摘要：Traditional farming, where livestock are raised on home-grown feed and livestock manure is recycled as a replacement for crop fertilizer, is the type of production system that both populist farming activists and environmental advocates would like to maintain and enhance. But midwestern agriculture is moving towards large, specialized production units where harvested crops are shipped off the farm and livestock are raised on purchased feed. Many people are concerned that increased manure concentration from specialized livestock units poses an unacceptable environmental risk, in part, because the nutrient supply from manure is greater than the nutrient demand from crops. The supply of nutrients available for crop production in a given region for a particular year includes purchased fertilizer nutrients, generated livestock nutrients, nutrients that become available from natural sources such as nitrogen from rainwater, and nutrients that are stored in the soil. Nutrients leave a region in the form of exported crops and livestock or when they are lost to the air and water. Increased livestock concentration in a region increases the proportion of crops used for feed locally and, in many cases, can result in the need to import feed into a region. The resulting increase in nutrient supply from livestock manure and the decrease in loss from crop exports worries environmental groups because it creates the possibility of increased nutrient contamination in the air and water.
