An increasingly common question asked by cooperators is "How many cows can I run if I don't fertilize anything"?
Legumes can provide an alternative to N fertilization, but they require a much higher degree of management to make a legume/grass system work compared to a grass/commercial N system. Depending on the species, legumes can fix anywhere from 50-200 lbs/ac N when grown in a pure stand. Grown in mixtures with grasses, nitrogen fixed will range around 20-80 lbs/ac dependent upon species and percentage of the legume making up the mixture. As a rule of thumb, target a percent legume composition in a grass of at least 30 percent in order to provide enough nitrogen to feed the grass.
To make legumes work, management must be focused on the legume and not the grass. Pastures will look different than they have in the past. Without the even distribution of fertilizer, pastures will look uneven and you may have more weeds. You must also understand the nitrogen transfer system in order to capture the full benefit of legumes.
Legumes do not freely give away the nitrogen that they fix. The majority of nitrogen transfer comes from decomposition of dead legume plant material. Therefore, the greatest benefit to a grass will be following a legume crop, such as growing cowpeas during summer, which are turned under and followed by winter pasture in the fall. Next would be grazing a legume/grass mixture and the decomposition of legume plant material through feces and urine. This increases the importance of rotational grazing, which creates a more even distribution of nutrients across pastures than you will have with continuous grazing. In bermudagrass hay systems, the challenge will be even harder since removing legumes with the hay crop will greatly enhance hay quality, but you will greatly diminish the benefits of nitrogen fixation.