Nearby Chicago wheat futures jumped three per cent on Wednesday and hit a four-month high on worries about dry conditions threatening yield prospects in the southern U.S. Plains. Corn futures rose on robust export demand for U.S. supplies while soybean futures declined after a choppy session. Chicago Board of Trade March wheat settled up 14-1/4 cents at $4.60-1/2 per bushel after reaching $4.61-3/4, the highest spot price on a continuous chart since Sept. 28