The degree to which a few large firms dominate total sales within segments of the meat packing industry, which, some farmers and other critics contend, can cause or at least contribute to lower prices for their animals. Market control by five large packers in the early 1900s led to passage of the Packers and Stockyards Act of 1921. Concentration declined after that, but has increased sharply in more recent years. For example, the four largest firms accounted for 80% of the steer and heifer slaughter in 1997, compared with 36% in 1980. Four-firm concentration in hog slaughter increased to 54% in 1997 compared with 34% in 1980, according to USDA. Numerous government-sponsored studies and investigations have been inconclusive on the relationship in recent years between concentration and prices.