Effects of the nitrification inhibitor dicyandiamide on soil mineral N, pasture yield, nutrient uptake and pasture quality in a grazed pasture system

Recent lysimeter studies have demonstrated that the nitrification inhibitor, dicyandiamide (DCD), can reduce nitrate (N03) leaching losses from cow urine patches in grazed pasture systems. The objective of this study was to quantify the effects of fine particle suspension (FPS) DCD on soil mineral N components, pasture yield, nutrient uptake and pasture quality under grazed pasture conditions. A field study was conducted on the Lincoln University dairy farm, Canterbury, New Zealand, from 2002 to 2006. FPS DCD was applied to grazed pasture plots at 10 kg ha(-1) in early May in addition to applied cow urine patches at a nitrogen (N) loading rate of 1000 kg N ha(-1), with DCD reapplied in early August. Soil mineral N levels in the urine patches were monitored. Pasture yield, N and cation concentrations and uptake were measured in treatment urine patches and inter-urine areas of the pasture. Comparisons were made with control plots which did not receive DCD. N03 levels under the DCD-treated urine patches (0-7.5 cm) were in the order of 10 kg N ha(-1) compared with 40-80 kg N ha(-1) under untreated patches, and soil ammonium (NH4+) levels were consistently higher under the DCD- treated patches. The DCD significantly and consistently increased pasture yield in both the urine patches, and inter-urine areas of the pasture in all 4 years of the trial. Mean annual dry matter (DM) yields over 4 years were inter-urine areas, 10.3; inter-urine + DCD, 12.4; urine, 12.4 and urine + DCD 16.0 t DM ha(-1), representing an average DM yield increase of 20 and 29% in inter-urine and urine patch areas, respectively. On a whole paddock basis, the increase in annual DM yield resulting from DCD application was estimated to be 21%. N, calcium (Ca), magnesium (Mg) and potassium (K) concentrations in pasture were unaffected by treatment with DCD; however, total annual uptake of these nutrients by pasture was significantly higher in all years where DCD had been applied. Pasture DM, protein, carbohydrate, metabolizable energy and fibre levels and sward clover content were not affected by treatment with DCD. The results demonstrate the agronomic value of the DCD treatment in addition to the environmental benefits in a grazed pasture system.