Evolution of N and P Mass Balances over Time on Four Dairy Farms in NY.

Whole-farm nutrient mass balances (NMB) can be used to monitor the impact of management changes on nutrient use efficiencies of dairy farms. A previous study set benchmark NMBs for milk production in NY at 118 N/ha (without N₂ fixation) and at 13 kg P/ha. The objectives of this study were to identify key management practices developed by farms that (1) consistently operated with NMB below the proposed NMB benchmarks and high milk productions; and (2) significantly improved NMB over time while maintaining or increasing milk production. Four farms were selected from a total of 27 dairy farms in NY that conducted NMB over ≥6 yr. Two farms operated with NMB per ha ranging from 11 to 113 kg N/ha and from -1 to 14 kg P/ha over 7 yr. A third farm reduced NMB from 253 to 84 kg N/ha and from 38 to 1 kg P/ha over 9 yr. The fourth farm increased NMB from -36 to 123 kg N/ha and from -7 to 10 kg P/ha over 8 yr. The 4 selected farms produced 62 to 92% of the feed at the farm. Three farms had animal densities below 2.6 animal units (AU)/ha and one farm operated with 2.8 to 3.1 AU/ha. For each farm, changes in the feed purchased were positively correlated to changes in the NMB (R² ranged from 0.78 to 0.98). Three farms reduced N and P feed purchases over time by either: (1) increasing the land base and thus producing more crops on the farm; and/or (2) implementing precision feeding: reducing the percentage of N and P in the purchased feed, resulting in lower percentages of N and P in the diet and higher N and P feed use efficiencies. Reduction in fertilizer use and crop exports also contributed to reducing NMB over time.