Chicago Board of Trade July corn ended the day down 6-3/4 cents, at $3.50-1/2 per bushel.
Traders said that the market has already factored in weather into corn pricing, though three traders said they were keep a close eye on forecasts of continued rainfall in parts of Michigan, Wisconsin and Illinois.
Crop-friendly weather in the US Midwest has weighed on prices of soybeans and also corn for weeks, traders said.
"This market is news driven right now, rather than weather," one trader said. "I saw the tweet from the President over the weekend and thought, 'There goes the grains market on Monday."
On Sunday, US President Donald Trump tweeted: "The United States is insisting that all countries that have placed artificial Trade Barriers and Tariffs on goods going into their country, remove those Barriers & Tariffs or be met with more than Reciprocity by the USA. Trade must be fair and no longer a one way street!"
Corn prices - and the grains sector as a whole - also felt pressure from the wider financial markets, where US stocks sank Monday in a broad sell-off.
The S&P 500 dropped more than 1.5 percent Monday, with technology firms bearing the brunt of an escalating trade dispute between the United States and other leading economies.
The CBOE Volatility index, known as Wall Street's fear gauge, spiked to 17.92, highest in two months.