Effects of oral administration of dicyandiamide to lactating dairy cows on residues in milk and the efficacy of delivery via a supplementary feed source

A metabolism stall study examined the fate of dicyandiamide (DCD) administered to dairy cows by either oral drenching or via a supplementary feed source (pasture silage) as a practical method to achieve targeted DCD excretion in individual urinations to reduce nitrogen (N) losses from grazed pasture systems. The study consisted of two experiments; firstly, lactating dairy cows were orally administered an aqueous solution of DCD at two rates (3 or 30 g cow-1 day-1) to examine the output in urine, faeces and milk, and secondly, non-lactating dairy cows were fed pasture silage amended with fine-crystalline DCD powder (30 g DCD cow-1 day-1) to investigate concentrations of DCD in excreta (urine and faeces) and the subsequent inhibition of nitrification of urinary-N in soil. Oral administration of DCD to lactating dairy cows in solution form resulted in DCD being predominantly recovered in urine at 61% relative to 19% in faeces and 1.2% in milk (SEM 2.3, 1.0 and 0.08, respectively). Increased DCD administration rate led to higher (P<0.01) concentrations of DCD in urine, faeces and milk, but had no significant effect on the total daily proportion recovered (percentage of that administered). After ceasing administration, concentrations of DCD in milk and excreta (urine and faeces) declined to undetectable levels after 5 days. In the second experiment, recovery of DCD in urine from cows fed DCD-treated pasture silage was higher at 82%, with 10% in faeces (SEM 1.9 and 0.6, respectively). Urine from DCD-treated cows applied to incubated soil (equivalent rate of 51 kg DCD ha-1) markedly inhibited nitrification of urinary-N relative to the untreated urine control and had the same inhibitory effect to that from urine with DCD applied directly to soil (50 kg DCD ha-1). This study highlights that oral administration of an aqueous DCD solution to lactating dairy cows is predominantly eliminated in urine with relatively low amounts voided in milk. Furthermore, provision of fine-crystalline DCD powder in supplementary feed is also a viable delivery method for excretion in urine to potentially reduce environmental N losses from grazed pasture systems.