The question of whether pasture can run on legume nitrogen (N) alone always arises when fertilizer prices increase, as is the case today. As with most serious management questions, this is a classic “it depends” scenario. Everything from geographic location to how often you rotate cattle can affect the answer.
There were only three occasions in which we ever put N-fertilizer on any of our pastures over the 22-year period we were on our Missouri grass farm. Otherwise, we ran on legume N and the natural nutrient cycle.
Most of the pastures at the University of Missouri Forage Systems Research Center were also legume-based. We used N-fertilizer at the research station for fertilizing stockpiled tall fescue and winter annual pastures for winter grazing. Very little N was ever used during the spring or summer seasons.
Some legumes can fix enough atmospheric N to provide N in amounts equivalent to around 200 lbs. of N-fertilizer/acre. Alfalfa and white clover are powerhouses of N-fixation. Many of the other clovers, trefoils and vetches can also fix substantial amounts of N. In many areas, this is a very cost-effective way of pumping N into the system without relying on N-fertilizer. So why doesn't everyone just rely completely on legumes and nutrient cycling?
Geography is an issue
The first big issue is geographic location. Conditions become less favorable for growing legumes as you move north to south across the U.S. Most perennial legumes have higher pH and soil nutrient requirements than most pasture grasses. Young soils tend to have a higher pH and are much richer in base minerals than older, highly weathered soils.
Northern soils are relatively much younger than southern soils due to the history of glaciers on the landscape. Some northern soils are only 7,000-10,000 years old and are rich in nutrients. Many southern soils were already leached of nutrients before farming began a couple of centuries ago. Crop farming further depleted them, making it much more difficult to grow legumes in the South compared to the North.
Adding lime and other fertilizer nutrients, including some micronutrients, may be necessary to establish and maintain legumes in your environment. Because many of these additional nutrients are more stable in the soil than N, once their level in the soil is brought to a certain sufficiency level, they're easier to maintain in the soil than N.
Most of the common legumes used in pasture are cool-season species that aren't well adapted to the hot, humid summers of the South. While some may do reasonably well in spring and fall, they don't have a long enough season to provide all the N the pasture can use. There are some warm-season legumes, however, and southern researchers continue to make progress in identifying new possibilities.
Add in higher rainfall and sandy soils, particularly in the Southeast, and much of the N leaches out of the soil regardless of the source. The bottom line is cattlemen in the South have historically been much more dependent on N-fertilizer than their northern counterparts, but there's a real debate on the relative profitability of grass+N vs. grass+legume pastures in the South.
While N fertilizer drives a high level of pasture production, it comes at a tremendous cost. Warm-season grass pastures with legumes typically don't produce near as much forage as N-fertilized pastures. They do, however, produce better individual gains at a lower cost, and several studies have shown them to be more profitable per acre than N-fertilized pastures.
As we move east to west across the country, we also see declining N-fixation rates until we reach irrigated land where fixation rates can equal those in the Midwest and Northeast. With the exception of alfalfa, most dryland legumes on the Plains fix relatively small amounts of N. On irrigated lands of the Intermountain West, N-fixation rates increase moving from north to south due to increasing length of the growing season.
Mixed grass-legume pastures under irrigation can produce tremendous yields without any N-fertilizer inputs. The pastures we manage under center pivots in Idaho are remarkably similar in composition to the pastures we had in Missouri. Where we have a strong legume component, pasture yields compare favorably to N-fertilized pastures, without the cost of N-fertilizer.
Whether you can run your pasture program entirely on N coming from N-fixing legumes depends a lot on your environment. In less favorable environments, added inputs of lime and other soil nutrients may be needed to allow legumes to thrive. In the long run, this may be more profitable than relying on N-fertilization to maintain pasture productivity, especially as N prices continue to rise.
Jim Gerrish is a grazing management consultant based in May, ID, and former lead pasture researcher at the University of Missouri's Forage Systems Research Center in Linneus. Reach him at 208-876-4067, email@example.com, or visit www.americangrazinglands.com.