“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.