Nitrogen fertiliser value of digested dairy cow slurry, its liquid and solid fractions, and of dairy cow slurry

An understanding of crop availability of livestock slurry nitrogen (N) is necessary to maximise crop N use efficiency and to minimise environmental losses. Results from field and laboratory incubation experiments suggest that first-year crop availability of slurry N comes mainly from its ammonium fraction because net mineralisation of organic N is often negligible in the short term. A two-year field experiment during 2011 and 2012 in northern Italy was undertaken with several aims: to estimate the N fertiliser value of raw dairy cow slurry, digested dairy cow slurry, and the liquid and solid fractions of the digested slurry, and to verify if applied ammonium recovery was similar both among slurries and between slurries and inorganic N fertiliser (ammonium sulphate). Different fertilisers were applied before silage maize cultivation followed by an unfertilised Italian ryegrass crop. The results showed that ammonium recovery was significantly higher in mineral-fertilised (75%) versus slurry-fertilised (30%) treatments, except in digested slurry (65%). This indicates that ammonium applied with organic materials is less efficient than when applied with mineral fertiliser. For the digested slurry and its liquid fraction, most of the applied ammonium was available to the maize during its application year (55%) due to a low carbon (C)/organic N ratio. The apparent N recovery of the raw slurry and digested slurry solid fraction increased substantially between the first (-1.4%) and second (20%) years, as these materials had high C/organic N ratios; they likely immobilised N for several months post application, producing residual effects during the Italian ryegrass and next maize crops.