Dietary effects on the composition of pig slurry and on the plant utilization of pig slurry nitrogen

Abstract

The composition of animal manure is influenced by the diet fed. Efforts are made to decrease nitrogen emission from pig production by optimizing diet composition. This normally results in a lower proportion of N excreted in urine relative to faeces, and may also influence the turnover and utilization of manure N after field application. The effects of pig feed composition on the excretion of urinary and faecal N, on the dynamics of manure N in soil and on the potential utilization of manure N in the field was studied. Growing pigs and sows were fed 12 different diets with variable contents of fibre and protein (with or without synthetic amino acids). Slurries consisting of a mixture of faeces and urine were stored according to common agricultural practice in Northern Europe. The plant availability of N in the resultant slurries originating from animals fed known diets was tested in small field plots with barley, under conditions with minimal N losses. Separate plots were fertilized with increasing amounts of mineral N. Nitrogen uptake in barley was determined and the utilization of slurry N was compared with that of mineral fertilizer N. The net release of mineral N and C from the slurries in soil was also measured in a parallel incubation study.

The mineral fertilizer equivalent of pig slurry N was 72–100% and significantly influenced by feed fibre composition, but not significantly influenced by the protein content. There was a significant positive correlation between enzyme-digestible organic matter in the pig diet (measurement used for feed evaluation) and the plant availability of pig slurry N (R²=0·90). The ammonium content of stored pig slurry could not be used for prediction of the N availability since the net mineralization of pig slurry N was variable, but there was a significant negative correlation between the pig slurry C/N ratio and the plant availability of slurry N (R²=0·86).

Increased dietary concentration of fermentable structural carbohydrates (e.g. by including sugar beet pulp in the diet) reduces the excretion of N in urine without affecting the availability of slurry total N, whereas an increased concentration of dietary fibre with a low fermentability (straw) results in less urinary N, but also a lower plant availability of slurry N.