Key nitrogen and phosphorus performance indicators derived from farm-gate mass balances on dairies.

Efficient management of N and P on dairy farms is critical for farm profitability and environmental stewardship. Annual farm-gate nutrient mass balance (NMB) assessments can be used to determine the nutrient-use efficiency of farms, set efficiency targets, and monitor the effect of management changes with minimal inputs required. In New York, feasible N and P balances have been developed as benchmarks for dairy farm NMB, alongside key performance indicators (KPI) that serve as predictors for high NMB. Here, 3 yr of NMB data from 47 farms were used to evaluate the main drivers of N and P balances and identify additional KPI. From the 141 farm records, 26% met both the feasible N balances per hectare and per megagram of milk produced. For P, 53% of the records met both benchmarks. Imports, rather than exports, drove NMB primarily by feed and fertilizer purchases, consistent with earlier findings. Linear regression analysis showed that a selection of KPI currently used, particularly animal density, nutrient-use efficiency, and the amount of home-grown feed, explained a large portion of variation in NMB. Heifer-to-cow ratio and the relative proportion of various forage crops may provide further insight into the drivers of feed and fertilizer imports and ultimately farm-gate NMB. This study provides avenues toward a better assessment of whole-farm nutrient management and means for farms to communicate progress to stakeholders and consumers.

Combining field phosphorus runoff risk assessments with whole-farm phosphorus balances to guide manure management decisions.

Phosphorus (P) loss from agricultural fields contributes to water quality degradation. A phosphorus index (PI) is a tool that scores fields based on P loss potential. Recently, a new transport × best or beneficial management practice (BMP) approach was proposed for the New York PI (NY-PI), which first scores fields using landscape-based transport factors (raw scores) and then offers various BMPs to reduce the score (i.e., risk of P transport). The final score is assigned a management implication (N needs based, P removal based, or zero P application), taking into account field-specific soil-test P (STP) and the farm's whole-farm P balance. With farmer and nutrient management planner input and data on field-specific transport factors and whole-farm P balances of 18 New York dairy farms, we set coefficients for transport factors, BMPs related to P application, and STP limits and determined the impact of implementation of the new NY-PI on manure management options. Based on raw scores, the proposed NY-PI initially limited manure application to 51% of the total cropland area of the participating farms (28% N-based, 23% P-based). Implementation of BMPs (i.e., changing the method and ground cover or timing of P application) allowed 43-98% of the land area to receive manure at N-based rates. For farms with whole-farm P balances within the feasible limits set for New York, an additional 0-50% of the land base was classified as N-based management, depending on BMP selection. These results show the ability of the new NY-PI to limit P applications on fields with high transport risk while incentivizing adoption of BMPs and improvements in whole-farm P management.

Towards optimal use of phosphorus fertiliser.

Because phosphorus (P) is one of the most limiting nutrients in agricultural systems, P fertilisation is essential to feed the world. However, declining P reserves demand far more effective use of this crucial resource. Here, we use meta-analysis to synthesize yield responses to P fertilisation in grasslands, the most common type of agricultural land, to identify under which conditions P fertilisation is most effective. Yield responses to P fertilisation were 40-100% higher in (a) tropical vs temperate regions; (b) grass/legume mixtures vs grass monocultures; and (c) soil pH of 5-6 vs other pHs. The agronomic efficiency of P fertilisation decreased for greater P application rates. Moreover, soils with low P availability reacted disproportionately strong to fertilisation. Hence, low fertiliser application rates to P-deficient soils result in stronger absolute yield benefits than high rates applied to soils with a higher P status. Overall, our results suggest that optimising P fertiliser use is key to sustainable intensification of agricultural systems.

Evaluating Management Implications of the New York Phosphorus Index with Farm Field Information.

Phosphorus (P) loss from agricultural fields can contribute to water quality degradation. The current New York P index (NY-PI) scores fields on the basis of P sources and field characteristics that reflect risk of P transport (a source × transport approach). Recently, a transport × best management practice (BMP) approach was proposed, which first scores fields using landscape-driven transport factors and then offers various BMPs to reduce the score (i.e., risk of P transport). To analyze the score distribution of the current NY-PI and the incentivizing potential of the proposed structure, a database of 33,327 agricultural fields in New York was assembled in collaboration with nutrient management planners and farmers. Under the current NY-PI, no additional P could be applied to 2% of the fields, while for 3% the application rates should not exceed annual crop P removal. Flow distance (field to stream) was a major driver for NY-PI scores. The current NY-PI relies heavily on soil test P to assess runoff risk, allowing some low-P fields to receive manure independent of transport risk. A scenario evaluation showed that the proposed NY-PI limits P application on fields with high transport risk while simultaneously incentivizing adoption of BMPs in such areas. In the absence of farm-level water quality data, a farm field database can help set P index coefficients and assess implications of a new P index. This study emphasizes the value of involving stakeholders in assessing nutrient management tools, as well as the importance of using an incentive-driven approach for protecting water resources.