Evaluation of separation plus acidification with nitric acid and separation plus dilution to make cattle slurry a balanced, efficient fertilizer for grass and silage

Abstract

SUMMARYCombinations of treatments of cattle slurry, which have previously been shown to have lowered ammonia volatilization by 75% of that from whole slurry, were compared in a field experiment with perennial ryegrass at the Agricultural Research Institute, Hillsborough in 1990. Whole slurry and slurries obtained by separation through screens with mesh sizes of 5·0, 3·0, 1·1 and 0·4mm were acidified with 1·4% by volume of 10M nitric acid. Slurries separated through 5·0 and 0·4 mm meshes were diluted 100% and 50% respectively by volume with water and amended with calcium nitrate to supply the same amount of nitrogen as the nitric acid. All slurry combinations were surface-applied at 100 kg (NH4+-N + NO3−-N)/ha to different plots on three occasions during the growing season. Volatilization of ammonia was measured on adjacent plots using ventilated enclosures. By comparison with inorganic fertilizer treatments, assessments were made of the efficiency of slurry nitrogen for herbage production, the fertilizer value of phosphorus, potassium and sulphur in the slurry, and the effects of slurry on silage quality.The mean ammonia volatilization over all applications of acidified slurries was 17% of that from whole slurry. The two separation plus dilution treatments, however, only lowered volatilization to about half of that from whole slurry. Within separation plus acidification treatments, the trend was for dry matter yield and nitrogen offtake to increase as mesh size decreased down to 1.1 mm. For slurries separated through 5·0 and 0·4 mm meshes, the effect of the additional dilution treatment on dry matter yield and nitrogen offtake was similar to the effect of acidification. Efficiency of nitrogen offtake over all three cuts, relative to inorganic fertilizer nitrogen, was 54% for acidified whole slurry and 88% for slurry separated through a 1·1 mm mesh and acidified. Denitrification in nitric-acid amended slurries and sward contamination from slurry solids may have been responsible for lowering nitrogen efficiency.Chemical analyses of herbage showed that the concentrations of phosphorus, potassium and sulphur from the slurry treatments were comparable to those in herbage receiving the recommended rates of inorganic fertilizers and were sufficient for optimum yield. Slurry application had a detrimental effect on the fermentation quality of the ensiled herbage. Chemical analyses indicated that fermentations from two slurry treatments at the first cut and all slurry treatments at the third cut were poorer than those on treatments receiving the equivalent rate of inorganic fertilizer nitrogen.