Fluxes of K, P, and Zn in a conventional and an organic dairy farming system through feed, animals, manure, and urine—a case study at Öjebyn, Sweden

Abstract

One aim of this paper was to present flows and balances of K, P, and Zn at a barn level for an organic and a conventional farming system, and compare the systems with respect to agronomic sustainability. A second aim was to identify internal on-farm flows of major concern for the environment. We define barn balance as the balance between element flows entering and leaving the barn. The flows of potassium (K), phosphorus (P), and zinc (Zn) through the barn compartments were separated between an organically and a conventionally managed farming system. The study comprised 1 year. The area of the organic system was 59 ha and that of the conventional system 45 ha, with 50 dairy cows in each system. Feed, bedding, tap water, and milk were recorded and sampled monthly. The manure and urine were sampled in conjunction with spreading. In comparison with the organic system, the conventional system produced more milk per cow and per hectare, but to reach that target, the conventional system was more dependent on land outside the farm for feed production than the organic system. As a consequence, the total internal flows from feed crop production were larger in the organic system. The concentration of K in organic silage was significantly lower than that in conventional silage. The concentrations of K, P, and Zn in the other materials studied did not differ between systems. The barn balances of the two systems were similar to each other. The study showed a large internal flow of K, equal flows of P, and smaller internal flows of Zn within a dairy farm as compared to the animal-associated flows through the farm gate. For K, about 70% of the total amount in farm feed crops and purchased feed was recovered in recorded outputs. For P the recovery was close to 100%, whereas for Zn, more of the element was found in the outflow than in the inflow. The imbalances of K and Zn represented flows that would not have been recognised in a farm-gate balance, but which could have a considerable impact on soil accumulation, long-term soil fertility, and losses to the surrounding environment.