U.S. farm incomes are expected to fall 13.8% this year to the lowest level in four years, the U.S. Department of Agriculture said, as expectations of a second straight bumper harvest push down prices for key crops.

Federal forecasters projected net farm income would slide to $113.2 billion this year from an estimated $131.3 billion in 2013, which was the highest level since 1973 on an inflation-adjusted basis.

The latest projected decline is significantly narrower than the 27% drop in 2014 net farm income that the USDA forecast in February. The change is due largely to 

an improved outlook for livestock farmers, who are benefiting from record prices for beef and pork as well as low prices for corn and other grains used in animal feed. The USDA on Tuesday said it expects livestock farmers' receipts to soar 15% this year, compared with a projection of a 0.7% increase in February.

Still, the overall projected income figure would be the lowest for U.S. farmers since 2010, and highlights a sharp turn in the fortunes of the Farm Belt after years of prosperity. Expectations for a bumper harvest have caused prices of corn, the nation's biggest crop, to fall 15% this year to near four-year lows, after plunging 40% last year. Soybeans, the No. 2 U.S. crop, also are trading near four-year lows.

"I'm certainly expecting my income to be down this year," said Shane Hanna, who farms 1,400 acres with his father in northwest Indiana, and noted that he and his father could see their largest corn crop ever this year. "We enjoyed the good times while they were happening but in back of my mind there's been a voice saying this can't go on forever."

Two years ago, a severe drought parched farmland and curtailed crop yields, sending corn prices to record highs. Growing demand for the grain from the ethanol industry and foreign countries such as China also fueled a run-up in farm profits, with net income almost doubling from 2006 through 2011.

Now abundant rainfall and cooler temperatures in the Midwest have paved the way for bumper crops for a second straight year. Earlier in August, the USDA estimated the nation's corn crop will reach 14.03 billion bushels in 2014, surpassing last year's record of 13.93 billion. Government forecasters also estimated the soybean crop would be by far the largest in history.

But with such expectations battering prices, the USDA on Tuesday projected that the annual value of U.S. crop production would slip 10.6% this year, including a drop of $12.8 billion in corn receipts.

Lower grain prices have been good news for livestock farmers who feed corn, soybeans and other ingredients to their animals. They also have benefited from soaring prices for meat-thanks to reduced supply because disease and drought in some regions have shrunk herds-and for milk, thanks to strong demand abroad. Government forecasters expect a rise of 21% in dairy receipts in 2014, as well as increases of 20% for hogs and 15% for cattle.

For grain producers, weak incomes are likely to persist through at least 2015, said Michael Boehlje, an agricultural economist at Purdue University. "Incomes will be weaker than we might have thought for the crop operation because prices are down much lower than anyone had anticipated," he said.

The USDA predicted a 4% rise in production expenses for farmers, with increased costs for seeds, fertilizer and pesticides. That marks the fifth consecutive year of increased costs, to the highest level on record.

The combination of lower prices and high costs has some farmers worried that they could see negative returns for the first time in years. Mr. Hanna said he and his father are carefully assessing the value of items such as fungicide and insecticide before making purchases for the next growing season.

"We'll still be in the black this year," he said. "But farming is a cyclical industry and a year like this is a real caution for us."