Soybeans reversed early gains to close back below $12 a bushel after Informa Economics forecast that US farmers had planted nearly 3m acres more of the crop than current official estimates.
The analysis group pegged plantings at 78.9m acres, compared with a current US Department of Agriculture figure of 76m acres, traders said.
That implies a potential extra 124m bushels (3.4m tonnes) of soybean harvest at the yield of 42.6 bushels an acre that the USDA is predicting this year.
Chicago's benchmark July soybean contract, which had topped $12.25 a bushel earlier, ended at 11.79 a bushel, down 2.9% on the day, with new crop contracts showing similar declines.
While Agrimoney.com was unable to obtain a copy of the report, many analysts have previously suggested that delays to corn and spring wheat plantings, coupled with buoyant soybean prices, might tempt farmers to switch to beans.
Indeed, Informa forecast that US corn sowings would come in at 83.1m acres, 1.9m acres below USDA forecasts, with spring wheat plantings pegged in line at 13.4m acres.
"Informa's numbers were considered bullish corn, bearish beans and neutral spring wheat," Vic Lespinasse at GrainAnalyst.com said.
However, other Chicago crops found it difficult to make much headway against such a weak soybean market. which has been supported in recent weeks by the prospect of US stocks falling to their lowest since 1977.
July corn ended 4.0 cents lower at $3.99 1/2 a bushel, its first close below $4 a bushel for six weeks, with wheat finishing down 5.0 cents at $5.55 1/2 a bushel.
The USDA will on June 30 release updated estimates for crop plantings.