Farmland boom alters 40-year Iowa corn yield model

In response to surging Midwest farmland prices, researchers in Iowa are overhauling a decades-old formula used to predict corn yields, which will allow farmers and tax officials to more accurately value property.

The corn suitability rating, or CSR, has long been a Midwestern benchmark to forecast how productive an Iowa farm will be if corn is planted on it.

Since 1971, CSR has been used to set cash-rent rates, help farmers gauge how much to spend on fertilizer and other inputs, and figure out how agricultural land is valued for property tax purposes.

Changes in the formula, which is expected to be released later this year, could raise or lower the value of the state's richest farmland by as much as $2,000 per acre.

The updated equation, dubbed CSR2, will allow farmers, appraisers and others to better understand why some parcels of land perform better than others, said C. Lee Burras, the Iowa State University agronomist who is leading the current effort to further refine the more than four-decade-old model.

"People felt that it was time for an update," said Burras, who is recalibrating the model with researchers from U.S. Department of Agriculture's Natural Resources Conservation Service in Des Moines. "The goal is to create a rating system that anyone can use."

The model predicts how productive a field will be on average over a 10-year period in Iowa, where farmland prices have hit record highs, rising nearly 72 percent over the past five years.

Among other elements, CSR2 weighs topography, the composition of organic matter in the soil, how sandy or clay-rich the soil is and microclimate and weather patterns.

Land is then given a numeric rating from five to 100, with 100 being best. For the best soils, each point roughly translates into $200 per acre or more in land value, according to Burras.

The new formula is able to get a more localized estimate of crop yields, because it takes into account more variables in the soil, said Burras.

In the past, CSR ratings generally were assigned on a state-wide or county-wide level. However, like the old formula, farmers who cross county lines may be vexed, as some CSR2 ratings change from one county to the next.

The older ratings, too, were altered to fit the researchers' knowledge of local farming practices -- a human factor that has made it difficult for modern-day farmers and investors to replicate the researchers' math.

The current formula will take out that human element, Burras said. "The goal is to create a rating system that anyone can use to calculate their own CSR," he said.

So far, Iowa tax officials anticipate seeing only subtle changes on the statewide and county-wide ratings, said Julie Roisen, Property Tax Administrator for the state of Iowa.

On the local or individual parcel level, however, "that's where we're expecting the value may change," Roisen said.

Burras said that CSR2's early findings have shown minor differences with the original CSR of between two to 10 points in much of Iowa.

Sizeable theoretical variances - up to 70 points -- have been found near urban centers. Those lands were never covered by the original rating system, Burras said.

Though Iowa is considered a leader in agricultural circles for this sort of research, several Midwestern states with land-grant universities have developed similar crop productivity indexes.

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