Where Do Herbicide Residues Hide?
Self-propelled sprayers are becoming more popular than pull-type sprayers with farmers, and they have become standard for agricultural retailers. In crops, sprayers are sophisticated machines that are outfitted with the latest technology including: rate controllers, chemical injectors, up to 120-foot booms, 1,800-gallon tanks, boom section controls, boom levelers, state-of-the art nozzles, advanced tires, computer recordkeeping, global positioning with light bars, and auto-steering. These advances in application equipment enable applicators to get more work completed more efficiently and effectively throughout the season.
Underneath these self-propelled sprayers, you will see a sophisticated system of interconnected valves, pumps, screens, hoses, nozzles, and boom sections. Although flushing the system with clean water will remove most remaining herbicides from the system, clean water will not remove it all. Most and all definitely have different meanings when we are talking about potentially damaging herbicide residues. This was one of the topics discussed in a report published by Purdue Extension titled “Removing Herbicide Residues from Agricultural Application Equipment.”
Clean water removes most suspended herbicide residues as they flow from the tank into the pump and out through the hoses, pump, screens, and nozzles. Some may assume that the water moving through the system has access to all of a sprayer’s plumbing components — that is, they think every valve is open and every inch of hose will have clean water running through it to remove leftover residue.
Introducing clean water will remove most remaining residues. However, if you only rinse with clean water, you will not remove larger, undissolved clumps of herbicide particles or areas of heavy residue buildup. Those particles could remain trapped on the interior of the boom’s end caps or in a screen no matter how many thousands of gallons of water have already run through the system.
Think of rinsing sprayer equipment like using just water to wash grease from your hands. If you just run your hands under water, the grease isn’t going to come off; nor will the residue come out of spray equipment if you just flush a few hundred gallons of water through it. Furthermore, to get the system completely clean, you must remove the filters and nozzles to remove the particles or paste from the screens at the ends of the booms.
There are a number of locations where residues can remain trapped within the sprayer:
Spray Tanks. A sprayer tank has a large surface area that can retain product on its interior surface. Sometimes, herbicide residue can embed in the tank’s lining. If you don’t clean the inside surface, residues can remain behind and accidentally become part of the next spray mix when you refill the tank with water.
A tank’s design can also play a factor in how much herbicide remains trapped inside it. Flat-bottom tanks have ledges and corners that can hold product. More importantly, many tanks do not drain completely. The suction or drain port leading out of the bottom of a tank is usually not flush with or below the tank’s bottom.
Typically, a tank’s drainage opening is slightly above the bottom of the sump. The size of the sump and the area below the drain opening will determine how much residue may remain on the bottom of a tank.
Hoses. Hoses, particularly rubber ones, may hold embedded residues. While plastic hoses may also trap residue, plastic is much less porous than rubber. In addition, rubber hoses may develop small cracks and rough edges on the inside, which can also hold residue.
A general problem with hoses is that many do not make a straight run from one attachment point to the other. Some hoses sag between sections of the boom pump, which creates a trap for liquid or suspended products in a manner similar to the trap under a sink. Many formulated herbicides can settle out of suspension in as little as 45 minutes. If this happens, the liquid remains in the trap until more water pushes it forward. Invert U-shaped hoses with the help of a tarp strap or zip tie so that they become self-draining.
Valves As ‘Blind’ Alleyways. Depending on the system, you can turn valves on and off manually or electronically. By opening and closing valves, clean water and tank cleaners can flow through all hoses, removing trapped products and residues.
Booms. Once the tank is empty, the pump will lose prime and cannot force all of the spray mix out of the booms. The only way to completely force the remaining herbicide mix out of the boom is to refill the tank with water and reprime the pump to build pressure. The clean water that runs through the system will push out any remaining herbicide from the pump to the hoses and booms, but any mixture that returns to the tank through the bypass could be contaminated with herbicide.
Many newer spray rigs are equipped with air systems that force air into the booms to push out any remaining spray mixture before adding clean water. Most of these systems clean one boom section at a time. These air systems do not necessarily help remove residues. Blowing out the booms is a first step, but you still need to flush the system.
Screens. Screens (also called strainers and filters) capture large pieces of herbicide products, undissolved bags, plastic shavings, sand, and grit before these large objects can reach and plug nozzle tips. Some spray systems also have suction screens on the fill lines going into the tanks. Suction screens pull liquid from the tank into and out of the pump and then the liquid passes through another screen. Whether this screen is before or after the pump depends on the manufacturer’s design.
Screens are sized and classified by the number of squares per inch or millimeter — the smaller the mesh number, the larger the mesh hole size. The mesh should be slightly smaller than the exit orifice of the nozzle. Fill screens often have relatively large holes designed to filter out larger objects. The screens before or after the pump are often 50-mesh, which means more holes that are smaller in size than the fill screen. As the fluid flows toward the nozzles, it will often pass through screens in the booms or at the nozzle tip.
End Caps or Cleanout Caps. Whether it’s one boom or a series of sectional booms, each “pipe” has caps on the ends to contain the flow of water within the pipe so that the spray mix is forced through the nozzle tips. The last nozzle is often 2 to 4 inches from the end cap, which means that the area beyond the last nozzle to the end cap allows suspended herbicide to collect and remain undisturbed. These undisturbed particles can collect, gather, and bind together and form a milky, gummy, and sticky substance with a toothpaste-like consistency.
Sprayers will normally have end caps on boom sections. End caps (also called cleanout caps) are designed to be taken off. You can clean the paste on the cap and inside the boom. If you do not remove the end cap and clean it, you can leave significant residue buildup trapped just inside the cap.