These grasses grow rapidly in late spring and summer and when managed properly provide high-quality forage. They are well suited as supplemental forages during hot, dry periods when perennial cool-season forages are less productive. Because the need for extra forage usually becomes apparent after row crops have been planted in early spring, summer-annual grasses are a good doublecrop option when planted by mid- to late July after a small grain harvest. They have the potential to produce forage yields of 3 tons of dry matter per acre within 45 to 50 days. With the exception of pearl millet, the summerannual grasses are members of the sorghum family and have the potential for prussic acid poisoning (see Animal Health Concerns section).
SUDANGRASS [Sorghum bicolor (L.) Moench.) is finestemmed, leafy, and grows 3 to 8 feet tall. Sudangrass regrows following each harvest, until cool temperatures or lack of moisture inhibit regrowth. It is the preferred summer-annual grass for pasture and can be used for hay, although it is a challenge to dry, so haylage harvest would be a better option. Solid stands grow shorter than when seeded in rows. Sudangrass usually contains lower levels of prussic acid and is usually lower yielding than the other sorghum family grasses.
Sudangrass hybrids are generally slightly higher yielding and have slightly higher prussic acid potential than sudangrass at comparable stages of growth. New varieties with higher digestibility are available, known as brown midrib (BMR) varieties. Those varieties have a characteristic brown discoloration on the main vein (midrib) of the leaves, which is a marker for the mutation for lower lignin content resulting in higher fiber digestibility at comparable growth stage as nonBMR varieties. The BMR varieties have greater animal preference and animal performance (intake and gains) compared with the normal varieties. The BMR varieties can be harvested at later stages of growth (60 to 80 days) with acceptable fiber digestibility and higher forage yields than non-BMR (normal) varieties, effectively widening the harvest window.
SORGHUM-SUDANGRASS HYBRIDS are crosses of sorghum with sudangrass. They resemble sudangrass in growth habit, but are generally taller, have larger stems and leaves, and are higher yielding. This grass can become coarse and unpalatable if not properly utilized. It is not as well suited for hay production as sudangrass. Sorghum-sudangrass hybrids regrow following each harvest, barring restrictive environmental conditions. Brown midrib varieties with higher digestibility are available, as described above for sudangrass.
FORAGE SORGHUM (Sorghum bicolor (L.) Moench.) grows 6 to 15 feet tall and has potential for high yields. It is utilized as a one-cut silage or greenchop crop. Forage sorghum produces silage containing more digestible energy than legume and cool-season grass silage. Making high-quality silage from forage sorghum is generally easier than from forage legumes because of the high levels of nonstructural carbohydrates, which enhance fermentation. The high-energy, low-protein characteristics of forage sorghum silage make it a good supplement for high-protein forage legumes. Because the feeding value of forage sorghum silage is considered to be about 85% that of corn silage, corn silage is usually the preferred high-energy silage grown in Ohio. Forage sorghum has the potential, however, to grow better than corn on light-textured, shallow soils that tend to be droughty. Brown midrib varieties with higher digestibility are available, as described above for sudangrass.
PEARL MILLET (Pennisetum glaucum L.) is not in the sorghum family, and prussic acid is not produced in the plant. It tends to have smaller stems and is leafier than the sorghum grasses. Pearl millet regrows after each harvest but not as rapidly as sudangrass or sorghumsudangrass hybrids. It may also be more sensitive to cutting height for regrowth than sudangrass. Other types of millets include: German, Foxtail, and Japanese millet. German and Foxtail millet do not regrow after harvest. Japanese millet grows best in wet soils.
TEFF (Eragrostis tef (Zuccagni) Trotter) is an annual grass native to Ethiopia that can be used for hay and silage. Its use for grazing is questionable because grazing animals can pull plants out of the ground. Teff produces several cuttings and can tolerate both drought-stressed and waterlogged soils. It is fairly easy to establish, provided the very small seed is placed 1/8 to 1/4 inch deep. It should be seeded in late May to early June once soils are warm. It emerges quickly and produces harvestable forage within 40 to 50 days (early heading stage), with subsequent harvests expected every 30 to 35 days in Ohio. When harvested in the early boot stage, it produces relatively high-quality forage. In Ohio, teff can be harvested three times with total forage yields reaching a total of 3 to 4 tons of dry matter per acre. A 4-inch cutting height will promote vigorous regrowth. Teff is very sensitive to frost, so growth ceases with the first frost. More details on managing this forage can be found in a fact sheet available at forages.osu.edu/forage-management/forage-species-varieties/annual-forages.
SUMMERANNUAL GRASS-LEGUME MIXTURES are marketed by some seed dealers. The legumes (e.g., field pea and soybean) generally improve protein content compared with summer-annual grasses grown alone. The annual legumes included in these mixtures would be present in the first growth only; regrowth would occur only from the grasses. The additional cost of the legume seed should be weighed against the improved forage quality potential.
Establishment
Summer-annual grasses require well-drained to moderately well-drained soils. They grow best in warm weather and should be planted from about two weeks after corn planting until the end of June in northern Ohio and midJuly in southern Ohio. Soil temperatures should be at least 60–65 F. Late plantings (after mid-July) shorten the growing season and may result in low yields because of poor establishment in dry soils in the summer followed by cool fall temperatures. Making two seedings about three weeks apart staggers the maturities and makes rotational grazing or harvest timing easier to manage.
Seeds should be planted on a well-prepared, firm, and moist seedbed at a depth of 1/2 to 1 inch, except teff, which should be planted 1/8 to 1/4 inch deep. The seed can be broadcast and harrowed, lightly disked (except teff), or seeded with a grain drill. Forage sorghums should be planted in rows with row-crop planters to facilitate harvest and minimize lodging. These summer-annual grasses may also be established in grass sods or stubble with no-till drills (except teff, which does not establish well into existing sods), but no-till is usually less desirable than conventional seedbed preparation. Refer to Table 7.4 for suggested seeding rates.
Fertilization
Determine lime and fertilizer needs by soil test. Fertilization is similar to that used to grow 100 to 150 bushels per acre of corn. Incorporate phosphorus and potassium fertilizer in the soil prior to seeding or apply at least six months before for no-till seedings. The soil pH should be maintained between 6.0 and 6.5 for best results. Nitrogen fertilization is critical to achieve high yields and varies by previous crop (Table 7.11). Nitrogen rates for teff are generally lower, about 50 to 60 pounds of nitrogen per acre at planting. For the other species, split applications of nitrogen should be made; half applied prior to seeding, and the remainder divided equally and applied after each cutting or grazing to achieve the most efficient use. Keep in mind possible volatilization losses of some forms of nitrogen when applied in the summer. Refer to bulletin 974 Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat, and Alfalfa for more information on nitrogen volatilization losses.
Harvest Management
Table 7.12 summarizes the guidelines for harvest management of the summer-annual grasses.
DIRECT-CUT SILAGE. Forage sorghum and sorghumsudangrass hybrids are well suited as silage crops. Harvesting forage sorghum in the dough stage and sorghum-sudangrass in the heading stage should provide sufficient dry matter content for ensiling without field wilting, but check the forage dry matter of the standing crop before ensiling to ensure it is indeed in the correct range for good fermentation.
GREENCHOP OR WILTED SILAGE. Sudangrass should be cut at 18 to 40 inches of growth. Sorghumsudangrass hybrids should be cut when at least 30 inches tall, teff in pre-boot to early-boot stage, and pearl millet in late-boot to early-bloom stage.
HAY. Sudangrass, sorghum-sudangrass hybrids, pearl millet, and teff can be cut for hay. Harvest when the grasses are vegetative (boot stage, before heading) or the plant reaches a height of 3 to 4 feet. Always use a hay conditioner to mow and crush the stems to improve drying. Even with a hay conditioner, it is difficult to field cure most of these grasses adequately for safe storage as hay. Teff can be made into hay more easily than the other warm season grasses mentioned here.
GRAZING. All of the summer annuals, except forage sorghum, are suitable for grazing. Sufficient height must be achieved before grazing to reduce animal health problems and to optimize production. Grazing plants that are less than 18 inches tall will weaken them, resulting in delayed regrowth. In addition, the chance of prussic acid poisoning is higher when grazing the sorghums before a full 18 inches of growth is present. Grasses in the vegetative stage are more palatable and nutritious. Trampling and wastage increases when grazing is delayed past the boot stage. Plants reach the grazing height of 18 to 30 inches about six to eight weeks after planting. Rotational grazing or strip grazing management should be practiced. A sufficient number of animals should be placed on the pasture to graze the grass down in less than 10 days. After grazing, clip the residue to about 8 inches if old stems remain. This improves forage quality for the next grazing period.
Animal Health Concerns
PRUSSIC ACID POISONING can occur when feeding sudangrass, sorghum-sudangrass hybrids, forage sorghum or grain sorghum. These species contain varying concentrations of cyanogenic glucosides, which can be converted to prussic acid, also known as hydrogen cyanide (HCN). As ruminants consume forage containing high levels of cyanide-producing compounds, prussic acid is released in the rumen and absorbed into the bloodstream, where it binds hemoglobin and interferes with oxygen transfer. The animal soon dies of asphyxiation. Prussic acid acts rapidly, frequently killing animals in minutes. Symptoms include excessive salivation, difficult breathing, staggering, convulsions, and collapse. Ruminants are more susceptible than horses or swine because cud chewing and rumen bacteria help release the cyanide.
Species and varieties differ in prussic acid poisoning potential―sudangrass varieties are low to intermediate in cyanide potential; sudangrass hybrids are intermediate; sorghum-sudangrass hybrids and forage sorghums are intermediate to high; and grain sorghum is high to very high. Piper sudangrass has low prussic acid poisoning potential. Pearl millet is virtually free of cyanogenic glucosides and no cases of prussic acid poisoning have been reported for teff.
Any stress condition that retards plant growth may increase prussic acid levels in plants. HCN is released when leaf cells are damaged by frost, drought, bruising, cutting, trampling, crushing, or wilting. Plants growing under high nitrogen levels or in soils deficient in soil phosphorus or potassium tend to have high levels of cyanogenic glucosides. Fresh forage is generally higher in cyanide than in silage or hay because cyanide is volatile and dissipates as the forage dries.
Reducing the Risk of Prussic Acid Poisoning in Sorghum Species
When grazing or greenchopping:
• Graze or greenchop only when grass exceeds 18 inches in height.
• Do not graze wilted plants or plants with young tillers.
• Do not graze plants during or shortly after a drought when growth has been reduced.
• Do not graze on nights when frost is likely. High levels of the toxic compounds are produced within hours after a frost occurs.
• Do not graze after a killing frost until the plants are dry. Wait five to seven days to allow the released cyanide to dissipate.
• After a non-killing frost, do not allow grazing because the plants usually contain high concentrations of toxic compounds. Once the first frost has occurred, grazing should not begin until five to seven days after a killing frost.
• Don’t allow hungry or stressed animals to graze young sorghum grass growth. To reduce the risk, feed ground cereal grains to animals before turning them out to graze.
• Use heavy stocking rates (four to six head of cattle per acre) and rotational grazing to reduce the risk of animals selectively grazing leaves that can contain high levels of prussic acid.
• Feeding greenchopped, frost-damaged plants has lower risk than grazing because animals have less ability to selectively graze damaged tissue; however, the forage can still be toxic, so feed with great caution.
• Always feed greenchopped forage of these species within a few hours (even in absence of frost), and do not leave greenchopped forage in wagons or feedbunks overnight before feeding.
• Split applications of nitrogen decrease the risk of prussic acid toxicity, as do proper levels of phosphorus and potassium in the soil.
When making hay or silage:
• Frost-damaged annual sorghum grasses can be made into hay with little or no risk of toxicity. When plants are wilted enough to make dry hay, most of the volatile cyanide gas will have dissipated.
• Normal silage making allows most of the cyanide to dissipate from frost damaged annual sorghum grasses. Delay feeding of silage for six to eight weeks after ensiling.
• Silage that likely contained high cyanide levels at harvest should be analyzed for HCN content before feeding.
NITRATE POISONING can occur under conditions of high nitrogen fertilization, heavy manure applications, drought, overcast weather, freezing or other stress conditions that retard plant growth. Under these stressful conditions, high nitrate levels accumulate in the crop. Once forage is fed, nitrate is converted to nitrite in the animal. When nitrite levels are high, the animal cannot metabolize it quickly enough, and nitrite inhibits oxygen transport in the blood. Symptoms include rapid breathing, fast and weak heartbeat, muscle tremors, staggering, and death if corrective steps are not taken.
The same management precautions for prussic acid poisoning help prevent nitrate poisoning. Pearl millet does accumulate high nitrate levels leading to nitrate poisoning. As mentioned above, pearl millet does not accumulate prussic acid. High nitrate levels persist when forages are cut for hay, but ensiling the crop reduces nitrates by one-half. If you suspect that forage contains high nitrate levels, have it tested before feeding.
POISONING OF HORSES fed sudangrass, sorghumsudangrass hybrids and forage sorghum has been reported. The exact cause of poisoning is not known. Do not feed horses any of these summer annual grasses.