Dairy Production Systems in the United States: Nutrient Budgets and Environmental Impacts.

Across the diversity of US dairy production systems, nutrient management priorities range widely, from feeding regimes to manure handling, storage and application to crop systems. To assess nutrient management and environmental impacts of dairy production systems in the USA, we evaluated nutrient budgets at whole farm and field scales for representative dairy systems in California, Idaho, Texas, Minnesota, Wisconsin and Pennsylvania. Dairy farming systems were simulated using the Integrated Farm System Model. The model is well documented and has been widely applied to dairy production systems, simulating crop and pasture production, feed use, manure management and other major activities related to nutrient management of dairy farms. Whole farm nutrient budgets were determined over 25 years of local weather conditions. Total N emissions, primarily as ammonia, are 40-50% of farm inputs. Ammonia emissions are greater from the farmsteads of western dairies with warmer ambient temperatures and more long-term manure storage, whereas they are primarily from the field soils of eastern dairies. Nitrate leaching is greatest in the humid conditions of the eastern dairies and is of least concern in the dry regions of California, West Texas and Idaho. Western feedlot dairies have low phosphorus use efficiency (<50% is exported in milk and animals) due to a reliance upon purchased feed and large amounts of manure export. Phosphorus use efficiency of the eastern dairies are >60%, reflecting better phosphorus cycling through on-farm feed production. However, liquid manure systems hamper manure export, resulting in net accumulation of phosphorus in farm soils and greater environmental losses due to erosion. Nutrient budget analyses suggest opportunities for increasing nutrient use efficiency and reducing nutrient losses. Depending on regional climate conditions and management system characteristics, nutrient losses may be mitigated by changes in feeding strategies, manure management technologies, or targeted soil and water conservation practices.