CHICAGO (AP) _ Grain futures were mixed in early trading Monday on the Chicago Board of Trade. Sept. corn was unchanged at $5.12 a bushel. Sept. wheat was unchanged at $7.67 a bushel. Sept. oats was down 14 cents at $3.35 a bushel. Sept. soybeans fell by 1.75 cents at $12.74 a bushel. Beef was...