The major revenue-generating sectors in the economy of Oklahoma are agriculture, industry, and energy. Oklahoma ranks in the top five states in the production of beef cattle and wheat. Other agricultural products include hogs, chickens, dairy products, hay, cotton, soybean, pecans, peanuts, rye, and watermelons. Major industries of the state are transportation equipment (especially aircraft and aerospace equipment), tires, meat processing, and oil and gas equipment manufacturing. American Airlines has its global maintenance and engineering headquarters in Tulsa. Also, the state is a leading producer of natural gas and crude oil. With its low tax burden, the state has been rated as one of the most business-friendly states in the nation.