The effect of the nitrification inhibitor dicyandiamide (DCD) on herbage production when applied at different times and rates in the autumn and winter

The high rate of urine excreted during animal grazing in late autumn provides a source of nitrogen (N) to the growing sward and also provides the potential for losses of N over the winter months. This study was established to evaluate the potential of applying a nitrification inhibitor, dicyandiamide (DCD), to urine patches to increase N use efficiency in grassland. Four simulated grazing plot experiments were undertaken across two experimental sites, one a free-draining acid brown earth (Experiments 1 and 3) and the other a moderate to heavy brown earth soil (Experiments 2 and 4). Experiments 1 and 2 received no fertiliser N application, and Experiments 3 and 4 received a split application of 350 kg N fertiliser ha−1 year−1. The effect of applying the nitrification inhibitor dicyandiamide (DCD) at 5 or 10 kg DCD ha−1 in autumn and winter to plots receiving synthetic urine or zero urine on spring and annual herbage production was examined in all experiments. The application of DCD did not increase spring herbage production in any of the experiments. Over the two years, the application of 5 or 10 kg DCD ha−1 increased annual herbage production in Experiment 1 when applied to October and November deposited urine patches. Urine application increased herbage production in spring and annually in Experiments 1 and 2, and increased herbage crude protein content and herbage N uptake in all experiments. The application of urine increased soil ammonium and TON content in the 0–100 mm horizon at both sites. The application of 10 kg DCD ha−1 reduced surplus N in Experiment 1 when applied to October and November deposited urine. Overall the effects of DCD on herbage production, surplus N and other parameters in this study were not consistent