A large percentage of crops are grown as feed for livestock. Grains, grasses, legumes, and other plants not desirable for human consumption are ideal feeds for cattle, horses, sheep and other animals. When animal eat hay or pasture grass, the ingestion of air and coarse material causes bloating. Therefore, farmers prefer to keep feed material in silos to allow it to ferment into silage, a more palatable and nutritious substance. For example, sorghums, which are poisonous when directly consumed, must be fermented. Silo storage also provides feed through months of cold or drought. The first improvement in the production of fodder--straw chopped small enough for livestock consumption--came with the invention of mechanical straw, or chaff, cutters during the 1800s. Perhaps the first of these was the Hotchkiss guillotine straw cutter of 1808. The first silo was built in the United States in 1873. By the 1890s, most farmers, especially dairymen, had silos on their farms, since high quality feed increased the quality and quantity of milk production. Originally constructed of wood, which was not very air or water tight, silos were later made of stone, brick, or concrete. Corrugated metal has also become a popular building material for silos and other types of storage facilities. In the absence of silos, some farmers store fodder in water-tight pits. The ensilage chopper, which combined silage cutting with the corn picking process, was developed in 1915. An improved version was introduced in 1928. Another machine used for silage production is the tedder, developed in the 1870s, which fluffed hay for drying prior to storage. This was an important development because hay, when stored wet, rotted in the silo, making it unusable. By 1914 the side delivery rake became the preferred method for drying hay. Today, 90 percent of silage is chopped in the field. The principle silage crop of the last half of the twentieth century is corn, making up three-fourths of the total in the United States.