Estimation of N2‐fixation by grass–white clover mixtures in cut or grazed swards

Abstract

Abstract. Formulation of nitrogen balances on farms requires accurate information on all inputs of the nutrient. N2‐fixation by legumes, particularly forage legumes, is an important input which is difficult to measure. Simple regression models have been established from the literature for predicting N2‐fixation by grass–white clover (Trifolium repens) mixtures using dry matter yields.Linear relationships were obtained between the N2‐ fixation (Nfix) and the extra dry matter production of mixed swards (Md) compared with pure grass swards. Nitrogen fixation was given by Nfix= A + 0.067Md where A is the intercept having a value of 6.8 for cut swards and ‐168.1 for grazed swards. A common slope was adopted because the F statistics showed that slopes fitted separately were not significantly different. The value of the negative intercept represents the supply of N to the grazed sward from soil and excreta‐derived N. The regression equation for the cut sward gave reasonable predictions (r2= 0.953) of values of N2‐fixed for experimental data not used in establishing the relationship. For grazed grass, the approach over‐estimated the N2‐ fixation by an average of 15%.