Interseeding summer annuals into cool-season grass sods: explaining the magic

By Dale Strickler

One of the real drawbacks of cool-season perennial grasses, like brome, fescue, orchardgrass, or bluegrass is that they become quite unproductive in the heat of summer, often not growing much at all, despite there being quite a bit of available sunlight in these months. This unproductive period is called the “summer slump”.

A few innovative producers are interseeding summer annual forages into cool-season grass pastures once those grasses have begun their “summer slump” and have achieved some fairly amazing results, some expected as well as some unexpected results.

What was expected was that the summer annual forages (like BMR sorghum-sudan, BMR 84 grazing corn, okra, sunn hemp, buckwheat, and cowpeas) would add greatly to the summer productivity of the pasture, which they did. It was also expected that these highly productive forages would be very competitive against the perennial grass base, and the growth of the brome or fescue would suffer. To the contrary, many of these farmers are discovering that the grass actually appears to be stimulated by this practice, rather than harmed. This seems to be completely illogical. How can adding highly competitive plants in the pasture improve the growth of the underlying grass?

The explanation for this mystery may come from understanding the physiology of cool-season grasses. Cool-season grasses have a photosynthetic pathway known as C-3, which becomes unproductive when leaf temperatures rise above 86 F. Since leaf temperatures of cool-season grasses exceed 86 F for much of the summer months in the Great Plains, there is very little cool-season grass production during these months. Adding the taller overstory of summer annuals acts to cool the underlying grasses from light shading and evaporative cooling, just like a shade tree, and allows them to perform photosynthesis for a higher percentage of the summer than they would otherwise. The net result is that not only is there added productivity from the yield of the summer annuals, but the grass itself also yields more, particularly in the summer and fall. If some of the interseeded annuals are legumes, the nitrogen they produce will boost pasture production as well. Finally, pasturing off the summer annuals results in the production of a great deal of manure and urine, which further adds to the productivity of the pasture over time.

Typically, a cool-season grass pasture in the eastern Plains will yield about 3 tons an acre if fertilized with nitrogen. Producers who have interseeded summer annuals have discovered that they can add several tons an acre of productivity to their pastures, and can do this without the need of nitrogen fertilizer by including legumes in their mixture. The end result is much more production with less actual cost than the standard pure stand of grass with nitrogen.

The best way to manage the interseeded annuals is to plant them into a hayed or closely grazed stand of cool-season grass at the end of their spring flush of growth (typically mid-June in most areas) and then let the forage accumulate until September to provide a large amount of grazing beginning in September. A solid canopy is undesirable, what is most beneficial is to achieve roughly a 50% canopy of summer annuals, to let filtered sunlight down to the understory of perennial grasses.

Although the main goal of interseeding summer annuals is usually to produce forage for grazing livestock, it is also possible to produce grain for a pastured poultry or swine enterprise, or to supply a nutritional boost for graziers.

An additional benefit of interseeded summer annuals into cool-season sods is the vastly improved habitat for wildlife, compared to cool-season grasses alone. The increased plant growth provides more cover, additional plant material for herbivores like deer, and attracts a diversity of insect life (usually lacking in grass monocultures) that can feed insect eating birds like bobwhite quail. These insect-eating birds can often provide good control of pest insects like armyworms if present in adequate populations, in addition to their recreational value. The value of such a mix for upland game birds can be enhanced by throwing in a few early maturing seed producing plants, such as buckwheat, mung beans, sunflower, proso millet, Japanese millet, browntop millet, and foxtail millet.

Plants that seem to perform well in this situation include the following:

Sorghums and sorghum-sudan grass are probably the most productive option for interseeding and are highly successful. They establish well, produce large amounts of biomass, are very drought tolerant, produce good quality forage, and offer a lot of root biomass for soil improvement. If the forage is to be summer grazed, a brachytic dwarf BMR sorghum-sudangrass hybrid is desired (see section on sorghums for more info on traits). If the forage is to be stockpiled for fall or winter grazing, a variety that does not produce grain is desired, such as a male-sterile or photoperiod sensitive. Photoperiod sensitives offer more yield and better drought tolerance, while the male-sterile is usually of better forage quality. The variety chosen should also be a brown midrib for better forage quality. One drawback of using sorghums for fall grazing is the risk of increased prussic acid at the time of frost (see the section in this resource guide on prussic acid). It may be prudent to have some paddocks that do not contain interseeded sorghum that can be grazed during frost risk, or a place in which animals can be confined and fed hay during risky periods. Sorghums are also mildly toxic to horses.

Pearl millet is probably second to sorghums and sorghum-sudan for total yield and is similar in palatability. If the pasture is to be summer grazed, a dwarf variety is preferred (such as Tifleaf 3). If the pasture is to be stockpiled for fall grazing, choose a brown midrib, photoperiod sensitive variety for better forage quality.

Foxtail millet (varieties include German and White Wonder) is a very early maturing forage that will produce seed within about 60 days of planting, upon which the forage becomes unpalatable. A small amount of foxtail millet in a stockpiled mixture can provide seed for upland game birds or pastured poultry. Foxtail millet forage is mildly toxic to horses.

Proso millet is very unpalatable as forage and matures very early. It’s sole value in a pasture interseeding is to provide seed for either livestock, wild birds or pastured poultry. It is perhaps the most water efficient grain crop known, producing a seed crop even in severe droughts.

Buckwheat is not a cereal grain, but rather a flowering broadleaf plant. The foliage is unpalatable to most grazing livestock, but the grain is readily eaten, and buckwheat is probably the earliest maturing seed producer among all domestic crops, often producing ripe seed as soon as 45 days after planting. Buckwheat has other endearing qualities. It has root exudates that render unavailable forms of phosphate in the soil into available forms, and the copious amounts of nectar and pollen make this one of the most beneficial plants to pollinators and honeybees. It also attracts ladybugs, lacewings, predatory wasps, and a whole host of beneficial insects. The seed can be utilized by livestock, wild birds, or pastured poultry.

Japanese millet is a cousin of the common weed barnyardgrass, with better forage production and quality. It does very well in wet soils (often found in pastures) and produces large amounts of seed that are relished by livestock, waterfowl, and wild birds. It maintains good forage quality after maturity, unlike foxtail millet. It matures early, in 60-75 days.

Browntop millet also maintains forage quality after maturity, and produces seed relished by livestock, wild birds, and pastured poultry. It matures early, in 60 days or so.

Teff is a forage grass that can be easily established by broadcasting and does not need to be drilled like most other forage grasses. Teff is very good quality forage but is shallow rooted and easily pulled up by grazing animals. These attributes make it better suited for hay than pasture. It is also very fine stemmed and cures quite rapidly in the swath.

Crabgrass can also be established by broadcasting, and does not need drilled, it is a very good choice for boosting summer grazing in cool-season sods. It is very high quality and productive and reseeds itself well if not overgrazed.

Sunnhemp is a tropical legume that produces more biomass and fixes more nitrogen than any other annual legume. Most varieties are not very palatable to cattle or horses, but are relished by sheep, goats, and deer. The variety Tropic Sun is quite palatable to cattle and appears to be slightly more productive than older varieties. Sunnhemp maintains forage quality very well until frost, after which the leaves dry up and fall off.

Cowpea is a viny legume that is a good nitrogen fixer and produces a fair amount of biomass. The forage is initially unpalatable to livestock that are unfamiliar with it, but once animals become familiar with cowpea the foliage is readily consumed. Early maturity varieties like Chinese Red can produce seed, while late maturing varieties like Iron & Clay or Black produce more forage. Cowpea has extrafloral nectaries that nourish beneficial insects even prior to bloom.

Mung bean is one of the cheaper summer annual legumes and is palatable to livestock, yields fairly well and is a decent nitrogen fixer. It does mature early, however, and once mature does not produce any more vegetative growth. The high protein seed is a good food source for upland game birds and poultry, as well as being retained in the pods well for livestock protein supplement after frost as well.

Guar is a very drought tolerant legume that produces a mucilaginous seed, used commercially to give ice cream and other foods a smooth, creamy texture. The seeds are high in protein and remain in the pod well after maturity. The foliage is reported to be of poor palatability while green, but becomes well accepted after frost.

Sunflower often has poor establishment into pastures (which is surprising, given how well wild sunflowers grow in pastures) but the seed is so cheap that we still recommend putting a pound or so an acre in the mixture. Sunflower is very attractive to many beneficial or benign insects, has a deep taproot, and has seed heads that are attractive to livestock, game birds, and poultry. Deer also relish the seedheads.

Okra is a deep-rooted forb with excellent grazing value and the stiff stalks left over after grazing do a great job of trapping snow in the winter.

Pumpkins, squash, and watermelon are unusual additives to a pasture interseeding, but actually do quite well in the situation. The vines are not eaten, but the fruits are eaten very well once animals finally figure out they are edible. It might take a sprinkling of salt or molasses on top of a broken fruit to get animals to eat the first one, but once tried these fruits become extremely palatable to livestock and many wildlife as well.

Below are some sample mixtures for interseeding into a cool-season sod:

Example mix for grazing livestock in fall on stockpiled forage

• 2 lb BMR photoperiod sensitive sorghum-sudan
• 1 lb bmr photoperiod sensitive pearl millet
• 1 lb Japanese millet
• 1 lb sunflower
• 1 lb okra
• 3 lb sunnhemp
• 4 lb cowpea

Example mix for both grazing and to attract upland game birds (or pastured poultry)

• 2 lb bmr photoperiod sensitive sorghum-sudan
• 2 lb bmr photoperiod sensitive pearl millet
• 1 lb buckwheat
• 1 lb Japanese millet
• 1 lb sunflower
• 3 lb sunnhemp
• 4 lb cowpea (use 2 lb each Red Ripper and Iron&Clay)
• 1 lb mung beans
• 1 lb browntop millet