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Variable rate technology adapted to soil for lettuce herbicide

Variable Rate Technology (VRT) — the melding of satellite guidance systems (GPS/GIS) for tractors, sprayers and harvesters and computers to create the new era of precision agriculture — is catching on fast in Arizona and California.

VRT has been used to apply varying rates of fertilizer, planting seed, plant growth regulators and soil amendments depending on plant and soil conditions with consider financial success.

Kurt Nolte, University of Arizona Cooperative Extension area Extension agent in Yuma County, has successfully taken VRT into a new arena, applying different rates of an old standby lettuce herbicide using soil types as the variable in an effort to prevent damage to seedling lettuce.

Nolte told the Desert Vegetable Crops Workshop in Holtville, Calif., recently that he reduced injury to seedling lettuce by 20 to 25 percent in a field by simply reducing the rate of Balan on sandy soils.

Nolte's divided up a Yuma County winter lettuce field into three segments based on the soil types — medium, heavy and light and applied three different rates of Balan based on soil texture (two pounds, 2.5 pounds and 1.5 pounds respectively).

Nolte told Pest Control Advisers and growers in Holtville at the workshop sponsored by the University of California, University of Arizona and Western Farm Press varying the rate of Balan made a difference in overall field plant injury.

VRT reduced the rate of injury to seedling lettuce measured just ahead of thinning by 20 to 25 percent using the three rates with good weed control.

The reduced injury was in comparison to an all-field rate of two pounds per acre, the conventional rate of the preplant herbicide.

Varying the rate in the medium and heavy textured soil did not mitigate injury compared to the all-field rate. However, in the sandy areas, “variable rate worked very well. There was quite a reduction in injury to lettuce plants.”

Nolte has been working with growers on VRT fertilizer application. Using variable herbicide rates based on soil types was actually simpler.

Factors unchanged

“Working with soil texture, field conditions do not change, unlike an (aerial) fertilizer map which can change with plant growth,” said Nolte.

The soil texture map was created using 2.5-acre grids.

The pre-plant herbicide was applied three weeks prior to planting. Balan was applied in the morning and disked in that same afternoon.

The Spra-Coupe was equipped with GPS for guidance. The soil texture map was loaded into global position system, which was connected to a Viper controller that varied the rate of herbicide applied. The herbicide was mixed in the spray mix tank to reflect a 2-pounds per acre rate. The applied rate was varied by the controller by opening and or closing valves to achieve the three desired herbicide rates based on soil texture.

“The controller handles everything,” he said.

“The interface of GPS/GIS and variable rate controllers now allows us to micro manage rather than macro manage fields,” said Nolte.

“Bottom line is we have the potential for reducing the amount of Balan injury in lettuce in non-uniform soils. If your soils are fairly uniform, this will not help, but if have a high rate of variability it is something you should consider,” said Nolte.

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