Improving Legume Persistence in Forage Crop Systems

Forage legumes benefit pastures and hay crops by fixing N, improving seasonal distribution of growth, and enhancing animal performance, but their lack of persistence is viewed as a major limitation. Stand persistence depends largely on plant persistence in crown-forming perennials that do not spread by stolons or rhizomes, but depends on seed production, timely germination, and seedling survival in annuals, biennials, and many short-lived perennials. Stolon- and rhizome-forming perennials can colonize unoccupied areas if management is favorable. Conversely, differentials in seed production, seed dispersal mechanisms, and seed survival allow reseeding annuals, biennials, and short-lived perennials to colonize areas that are more widely dispersed. Several pathogens and insects invade the stand each year, but to different intensities depending on climatic and crop management conditions. Other pathogens and insects reside in production fields and pastures, gradually increasing in population while reducing plant persistence, the seed bank, and seedling survival. Environmental and management stresses weaken plants, which are subsequently killed by combined influences of environmental stresses, resident insects, and pathogens. Improving disease and insect resistance is a major breeding objective for crown formers, but these efforts have to be supplemented by physiological improvement in stress resistance. To improve stand persistence of annuals, management and genetic information is needed on seed production capacity, hard seed content, seed bank management, and optimizing conditions for seed germination and seedling survival. The long-term goal is to improve cultivar persistence, and develop management systems to aid legume persistence in a wide range of grassland ecosystems. Educational programs are also essential. Research QuestionMost producers understand the importance of legumes in forage systems, but management decisions that enhance short-term yields or quality of a grass-legume association can reduce persistence of the legume component. Our objective was to review the biology of legume persistence and evaluate management strategies associated with enhanced plant persistence of crown-forming perennial legumes and reseeding properties of annual legumes and short-lived perennials. Interactions of legumes with biological pests, environmental stresses, and associated grasses were considered. Literature SummaryLegumes enhance animal performance and reduce N needs for grass pastures, but legumes seldom dominate in agricultural or natural ecosystems. Thus, the association must be managed to enhance vigor and persistence of the legume. Legume breeders have emphasized temperature resistance, especially cold adaptation, and disease resistance for crown-forming perennials like alfalfa, but enhanced disease resistance and improved productivity may be at the expense of winterhardiness. Conversely, annual legumes are generally improved by increasing seed production and seedling vigor. The role of hard seed and dynamics of the seedbank are not well understood. An integrated assessment of current approaches and future needs for enhancing legume persistence is needed. Study DescriptionAs part of a national symposium on forage legume persistence, we evaluated the published literature from different perspectives and established futuristic needs for legume persistence in forage systems. Special efforts were made to evaluate potentials for developing persistent cultivars and improving the production environment through crop management. Cutting management, grass associations, soil management, potentials for reseeding, and the disease and insect complexes were considered in terms of the legume life cycle. Applied QuestionsHow important is legume cultivar selection? Cultivars are rarely evaluated for persistence over a long time, but clearly do differ in yield potential and forage quality. Disease complexes build up during the life of the stand. Disease resistance is generally touted as being important for persistence of crown forming legumes such as alfalfa and red clover, but persistence involves more than disease resistance, especially in grazing situations. Foliar feeding insects rarely kill plants, but weaken the stand. Winter injury is related to cultivar, but is also strongly affected by crop management. Annuals and short-lived perennials depend on natural reseeding or a managed overseeding at regular intervals (Fig. 1). The role of hard seed is unknown, and breeding and management efforts for seed production and seedling survival are just emerging. How can I manage a grass-legume stand to enhance persistence? Generally the legume is the most fragile species in the system, so emphasis is placed on managing to improve its well-being. Proper pH and mineral nutrition, especially P and K, generally benefit the legume more than the grass component of mixtures. Cutting frequency or grazing management should favor the legume component. Annual legumes depend on reseeding, which requires management of the grass canopy to allow sufficient seed production and annual seedling emergence and survival for perennation. Applying seed at regular intervals may be the most feasible persistence strategy for some legumes. Will legume persistence continue to be an important issue? Longevity of grass-legume stands will increase in importance as production agriculture is affected more by issues related to sustainability and environmental quality. This will place more emphasis on legume persistence, especially in grazing situations. A need to control erosion on fragile landscapes and changes in government programs on feed grains will continue to place emphasis on long-term forage systems to improve environmental quality, provide wildlife habitat, and restore soil productivity. Legumes will be important components of these systems to provide N and species diversity to the mixture. Persistence will be especially critical in locations where reseeding is not practical. Fig. 1True annuals, such as annual lespedeza or subterranean clover, perennate by reseeding only, whereas crown-forming legumes like alfalfa and red clover depend almost exclusively on longevity of original plants. The solid line delineates the minimal combination of strategies needed for stand maintenance.