Keith Alverson and his family started to variable-rate plant in the 1990s, and now find benefit with the practice on every corn acre. He, his father Ron and uncle grow corn and soybeans on rolling land near Chester, S.D. In the early 1990s, they started reducing seeding rates manually in the dry corners of pivot-irrigated fields, where yield potential is always much lower. The practice cut seed costs by 25% in unwatered sections, and was especially beneficial in dry years, Alverson says.

Eventually, they extended the practice inside their irrigated circles and then to non-irrigated fields, where soils are characterized by bands of gravel glacial outwash and eroded clay knobs that lend themselves to variable-rate planting.

Now, they use it on every corn acre, Alverson says. “We’ve found it pays off in good years and bad years. It not only reduces seed costs, but makes us money by reducing lost yield and placing seeds where they need to be.”

There’s a lot of interest in variable-rate seeding (VRS) for corn, which tries to match planting rates and yield potential, says Gregg Carlson, South Dakota State University (SDSU) plant scientist. “The concept makes good sense to producers.”

Yield maps clearly show significant spatial variability within fields. Valuable seeds and crops boost potential returns from VRS. And many farmers already own software that can generate prescription maps and planters that can do variable-rate seeding on-the-go.

In addition, plant population studies across the Corn Belt have shown the value of higher seeding rates in high-yield environments and lower rates in less-productive soils, such as droughty hilltops, says Kurt Reitsma, SDSU Extension precision agriculture specialist. “For example, in South Dakota, our main yield-limiting factor is rainfall, and variable-rate seeding is one way to manage water.”

Yet, practical questions abound about VRS, says Pat Reeg, technology manager for the Iowa Soybean Association On-Farm Network: What’s the best way to delineate consistent VRS management zones? What seeding rates should you use? Does VRS increase yield, cut costs and raise profits?

“Variable-rate planting has the potential to do all of these things,” Reeg says, “but research demonstrates that profits can also be reduced by adjusting seeding rates. Predicting where and by how much you should vary seeding rates is challenging and may not be repeatable from one year to the next.” (His study is at http://bit.ly/ZMC055)

Bob Gunzenhauser, mapping services manager at DuPont Pioneer, agrees: “There are opportunities available for variable rate seeding, but also limitations and confounding issues.”

Carlson adds that these issues make variable-rate planting one of the most significant management topics for producers today.

Match inputs to yield potential

“Variable-rate seeding is one of the fastest-growing segments of our precision business,” says Agronomist Norman Mieth, Advanced Cropping Systems, the precision farming division of Central Valley Ag Cooperative, Wayne, Neb. He says co-op customers have about 80,000 acres of corn under variable-rate seeding management.

Over the past eight years, Advanced Cropping Systems has demonstrated the value of matching seeding rates to yield potential, Mieth says. “We started on an irrigated field that had a lot of variability in soils, ranging from heavy gumbo clay to sand.”

The sandy areas produced poorly every year, despite adequate fertility, he says. “If those areas never make over 100 bu./acre, why do we need inputs for 200 bu./acre?”

When Mieth dropped the seeding rate from 34,000 to 24,000 in those sandy pockets, he saved $30/acre in seed costs. “Yield jumped from 100 bu./acre to 170 bu./acre, because of less competition for water,” he adds.

The first step in variable-rate planting is delineating management zones based on crop yield potential. That can be quite a challenge, Reeg says. “The highest yielding area of the field one year may be the lowest the next, due to varying amounts of rainfall.”

For example, in a four-year study of 20 fields in north-central Iowa, only 40% of the high-yielding areas in Year 4 matched those in Year 3, according to a 2011 report by Pioneer’s Gunzenhauser. When yield data from years 1, 2, and 3 were combined, a little over half of the high-yielding area matched Year 4’s.

In 146 Iowa On-Farm Network fixed-rate corn population trials from 2009 to 2011, “Yield response from the population treatments did not have a high correlation to previous yield, soil map units, corn suitability ratings, soil conductivity, or bare soil reflectance, which are commonly used to generate management zones for variable-rate seeding, Reeg says” Growers should be especially cautious about using soil survey maps as the basis for VRS zones, he adds.

Work around inconsistencies

In an effort to deal with these inconsistencies, SDSU’s Carlson is developing variable-rate seeding algorithms based on long-term average yields and the degree of yield variation within the field. He says growers should think of productivity potential as a continuum, rather than as discrete zones.

At Advanced Cropping Systems, Mieth combines yield maps and other layers of information, such as aerial imagery, topography and slope, to estimate yield potential. In eastern Nebraska, for example, “It’s almost inevitable that south- and southwest-facing slopes have a lower yield potential than southeast-, northwest- or north-facing slopes. We process all the data available on the field, and let that determine productivity potential on a 5-meter resolution.”

Despite the high-tech analysis, though, there’s a great deal of subjective knowledge that goes into defining these zones, Mieth says, especially in areas with inconsistent yields. “We sit down and discuss what the grower knows about the field. There’s a lot of agronomic judgment involved.”

Farmer Alverson agrees. Until just a few years ago, the family varied seeding rates by an educated guess. “It was all visual, based on the slope of the ground, the color of the soil, and our knowledge of the field. It was an art.”

They’ve gone digital now, adjusting planting rates from 24,000 to 36,000 seeds/acre on pretty much every planter pass, Alverson says. Still, he calls their GPS-based productivity zones, which are delineated from yield and topography maps, a work in progress. “They’re not as accurate as what we can see out in the field. The challenge is getting what’s in our eyes and heads into our prescription maps.”

Crop consultant Kim Retzlaff, Applied Agronomics, Aberdeen, S.D., sorts through 80-100 satellite images to locate high- and low-potential areas within fields. He says satellite imagery is often a better indicator of productivity differences than yield monitor data, which can be misleading if not collected and filtered correctly.

Fields that show little yield variation will not be good candidates for VRS, adds Joel Wipperfurth, Winfield agronomist in Owatonna, Minn. “In general, you need to see more than 5% yield variability.” Look for areas of the field that yield consistently above or below average, he says.

How to adjust seeding rates

Allen Walter, a farmer from Groton, S.D., has used VRS for three years. He starts with an average planting rate of 33,000 seeds/acre, bumps it up by 3,000 in above-average areas of the field, and drops it by 3,000 in below-average areas, which tend to be wet. But he admits that when it comes to assigning seeding rates to different management zones, “I’m just kind of winging it.”

Reeg suggests that, before you adjust seeding rates, consider both the cost of the seed and the expected yield differences. “Raising or lowering seeding rates can impact the bottom line, and not always in a positive way.”  As a first step, he suggests planting two different uniform population rates in alternating strips across the field. “This can help you identify if there’s enough difference in yield response to justify changing rates as you go across the field.”

At Advanced Cropping Systems, Mieth says they’ve developed their own algorithm for yield response to population across environments. A typical seeding prescription calls for eight to 16 different rates in a field. However, a good part of what goes into the Rx are the agronomist’s and grower’s experiences, he adds. “We work with each grower to determine what he’s comfortable with, as far as minimum and maximum seeding rates.” Because population response is influenced by the entire cropping system, “we take into account hybrid, tillage, fertility, rotation – everything.”

Advanced Cropping Systems’ strip trials suggest a 7- to 8-bu. average yield increase over the whole field using VRS, Mieth says, and sometimes more. He points to one example where the grower was seeding corn at a low, uniform rate of 22,000 plants/acre because the field had a lot of sandy pockets. Yet, that low rate was still too thick for the bad areas, where plants were often stunted, and not thick enough for the good areas, he says.

“So we cut rates to 15,000 seeds/acre in the poor areas, and in good areas, we increased from 22,000 to 28,000. The whole-field average yield came up by 15 bu./acre,” he says. The whole-field average seeding rate also went up by about 2,000 seeds/acre, or about $6/acre. “Population was allocated to where it would get the best return.”

On his South Dakota farm, Walter says it has been hard to nail down the return from VRS. “To put a value on it – to say I gained 3 or 5 bu./acre – I can’t do that. But I already had the equipment, software and zone maps to do variable-rate seeding. So I’m not investing any additional money,” he says. “I see only upside potential.”