Nitrogen excretion of adult sheep fed silages made of a mixed sward or of pure unfertilised grass alone and in combination with barley

Four adult rumen-fistulated wether sheep were fed silages combined with barley. The silages consisted of 48% grasses, 28% legumes and 24% other forbs (GCF) or of pure grass (G). The swards received no mineral fertiliser. The dry matter (DM) and fibre contents were lower in GCF than in G. Crude protein content of DM in GCF and G were 145 g/kg and 102 g/kg respectively. DM content as ash, lipids and non-fibrous carbohydrates were rather similar in both silages. About 40g DM were offered per kg BW0.75 and day either as silage alone or as a mixture of silage and barley (60:40). Faecal N excretion was greater with GCF than with G. The proportion of faecal bacterial and endogenous debris N reached 75 and 73% when GCF or G was fed, respectively. Undigested dietary N represented about 20%, and water soluble N accounted for 5-6% of faecal N. GCF caused more urinary N than G. Barley reduced urinary N excretion when supplemented to GCF. No dietary influence on urinary non-urea nitrogenous compounds was shown. GCF caused higher urinary urea N excretion than G and barley reduced this fraction when replacing part of GCF. Based on the urinary urea N proportions, it is concluded that N intake exceeded N requirement for any of the four diets fed. Dietary supplementation of ruminally fermentable carbohydrates can reduce urinary N excretion and this improves the efficiency of utilisation of N in N-unfertilised biodiverse grassland/ruminant farming systems.