Corn growers who also grow alfalfa and other legumes can save a lot of money by cashing in on the nitrogen credits available from older stands.

“As fertilizer costs go up, the value of nitrogen credits go up accordingly,” points out Glenn Shewmaker, University of Idaho extension forage specialist.

Shewmaker says a variety of factors influence the level of N credits available from alfalfa stands.

“In general, you can figure a credit of anywhere from 100 to 200 lbs/acre, depending on things like soil type and stand density at the time of plow-down,” he says. “At the higher end, the credit will provide most or all of the nitrogen needed for a following corn crop. You also get some nitrogen carryover for the second year as well (typically about half the credit of the first year).”

How you calculate the nitrogen credit will vary by state and region. For example, University of Wisconsin researchers figure that, on medium or fine soils with a stand density of more than four plants per square foot, the nitrogen credit from alfalfa works out to 190 lbs/acre (see table). Credits decrease on sandy/light soils and as stand densities drop.

A similar table from the University of Minnesota puts the credit for a stand with more than four plants per square foot at 150 lbs/acre, while researchers at Kansas State establish the credit at 100-140 lbs/acre.

“You need to check to see what's specifically recommended for your area,” says University of Wisconsin extension agronomist Dan Undersander.

Depending on your location, you'll also want to base calculations on the amount of regrowth after the last harvest the previous year. The Wisconsin numbers are based on at least 8” of regrowth. If it's less than that, subtract 40 lbs from the credit.

“Last year we had very good yields in many parts of the Midwest, so a lot of producers decided not to take a fall cutting,” says Undersander. “As a result, there was a lot of residue out there when we had a killing frost. That means it should be a good year to consider cashing in those N credits.”

Idaho's Shewmaker says the 8” guideline may be less of a factor in areas where alfalfa is irrigated. “With irrigation, we end up with a fairly substantial root mass and that's where the nitrogen is,” he says. “With shorter stubble in the field, you also have less potential for damage from rodents. They don't have as many places to hide.”

As you calculate the credit, remember that not all of the nitrogen from an alfalfa plow-down will be immediately available at the start of the season.

“You'll want to consider using a starter fertilizer,” Shewmaker says. “It gives you a little bit of extra insurance that you'll get the crop off to a good start. If you're working with a custom operator, most of them are set up for that, anyway.”

He notes that alfalfa growers in his region typically keep fields in alfalfa for five or six years, slightly longer than their counterparts in other parts of the country. Shortening the rotation by a year or so to capitalize on nitrogen credits is worth considering.

“People use the excuse that re-establishing a crop is expensive in terms of time and money,” he says. “But new alfalfa varieties are coming onto the market all the time. After a stand has been established for four or five years, it's likely that you'll be able to find a better variety than you have out there now. Couple that with the nitrogen credit and it's pretty easy to justify re-establishment costs.”