Corn tumbled as traders looked past potential crop stress from the building heat dome to a U.S. Department of Agriculture report due Tuesday that’s expected to show the largest American stockpile since 1988.

The agency is set to publish quarterly stocks data along with annual acreage data showing how much U.S. farmers were able to plant in the spring. While growers pared back on corn planting in favor of soybeans, existing supplies of the yellow grain are seen 17% higher than a year ago after a record autumn harvest, according to a Bloomberg survey of analysts.

The excess supply has contributed to a bearish sentiment on corn with the growing season off to a relatively smooth start prior to the arrival of warmer-than-normal temperatures this week. Still, even with hot conditions that are boosting energy demand, there still could be precipitation to benefit fields.

Corn for December delivery fell as much as 2.5%, the biggest intraday decline in about six weeks, to a fresh contract low of $4.305 a bushel.

July is considered the most crucial month of the U.S. corn crop’s development. USDA will also publish weekly condition rates later Monday.

“While weather is hot, there are chances for rain,” Naomi Blohm, grain analyst at Total Farm Marketing, said in a note.

The heat is “helping to ease pockets of excessive wetness and are promoting a rapid pace of corn and soybean development,” USDA said in a weather outlook. “However, heat has begun to reach stressful levels for some summer crops in the western Corn Belt, where today’s high temperatures will approach or reach 100 degrees Fahrenheit as far north as southwestern Minnesota.”

Soybeans and wheat futures also slumped in Chicago.

Traders are still pondering how much China might buy after the White House in May said the Asia nation committed to $17 billion in new agriculture purchases beyond a vow to bring in 25 million tons of U.S. soybeans annually.

The USDA said exporters sold 136,000 tons of U.S. soybeans to unknown destinations but the agency disclosed no deals to China.

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—With assistance from Dominic Carey, Eleanor Thornber and Abigail Tobias.

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