<P> In the United States, farms spread from the colonies westward along with the settlers . In cooler regions, wheat was often the crop of choice when lands were newly settled, leading to a "wheat frontier" that moved westward over the course of years . Also very common in the antebellum Midwest was farming corn while raising hogs, complementing each other especially since it was difficult to get grain to market before the canals and railroads . After the "wheat frontier" had passed through an area, more diversified farms including dairy cattle generally took its place . Warmer regions saw plantings of cotton and herds of beef cattle . In the early colonial south, raising tobacco and cotton was common, especially through the use of slave labor until the Civil War . In the northeast, slaves were used in agriculture until the early 19th century . In the Midwest, slavery was prohibited by the Freedom Ordinance of 1787 . </P> <P> The introduction and broad adoption of scientific agriculture since the mid-19th century contributed to economic growth in the United States . This development was facilitated by the Morrill Act and the Hatch Act of 1887 which established in each state a land - grant university (with a mission to teach and study agriculture) and a federally funded system of agricultural experiment stations and cooperative extension networks which place extension agents in each state . </P> <P> Soybeans were not widely cultivated in the United States until the early 1930s, and by 1942 it became the world's largest soybean producer, due in part to World War II and the "need for domestic sources of fats, oils, and meal". Between 1930 and 1942, the United States' share of world soybean production grew from 3% to 47%, and by 1969 it had risen to 76% . By 1973 soybeans were the United States' "number one cash crop, and leading export commodity, ahead of both wheat and corn". </P> <P> Significant areas of farmland were abandoned during the Great Depression and incorporated into nascent national forests . Later, "Sodbuster" and "Swampbuster" restrictions written into federal farm programs starting in the 1970s reversed a decades - long trend of habitat destruction that began in 1942 when farmers were encouraged to plant all possible land in support of the war effort . In the United States, federal programs administered through local Soil and Water Conservation Districts provide technical assistance and partial funding to farmers who wish to implement management practices to conserve soil and limit erosion and floods . </P>

What is the number one cash crop in the united states