Nitrogen (N) fertilizers are critical in today's agriculture especially in the United States. Leaching, methane and nitrous oxide emissions from N fertilizer use and the implications for global climate and water pollution raised serious concerns among environmentalists and agronomists. Prices of N fertilizers increased geometrically over the past few years. The projected increase in maize (Zea mays L.) ethanol production is expected to increase N fertilizer demand and prices. Hay prices are, however, staggering and producers are looking for alternatives to N fertilizers. This review paper assessed trends in fertilizer use and prices as well as factors affecting fixation and transfer of dinitrogen (N-2) in forage production systems. Additionally, economic implications of using N fertilizers and grass-legume mixtures are discussed. From the review, it was apparent that legumes have the potential to replace N fertilizers or at least complement the use of N fertilizers in forage production systems. However, N transfer in forage production systems is low. A lot more research is needed to answer the question Why legumes fix so much N-2 and transfer just a small proportion to other crops. Similarly, identifying compatible grass-legume species that enhance N-2 fixation will be a giant step towards reduced N fertilizer use and ensuring environmental sustainability. With the sharp increase in N fertilizer prices, the use of legumes in forage production systems seems promising.