Over the past two years, as the price of nitrogen has risen steadily, there’s been more interest in legumes on pasture.

Last December, ammonium nitrate was running about $450 per ton and urea was a bit higher. Since then, both have gone up another $150 per ton. In December, diammonium phosphate (18-46-0) was at $575 per ton. Now, it costs over $850 and projections claim that, before year’s end, it will hit $1,000.

With such hefty input costs, “you’ll have to better manage your pastures, hit the target when you do have to fertilize, make use of any natural source of fertilizer — like legumes — and try to get as much as you can without overgrazing,” said John Jennings, University of Arkansas forages professor at the Livestock and Forestry Branch field day outside Batesville, Ark., on April 15. “We can grow annual legumes or perennials. Some are clovers, some not. And there are differences in how they should be managed.”

Jennings tackled a common misperception: it isn’t legumes that fix nitrogen, but the rhizobia bacteria living in root nodules. The legume — hairy vetch, clover, lespedeza, alfalfa — provide a home for that bacteria.

“Check out the roots on this alfalfa plant,” said Jennings to the crowd. “The little knots all over them are where the bacteria live. If you pinch one open and it’s pink inside, that means it has living, active N-fixing bacteria. If it’s green or white, the nodule isn’t doing anything.”

The N fixation is a symbiotic process. The legume produces a carbohydrate that feeds the bacteria. The bacteria can then fix N out of the air and soil. The extra N is available for the legume.

As for fertility management, the bacteria has nutritional requirements and so do the legumes. “You’ve got to take care of both.

“Lots of our legumes can grow in more acid soils than the bacteria can survive in. That’s why soil pH needs to be at 6, or above, to care of the bacteria even though the clover might be find growing in a more acid pH. If we want the free N fixation, you have to take care of the bacteria.

“Another point: if you’re growing legumes to get the N value from fixation, you have to manage for it. These bacteria aren’t necessarily generous. They won’t fix tons and tons of N and exude it into the soil for every plant surrounding. They take care of themselves first and the clover gets the remainder.

“The way you make N fixation work in a field is to get the plant to go through a cow and cycle back onto the field through animal waste. You need to do a little more rotation in your grazing — rotate to where the waste needs to go. That’s important to keep in mind. Spread the manure out as much as possible — improve the uniformity of the nutrients in the plant.”

Research at Texas A&M shows that in crimson clover, 90 percent of the N is in the plant’s top growth. While root nodules are the N “factory” — that isn’t where it’s stored.

“How to get it cycling back to the field? Grazing is one way. You can cut it for hay and take it off. There’s some in the roots, of course, and after harvest the plant left will break down and release the small amount of N. It’s a slow-release.”

Cattlemen can get a lot of dry matter from a legume and from incidental grasses in the pasture. Research in Minnesota showed that legumes can share N within an 8-inch radius. “If you’ve got a good distribution of legumes in a field, you can get some N sharing that’s beneficial.”

However, “if you put on N fertilizer or manure, you’ll shut down the legume’s fixation process. The N isn’t toxic to the legume. The bacteria in the plants will use the (applied) N instead of fixing its own. That means if you’re managing for legumes, then manage for them. Don’t put unnecessary N on the fields. It’s one or the other — halfway is a waste of money.”

Standing in a pasture largely planted in just-blooming hairy vetch, Jennings said the winter annual is easy to grow. It has large seeds, is easy to plant into bermudagrass or even fescue and germinates quickly.

Although it doesn’t produce a lot in the fall, it makes up for it in the spring. From April to early May, it does very well.

As far as nitrogen fixation, hairy vetch has “a moderate N fixation capacity. The reason I say ‘moderate’ is its growing season is fairly short. It doesn’t produce much in the fall. The spring is when its N fixation begins. It matures out and dies in May and N fixation is over.

“So there’s a short fixation window. It may have a rapid rate of N fixation for a while, but it only keeps that up for a short time.

“It makes a lot of forage and grazing on it works well. A lot of dairymen used to plant it with wheat. The wheat would grow up and provide the vetch something to vine up on. It would be off the ground a little, and later they’d chop it for silage.”

Chopping vetch works fine. “But if you’re trying to bale this stuff for hay, it’s going to be a problem.” The plant’s vine-like structure means, “it’s tough to rake. It’s almost like rolling up a rope. It’s really difficult to bale it properly. It’s a better grazing or silage crop than a baling crop.”

Crimson clover, which turns bright red in the spring, is almost ubiquitous alongside Mid-South highways. It’s a tap-rooted plant and is just beginning to bloom now. The clover is vigorous and has large seeds so it’s very easy to grow.

It can be planted by broadcast although it’s “best if you do some residue scuffing in the bermudagrass first. It comes up vigorously and grows from mid-March to late April when it blooms. Then it dies.”

Crimson clover will fix a “fair amount of N, but because it has a short window to grow, it doesn’t fix a tremendous amount. Some research shows it can fix up to 75 to 100 pounds worth of N per acre, per spring.”

Because it grows so early, crimson clover often works well in bermudagrass pastures. “By the time you get it off through grazing, hay or silage, the bermudagrass will have opened up. And whatever N the clover contains will then be available to the bermudagrass. That’s also true with vetch.”

One key difference between hairy vetch and crimson clover is in seed type and germination periods. Vetch sets seed fairly well, even under grazing. It also does a good job of reseeding.

On the other hand, crimson clover has a lot of soft seed. “That means every time a summer rain hits, that soft seed may want to germinate. If it does so during the summer, the clover plant will die — it’s too hot and dry. So there needs to be a lot of seed set so some will be available when fall conditions arrive and the seed can germinate and grow.”

Arrowleaf clover is another annual cattlemen might consider.

When the tap-rooted legume first comes up it resembles white clover. “But the stems are bigger than any white clover stem you’ll find. That’s one way to tell the difference early on. As the plant gets older, though, the leaves begin to elongate and take on an arrowhead shape.”

Arrowleaf clover is planted in the fall and produces a lot of small, hard seed. Once a good seed crop is in the ground, “all you have to do is scuff the soil surface a little. That stimulates the seed to break dormancy and germinate in the fall. You can keep that up year after year. It works well for hay, for wildlife, for grazing.”

Another winter annual is subterranean clover — or “sub clover.” The architecture of the plant means it’s low growing and doesn’t get tall enough to be a good hay crop. But it’s good for grazing and fixes a “good amount of N. If you can get a good stand and don’t overgraze it, it’ll reseed fairly well.

“The seed is fairly large. Once established, it grows densely and works especially well in bermudagrass. It grows in the spring when bermudagrass is dormant.”

However, sub clover is a bit sensitive to cold weather. “It’s better adapted to the southern half of Arkansas.”

e-mail: dbennett@farmpress.com