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April 2008

Utilizing Forage Legumes as Nitrogen Sink and Source

With nitrogen and chemical fertilizer prices high, legumes such as alfalfa and clovers are an attractive crop becasue they provide free nitrogen to grasses and pasture and some nitrogen credit to following crops such as corn and small grains. In addition to being an integral part of sustainable agricultural systems, forage legumes have potential to increase forage quality and reduce summer slump, susceptibility to pests and disease, and lower unwanted toxicity. Used effectively, legume plants can make your farming more sustainable economically and environmentally.

Doo-Hong Min
Richard Leep
Dept. of Crop and Soil Sciences

Nitrogen prices as well as other chemical fertilizers are at historic highs. This can be a huge financial burden for dairy producers in their forage-based farming across the country. Legumes have been playing an integral role in sustainable agricultural systems by providing high quality animal feed, nectar, seed, green manure, and wildlife habitat. Forage legumes such as alfalfa and clovers can provide free nitrogen to grasses on pasture and some nitrogen credit to following crops such as corn and small grains. That’s why the importance of legumes as a way of lowering the nitrogen fertilizer cost can’t be emphasized enough. Legume plants can play a role as a nitrogen fixer as well as a nitrogen provider. Legume plants fix their own nitrogen using rhizobium bacteria and make ammonia by combining nitrogen with hydrogen in the air and eventually the ammonia is converted to nitrates. This nitrate nitrogen is a plant available form. Legume plants can utilize nitrogen from two sources and there is a mutual relationship happening between nitrogen fixation and manure nitrogen. If dairy manure is applied to alfalfa fields, alfalfa prefers to utilize the free nitrogen from manure first and then fixes its own nitrogen. Therefore, applying dairy manure to alfalfa is an option to increase the acreage of manure application; otherwise, excessive manure might be applied to corn fields. This could result in environmental impacts to ground- and surface water.

In legume-grass mixture, nitrogen fixed by legumes is naturally free nitrogen that can be transferred to the grasses such as orchardgrass, timothy, tall fescue, and even subsequent crops such as corn or small grains. Think about the dollar value of legumes. For example, alfalfa fixes 150 – 250 lb N/acre/yr which is equivalent to $90 – $150 (@ 60 cents per pound of nitrogen) and red clover and white clover fix 75 – 200 and 75 - 150 lb N/acre/yr, respectively. Nitrogen fixation rate in legumes varies with age of stand, soil pH, soil macro-/micro nutrient levels, and degree of pest resistance. As a rule of thumb, if there is more than 40 – 50% of legumes, no nitrogen is required in legume-grass mixtures. That’s why the legume-grass mixture is one of the most efficient systems in terms of utilizing fixed nitrogen. Although there is concern about the possibility of bloat from legumes, bloat is very rare on legume-grass pasture when legumes represent 50% or less. Legumes with high potential of bloat are alfalfa and clovers whereas birdsfoot trefoil and crownvetch are non-bloating legume species.

Legumes generally contain about five times the calcium, 30-50% more phosphorus, and twice the magnesium of grasses. Calcium, phosphorus, and magnesium are very important minerals that are involved in physiological and biochemical processes in the animal’s body because these minerals help maintain milk production, growth, reproduction, and activation of enzymes.

Besides this, including legumes in your pasture will have the following impacts: 1) less summer slump, 2) higher forage quality and animal performance, 3) less plant disease/insect susceptibility, and 4) lower tall fescue endophyte toxicity by diluting ergot alkaloids. Therefore, revisiting the value of legumes and strategies of using legumes will help make your forage-based farming systems more sustainable and profitable.

In summary, legumes are very important forages in terms of nitrogen source and sink in forage-based dairy systems. If legume plants are utilized efficiently, this would make your farming more sustainable economically and environmentally.





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