Legumes such as clovers and alfalfa make immeasurable contributions to forage agriculture – yield, nutritional quality and improved animal gains. Astoundingly they do all these things while supplying themselves with nitrogen converted (‘fixed’) from the air via their root nodules.
Managing grass-legume stands over time presents farmers with tough questions, such as ‘do I have enough clover to withhold nitrogen?’ Another common question is whether to control broadleaf weeds when doing so will likely take out the clover. University experts commonly recommend withholding N-fertilizer from mixed stands when legumes make up 25% of the stand. I have said it myself. But it is not much of an answer, and this has bothered me for years.
Here are some important findings from the PhD of Dr. Chuck West:
- Legumes do fix large amounts of N, but the highest numbers are from grass-white clover stands in temperate regions with long growing seasons and near ideal growing conditions.
- The amount of N fixed per season shared directly with companion grasses is between 20 to 50 lb/N/A/year, a fraction of total N fixed.
- White clover turns over more N during the growing season because it sloughs root nodules every time it is defoliated. Nodule sloughing is the main way fixed legume N is released directly to the organic soil N pool. This pool is converted to nitrate-N which can be used by the companion grass. In contrast, alfalfa does not slough nodules after harvest. In fact, alfalfa only sloughs its nodules at the end of the growing season.
- The N benefit to the companion grass is more closely related to legume growth and yield in the previous rather than current year (read this again, I had to).
- Adding N to mixed stands increases yield by increasing the yield of the grass (in other words, the grass is not getting enough N in mixed stands).
Fixed N absorbed by the grass increases as legume yield per acre increases AND as stands get older. This yield increase in the grass is due to the buildup of the soil N from the sloughing of N-fixing nodules and legume residue decomposition over multiple years (and from the manure and urine of cattle grazing legumes in pastures).
Another, somewhat controversial ‘so what’ – Grasses in mixed stands are going to be N-limited, guaranteed. Therefore, nitrogen application to mixed stands can be justified (from increased grass yield), even those with good legume content.
Don’t confuse this with N application in the establishment year for clover. Nitrogen should not be applied while clover is trying to become established in existing grass.
What about weeds? Still a tough question. But the downside of clover loss when broadleaf herbicides are used is mitigated by the release of N from the killed legume. The companion grass gets the double benefit of weed removal and a burst of N.
Focusing solely on modest rates of N transfer directly from legume to grass is missing the point, especially in pastures. Most (90% plus) of the nitrogen consumed by the grazing animal is returned in manure and urine.
So legumes are still good and desirable and vital in forage systems, even if they do present some management dilemmas. Producing economic yields in mixed stands means keeping legumes present in high quantities (even 30 to 50%) by weight, year after year. ~ Dr. Jimmy Henning, excerpt from Farmers Pride article, June 20, 2019. Read the full article here.
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