When coupled with the cattle’s overgrazing, invasive Eurasian plants began beating out native grasses and plants while retaining less water and carbon in the soil. Concentrations of cattle increased throughout the 20th century, and instead of letting them graze, farmers began fattening them up in crowded pens called feedlots, which in turn increased the need to cultivate wheat, corn, and soy to feed the cows. Ancient, well-managed prairie was aggressively tilled for the first time, exposing the soil to the air. Some experts have argued that it resulted in a period of severe and destructive dust storms on the Prairies in the 1930s. Today, farmers help prevent this erosion and increase their crop yields with intensive irrigation from underground aquifers that are nearly bled dry – a practice that some studies estimate will leave millions on the Great Plains without water for crops, livestock, or human settlements within a century.