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Variable-rate water

Given today’s technology that permits variable-rate seeding plus fertilizer and herbicide application, it would seem that there is only one variable left – water. That’s kind of the way Roric Paulman looks at it, too. 

Of the nearly 7,000 acres Paulman farms south of the Platte River near Sutherland, nearly 6,000 acres are devoted to white and yellow field corn, popcorn, sugar beets, and dry beans. Consequently, Paulman was among the first producers in the nation to test variable-rate center pivot irrigation via eight of the 40 pivots that cover Paulman Farms. 

As most every farmer knows, crop yields are often directly proportional to the most limiting growth factor. Even when water isn’t a limiting factor, its use is far from uniform due to variables in soil type, topography, water-holding capacity, water infiltration, and drainage rates. Paulman says that is particularly true in much of Nebraska, where the rolling terrain literally ruled out irrigation until center pivot systems came along.

“We’ve already implemented a number of tools to manage water usage, including soil moisture sensors, meters on the water pumps, and weather stations scattered around the farm,” Paulman says, noting that he already faces water-use restrictions on part of his farm. “Until now, we’ve never had a way to vary the amount of water we apply, short of adjusting the time it takes the pivot to make a circle.”

Paulman says one of the solutions has been an option from Valmont Industries in cooperation with CropMetrics. “Actually, they offer two different types of variable-rate systems,” he relates. “One is VRI Speed Control and the other is VRI Zone Control.”

The speed-control system, which was added to seven of Paulman’s existing pivots, allows the field to be divided into sectors, similar to pie slices, as narrow as 2° for a maximum total of 180 sectors. Water variability is controlled by simply varying the speed through those sectors via a preprogrammed prescription map. The zone-control system, which accounted for one new pivot on Paulman Farms, goes a step further by adding individual control of each sprinkler head, for up to 30 different VRI zones along the pivot span. When combined with speed control, zone control provides the potential for more than 5,000 unique management zones. 

“Unfortunately, we had a really bad hailstorm over most of the farm last August, which tempered the results,” Paulman says. “Even with the hail, we saw a few surprises. For example, we discovered some of the corn hybrids don’t like wet feet. And with the speed control systems, we were really able to pick those out.”

10-bushel bump with less water

In fact, he says they saw instances where they had a 10-bushel advantage with less water. “Instead of putting on 1 inch across the field,” he says, “we had areas where we cut the irrigation rate by as much as 20% to 30% in 30° slices around the pivot, based on soil type, slope, and past yields. Ironically, Monsanto has been seeing some of the same results at their research facility in Gothenburg.”

Paulman says he saw another benefit of variable-rate irrigation in dry beans that had been drilled in narrow rows under a VRI system. 

“Interestingly enough, variable-rate irrigation appears to be another management tool for controlling white mold,” he says. “By using the VRI in conjunction with moisture probes, we were able to keep the top layer of soil a little dryer so it didn’t accelerate the development of mold spores.”

Paulman says his goal for 2012 is to see how much difference VRI can make on yields and water usage across a variety of crops. 

“What we’re ultimately after is the ability to manage variability,” he says. “And with the moisture probes we have now, we’re able to see what is really happening below the surface as we vary the irrigation rate over different soils and terrain.” 

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