March 2010 Articles

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Legumes in the Forage System

Legumes in the Forage System Nitrogen is required in large quantities for high levels of forage production, & nitrogen deficiency is a common limitation to forage/livestock production. There are several sources of nitrogen for plant growth. Minute quantities are supplied by free-living soil bacteria and blue-green algae. Rainfall typically adds a few pounds per acre, mainly converted from atmospheric nitrogen by lightning. Most commercial nitrogen fertilizer is produced by, under conditions of high temperature and pressure, utilizing natural gas for the production of ammonia. Legume plants (clovers fall into this variety of forage) also produce substantial quantities of low-cost nitrogen. The unique ability to obtain nitrogen from the air helps make legumes especially valuable in forage programs. The nitrogen they produce can be used for their own growth, by associated grasses, or by other grasses grown in rotation. In addition, pasture legumes can ultimately provide nitrogen for plants a considerable distance from where they are growing because much of the nitrogen taken into the bodies of livestock is recycled in manure and urine. Most legumes have a mutually beneficial relationship with bacteria in the genus Rhizobium. The bacteria infect the roots of legume plants from which they obtain food, and the bacteria obtain nitrogen from soil air and fix it in a form usable by the plants. The nitrogen is accumulated in small appendages called nodules that form on legume roots. Nitrogen fixation is influenced by many factors including Rhizobium population in the soil, temperature (77̊ to 84̊ is optimum), soil moisture, soil pH (less than 5.8 can quickly reduce Rhizobium numbers), soil fertility, amount of shading, and legume species. Good nitrogen fixation requires that large numbers of live Rhizobium bacteria are present on legume roots. Assurance that this will occur can be obtained by inoculating the seed (putting bacteria on the seed). Only packages of Rhizobium with labels which specify the legume that is being planted, and for which the expiration date has not passed, should be used. Using an adhesive in applying the inoculant increases the effectiveness of inoculation. Prior to use, the inoculant should be stored in a reasonably cool place (refrigerated) and out of direct sunlight. Many inoculation failures have resulted from inoculants being carelessly left in the heat of a pickup truck cab or similar locations. Much of the commercially marketed seed is pre-inoculated and thus does not require inoculation by the producer. This seed is coated using a commercial process that involves putting an adhesive on the seed, then applying the proper legume inoculant. This coated seed offers the advantage of higher populations of Rhizobium bacteria attached to the seed than would normally be the case with on-farm inoculation. In addition, the coating helps protect the inoculant and ensures longer bacteria survival in the soil. Amounts of nitrogen (lb/A/yr) fixed by various legumes vary greatly depending on conditions, but good legume stands range from about 50-150 for annual clovers and vetches, and 75-200 for white clover and red clover. With ammonia nitrate currently trading at 17¢/lb, we can associate a value of $9-$26 (A/yr) with annual clovers and vetches, and $13-$34 (A/yr) with white clover and red clover. In closing, it is important to keep in mind these thoughts when making the decision to incorporate legumes into your forage system: 1) legumes generally produce higher quality forage than grasses and digestibility falls less rapidly with maturity, 2) legumes generally contain higher levels of protein than grasses, 3) the hot, dry conditions often encountered at planting time in the South can greatly reduce numbers of Rhizobium bacteria very quickly. Poor inoculation and resulting poor nodulation and nitrogen fixation are major reasons why production of legumes is often difficult in the South, and 4) take precaution that many popular pasture herbicides used in the South are not labeled for use on permanent pasture with legumes. To help farmers make logical choices among varieties, the LSU AgCenter conducts variety trials and makes suggestions each year. These 2009-2010 suggested varieties of clovers are as follows: More information on these varieties including seeding rates, planting depth and seeding dates can be found in LSU AgCenter publication number 2334 “2009-2010 Cool Season Pasture & Forage Varieties” available at parish LSU AgCenter offices, or downloadable from . Some information is this article was obtained from the 3rd edition of Southern Forages by D.M. Ball, C.S. Hoveland, and G.D. Lacefield. Source: Jason E. Holmes, County Agent, Union Parish