The average price of Iowa farmland slumped 8.9 percent in 2014, the largest annual decline since 1986 as crop prices fell, according to the Iowa Land Value Survey released on Thursday.

It is only the second year since 1999 that the benchmark survey, conducted in November by Iowa State University researchers, has shown a decline in farmland values. The findings are similar to those of the Realtors Land Institute and the Federal Reserve Bank of Chicago.

The value of farmland in Iowa, typically the top U.S. corn and soybean growing state, is closely tracked by farm economists, bankers, the Federal Reserve, farmers and investors as a gauge for the health of the farm and rural economy.

The average price of Iowa farmland fell to $7,943 an acre in 2014, after hitting a record peak at $8,716 in 2013, the survey said.

Values have more than doubled in 10 years and were up 18 percent just since 2011 given the historic boom in land and grain prices driven by demand for biofuels and food exports.

"We had a reality check that $6 corn wasn't what we were going to settle in at and the decrease in land values is just a reflection of that," said Michael Duffy, a retired Iowa State University farm economist who conducted the survey.

Corn prices fell to $3 a bushel during fall harvest, down sharply from the summer of 2013 when corn was $6 to $7.

The Iowa State survey, begun in 1941, is based on transaction reports from licensed real estate brokers and selected individuals considered knowledgeable on rural land market conditions. The 2014 survey is based on 608 county land value estimates from Iowa's 99 counties.

Farmland remains the basic collateral for farmer loans. Economists have worried that a farmland "bubble" may pop as it did in the 1980s, hurting a rural economy which has been one of healthiest sectors of the U.S. economy.

"A lot of people are concerned that we're heading into big troubles. I don't share that pessimism," Duffy told Reuters.

"The lower grain prices are going to cause pain and everyone is going to have to adjust. But because we have so much equity left I think most people will be able to refinance or readjust," Duffy added.

Northwest Iowa reported the highest average values at $9,615; the lowest average was in South Central Iowa at $4,475.