The United States is the world's biggest corn supplier and the No. 2 exporter of soyabeans and wheat. The drop in soyabean acreage, if realized, would follow two years of record-high US plantings of the oilseed, while corn acreage would be the largest since 2016.
Expanding global soya stocks will likely discourage soyabean plantings, Robert Johansson, the USDA's chief economist, said at an annual outlook conference.
The USDA has projected that US soyabean supplies at the end of the 2018-19 marketing year will balloon a record 910 million bushels, doubling from a year earlier, due to rising South American soya output and a US trade dispute with China, the world's top soya importer.
"(US soyabean) sales to the EU, Egypt, Argentina, and many others are up this year. ... However, sales to other countries have not been enough to make up for the lost exports to China," Johansson said.
Nonetheless, he added, "the decline in soyabean acres is tempered by current forward pricing opportunities." New-crop November soyabean futures on the Chicago Board of Trade settled Wednesday at $9.48-1/2 per bushel, about 2.4 times the price of new-crop December corn futures at $3.98-1/2.
Johansson noted that the soya-to-corn price ratio is the lowest for this time of the season in three years, but still higher than it was in 2015 and 2016.
The USDA projected total US wheat plantings for 2019 at 47.0 million acres, down 1.7 percent from a year earlier. Johansson attributed the decline in wheat to wet weather this past autumn that inhibited winter wheat seeding.
"Winter wheat planting conditions proved unfavorable across several states," Johansson said, adding that plantings of spring wheat, typically seeded in April, could expand as the contraction in winter wheat seedings is expected to support wheat prices.
A Reuters poll of analysts estimated that corn plantings would rise to 91.5 million acres and soyabean seedings would drop to 86.1 million acres. The poll put US wheat plantings at 47.2 million acres.