Most farmers already have fields, they dont need to buy them.
Farmers that grow food have fields. Cattle farmers typically do not. It is much cheaper and easier to raise cattle in a confined area and feed them corn than it is to buy and maintain huge tracts of grasslands. Grass-fed beef is available to the consumer at a higher cost than corn-fed beef because of the extra expense in producing it.
Before World War II, most Americans had never eaten corn-fed beef. Raised on pasture, cattle reared before the 1950s usually took two or three years to be ready for the slaughterhouse. Steers were fed grain only occasionally and in small quantities, and farmers tended to use corn as a supplement—not a staple—of their livestock’s diets. http://www.pbs.org/independentlens/kingcorn/cows.html
A close-up of two cows confined with several other cows, each with a red numbered tag hanging from one ear
But as American corn production skyrocketed in the post-War era, and as the economic boom of the 1950s prompted higher consumer demand for meat, farmers and ranchers turned to a new practice: fattening their cattle on corn. Cheaper and more efficient than grass, corn enabled cattle to be brought to market in as few as 15 months. Moreover, it allowed farmers to feed cattle in confined pens or lots, reducing ranchers’ land costs and limiting their risk of losing livestock to predators and bad weather. With cheaper feed in the equation, beef prices fell, and Americans began to purchase more and more beef, most of it corn-fed. By 1960, Americans ate a yearly average of more than 66 pounds of beef each. By 1975, that number had grown to 88.5 pounds of beef per person, per year.
In 2008, corn-fed cattle are the norm. While most cattle still begin their lives grazing on grass, the vast majority—an estimated three-quarters of them, according to the U.S. Department of Agriculture—are “finished,” or fattened for market, in feedlots. There, they spend three to six months eating a diet composed of 70 to 90 percent corn.