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Nitrogen Fertilizer Giveaway

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Posted Jun. 30, 2001

Now that we have your attention, we'd like to talk about overseeding legumes into grass pastures. We will discuss some success stories, benefits, drawbacks, and ways to make it work. Conservative estimates are that legumes can provide 20 to 80 pounds of nitrogen per acre during a growing season after the establishment year. So why are we not taking better advantage? There are many reasons, including the need for increased management of pastures and livestock, bad experiences, inability to try new practices, perceived lack of an economic benefit, and perceived risk of bloat.

Overseeding legumes will potentially benefit the higher-rainfall regions east of interstate 35 the most. West of that line, overseeding may not be as successful. Some of the success stories include a five-year test at the Oklahoma State University Haskell Research Station and a twelve-year study at the Texas A&M Overton Experiment Station.

We'll discuss the results at Haskell first. The researchers were able to run 60 cow-calf pairs on 131 acres of a grass and clover mix year-round without supplementing the cattle's feed. Hay fed to the cattle was produced from the same acreage when there was excess. Beef produced averaged 291 pounds per acre. The researchers estimated that equivalent production on bermudagrass or fescue would have required 150 pounds of nitrogen per acre. At $0.30 per pound of nitrogen, the savings would have been $45 per acre.

This project was sustainable for the given stocking rate. It was discontinued to allow space for other research projects but will probably be restarted soon. This research is described in detail in Oklahoma Forage Management Topics (March 1999, vol. 11, no. 13, pp. 97, 103, 192, 195).

Dr. Monte Rouquette, working at Overton, Texas, used bermudagrass that was overseeded with ryegrass and fertilized with nitrogen or overseeded with clover and fertilized with potassium. The twelve-year average gain per acre for suckling calves was 640 pounds with ryegrass overseeding and 450 pounds with clover. The twelve-year average cost per pound of gain on suckling calves was $0.210 cents with ryegrass overseeding and $0.115 cents with clover. Therefore, gain per acre from the clover system was only 70 percent as much as that from the fertilizer nitrogen system, but production cost per acre was 55 percent lower. This research shows that bermudagrass pastures can be sustainable and productive without nitrogen fertilizer. The results are in the TAMU-Overton Beef Field Day Report, April 2000, Research Center Technical Report No. 2000-1 (Texas Agricultural Experiment Station, pp. 97-98).

Benefits of legumes in pastures include the following:*

  1. Lower nitrogen fertilizer cost. Legumes fix nitrogen, reducing the amount of commercial fertilizer nitrogen that is needed.
  2. Better forage quality. Legumes generally have higher crude protein and nutrient content than most grasses.
  3. Better distribution of growth. Legumes produce forage earlier or later than grasses, extending a pasture's growing season.
  4. Increased forage yield. Legumes increase total forage when added to a grass pasture and increase grass production because of the nitrogen they fix.
  5. Reduced risk. If you lose a species in a monoculture, you have lost the entire pasture. If you lose a species in a mixed pasture, you still have the other species.
  6. Benefits in crop rotation systems. Legumes improve soil structure and tilth.
  7. Reduced animal toxicities. Legumes mixed with fescue can dilute the effect of the endophyte, a fungus that lives in the grass.
  8. Environmental acceptability. Legumes provide nitrogen without depleting the earth's natural resources. They also provide food and shelter for insects such as bees and wildlife such as birds and rabbits.
  9. More interesting and attractive pastures. Flowers on most legumes are colorful.
  10. Increased profit. Legumes can decrease feed and fertilizer cost and increase production and animal performance.

* From 10 Great Reasons for Growing Clover by Don Ball and Garry Lacefield (Salem, Ore.: Oregon Clover Commission, 1997).

There are things to think about when you're planning to use legumes in a mixed-pasture program. Legumes need a higher pH (6 to 8) than grasses do, in part so that the bacteria that fix the nitrogen can function effectively. Since legumes produce their own nitrogen, growers should shift fertilizer emphasis to phosphorus and potassium. Lime, phosphorus, and potassium are three factors that Tommy Pickard at the Haskell Research Station cites as important. John Spain, well known in the rotational grazing world, stresses that you have to feed clover, give it light, and let it reseed. Feeding refers to using adequate amounts of phosphorus and potassium. When grasses get more than 8 inches above the clover, remove them by grazing or haying so the clover can get light. For annual clovers to perennate (survive from season to season) by reseeding, they need to be deferred for a time each year so they can flower and drop their seed. If you can't allow the plants to reseed, add seed at a reduced rate each year to maintain the stand. According to Bob Woods, an Oklahoma State University extension agent, the more legume species you have, the better, because you never know which species will do well in a given year.

So what does it take to introduce and sustain legumes in a pasture? The answer is management, pasture and soil management. For legume establishment, you'll need these conditions:

  1. soil phosphorus and potassium above threshold levels,
  2. pH within optimum ranges,
  3. pastures grazed or mowed short before planting,
  4. adequate soil moisture,
  5. moderate temperatures, and
  6. a variety of legumes that will do well in the local soil type and climate. Apply proper inoculants to seed just before planting (different species of legumes require different inoculants).

The table below, compiled by Gerald Evers and Ray Smith and published in the Texas A&M University Agricultural Research and Extension Research Center Technical Report No. 98-3, lists several broad categories of clovers and their preferred soil characteristics, some plant characteristics, and recommended seeding rates. Other cool-season legumes are alfalfa, birdsfoot trefoil, hop clover, singletary pea, vetch, and winter pea.

Don't forget that legumes tend to be more successful east of I-35 because of higher average rainfall. Adequate moisture is especially important at planting. Most legumes are planted in the fall, preferably in September or early October, once average daily high temperatures have fallen below 90°F.

Soil pH is the next most limiting soil characteristic for stand establishment. It should be above 6.0 for most legumes (see the tables). If the pH is below 6.0, you can add lime to raise it. It's not prudent to attempt legume establishment until the soil pH is within the prescribed range for a specific variety.

The most critical soil nutrients for legume establishment are phosphorus and potassium, with phosphorus being the most limiting. Ideally soil phosphorus levels based on the soil test index should be 40 or higher. Soil potassium levels should be 220 or above. If levels are low, you can add soil nutrients at or before planting. Test the soil well before the intended planting date.

Assuming you've selected a suitable variety and properly inoculated it, you need to consider seeding rate and depth. The tables indicate seeding rates for clover varieties. The seeding depth should never be more than four times the diameter of the seed, which means most legumes are planted at or near the soil surface. Seeding methods for pastures may vary from broadcast planting to drilling by using a small seed box. If you broadcast plant, pulling a drag or harrow over the seeded area will enhance seed-soil contact. Pack prepared seedbeds before planting. A Brillion seeder or similar product developed specifically for planting small seeds works best on prepared seedbeds.

Minimize grazing on planted sites until the stand is established. After establishment, moderate grazing during the fall and winter is acceptable. However, most legumes produce little grazeable forage until the late winter or spring. Grazing pressure may be required when winter annual grasses are abundant and threaten to block the legumes' light. For the greatest fertilizer benefit, stockpile legumes or graze them moderately throughout the spring until they set seed. After seed set, grazing the legumes helps reseed the stand and removes the legume forage, preventing shading of the pasture grasses late in the spring. On large acreages of legume pasture, you may have to hay or mow the legumes in the late spring to allow warm-season grass production. Legumes work well under rotational grazing management, since pastures can be managed for grass residue height, fall establishment of legumes, grazing intensity, rest and recovery, and reseeding, all of which are important controllable factors in legume management.

Unless there is a potential for seed harvest, planting a mixture of legumes in pastures is a good practice. For example, arrowleaf clover, crimson clover, and hairy vetch would be a good mixture for sandy soils, especially in central Oklahoma and north central Texas. White clover, ball clover, berseem clover, and subterranean clover would be a good mixture for clay soils that are fairly or poorly drained. On good loamy soils, most legumes perform well. A recommended mixture for eastern Oklahoma is red clover, ball clover, white clover, and crimson clover. In northeast Texas on loamy soils, a suggested mixture is crimson clover, arrowleaf clover, hop clover, and rose clover. Specific mixtures and varieties are determined by local availability and seed prices. A good forage reference book that addresses legumes and grasses is Southern Forages by D. M. Ball, C. S. Hoveland, and G. D. Lacefield (Norcross, Ga.: Potash and Phosphate Institute, 2nd ed., 1996).

Once legumes are established in a pasture, planting additional seed annually or every other year but at a much reduced rate is a good practice, especially if seed production was poor because of poor moisture or excessive grazing pressure. Legumes can be a substantial component in the total forage production system if you pay proper attention to their management. Since legumes are more susceptible to mismanagement than most perennial pasture grasses, to sustain them in pastures, you have to carefully manage soil, forage, and grazing.

Most of this article is geared toward cool-season legumes. The same principles apply to summer legumes, but because of the dominance of warm-season grasses and the extreme heat and dryness during the summers, warm-season pasture legumes are usually less successful than cool-season pasture legumes. There are several available, such as the striate lespedezas (Korean, Kobe, and Marion), cowpeas, and alyceclover. Most warm-season legumes are planted in the late winter (lespedezas) and spring (cowpeas and alyceclover).

Legumes are not a quick fix. They need at least a year of establishment before nitrogen fixation does the pasture any good. They are also not a cure-all, but with management they can enhance forage production systems.

We would like to thank Tommy Pickard, Bob Woods, John Caddel, Larry Redmon, Monte Rouquette, and John Spain for their contributions to this article.