Minimizing Pesticide Waste

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Waste (noun): an act or instance of using or expending something to no purpose.

In agriculture, environment and economy are intertwined. Producers strive to obtain the maximum return on their inputs. They study incremental returns and avoid applying more inputs than necessary, especially if conditions don’t warrant it.  The financial incentive is powerful, and waste is a four-letter word. This applies to seed, fertilizer, and pesticide. Pesticide labels identify the rate needed to obtain the desired result, and there is no incentive to over-apply. In fact, it’s illegal.

But there are plenty of other places where applications incur waste. As with time efficiency, it’s a good idea to identify where this waste occurs, and the only tool needed is a sharp pencil.

When might we incur waste in the spray application process?

  • Mixing more than we need because we don’t trust the flow meter or the tank gauge entirely, or don’t know the exact field size.
  • Priming the boom before the first swath.
  • Overlapping due to curvy terrain and coarse sectional control.
  • Spray drift away from the intended swath.

How big are the losses?

Let’s say we have a clean sprayer and need to spray 160 acres before moving to a new crop and product. We plan to apply 10 gallons per acre and have a 1,200 gallon tank with a 120 foot boom. That means we need 1,600 gallons of spray mix in total.

Once we’re at the field we prime the boom. Each sprayer is different, but depending on operator experience, 30 to 50 gallons are usually needed to push product from the tank to the last nozzle. Only part of that is lost to the ground, as boom sections can be shut off as soon as product has reached every nozzle of that section. We’re assuming 0.2 gallons per foot of boom is lost.

Spraying itself is relatively straightforward. Swath and sectional control handle the overlaps, but in less ideal terrain, double application is known to account for 4 to 5% of the area to reach non-square parts of the field. This is even more likely when the outer section is 10’ or more. Early turn-on of the boom prior to leaving the headland, to allow boom to reach operating pressure, adds to this.

Air-activated shutoff for individual nozzles reduces section size at a reasonable cost.

With an average nozzle, we can expect about 2% of the product to airborne drift. Most airborne won’t return to the ground within the field borders, so it’s a complete loss.

Most of the spray that travels more than 5 m after leaving boom stays airborne and should be considered a total loss from the field.

As we finish, the pump will draw air before the tank is empty due to sloshing or foaming, and a 50 to 60 gallon remainder may not be unusual. This simulation has assumed 5% of tank volume remains.

We also need to purge spray from the boom at cleanout, consuming approximately 0.4 gallons per foot of boom. This occurs after the field is completely sprayed and is therefore considered waste.

So how does this add up? The following table shows the approximate losses associated with five setups.

Table 1: Spray mix losses during a sprayer operation. Setup 1 = baseline, Setup 2 = low application volume, Setup 3 = baseline with recirculating boom, tank level monitor, and low-drift nozzles, Setup 4 = large area between cleaning, Setup 5 = large area with recirculating boom, tank level monitor, and low-drift nozzles.

In the first scenario, we spray just 160 acres at 10 gallons per acre. Priming the boom with 0.4 gallons per foot (allowing for all associated feed lines) consumes 48 gallons, but only wastes half of that, or 1.5% of the total volume needed for the field.

Four percent overlap consumes another 64 gallons.

If we have 5% of the tank volume left over, that’s 60 gallons. That amount is so small it doesn’t even register on the sight gauge but nonetheless it represents another 4% of the total sprayed amount.

Upon cleaning the boom, we need to push the spray mix out of all the  plumbing after the pump, as it has nowhere else to go. At an assumed 0.4 gallons per foot, that’s another 48 gallons or 3%.

If we add to that a conservative 2% drift loss, it sums to a surprising 14% of the total spray volume. For those that use lower water volumes (the second scenario), the volumetric losses are slightly less, but their proportion is higher, now accounting for 23% (!) of the total spray mix.

In the third scenario, let’s assume we use a recirculating boom that returns the initial prime volume to the tank, eliminating any waste. We’ll also upgrade to individual nozzle sectional control, reducing overlap to 1%. And, since we want to know exactly what’s left in the tank, let’s invest in an AccuVolume system to precisely monitor tank volume. This allows us to make small rate adjustments up or down to be sure as much of the mixed product goes onto the sprayed swath as possible.

Recirculating booms allows the spray mix to pass through entire length of boom without being sprayed, saving waste during priming and allowing waste-free boom rinses.

When the sump begins to empty, we can introduce some water from the clean water tank to push the last of the mix to the boom (a continuous rinse system makes this easy).

An AccuVolume sensor shows the exact volume left in the tank at any slope position and with 1 gallon resolution, allowing greater accuracy when filling and emptying.

We’ll assume our sump waste is now reduced to 12 gallons. We still need to dispose of the content of the boom somehow, so the recirculating boom offers no saving there. But let’s also add better low-drift nozzles to reduce drift by 50% (now 1% total volume). Total loss is now just 6%.

Low-drift nozzles such as this AirMix (Agrotop) SoftDrop reduce airborne drift by 50 to 90%.

The last two rows in the table repeat the first and third scenarios for a larger sprayed area (1000 acres) before a tank cleaning is needed. This doesn’t change the magnitude of the volumetric loss, but reduces its proportion. Percent loss is down by a factor of two from the 160 acre interval, to 3 to 7%.

Experienced operators might cheat the system a bit by mixing the required pesticide with some extra water to make up for the plumbing waste. Doing so prevents extra pesticide from being consumed, but it doesn’t reduce the inherent inefficiency.

Lessons

This exercise suggests that waste from spraying is probably higher than we assumed. If we average the scenarios, there is 10 to 15% waste. At, say, $200,000 spent on pesticide for a single spraying season, that’s $20 – $30,000 worth of product and water hauled that ends up where it doesn’t belong. Beyond the time and money, there can also be environmental consequences depending on how that waste is treated.

There are some things that can be done.

  • Know the exact area of the field to be sprayed.
  • Study your sprayer plumbing and consider improvements such as recirculating booms and continuous cleanout.
  • Improve monitoring of tank content to allow lower remainders.
  • Consider individual nozzle shutoff to improve sectional control. These are part of Pulse Width Modulation (PWM) systems, but can also be achieved with less expensive valves.
  • Plan spray operations to minimize the amount of product changeovers.
  • Consider direct injection.

The return on investment for plumbing improvements can be high and result in considerable future savings over the life of the sprayer. It’s worth thinking about.