Evaluation of intermediate crops to trap nitrogen and produce forage. Experiments on catch crops were carried out over a period of three years in the loamy region of Belgium at the Catholic University of Louvain. The results were used to evaluate the performance of certain species of grass and legume, in pure or mixed culture. The performance of each species was judged according to its ability to take up nitrogen (catch crops), to produce forage of a sufficient quality and to meet certain agronomic criteria for intercropping. The results showed that grasses in pure culture or when mixed with legumes were able to significantly reduce soil nitrogen at the beginning of the drainage period, provided that the proportion of non-legumes constituted at least 50% of the mixture. Biomass produced by most of the tested crops can be harvested as complementary forage, which can be useful in times of shortage.