An evaluation of the use of the nitrification inhibitor dicyandiamide (DCD) to reduce nitrogen losses from intensive sheep winter grazing systems : a dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Agricultural Science with Honours at Lincoln University

Abstract

The intensification of modern pastoral agriculture has led to an increase in nitrate (NO₃⁻) leaching and nitrous oxide (N₂O) gas emissions from animal grazing systems, leading to environmental degradation. The use of nitrification inhibitor technology has recently been shown to reduce NO₃⁻ leaching losses and N₂O emissions from dairy pasture systems. However, there is no published data on the effect of using an inhibitor to reduce these losses in sheep winter grazing systems. This research project was therefore conducted to quantify the effect of using dicyandiamide (DCD) to reduce NO₃⁻-N leaching and N₂O emissions in an intensively winter grazed pastoral sheep system. A trial was conducted at Lincoln University comprising of sixteen lysimeters of Templeton silt loam soil, sown in a ryegrass (Lolium perenne) and white clover (Trifolium repens) pasture mixture. Four treatments (control, control + DCD, urine, urine + DCD) were applied in May 2009, with urine patches applied at an N loading rate equivalent to 300 kg N ha⁻¹. DCD treatments were applied at 10 kg DCD ha⁻¹. Simulated rainfall was applied in the spring to supplement natural rainfall in order to produce a similar water input to Southland averages conditions. Nitrous oxide gas collections and analyses were made twice a week over the first month, followed by once a week for the remainder of the experimental period. Drainage water was collected on a once or twice weekly basis depending on leachate volumes and was analysed to measure the NO₃⁻-N concentration of the water. Results showed that the application of DCD reduced the N₂O emissions by 72% from sheep urine applied in the late autumn, from 4.55 kg N₂O-N ha⁻¹ without DCD to 1.31 kg N₂O-N ha⁻¹ over the four month experimental period. The application of DCD also reduced the amount of NO₃⁻-N leached by 70% from sheep urine with a reduction from 147 kg NO₃⁻-N ha⁻¹ to 44 kg NO₃⁻-N ha⁻¹. The data collected from the trial can be easily scaled up to a farm scale situation. Canterbury and Southland sheep farmers’ ‘break feed’ approximately 1800 to 2000 s.u. ha⁻¹ over the winter period. Sheep urine patches (0.03 m² patch⁻¹) cover approximately 12% of the grazing area over 24 hours, therefore the N leaching and emission losses could be up to 18 kg N ha⁻¹ day⁻¹ and 0.54 kg N ha⁻¹ day⁻¹ respectively on grazed areas. The use of a nitrification inhibitor has been shown to be beneficial environmentally in intensive sheep grazing situations, as it reduced N leaching and emission losses by up to 5.4 kg N ha⁻¹ and 0.15 kg N ha⁻¹ respectively in these conditions. In the future, intensive winter grazing systems may become a ‘hot issue’, where regional councils may target highly intensive farming systems. Results also indicate that the use of DCD may be an important tool to sustain intensive sheep grazing systems in the long term.