Manure is an important resource for meeting the nutrient needs of corn and soybean grown in Iowa. Land application is the most widely accepted and best economic and agronomic use of manure. Concurrently, however, is the environmental concern when manure nitrogen (N) and phosphorus (P) is not adequately accounted for or utilized by crops. Use of manure as a crop nutrient source requires producer confidence in nutrient availability and maintenance of high crop yields. When that confidence is lacking, either because of unknown application rates or uncertain nutrient content and crop availability, then additional fertilizer or higher manure rates are applied to ensure adequate soil fertility levels. This leads to over-application of crop nutrients, reduced profits, and potential for off-site movement and water quality degradation.