美国是世界上最大的玉米生产国,也是玉米科研和生产水平最高的国家。20世纪30年代,玉米种植密度每亩不到2000株,亩产只有100多千克。20世纪60-70年代,密度增加到3000株左右,亩产也提高到300千 The United States is the largest corn producer in the world and also has the highest level of scientific research and production of corn. In the 1930s, the density of maize planting was less than 2,000 per acre and the per-mu yield was only over 100 kilograms. During the 1960s and 1970s, the density increased to about 3,000 and the yield per mu also increased to 300,000