Dairy diet effects on phosphorus cycles of cropland

The long term importation of feed and fertilizer has resulted in soil nutrient accumulation and subsequent loss from many dairy farms. This study examines how excessive supplementation of dairy cow diets with inorganic P affects the land required for recycling manure P through crops and the ability of dairy farms to recycle manure P in view of new federal guidelines that limit land application of manure based on crop P requirements. Approximately 0.71 ha of cropland is required to recycle manure P excreted by a lactating cow fed a P adequate diet. Addition of supplemental P to raise dietary P from 3.8 g kg−1 of diet dry matter, a level adequate for lactating cows, to 4.8 g kg-1, typical of diets currently fed in North America, increases the area of cropland needed for recycling manure by 39%. For a representative Wisconsin dairy farm having 78.3 ha of tillable land, 9.5 ha of which already has excessive P, the annual addition of 8.1 kg P per cow to increase dietary P from 3.8 to 4.8 g kg−1 for a 90 cow herd would create excessive soil test P levels in all fields within 11 years. More strategies for integrating feed manure, fertilizer, and land management are needed to decrease environmental risks while maintaining farm profits.