Changes in the cycling of nitrogen, phosphorus, and potassium in a dairy farming system

The objective of this study was to quantify nitrogen (N), phosphorus (P), and potassium (K) use and cycling in a dairy farming system. The data were collected from the experimental farm at the National Institute of Livestock and Grassland Science in Tochigi Prefecture, Japan, using about 11 ha of forage crop fields and about 30 dairy cows. Forage crops grown in the field were ensiled and offered to the cows, and the subsequent compost from the animals’ excretion was applied to the field. The dairy farming system consisted of soil/crop, feed storage, animal, and compost components. Nutrient inputs and outputs and flows of the soil–plant–animal pathway for the whole farm and each component were measured for 5 years. Nutrient utilization was evaluated using nutrient balances, use efficiencies, and cycling indices. The 5 year average nutrient balances and nutrient use efficiencies of N, P, and K for the whole farm (kg ha−1 year−1) were 378, 97, and 199 and 0.25, 0.19, and 0.18, respectively. The characteristics of nutrient balances and use efficiencies for each component differed among N, P, and K. The average cycling indices of N, P, and K were 0.12, 0.11, and 0.37, respectively. Significant positive relationships between use efficiencies and cycling indices were observed in N and K. Year-to-year variations in flows were relatively large for compost application. The results suggested that improving N balance would be the most effective option for solving many of the environmental problems related to dairy farming.