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Farmer-friendly herbicide applicator.

How low can it go? That's the question Chester McWhorter and co-workers were asking themselves nearly a decade ago.

In 1983, McWhorter, an ARS plant physiologist at the Jamie Whitten Delta States Research Center, was searching for a way to reduce the amount of water that is mixed with herbicides used to kill weeds on agricultural lands. Typically, 5 to 20 gallons of water are used per acre.

"Hauling water to the field can be expensive and time consuming for farmers," explains McWhorter, now head of the Application Technology Research Unit at the Stoneville, Mississippi, center. "We thought cutting the amount of water in the herbicide mix would give us a substantial saving."

Each year the researchers cut back. Eventually, they decided to go for broke and eliminate all the water. The mix consisted of a herbicide and a spreader to boost the weed-killer's effectiveness. But when it came time to apply the herbicide at less than 1 gallon per acre - ultra low volume (ULV) application - they encountered problems with the spray equipment. Conventional hydraulic nozzles did not adequately distribute the spray.

So Floyd Fulgham, an agricultural engineer in the center's Field Mechanization Research Unit, developed an application system called the T-miser. Resembling an inverted "T," the nozzle uses air to atomize the herbicide mixture - instead of liquid pressure, like conventional hydraulic sprayers.

As research progressed, scientists found two problems with the sprayer: The holes in the disk were so tiny they became clogged, and the plastic tubing used as a nozzle orifice did not adequately distribute the spray in a swath.

To remedy the problems, Fulgham replaced the disk orifice, a round piece of metal with a hole in the middle, with a positive displacement, piston-type pump to meter the liquid. Commercially available flood-jet nozzles replaced the plastic tubes. With a flood-jet nozzle, the liquid comes out of the nozzle in a stream, then hits a hard surface to create a fan-shaped spray.

Fulgham is now retired. James Hanks, an agricultural engineer at Stoneville, continues to work with McWhorter to find the best possible application method for herbicides at low and ultra-low rates.

"This is the best system we know of to apply herbicides in oil or water at total spray volumes of less than 1 gallon per acre," says Hanks. "We started out with a rather crude sprayer system and modified it, so now it's farmer-friendly."

From 1988 to 1992, the scientists conducted field tests using ULV application. They used several different herbicides for postemergence control of johnsongrass and barnyardgrass in soybeans.

These tests show that herbicide mixed with paraffinic oil - which is similar to mineral oil - controls johnsongrass better when applied at 1 gallon per acre than it does when mixed with water and applied at 20 gallons per acre.

There may be several reasons for the increased weed control, explains McWhorter. For starters, the T-miser applies smaller, more concentrated droplets of the herbicide mixture. These droplets are more toxic to plants than the larger, less concentrated droplets dispersed in water by conventional sprayers. Oil also spreads better than water on leaf surfaces.

"Apparently, the greater spread activity leads to greater control," says McWhorter. And solubility of the herbicide in oil may also be a factor.

Overall, paraffinic oil outperformed soybean oil, cottonseed oil, No. 2 diesel fuel, kerosene, and jet A fuel as a herbicide-spreading agent.

McWhorter says ULV herbicide application is a win/win situation for everyone.

"Reduced herbicide use should improve both the farmer's bottom line and the environment," he says.

Farmers would be able to easily construct their own sprayer for ULV use. However, these low application rates are not approved by the U.S. Environmental Protection Agency.

According to McWhorter, chemical industry officials say that they plan to request registration for these extremely low application levels as new products come on the market.
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Author:Gerrietts, Marcie
Publication:Agricultural Research
Date:Feb 1, 1993
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