The U.S. Department of Agriculture (USDA) said 30% of the corn crop has been harvested, compared with 22% a week ago, but below the average pace of 47%.
U.S. soybean harvest stood at 46%, up from 26% a week ago, but lower than the average pace of 64%, the agency said after the market closed on Monday.
Lower yields for both corn and soybeans are supporting prices.
Six more U.S. Crop Watch corn and soybean fields were harvested in the latest week, the busiest one yet for the participating growers, and the results reduced the overall yield scores for both crops, Karen Braun, a market analyst for Reuters, wrote in a column.
"Particularly for corn, it appears that largely favourable weather during the growing season could not outweigh some of the earlier problems with moisture during and just after planting," Braun wrote.
Snowfall and freezing temperatures in northern U.S. states have raised additional risks.
The USDA has said it will collect extra data on corn and soybean acreage in Minnesota and North Dakota following the recent snow.
"The harvest is progressing now, which is good but not great if you look at the historic pace," said one Singapore-based trader. "We will have an idea about the size of the crop in a few weeks time."
Brisk weekly U.S. export sales reported on Friday are supporting the soybean market.
The wheat market has been underpinned by expectations of lower production in the Southern Hemisphere.
Australia's wheat production is expected to drop to 15.5 million tonnes, National Australia Bank said on Friday as it cut its forecast to well below recent market estimates.
In Argentina, recent rains have not been strong enough to help wheat fields after weeks of dryness, the Buenos Aires Grains Exchange said on Thursday.
Commodity funds were net sellers of CBOT wheat, corn, soybean and soymeal futures contracts on Monday and net buyers of soyoil futures, traders said.
(Reporting by Naveen Thukral; Editing by Subhranshu Sahu and Shounak Dasgupta)
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