Proper weed control stewardship practices begin with considering the options to control unwanted plants.

When herbicide applications are the preferred option, it’s important to carefully follow product label instructions that include applying correct herbicide rates at the right times to avoid unintended negative consequences.

If application rates are too low, weed control may be poor and surviving plants may pass potential resistance traits to the next weed generation(s). If rates are too high, chemical costs increase and there may be injury to non-target plants.

Other issues may include contributing to pesticide soil carryover and environmental contamination in field water runoff and/or leaching into groundwater.

It’s important to have the correct sprayer calibration to comply with best management herbicide stewardship and to be certain that label rates are accurately applied.

New equipment with on-off sensor technology is increasing spraying efficiency and lowering application costs.

Drones also are emerging as potential sprayer improvements. In some cases, a drone may apply chemicals to weeds using herbicide products with very little water, lowering herbicide drift risk.

For most applications, sprayer calibration can be calculated using standard liquid or dry pesticide rate equations.

The most common rate application errors likely occur when hand and backpack sprayers are used for spot applications. There can be a temptation to guess when applicators are unsure how much product is being applied. That’s not part of good environmental stewardship. Here are some calculations to improve application rate accuracy:

- Step one. Measure a square area 18.5 feet long by 18.5 feet wide, which equals 1/128th of an acre. An acre is equivalent to 43,560 square feet.

- Step two. Use a stopwatch and record the time in seconds that it takes to spray the measured and marked square.

- Step three. Place the hand or backpack sprayer nozzle into a container that can measure in fluid ounces. Then, spray the nozzle into the ounce container, using the stopwatch to determine the length of time during step two.

- Final step. Record ounces applied into the measuring container. Because 1 fluid gallon equals 128 ounces, the ounces applied in the container equals the approximate number of gallons per acre applied with the sprayer.

This equation will be most accurate when the spot application rate while walking or driving matches the travel speed used in the step two application measurement.

More sprayer information

The Nebraska Extension publication EC130, “Guide for Weed, Disease and Insect Management in Nebraska,” has several options for calibrating boom sprayers.

Three factors determine sprayer application rates: equipment speed, nozzle spacing on booms, and nozzle (tips) output determined by orifice size, pressure and sprayer solution density.

For calibration equations, speed is measured in length of distance covered divided by time. Nozzle spacing equals inches between nozzle tips and nozzle output equals the quantity applied divided by unit time.

The most common method to hand calculate travel speed is to make a distance of 88 feet and time how long it takes to move that distance. The speed in mph equals the distance in feet multiplied by 60, divided by time in seconds, and multiplied by 88.

The standard formula to determine gallons per acre is: gallons per minute multiplied by 5,940, divided by mph, and multiplied by sprayer nozzle width (inches between nozzles).

Other sections in the Extension publication include: “How to Determine Rates/Acre,” “How to Determine the Acres Sprayed Per Minute,” “How to Determine Nozzle Size Needed to Achieve the Operational Goal” and “How to Adjust Pressure (psi) to Match Nozzles.”

Applicators of restricted-use herbicides must be safety certified before buying or applying such products. Nebraska Extension’s statewide certification training session locations are listed at http://pested.unl.edu/privateschedule.

Other re-certification options include the Nebraska Crop Management Conference Jan. 21-22 at the Younes Conference Center in Kearney.

Todd Whitney is a University of Nebraska-Lincoln Extension educator with Phelps, Gosper, Harlan and Furnas counties as his focus region.

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