Chicago Board of Trade corn and soyabean futures tumbled early on Friday reacting to the government's huge US crop estimates released before the open, traders said. December corn hit a contract low of $1.95-1/2 early, while CBOT soyabeans fell 5 to 6 cents. December corn was down 1-1/2 cents at $1.95-3/4 by 9:34 am CST and January soya was 7-1/4 weaker at $5.12-1/2.