Among ranchers, opinion as to the value of forage legumes varies widely. The graziers who can grow them and who understand how to manage them, love legumes. Those who have yet to learn the tricks tend to shy away from legumes as dangerous, not worth the trouble, too expensive or “the darn things won’t grow here”. The following is an attempt to look at forage legumes as they can fit into a graziers’ forage plan. Truly successful forage plans rely on the concept of complex forage swards maximizing days of quality graze over time. This is considerably different from the more common concept of judging pasture solely on the number of tons of forage produced per acre per year. The value of a pound of green forage at any point in time is directly proportional to the abundance of forage at that time. The extension literature is full of figures showing exactly what it costs to grow forage and how to reduce the cost of forage by applying nitrogen fertilizer to increase yields. These figures reflect the short-term cost of the forage but do not reflect the value of this forage in a grazing operation. If coastal Bermuda grass is nitrated in May or if fescue is nitrated in March, yields of forage will be greatly increased. In both cases, the increased production comes during the time period of peak production and aggravates an already severe problem of forage distribution over time. To be consistently profitable, grazing operations must maximize the percentage of the total ration that comes from grazing. No mechanical forage harvest system can come close in cost to the grazing animal. In an on going grazing operation, a major goal should be to provide, in a graze able form, both the quantity and the quality of forage needed to supply the needs of the livestock present for as many days of the year as possible. To do this requires having green and growing forage for as much of the year as is feasible. Plants are classified as warm or cool season according to their growth habits but there are also differences by species and within species as to time of growth within a season. Complex mixtures of forage plants consisting of different species of warm and cool grasses as well as forbs and shrubs both legume and non-legume can capture more solar energy and provide more days of quality grazing than is possible with monocultures or with simple mixtures. The complex mixtures are also much more stable in their ability to deliver forage in the year-to-year time frame.
Legumes are unique among plants in that they have the ability to form a symbiotic relationship with a class of bacteria known as rhizobia whereby the legume furnishes energy to the bacteria and in return the bacteria takes nitrogen from the air and converts it to a form that is useful to the plant. This occurs by the bacteria entering and colonizing a plant root where it draws nourishment from the plant while extracting nitrogen from the soil air surrounding the root hair. Species of rhizobia are specific as to the group of legumes with which they can form mutually beneficial relationships so it is very important that the correct inoculating material is used when planting various legumes. Nitrogen is directly available to the legume hosting the bacteria and becomes available to the plant community at large in a number of different ways. When legumes are consumed by grazing animals a portion of their nitrogen content is excreted by the animals in urine and to a lesser extent manure. The percentage of this nitrogen that becomes useful to the plant community depends upon the condition of the soil surface upon which it falls and upon the amount of biological activity present in the soil. In covered soil well populated with all forms of soil life from bacteria and fungi up through earthworms and dung beetles much of the excreted nitrogen will be captured by some life form while in a bare lifeless soil most of the nitrogen will be lost to the atmosphere or to water runoff. Nitrogen also enters the nutrient cycle anytime that tissue from the legume dies and is broken down by decomposer organisms such as pill bugs, termites and fungi. There is also evidence that nitrogen can be transferred directly from legumes to other plants when the filaments of mycorrizial fungi connect the plants. The amount of nitrogen fixed by legumes varies tremendously depending upon a number of factors but in well managed pastures it can be more than enough to supply the nitrogen needs of all members of the soil-plant-animal complex. An advantage to this method of supplying nitrogen is that the nitrogen is released, as plants need it, slowly over time, being held either by living organisms or in organic matter. This prevents the bursts of growth seen with nitrogen fertilizers and also the loss of nitrogen to either the atmosphere or the ground water in any significant amounts.
Walt Davis © 1998
262 SR 70E
Calera, OK 74730
580-434-4021
wwdranch@brightok.net