Annual grasses are normally used as emergency forage. Emergency forage may be needed during a drought or after crop damage from other severe weather, such as flooding.
Summer annual grasses are used for summer pasture, green chop, hay and silage. The most common annual grasses used in Michigan are sudangrass, hybrid sudangrass, sorghum-sudangrass hybrids and forage sorghum. Desirable characteristics such as rapid growth, excellent drought resistance and good response to fertilizer and water make summer annual grasses attractive to use in an overall management scheme for forage production.
Sorghum-sudangrass hybrids produce about the same amount of feed as sudangrass when used for pasture. When used for green chopped forage, yields of sorghum-sudangrass hybrids usually exceed sudangrass or forage sorghum. Forage sorghums are best suited for silage. Making sorghum-sudangrass into hay is difficult because of the slow drying time.
Sudangrass and Brown Mid Rib (BMR) sudangrass
True sudangrasses have fine stems, tiller extensively when conditions permit and can regrow rapidly. Thus, they are more suited to pasturing than other types of sorghum and are more popular for annual hay and late summer pasture. Piper sudangrass is low in prussic acid content and has good drought and disease tolerance. It is a Wisconsin release that has good regrowth after pasturing and is the leading sudangrass hybrid. Brown Mid Rib sudangrass is more palatable and contains significantly less lignin, making it more digestible than normal sudangrass.
Hybrid sudangrasses result from a cross among true sudangrass strains that are available primarily as commercial varieties. They are similar to true sudangrass varieties, but yield slightly more in a three-cut green chop or hay system. Research from the University of Wisconsin forage team reported yields from 3 to 5 tons per acre dry matter. It can be ready for harvest as early as 45 days after planting. Their prussic acid content is generally between that of true sudangrass and sorghum-sudangrass hybrids.
Sorghum-sudangrass hybrids are the most numerous of the various types of summer annual grasses. Most of these are available as commercial hybrids. They are high producing forage grasses, but more than 50 percent of their yield usually comes from their stems. Their rate of regrowth after repeated clippings or grazing is lower than that of sudangrass. Thus, animals graze or being fed sorghum-sudangrass hybrids sometimes result in less gain or milk production than those consuming other summer annuals, apparently due to lower energy content. When these hybrids are cut at immature stages, quality is higher, but yields are much lower.
Sorghum-sudangrass Brown Mid Rib
Brown Mid Rib increased digestibility of the stems by reducing the quantity of digestible lignin. Lignin content is reduced approximately 40-60 percent, depending upon environmental conditions. The reduction in lignin increases cellulose and hemicellulose content, both are more digestible than lignin. Since lignin is a structural component of the stem, its reduction stems are somewhat softer and more limber. Brown Mid Rib annual forage grasses should be planted at the same rate as Sorghum-sudangrass.
Forage sorghums are usually tall growing and mature late in the growing season. Often called “sweet sorghum,” forage sorghums often have sweet and juicy stems, and many have relatively small grain heads.
Forage sorghums usually yield more silage dry matter per acre than corn without irrigation. However, yields of total digestible nutrients per acre are usually lower from forage sorghums than from corn. Research from the University of Wisconsin forage team reported yields from 3 tons per acre in cool years to 11 tons per acre dry matter in years with above-average temperatures. Feeding value of sorghum silage is 80-90 percent that of comparable corn silage. Some long-season or non-flowering types will need to be killed by frost to dry down enough for ensiling.
Grazing forage sorghums is not recommended. They usually contain much higher levels of prussic acid than other summer annual grasses and can be dangerous to graze even when plants are completely headed, especially when young shoots are present. Forage sorghums can be cut for hay, although their stems dry very slowly after cutting.
Note: The content of this series of articles is adapted from Rich Leep’s 2008 article, “Summer annual forage grasses for emergency crops.” Leep was the Michigan State University Extension state forage specialist and is now retired.