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Corn+Soybean Digest

Prescribe Your Water | Variable-Rate Irrigation Applies Rates by Soil Type, Topography, Yield goals

Think Different   Precision irrigation complements other site-specific management practices, says Tim Schmeeckle, a tech-savvy farmer from Gothenburg, Neb., and a self-described early adopter. Schmeeckle has been yield mapping since the late 1990s. “Sometimes, I would wonder: Why am I doing this? It makes a nice map at the end of the year, but how do I use it?” Now, he says, all those layers of data he has collected over the years – yield maps, grid soil sampling, variable rate fertilizer maps – are “finally coming together in precision irrigation.” 


Tim Schmeeckle is learning to grow corn with less water. Precision-irrigation management is helping him and other farmers apply the right amount of water on every part of the field. Variable-rate irrigation (VRI) adjusts water application depth for differences in soil water-holding capacity, topography and yield potential. The goal is to boost crop production while conserving an expensive and increasingly scarce resource. “Even in a drought year,” Schmeeckle says, “I’m conserving water.”

Schmeeckle grows corn and soybeans on 2,000 irrigated acres near Gothenburg, Neb., in the Platte River Valley. He runs 11 center pivots, pumping water from the Ogallala Aquifer. “Water is our main yield-limiting factor,” says Schmeeckle, whose operation is three-quarters corn. He has no watering restrictions now, but he’s convinced that water allocations are on the way. “It’s not if, but when. I want to know how to raise the same amount of corn with less water.”

Interest in variable-rate irrigation is booming, says Tim Shaver, a soil scientist at the University of Nebraska West Central Extension and Research Center in North Platte. “We’re seeing big potential benefits in semi-arid regions where a lot of growers have water allocations. We think it’s a real promising technology.”


Mapping field variations

In 2011, Schmeeckle began working with CropMetrics, a variable-rate irrigation management company founded by North Bend, Neb., farmer Nick Emanuel.

The first step in VRI is mapping field variations. Variable fields offer the most opportunity for improving irrigation management, Emanuel says. “Try it first on your toughest field with the most soil variability, sandy ridges or waterlogged depressions.”

Many of Tim Schmeeckle’s fields vary in both topography and soil texture, ranging from fine sand soils, which can hold about 1 in./ft. of water, to heavier sandy loam and sandy clay loam soils, which can hold 1.4-2 in./ft. water. In fields such as these, “if you water the sandy soils right, you may flood out the better soils,” says Jake LaRue, research and development director for Valmont, maker of Valley Irrigation equipment. “But if you water the better soils right, the sandy soils dry up. So uniform application is not the answer.”

Using RTK elevation data and soil electromagnetic conductivity surveys of Schmeeckle’s fields, CropMetrics created a map of soil water-holding capacity and areas prone to runoff or waterlogging. This data was combined with aerial imagery and yield maps to generate a watering prescription, which was uploaded wirelessly to Schmeeckle’s Valley Pro2 control panel. Once the Rx is loaded, VRI is automatic, Schmeeckle adds – no babysitting required.

As the pivot rotates, the controller adjusts the base walk speed every few degrees. Water flow remains constant, while application depth increases or decreases with walk speed. On heavier soils or low spots prone to ponding, the pivot travels faster, applying less water. On lighter soils or droughty ridges, the pivot slows down, applying more water.

Each center pivot irrigation circle can be sliced into as many as 180 pie-shaped prescriptions. VRI speed control is “a low-cost way to match irrigation application depth to soil type,” Emanuel says. The software works with any electric-drive center pivot that has position-based speed control. Older pivots can be upgraded with GPS and wireless telemetry for $1,500-2,500, LaRue adds.


Evening out yield variations

Schmeeckle first tried VRI on a field that falls 45 ft. in a quarter mile, with sandy uplands and heavier-soiled lowlands. In a good year, the entire field would average around 175 bu./acre of corn, but the droughty tops rarely yielded more than 60-75 bu./acre. At the same time, over-watering was an issue in the lower half of the field, resulting in runoff and N loss, Schmeeckle says, not to mention wasted water and energy.

In 2011, Schmeeckle watered the field using a CropMetrics speed control Rx. His Valley irrigation system applied 0.60 in. water as a base rate on the majority soil type, dropped to 0.45 in. on the heavier soils and increased to 0.85 in. on the sandy ridges.

“That may not sound like a big variation,” says Shaver, the University of Nebraska scientist, “but on a 160-acre pivot over time, it adds up.” And in more arid farming regions like western Nebraska, Kansas and Texas, where irrigation is restricted, “every inch of supply is significant,” Emanuel says. “Optimizing water use helps them make the most of their water allocation.”

Schmeeckle’s field yielded 205 bu./acre last year, up 30 bu./acre from the historic average. And yields in the sandy pockets were 100 bu./acre higher than in 2009, he says. VRI “evened out yields across the whole field.”

In 2011, he used VRI management on three pivots. This year, he put another five on VRI. As the 2012 drought worsened, CropMetrics revised Schmeeckle’s watering prescriptions, narrowing the variations from the base rate.

At the end of the season, CropMetrics does a yield analysis to determine VRI benefits for each soil type and landscape position. “It takes a couple of years to fine tune the prescription,” Emanuel says.


Soil probe synergy

Schmeeckle combines precision watering with AquaSpy soil-water sensors, which monitor moisture and root activity in real time down to about 36 in. “The key to water-use efficiency is the soil-moisture probes plus variable rate irrigation working in conjunction,” Schmeeckle says, “because without the probes, you could still be overwatering.” Nebraska’s Shaver seconds that. “We think it’s important to use both together. The two technologies need to go hand in hand.”

In August 2011, the water sensors gave Schmeeckle confidence to turn off his pivots during a 105-degree heat wave, while his neighbors took their wells off load management and continued to water. “I could do that because I could see I had moisture at the 16-24-in. level. So I let the first 12 in. dry up and let the corn drink from the 24-in. level that week.”


Cost and payback

Schmeeckle pays $10/acre/year on a three-year contract for CropMetrics services, and $1,800-2,200/field/year for AquaSpy soil moisture probes and data service. His costs for pumping and pivot maintenance run $800 to $1,000/revolution. In 2011, he figures his new water management technology saved about seven pivot revolutions. In addition to water savings, VRI boosted his average 2011 corn yields by about 20 bu./acre, he says. “So I feel I’m coming out well ahead on my investment.”

In variable fields, the payback from VRI speed control can be very quick – one to three years, says Valmont’s LaRue. “But in some fields,” he cautions, “it doesn’t make 

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