In a 2020 survey evaluating the perception of alfalfa production in the Southern United States, many producers easily recognized the value of legumes for improving forage quality and seasonal yield potential. Interestingly, only 10% of the respondents expressed an interest in the inclusion of alfalfa to reduce Nitrogen (N) fertilizer inputs in their operation (Silva et al. 2021). While legumes are commonly recommended and easily implemented to improve pasture productivity, increase plant diversity, and enhance forage quality, many of the secondary and tertiary benefits remain underappreciated or poorly understood.
Benefits of planting Legumes
The addition of legumes provides many benefits in our livestock-forage operations. Growing compatible legumes with thriving grasses helps our pastures operate as our high-functioning forage factories by increasing forage quality, pasture yield, pasture diversity and decreasing the impacts of certain animal related disorders*. An important example of combatting an animal disorder is when dealing with fescue toxicosis. The inclusion of white clover in Kentucky 31 (KY31) tall fescue has been proven to decrease the impacts of toxicosis by diluting the high amounts of alkaloids that are ingested when animals are grazing KY31 tall fescue monoculture pastures. Additionally, recent research from the USDA-FAPRU unit in Lexington, KY has identified a positive associative response to animals grazing red clover in KY31 tall fescue pastures linked to the phytoestrogens provided by the red clover in the mixture. Moreover, legumes have the unique and extremely valuable trait of having the ability to capture and fix nitrogen that can be utilized by non-legumes in forage production. Now that is an awesome benefit to our high-functioning factory.
The Belowground Partnership
How exactly legumes provide Nitrogen to the system can get a little bit more technical, hence why it is often accepted as a benefit but maybe not as well understood. Legumes, such as alfalfa, clovers, vetches, and lespedeza for example, have formed a mutually beneficial partnership with soil bacteria, Rhizobium. These bacteria infect legume root hairs forming small nodules near the point of root elongation. These bacteria pull nitrogen gas (N2) from the atmosphere and convert it into plant-available forms of nitrogen through a process known as nitrogen fixation. The relationship benefits both partners. The bacteria provide the plant with nitrogen and essential nutrients for growth, and the legume host supplies the bacteria with sugars and other compounds they need to survive and fix nitrogen.
When legume seeds are properly inoculated and planted in favorable soil conditions, root nodules can begin forming within three to four weeks after emergence. As the plant grows, the number and size of these nodules provide a good indication of how much nitrogen fixation is occurring. Nitrogen fixation is highly dependent on soil condition and responds best if conditions are adequate including proper soil moisture, temperature, pH, and fertility. Soil fertility and pH are directly correlated to number of Rhizobium present. Making sure the pH is between 5 and 8 and that calcium, phosphorus, and potassium levels aren’t limited will help encourage healthy Rhizobium populations. If soil conditions are suitable for growth of the legume, they are typically suitable for bacterial infection and nitrogen fixation. Rhizobium populations can be reduced by extreme weather conditions like drought and flooding which will impact nitrogen fixation in the system.
Not all legumes contribute nitrogen to surrounding plants in the same way. Grain legumes, such as soybean and peanut, use most of the nitrogen they fix to support their own growth and production. Forage legumes, including alfalfa and clovers, often produce more nitrogen than they need and are the best companion crops for pasture plantings. Some of this excess nitrogen is released through the roots of the legume and is available in the soil for plant uptake by the grasses through nitrogen transfer. The amount of nitrogen transferred depends on species of the plants involved, plant productivity, and duration of the growing season. Planting mixed pastures of grasses and legumes is a feasible, low input management practice to help our pastures operate as high-functioning forage factories.
The Aboveground Factory
Keep in mind that legumes and grasses benefit one another while growing together in the pasture; legumes do not have to die before they can contribute nitrogen to neighboring plants. Most producers utilize forage legumes through harvesting, either via grazing livestock or mechanically cutting hay or baleage. These harvesting events, collectively referred to as defoliation, influence the legume’s ability to fix atmospheric nitrogen. Defoliation stimulates root die back in legumes, which can lead to the release of nitrogen-fixing nodules into the soil. Removing above ground plant material, particularly leaves, results in the removal of the sources of energy for the legume plant and root nodules. When these nodules are cut off from the legume root they will degrade and release nitrogen into the soil that is then available for uptake by nearby pasture plants. Once the legume plant recovers from defoliation and regrows leaves, the formation of new nodules will occur, and biological nitrogen fixation will continue. This cycle allows legumes to provide a season-long, time-released source of nitrogen to the pasture system.
The amount of nitrogen fixed by legumes fluctuates considerably depending on legume species, soil conditions, proper inoculation, amount of bacteria available, defoliation, etc. Expectations of different pasture legumes are listed in Table 1 as a guideline:
Table 1. Estimated annual nitrogen fixation by selected forage legumes (lb N/acre/year).
The Complete Forage Factory
Grazing mixed grass-legume pastures is a common practice and can be very beneficial to both the growing animal and the pasture. Not only is nitrogen released in the soil through defoliation by grazing, but some of the nitrogen is also returned to the land through the excrement of the animal in the form of urea. When a grazing animal consumes legume forage it is taking in nitrogen fixed by the legume, much of which is recycled in the form of dung and urine. With proper grazing management, much of this recycled nitrogen will eventually again be available for uptake by pasture plants. In addition, legume leaves that shatter eventually decompose and provide some nitrogen and other nutrients to the soil. A good rule of thumb is if legumes make up at least 30% of the ground cover in a field, no additional nitrogen should be applied. Nodulation and nitrogen fixation is highest in soils that contain minimal amounts of nitrogen. To realize the full benefits of biological nitrogen fixation, application of nitrogen to fields containing substantial amounts of legumes should be minimized or avoided.
When we view our pastures as high-functioning forage factories, it becomes clear that maintaining efficient production depends on both sound forage management and proper resource stewardship. Legumes provide more than high-quality forage for grazing livestock; they also enhance nutrient cycling, improve pasture productivity, and supply biologically fixed nitrogen that supports the entire forage system.
While some legumes can increase the risk of bloat, the likelihood is generally low when legumes make up less than 50% of pasture ground cover. If bloat potential exists, animals need to be monitored and managed to avoid this problem.