Corn buyers and sellers have their eyes on the skies, because if rain doesn't start falling soon, prices could be on the rise.

Monday's rally for America's biggest crop highlighted the risk.

Corn for July delivery climbed 3.5%, but the more telling move was the 5.5% leap, to $5.34 per bushel, in the contract to deliver corn in December -- after the crop currently growing in the fields is due to be harvested.

Prices for December corn had come down sharply in recent months, as it became clear that America's farmers were planning to sow a great number of acres with corn seed this year.

Now, however, attention has shifted to whether those seeds produce a vast amount of corn, and to do that, they need rain as they enter the crucial pollination phase.

That's especially true given that the government is forecasting that farmers will produce 166 bushels per acre, which would be a record. If unfavorable weather cuts into that significantly, a lot of corn could disappear.

Even if moisture levels aren't ideal this season, the current crop may still turn out to be huge. But Monday's price increase suggests the crop may need to be nearly as big as originally expected in order to prevent prices rising from here.

-For continuously updated news from The Wall Street Journal, see WSJ.com at http://wsj.com.

(END) Dow Jones Newswires

June 18, 2012 16:35 ET (20:35 GMT)