Dietary effects on the turnover and plant utilisation of animal manure nitrogen

Animal manure is an important source of N for crop production. Part of the manure N is bound in organic compounds and must be mineralised in the soil before it can be utilised by plants. The composition of animal manure is influenced by the diet fed to the animal. We have studied the effects of pig and dairy cattle feed composition on the turnover and potential utilisation of manure N. Dairy cows were fed seven different forage types with or without supplemental concentrates. Growing pigs and sows were fed 12 different diets with variable contents of fibre and protein (with and without synthetic amino acids). Cattle and pig slurries consisting of a mixture of faeces and urine were stored according to common agricultural practice in Northern Europe. The plant availability of N from the animal slurries was tested in small field plots with spring barley, under conditions with minimal N losses. Separate plots were applied with increasing amounts of mineral fertiliser N. Nitrogen uptake by barley plants was determined and the mineral fertiliser equivalent (MFE) of slurry N was calculated. The net release of mineral N from the slurries in soil was also measured in a parallel incubation study. The MFE of cattle slurry N was 56-81%. The plant availability and net mineralisation of cattle slurry N was influenced by forage type and concentrate supplementation, and the MFE was negatively correlated with the concentration of crude fibre in the diet. The MFE of pig slurry N was 72-100% and significantly influenced by feed fibre composition, but insignificantly influenced by protein level. There was a significant positive correlation between enzyme-digestible organic matter in the pig diets (measurement used for feed evaluation) and the plant availability of pig slurry N. The ammonium content of stored pig slurry could not be used for prediction of the N availability, since the net mineralisation of pig slurry N in soil was variable. For both pig and cattle slurry there was a highly significant negative correlation between C/N ratio and the plant availability of slurry N (r = 0.91). The results suggest that the fibre content of both pig and cattle diets has significant influence on the amount of residual organic manure N remaining in soil after a few months of decomposition.