Chicago Board of Trade wheat futures climbed nearly 3 percent, led by outlooks for more hot and dry weather in the northern Plains spring wheat region.
As of 12:11 p.m. CDT (1711 GMT), CBOT September corn futures were up 8-1/2 cents at $4.01 per bushel, with new-crop December corn up 9 cents at $4.13-3/4 after reaching $4.15, its highest since June 2016.
CBOT August soybeans were up 25-1/4 cents at $10.26-1/4 after setting a four-month high at $10.29-1/2.
CBOT September wheat was up 15 cents at $5.50 a bushel while Minneapolis Grain Exchange (MGEX) September spring wheat was up 26 cents at $7.92-3/4 a bushel.
Corn and soybeans firmed on fears that rising temperatures this week and dry conditions in parts of the western Midwest could curb yield prospects. Corn in the heart of the Midwest is particularly vulnerable to weather stress this month during pollination, its key reproductive phase.
Based on historical trends, the US Department of Agriculture in May projected the 2017 US corn yield at 170.7 bushels per acre. But some analysts think the this season's yield ultimately could fall short.
"It is possible that we could stress the crop enough to produce a sub-160 bushel per acre crop," INTL FCStone chief commodities economist Arlan Suderman said in a note to clients.
The USDA is scheduled to release updated monthly supply/demand reports on Wednesday, although typically the government waits until August to revise its corn and soy yield forecasts.
Ahead of its weekly crop progress report due later on Monday, analysts expected the USDA to lower its condition ratings for corn, soybeans and spring wheat.
Traders also appeared to be covering short positions in corn and soy. Weekly commitments data from the US Commodity Futures Trading Commission on Friday showed speculators held larger-than-expected short positions, Suderman said, leaving those markets vulnerable to bouts short-covering.
MGEX spring wheat futures led CBOT wheat higher on forecasts for more sizzling temperatures in the northern US Plains.