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How legume and grass systems actually work

Chad Hale for Progressive Forage Grower Published on 30 March 2016
Two week old hairy vetch seedling is already fixing nitrogen

There are two schools of thought when it comes to adding nitrogen to mixed stands of legumes and grasses. The first school of thought is this: legumes provide all the nitrogen that the accompanying plants need, so additional nitrogen fertilizer is not needed.

The second school of thought says “to produce top yields, nitrogen must be added to mixed stands, with the amount of nitrogen decreasing with more legumes in the stand.” Which is correct?

By way of introduction, let’s consider why we plant alfalfa or clover for forage. We are hoping in some way to capitalize on the ability of these legumes to convert atmospheric nitrogen gas into a form that is usable as a plant nutrient.

The producer may want to reduce the need for nitrogen fertilizer by planting a legume, may want to take advantage of the higher protein levels in legume forage for their livestock or both. This article will deal mainly with the fertilizer aspect, but as we will discuss, the high-protein forage aspect is related.

As we think about how this system works, there are some key facts that we need to keep in mind and some misconceptions to clear up. In biology class we learned that it is actually the bacteria living in the root nodules that do the work of nitrogen fixation. In this symbiotic relationship, the bacteria are given sugars for energy by the host plant and in return, the plant is given nitrogen it can use for growth.

Misconception #1

The problem with our definition is the word “given.” That implies something for free. Nothing about nitrogen fixation is free to the plants or bacteria! At peak growth, many legumes direct 50 percent of their total energy derived by photosynthesis to the nodules to fuel nitrogen fixation. This leads directly to the next item to clear up.

Misconception #2

We act as if legumes will freely share their nitrogen with neighboring grass plants. If plants are devoting half of their resources to manufacturing their own nitrogen, they cannot afford to simply give it away.

If legume plants aren’t freely giving all of this nitrogen to their neighbors, where is it? Again, we visualize a system where the soil within a few feet of a growing alfalfa plant is just chock-full of plant-available nitrogen.

The truth is that 60 to 70 percent of the nitrogen in legumes is in the leaves and stems of the top growth. Also, 30 to 40 percent or so of the nitrogen that is in the roots and crowns is being saved as a reserve for later plant growth.

Maybe you are like me – I didn’t have the right picture of how this legume system was really working. However, taking a step back, the system does work and has for thousands of years in agriculture all over the world.

Somehow, nitrogen from legumes is getting transferred to neighboring plants. The final piece of the puzzle is how our individual management affects the system.

The bottom line is that to get nitrogen from the nodules to a neighboring grass, we have to stress the legume. Consider a grass-alfalfa hayfield with the alfalfa at bud stage. The alfalfa plant is devoting half its energy from photosynthesis to the nodules at that point.

If we come along with a mower and cut the plants off, they can no longer conduct the photosynthesis to feed all those nodules and roots, so some of them die. The dead roots and nodules decompose and then the nitrogen from the legume is available to neighboring grass plants.

The same thing would be true of a cow grazing off a grass-clover pasture. The stress of defoliating the clover is the action that facilitates the transfer of nitrogen to the grass. So what we are actually getting from the legume component of our mixed stand is the nitrogen from some of the roots and nodules as they decompose. But there is one last piece of the puzzle.

Getting back to the beginning of the article, do we need to add nitrogen to mixed stands or not? The biggest factor is our harvest management. If the vast majority of the nitrogen in a legume is in the leaves and stems, following where the forage goes will tell us where our legume-based nitrogen is going. If I mow the field and sell the hay, I just sold the majority of my legume nitrogen.

If I graze the pasture, some of that nitrogen is leaving the field as meat or milk, but the amount is minuscule. Nearly all that nitrogen is going back onto the pasture in feces and urine. As a middle ground, hay that is fed back onto the field returns some of that legume nitrogen back to the field.

In conclusion, we almost always see an economic response to nitrogen fertilizer in mixed hayfields with split applications totaling 100 pounds of nitrogen or more. Thinking we can sustain a hayfield where so much nitrogen is exported is flawed thinking.

In pasture systems, we can get closer to a sustainable closed system using legumes in place of nitrogen fertilizer, but the truth is there is often an economic response to adding 50 pounds of nitrogen or more per acre per year in pastures too. Obviously, as the percentage of legume in the stand goes up, the need for nitrogen fertilizer decreases.

Experiment on your own operation and conduct soil testing to evaluate what you need to do, but don’t just assume having a legume present will eliminate the need for extra nitrogen inputs.  FG

PHOTO: This two-week-old hairy vetch seedling is already fixing nitrogen. Notice the pink-colored nodules along the main taproot. While these nodules are what we think of when we think of legumes, they don’t contain nearly as much nitrogen as the top growth. Photo by Chad Hale.

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