Agriculture producer confidence in the U.S. has fallen to its lowest level in four years, according to a survey released Monday.

The confidence rating, which includes livestock and crop farmers, dropped from 106.9 in March to 99.8 in August, marking the first time the overall value has fallen below 100 since the survey began in 2010. Values of 100 are considered neutral, while those above 100 are considered optimistic, and those below 100 signal pessimism.

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Leading the decline are crop producers. Their confidence level dipped to 96.3 in August from the previous low of 102.7 set this past March. The lower number is due to falling crop prices, which have been in a free fall for months, said Katie Micik, DTN markets editor and director of the confidence index.

"Prices are really an important factor in the psychology of farming," said Micik in a phone call with CNBC. "Corn future prices have fallen from about $8 a bushel in 2012 to the upper $3 range now."

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"It's not a crisis, but crop farmers are pretty stressed out," Micik explained. "Their profit margins are pretty tight."