CBOT December corn settled up 2-1/2 cents at $3.61 per bushel after reaching $3.62, its highest since Aug. 23.
The USDA late Tuesday said 12 percent of the US corn crop was mature, behind the five-year average of 18 percent. The slow maturity could delay the harvest, which should begin this month.
The USDA also rated 61 percent of the US corn crop in good to excellent condition, down from 62 percent the previous week. Analysts had expected no change.
Separately, the USDA said private exporters sold 253,300 tonnes of US corn to Mexico for delivery in the 2017/18 marketing year that began on Sept. 1, the second such sale to Mexico in as many days.
Traders continue to monitor forecasts for the path of Hurricane Irma, which could bring flooding rain to crop areas in the US Southeast.
The CBOT reported 215 deliveries against CBOT September corn futures.