides may precipitate out of solution and many dry herbicides use clay as a carrier. These particles can become trapped in some parts of the sprayer or plumbing. Visual inspection can identify these problem areas and ensure that they are cleaned properly.
Apply direct pressurized spray to all parts of the tank wall.
Empty the sump as completely as possible by spraying it out. The only way to remove the remaining herbicide is through dilution by repeatedly adding water and each time draining the sump as much as possible.
Pump clean water through the boom. Check that all return and agitation lines also receive clean water. Flush all residue. This may require opening and closing various valves several times and repeating the process with new batches of clean water.
Many small washes are better than one big one. For example, a single 600 gallon wash is as effective as two washes with 70 gallons each, and three with 30 gallons each, assuming a 10 gallon sump remainder. Even better would be continuous clean-water rinsing, when it becomes available.
Clean filters and screens. Nozzle screens and in-line filters can be a significant reservoir for undiluted or undissolved herbicide and are one of the most overlooked parts of sprayer decontamination. Remove all filters and nozzle screens and thoroughly clean these with fresh water. Run clean water through plumbing leading to the screens. Consider having a second set of screens on hand to allow soaking and cleaning time with the first set.
Clean all nozzle bodies. When rinsing the boom, rotate through all nozzles in a multi-outlet nozzle body. Remove screens that may have been used with herbicide, even if just for a short while.
Consider tank cleaning additives. Check herbicide product labels to see which cleaner is recommended. If this is not on all labels, call the chem company. Some farmers and sprayer operators keep a list of clean out products specific to each herbicide they use. For some herbicides, ammonia alone is enough. For some, a detergent (surfactant) alone is enough. For others, a combination of both is required. All-Clear is one example of a detergent product. Finish and Flush are examples of ammonia-based products.
Safely dispose rinsate. Always spray out the tank in the field. Do not drain the tank while stationary unless you are certain it is free of pesticide and that you are away from sensitive areas and waterways. Another option is to build a biobed for rinsate disposal near the yard. A biobed is a clay-lined hole filled with a mixture of topsoil, peat or compost, and straw. This rich microbial environment can speed the breakdown of pesticides. Biobeds are a new concept in Canada but are considered a best management practice in Europe. More on biobeds, including research from AAFC.