U.S. farmers plan to boost their corn seedings by 6.4 percent in 2016, above even than the highest estimate, and dial back soy plantings, the U.S. Agriculture Department said on Thursday, pushing futures in the yellow grain to contract lows.

The Chicago Board of Trade December corn futures contract that tracks the crop that will be seeded in the coming weeks sank to an all-time low. The most actively traded corn contract dropped 3.5 percent to its lowest in 2-1/2 months.

"The corn acreage was clearly the outlier, way over the bell curve of expectations, and corn is reacting to it," said Charlie Sernatinger, global head of grain futures at ED&F Man Capital.

Corn acreage was seen at a bigger-than-expected 93.601 million, which would be the third-highest level since 1944, in the government's closely watched prospective plantings report. USDA also said corn stocks as of March 1 stood at 7.808 billion bushels, the second most on record and up from 7.750 billion bushels a year earlier.

The corn acreage outlook topped analysts' expectations, which ranged from 89.000 million to 91.000 million, with an average of 89.972 million, according to a Reuters poll. In 2015, U.S. farmers seeded 87.999 million acres of corn.

Analysts on average had expected corn stocks of 7.801 billion bushels.

The supply glut has weighed on corn prices for months and is already threatening the profitability of the crop even before most farmers begin running their tractors through fields.

USDA said farmers planned to seed 82.236 million acres of soybeans, which would be the third-highest recorded. That compares with 82.650 million a year ago and an average analyst estimate of 83.057 million.

Soybean stocks as of March 1 stood at 1.531 billion bushels compared to 1.327 billion a year earlier.

USDA said all-wheat seedings would be a smaller-than-expected 49.559 million acres, with the outlook for all classes except for durum falling below the low end of trade forecasts. Winter wheat acreage was pegged at 36.216 million, spring wheat other than durum acreage at 11.348 million and durum acreage at 1.995 million.

If realized, the other spring wheat acreage would be the lowest since 1972.

Wheat stocks as of March 1 were 1.372 billion bushels, a five-year high and up from 1.140 billion a year earlier. Analysts on average had expected quarterly wheat stocks of 1.356 billion bushels.