Smallholders of poultry production systems in developing countries are commonly found in rural, resource-poor areas, and often face food insecurity. The main constraints for smallholders in poultry production in rural, resource-poor areas are the shortage of available commercial dietary protein and the high cost of commercial diets. The beneficial effects of legume and forage cultivation are economic, through providing protein for animals, and ecological, such as soil amendment, nitrogen fixation, and stripping control which participate to increase cropping efficiency. The potential nutritive value of a wide range of forages and grain legumes is presented and discussed. The impacts of dietary protein, fiber, and secondary metabolites in plant content, as as well as their consequences on feed efficiency, animal performance, and digestion processes are enclosed in this review. Lastly, approaches to reduce the anti-nutritional factors of the secondary metabolites of plants are explained.