January soybeans were down 2.75 cents at 9.8425 USA dollars per bushel as of 1610 GMT in the morning trading, December corn was down 0.75 cent at 3.4275 dollars, while December wheat was down 7.25 cents at 4.2425 dollars.
The most active wheat futures were up 0.2 percent at $4.30-1/2 a bushel, having closed up 0.6 percent on Friday. The CBOT December contract stayed inside of Friday's trading range but held above last week's contract low of $3.40-3/4.
The CME Group reported 137 deliveries against CBOT November soybean futures, which expire on Tuesday. The Agriculture Department's latest estimates put South Dakota's corn crop at 788 million bushels, down 5 percent from past year. This pushed up in advance the soybean prices while dragging down the corn futures earlier in the week.
Another record-high yield in the USDA November report pulled corn futures further to new lows, but this only lasted for one session and massive technical buying pushed up the prices again.
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In its weekly Crop Progress Report Monday, the USDA pegged the US corn harvest at 83% complete, compared with 70% a week ago and a 91% five-year average.
As for wheat, short-covering boosted its prices. The U.S. Department of Agriculture last week trimmed its forecast of global 2017/18 wheat ending stocks, but the figure, at 267.53 million tonnes, remains at a record high. In response, the Soybean futures took back nearly all of the early week gainers and closed the trading week fractionally higher.
Still, the January soybean contracts are above the 200-day, 100-day and 50-day moving averages, which will provide support to the market.