Farmland prices in Iowa, the top U.S. corn and soybean state, are down by an average 7.6 percent over the past six months and 11 percent from a year ago, pressured by lower grain prices and firmer interest rates, according to a survey released Tuesday.

The report by the Iowa Realtors Land Institute said that the nine Iowa crop-reporting districts all showed a decrease in value, ranging from 4.6 percent in the southwest to 11 percent in the northeast, since September 2014.

Farm economists, farmers, bankers and the Federal Reserve keep a close eye on Iowa, a bellwether for the U.S. farm economy.

The twice-yearly survey, carried out in March and September, said factors contributing to current farmland values include lower commodity prices and a limited amount of land on the market, as well as lack of stable alternative investments and increasing interest rates.

Highest prices for top-quality crop land averaged $11,619 an acre in the northwestern district of Iowa, down from $12,385 six months earlier. Lowest prices for high quality grain acreage were in the south central district at $7,775, down from $8,267. Highest pasture prices were in the southwest district at $3,400, up from $3,394. Pasture land prices ticked higher in all but two districts, buoyed by livestock farm demand.

U.S. farmland values have set records in recent years following the strength in grain prices amid short supplies and the booming demand for biofuels and food exports.

Economists and bankers have been expecting land prices to hold at current levels or soften some but not burst a farmland bubble similar to what happened in the 1980s, when over-leveraged farmers lost their land. Crop farmers are in a much healthier cash and debt position now than they were 30 years ago.

The Iowa realtors farmland survey has been conducted since 1978 and is compiled with information from farm real estate brokers, lenders and others with knowledge of land prices.