Nitrogen and phosphorus in the United States food supply chain, focusing on dairy: efficiency and potential for improvement

Analyses of global nitrogen (N) and phosphorus (P) flows have identified the potential for increased nutrient use efficiency from livestock. This work created national-level N and P budgets for the US, accounting for 9 major... [ view full abstract ]

Analyses of global nitrogen (N) and phosphorus (P) flows have identified the potential for increased nutrient use efficiency from livestock. This work created national-level N and P budgets for the US, accounting for 9 major sectors (from fertilizer production to waste treatment), examining the role of agriculture, and specifically dairy production, in national nutrient flows, quantifying the effects of dairy on nutrient cycling and associated impacts, and looking across all sectors to identify hotspots and potential improvements in terms of use efficiencies.

For N, results show that the combined fertilizer and chemical industry is the major fixer of N₂ from air, with an annual fixation of 9,350 kt N/ yr.  Agriculture is the second largest sector, with an N₂ fixation of 5,156 kt N/yr for US. Agriculture is the largest source of reactive nitrogen emission to air, predominantly due to ammonia emissions of 3,047 kt N from fertilizer application to field crops. Many feedback loops exist in both the dairy food supply chain and the ‘non-dairy’ food supply chain. For example, 104 kt N is produced as a by-product at the crop-based food industry and fed to the national dairy herd. N-efficiencies range from 71% for the poultry processing industry to 95% for the dairy processing industry. In total 1,203 kt N in synthetic fertilizer is used to produce 445 kt N in milk and 1 kt N in meat (boneless equivalent, veal calves only). Thus, 37% of total N applied with synthetic fertilizer ends up in milk supplied to the processing industry. In terms of nutrient use efficiency (NUE), the dairy processing industry is the most efficient part of the dairy food supply chain with an efficiency of 97%.

‘Hotspots’ in the dairy food supply chain occur predominantly in the crop production stage and the ‘dairy herd’ (milk production) stage, suggesting that improvement should focus in particular, on reducing ammonia emissions in manure management systems and at fertilizer and manure application on the field. Scenarios were developed to determine to what extent a) anaerobic co-digestion of dairy manure and food waste, b) no-till cropping practices, or c) reducing consumer waste can contribute to improve nutrient cycling efficiency. The co-digestion scenario suggests that 22% and 64% of synthetic N and P (respectively) fertilizer used for dairy might be avoided if all recuperated N and P from co-digestion is diverted to dairy production systems. The no-till scenario shows potential reductions of 6% and 3% of N and P synthetic fertilizer use for all agriculture.  Reducing consumer waste of dairy products by 50% could increase dairy fertilizer use efficiency from 20% to 23% for N, with similar potential improvements for P.