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Getting pesticides to the intended target is the goal of Dr. Roberto Barbosa, LSU AgCenter assistant professor of agricultural and biological engineering.
He said spraying chemicals on soybeans becomes more challenging after the plant canopy develops because "the top canopy acts as an umbrella, preventing the spray from reaching lower parts of the plant."
Barbosa explained insects often hide on the undersides of leaves, and some diseases usually develop from the bottom up.
A higher volume of water can help get the material to the lower parts of the plant, he said, even though it means the chemical is more diluted. A 10-gallon-per-acre application rate delivers twice the amount of water when compared to a 5-gallon-per-acre rate, but the chemical rate should be the same.
"What we are finding with aerial and ground application is that increasing application rate increases the chances of the product getting to the bottom of the canopy," he said of his research that has been funded in part by a grant from the Louisiana Soybean and Grain Research and Promotion Board.
Nozzle type also will affect canopy penetration, he said.
Systemic chemicals such as glyphosate can be applied with air induction nozzles. These nozzles produce large droplets (larger than 400 microns) and are very good at preventing drift, Barbosa said. But they do a poor job in breaking up the spray.
Nozzles producing medium-sized droplets (300 microns) are needed for contact chemicals, he said.
Turning to other nozzles that result in smaller droplets, Barbosa said, "These nozzles break the liquid jet thoroughly." The negative side of such breakup, however, is the nozzles produce droplets that are small (less than 150 microns) and prone to drift, he said.
"We should always exercise caution when spraying," the LSU AgCenter researcher added. "Observe meteorological conditions and spray only when winds are calm and there’s no sign of a temperature inversion."
Barbosa’s work includes helping pesticide applicators who use ground rigs and aircraft to calibrate their spray equipment. "We do that for liquids and solids," Barbosa said.
He travels around the state with a mobile lab to analyze spray patterns and help make sure nozzles are working properly and providing an even spray pattern.
"The best efficiency comes from doing a task one time, which can be done if you have your equipment in order and get the product in the field on a timely basis," he said.–Bruce Schultz
(This article was published in the 2008 edition of the Louisiana Soybean & Grain Research & Promotion Board Report.)