Long-term trends of nitrogen and phosphorus mass balances on New York State dairy farms.

The whole-farm nutrient mass balance (NMB) is an adaptive management tool that can be used to identify areas for improvement in nutrient management and to monitor progress over time. The objectives of this study were to (1) evaluate the trends of nitrogen and phosphorus mass balances of 27 New York State dairy farms over 6 to 10 yr, (2) identify specific management changes made by 4 case study farms that improved NMB over time by shifting NMB up or down depending on the initial NMB, and (3) evaluate the potential of key indicators to identify opportunities for improvement in NMB. During the study period, milk price fluctuated whereas costs associated with feed and fertilizer increased substantially. Of the 27 farms, 67 to 74% (depending on the nutrient) decreased NMB per hectare over time, whereas 63 to 67% decreased NMB per megagram of milk over time. In general, changes in NMB were directionally correct, with 43 to 56% of farms operating in the optimum operational zone (with both NMB per hectare and per megagram of milk below the feasible levels suggested for New York) toward the end of the study versus 22 to 26% in the first 2 yr of the assessments. The 4 case study farms improved their NMB, whole-farm nutrient use efficiencies, and feed nutrient use efficiencies while maintaining or increasing milk production per cow. The case study farmers made the largest changes in precision feed management, reducing protein and P in purchased feed by replacing concentrates with blends with lower nutrient concentrations. Total nutrient imports, feed imports, the percentage of homegrown feed and nutrients, the concentration of nutrients in the purchased feed, fertilizer imports, and overall crop yields were useful in identifying potential areas for improvement in NMB.