The extraction of leaf proteins from perennial forage crops within a green biorefinery concept represents a promising approach to face the increasing demand for protein arisen in the organic farming sector to feed monogastric animals. Given the background, the present research aims at assessing the protein extractability from three plant species i.e. chicory, red clover and timothy, at different development stages and investigating lactic acid fermentation as the key method for the extraction of leaf proteins. Based on our results, up to 86% of the proteins in the green juice were recovered in the leaf protein concentrate (LPC) by means of lactic acid fermentation. Red clover presented the highest protein content and resulted in the extraction of 65–98 kg crude proteins per ton dry matter and the production of 186–235 kg dry LPC per ton dry matter. The plants development stage significantly influenced the process figures i.e. protein extractability and production of protein concentrate were reduced with maturity. Accordingly, the maturity of the plants should be addressed when utilized as feedstock for producing protein concentrates for animal feeding in order to optimize the process yields.