Have you ever wondered where the reduced nitrogen comes from on Earth? It is so important in agriculture and all plant life; it is derived from the atmosphere which is 78% nitrogen. As far as reduced nitrogen on earth, 90% comes from biological nitrogen fixation, and 8% comes from lightning reducing the nitrogen gas in the atmosphere during storms. The remaining 2% is derived from ozone photochemical reactions.
The first thing that comes to mind when a livestock farmer/rancher sees the word “legume”, is nitrogen (N). Legumes are a category of plants that can associate with a specific group of bacteria that can take atmospheric, gaseous nitrogen (N2) and reduce it to ammonium (NH4). Plants can use reduced nitrogen to build amino acids; gaseous nitrogen found in the atmosphere cannot be used for this function.
Nitrogen give and take
I think the part that people overlook is that this relationship is a give-and-take between the plant and the bacteria. The bacteria that can reduce nitrogen is called Rhizobium. They are housed by the legume roots and create nodules to reduce the nitrogen for the plant. The bacteria will require energy, sugars, and housing from the plant. In return, the bacteria will provide reduced N and the plant will engineer amino acids with the nutrient. If you have ever wondered why plants “green up” when N is applied, it is due to the increase in the Rubisco protein and consequently the pigment chlorophyll.
In a situation where there is plenty of available reduced nitrogen, the legume will reject the Rhizobia bacteria because it will not spend sugars and house the bacteria if it will not benefit. A scenario that is overlooked sometimes when the legume has plenty of available nitrogen around it. Many times, in very fertile soils, legumes will not have nodules due to the lack of necessity for reducing nitrogen. I have seen clover seedings fail to nitrogen applied after planting clover due to the grass smothering it out. Grass will take in N faster than most plants and respond accordingly.
There are some differences in what legumes and grasses provide for livestock. In general, grasses are highly adaptive to many seasonal fluctuations. They reduce erosion due to their interconnected fibrous roots, hold in plants that have heaving potential and are great at smothering out weeds when not overgrazed. The legumes will again provide nitrogen to the pasture, are typically higher quality than grasses, can increase the carrying capacity of the pasture-based on the root system and plant structure and will reduce grass tetany.
As livestock owners, certain disorders such as grass tetany or bloat can cause a sudden and serious issue. Either one can result in multiple animal losses. Without going into too much detail, grass tetany can be caused by a potassium and magnesium imbalance. Bloat can be caused by forge quality that is very high in energy and very low in fiber. A well-balanced grass and legume pasture will mitigate these harmful disorders.
As far as maintaining a pasture balance, the way we graze can affect a clover, legume, grass ratio. One of the points mentioned earlier was that grass can smother out weeds. Grass can also smother out clover if kept high in a pasture setting. Grazing down to below 4” allows the clovers present to grow when the conditions are right. You do not have to over-graze to make this happen. Clovers and many legumes do well in the hot summer months.
One strategy is to allow the clover to grow in areas that are desired by allowing the grass canopy to open a bit when it is starting to get warm in the summer. On the flip side, in areas of high weed pressure, taller grasses can help smother out weeds for a more productive area.
Weighing your options
In most pasture settings, the most common legumes are alfalfa, red and white clover and birdsfoot trefoil. Each will offer different advantages and disadvantages.
Alfalfa advantages: excellent quality; very productive; good summer growth; good drought tolerance; excellent option for dual-purpose hay-n-graze (graze in summer).
Alfalfa Disadvantages: requires high pH and fertility (potassium and phosphorus); requires good drainage; prone to insect damage; sensitive to trampling damage; bloat hazard.
White clover advantages: excellent quality; palatable; grows rapidly; spreads by stolons & fills gaps; tolerates poor drainage; can be no-till or frost-seeded.
White clover disadvantages: low yield; lacks heat and drought tolerance; bloat hazard.
Red clover advantages: vigorous establisher; high yields; tolerates wet soil and low pH better than alfalfa; excellent for renovation by no-till or frost seeding, fairly heat and drought tolerant.
Red Clover Disadvantages: generally short-lived; difficult to dry for hay; bloat hazard.
Birdsfoot trefoil advantages: can grow in adverse soil conditions like hard clay soils; salt tolerant (close to roads); does well in Wet and Acidic soils (best at pH 6.5); prevents bloating in livestock.
Birdsfoot trefoil disadvantages: poor seedling vigor; requires a reduction in competition for good establishment.
My recommendation is to work with what is there and manage for the best that you have. In a situation where renovation is desired/needed:
1. Prepare the soil (lime, fertilizer)
2. Control problem weeds the year before seeding.
3. Purchase high-quality seed of adapted varieties.
4. Inoculate legume seed with the correct and viable inoculant.
5. Suppress existing vegetation prior to seeding.
6. Calibrate and check all equipment settings prior to planting.
7. Use appropriate planting equipment and techniques that ensure good seed-soil contact.
8. Plant on time!
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