Kellogg Co posted its sixth straight quarter of weaker net sales in its U.S. cereal business as demand for products such as Corn Flakes and Rice Krispies wane in its biggest market. The company also said sales in its U.S. snack business dropped by the most in seven quarters, highlighting how processed food makers such as Kellogg, ConAgra Foods Inc and General Mills Inc are struggling to compete against healthier foods and cheaper private-label brands.