Effects of prolonged oral administration of dicyandiamide to dairy heifers on excretion in urine and efficacy in soil

Oral administration of the nitrification inhibitor dicyandiamide (DCD) to grazing ruminants for excretion in urine represents a targeted mitigation strategy to reduce nitrogen (N) losses from grazed pastures. A field trial and allied laboratory incubation study were conducted to examine the effects of oral administration of DCD to non-lactating Friesian dairy heifers on excretion of DCD in urine and efficacy in soil. Dairy heifers were orally administered DCD daily at three treatment levels (low, medium and high; 12, 24 and 36 g DCD heifer−1 day−1, respectively) and compared to a nil-DCD control group over a 90-day continuous dosing period. There were no adverse effects of DCD administration on heifer health or growth, as inferred by live-weight gain and measured blood metabolite levels. Prolonged administration of DCD to dairy heifers resulted in the sustained excretion of DCD in the urine over 90 days and inhibition of nitrification of urinary-N in the silty peat soil for up to 56 days (incubated at 20 °C; P < 0.001). Field soil sampling (0-75 mm depth) of individual urine patches for DCD analysis revealed that a 3-fold increase in the rate of DCD administered resulted in a similar increase in the concentration of DCD voided in the urine and subsequently deposited in urine patches (median equivalent DCD application rates of 22, 36 and 59 kg ha−1 for the low, medium and high DCD treatment levels, respectively; P < 0.001). However, large differences (up to 40-fold) existed between individual urine patches in the rate of DCD deposited at each treatment level, which showed a positively skewed distribution. This study highlights the viability of prolonged daily administration of DCD to ruminants for sustained excretion in urine and effective inhibition of nitrification in soil as a practical targeted mitigation technology to reduce urinary-N losses from grazed pastures.