The nutritional requirements of cattle are usually divided into maintenance requirements, which depend on the cow's weight; and milk production requirements, which in turn depend on the volume of milk the cow is producing. The nutritional contents of each available feed are used to formulate a diet that meets all nutritional needs in the most cost effective way. Notably, cattle must be fed a diet high in fiber to maintain a proper environment for the rumen microbes. Farmers typically grow their own forage for their cattle. Crops grown may include corn, alfalfa, timothy, wheat, oats, sorghum and clover. These plants are often processed after harvest to preserve or improve nutrient value and prevent spoiling. Corn, alfalfa, wheat, oats, and sorghum crops are often anaerobically fermented to create silage. Many crops such as alfalfa, timothy, oats, and clover are allowed to dry in the field after cutting before being baled into hay.