Comparison of methods to assess nitrogen, phosphorus and potassium excretion in pig farms

This study consists in comparing three assessment methods of nitrogen, phosphorus and potassium excretion in 13 pig farms. Two methods are based on calculation (Corpen average reference values and the Corpen balance model, socalled ”simplified actual balance”). Another way consists in quantifying and analyzing the liquid manure according to three techniques of sampling. The results show less excretion of N, P and K by direct measurement than the values mentioned in the environmental impact studies. The differences of excretion reach 15 to 17% for nitrogen, 41 to 42% for phosphorus and 8 to 13% for potassium, according to the sampling technique. Between field measurements and the balance model, the relative differences stay high and closely the same for N and P; the estimates for K become nearly equivalent (from -3 to +4% on average for the 13 pig farms). Then, we can make the assumption that the quantity of liquid manure is correctly evaluated whereas the field sampling techniques here tested remain poorly efficient, especially for the elements linked to the dry matter such as phosphorus. Further investigations are however necessary in order to precise the differences observed in nitrogen contents.