Legumes have long been used in forage production to provide an organic source of nitrogen (N) as well as to improve forage quality. In native grasslands of the eastern U.S., there were an abundance of forbs, including legumes, that were a part of these plant communities. Although grasses produced the overwhelming majority of the biomass, in terms of numbers of species, the forbs were far more dominant. Inclusion of introduced legumes, species such as red or ladino clover, and native forbs have both received increased attention in recent years. I am often asked whether or not such increased diversity in the pasture is beneficial, if it can be done and how it can be done. These subjects are addressed in this [article].
Introduced legumes
Perhaps because we are most familiar with them, let’s start with the legumes used so often in forage production. In a series of studies conducted at the University of Tennessee, red, crimson, white and arrowleaf clovers, hairy and common vetch, and alfalfa were drilled into established switchgrass stands. These stands were managed as either a biomass crop (single, post-dormancy annual harvest) or were harvested once for hay in early summer (typically mid-June) and then as a biomass crop that fall.
In this environment, with limited light able to reach the seedlings beneath a robust switchgrass canopy, most of the legumes did not make a large contribution to the stand. For example, alfalfa seedlings were abundant initially, but few if any plants recruited into the stand canopy. Of the seven species included in these trials, the most successful was red clover, perhaps because of its upright growth habit and seedling vigor. However, ladino clover, although less abundant, had the best persistence among these species, likely because of its stoloniferous growth habit.
On the other end of the spectrum, with crimson clover and the two vetches, a serious problem became apparent. With fall-planted crimson clover, stand development by the following spring was very good and the canopy shaded the switchgrass enough to retard growth and thin the stand. Similarly, the vetches could quickly overtop the switchgrass because of their vining growth habit and as a result had a negative effect on the grass. Comparable studies conducted in Iowa and Virginia had similar results. In these studies, red clover also proved to be a good choice in terms of establishment. But studies in both states reported substantial reductions in grass yield, roughly 60% with red clover, as a result of competition with the legumes. In the Virginia study, switchgrass tiller density was reduced as well, in some situations by as much as 50%.
The studies in Iowa evaluated some additional introduced legumes that were not included in the Tennessee studies. These were white and yellow sweet clovers, crown vetch and bird’s-foot trefoil. Because these four forages, like the seven studied in Tennessee, are all cool-season legumes, they grow well in fall and again in early spring. As a result, if they develop vigorous stands, they can present substantial competition to the warm-season grasses. This is exactly the same problem mentioned in regard to cool-season competitors encroaching into warm-season grass stands. Whether that competitor is a cool-season perennial grass (e.g., tall fescue), winter annual broadleaf weed or interseeded cool-season legume, it can have the opposite effect of prescribed fire by shading the warm-season species and maintaining ground cover that keeps soils cooler for longer. The natural consequence will be delayed dormancy break, slowed growth and, eventually, weakened warm-season grasses.
Warm-season legumes would avoid these issues and be more compatible with the growth season of the warm-season grasses. Unfortunately, the options for warm-season legumes are much more limited. While I have not ever tried to use cowpeas, I suspect their trailing habit could become a serious problem much like the vetches. Sericea lespedeza, which is used on a limited basis as forage in parts of the southeastern U.S., can quickly become a pest. In fact, there are a number of states that consider sericea lespedeza a noxious weed. The Virginia study mentioned above evaluated annual lespedeza, which is a warm-season species, and found that it established well in switchgrass and proved to be a good reseeder producing well in the second year of their experiment. Because of its seasonality and small stature, it did not result in undesirable competition to the grass.
Across these various studies, the interseeded legumes provided organic N. However, where establishment was poor or the seedlings did not recruit well into the canopy, the amount was negligible. On the other hand, well-established legumes generally provided enough N to replace inorganic input requirements, about 60 pounds per acre. Likewise, crude protein (CP) increases in the sward were only evident where substantial volumes of legumes were present in the canopy. Unfortunately, the observed benefits with respect to providing organic N and improved CP were only apparent with stands of legumes that were near or at the point where they had a detrimental impact on the grasses.
When and how to interseed
Our experience with establishing cool-season legumes confirms existing forage production recommendations: Fall establishment of cool-season legumes is considerably more effective than similar plantings conducted during early spring. However, such plantings also come at greater risk to the warm-season species because the legumes may become very well established and by spring overtop the yet-dormant grasses. Moderating seeding rates appears to be of some benefit in minimizing this risk, but stand density can vary a great deal for any given planting rate.
Another consideration for timing interseeding legumes into native grasses is that it should only be done once the grasses are well established, following the third summer. Such well-established stands will be much better able to compete with cool-season legumes should they become too rank. Consequences to long-term vigor and productivity of the native grasses will be much greater in younger stands. Also, do not attempt to establish both the grasses and the legumes at the same time. The slower (and later) developing native grass seedlings will be outcompeted by the legumes, and stand failure is likely.
Sowing into existing native grass stands may not be effective due to the heavy thatch that could minimize seed-soil contact. For that reason, drilling is a preferable approach. Where growth and/or thatch is heavy, some treatment may be required prior to drilling to ensure proper drill operation. Although haying or clipping could work in preparation for fall planting, grazing would be preferable at this time of year because of the less severe impact to the canopy compared to mechanical treatments.
Another excellent option for spring plantings, one that would make sowing more likely to succeed, would be to plant in March following a burn. Because such a burn would have to be conducted in February or early March to ensure timely planting, this approach may not be advisable where there is already a problem with cool-season weeds.
As with any legume planting, be certain the field into which you plan to plant has not been treated with herbicides with soil residual activity that may be detrimental to the legumes. Also, be sure that if legumes have been absent from the field for a number of years, you identify and use the appropriate inoculant for the legume species you are planning to plant.
One other important consideration is soil pH. For native grasses, so long as pH is not extremely low, growth is not impeded. However, many legumes require considerably higher pH to thrive. Therefore, if you plan to interseed legumes, be sure that pH is above 5.8 for red and white clovers and near 6.5 for alfalfa. .png){kind=icon}
This article was derived from Patrick Keyser’s new book, Native Grass Forages for the Eastern U.S.
