Signs of easing US-China trade tensions also buoyed the market after confirmation on Monday of more Chinese purchases of US soybeans that triggered a rally last week.
Wheat firmed in tandem with rising corn and as dry conditions in some Southern Hemisphere wheat areas stoked concerns about smaller harvests in key exporters such as Australia and Argentina.
Grain prices were broadly supported by Monday's crude oil rally of about 12% after weekend attacks on Saudi Arabian oil facilities.
The market drew additional support from a tentative White House plan to increase the amount of biofuels that oil refiners are required to blend each year to compensate for exemptions handed out to small refiners. Chicago Board of Trade December corn rose 3-3/4 cents to $3.72-1/2 a bushel by 1155 CDT (1655 GMT), having earlier touched its highest since Aug. 30.
November soybeans touched a 1-1/2 month high of $9.04-3/4 a bushel before slipping to $8.97, down 1-3/4 cents.
CBOT December wheat jumped 5 cents to $4.88-1/2 a bushel. The US Department of Agriculture (USDA) on Monday confirmed private sales of 256,000 tonnes of US soybeans to China in what traders said was a further confirmation of sales of at least 600,000 tonnes reported by Reuters last week. Grain traders also focused on yield prospects in the run-up to the US harvest and forecasts for mild Midwest weather. The improved weather raised expectations that crops planted very late in a soggy spring could still achieve decent yields.
Grain markets will receive a further indication of field conditions later on Monday from the USDA's weekly crop report, due for release after the market closes.