Forage Sorghum

D. J. Undersander1, L. H. Smith2, A. R. Kaminski1, K. A. Kelling1, J. D.Doll1

1Departments of Agronomy and Soil Science, College of Agricultural and Life Sciences and Cooperative Extension Service, University of Wisconsin-Madison, WI 53706.
2Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, MN 55108.
September, 1990.

I. History:

Sorghum (Sorghum vulgare Pers.) is indigenous to Africa, and many of today's varieties originated on that continent. Sorghum was also grown in India before recorded history and in Assyria as early as 700 B.C. The crop reached China during the thirteenth century and the Western Hemisphere much later.

Sorghum was introduced to the United States from Africa in the early part of the seventeenth century. It was not grown extensively in this country until the 1850s, when the forage variety Black Amber (also called "Chinese sugarcane") was introduced by way of France. Since then many other varieties have been introduced from other countries and developed domestically.

Sorghum was grown primarily as a source of sugar for syrup until the settlement of the semiarid West created a demand for drought-resistant forage crops. By the 1950s, about 90% of the acreage of sweet sorghums in the United States was grown for forage.

Currently there are five major types of sorghum grown:

1) Grain sorghum with dwarf varieties that grow 2 to 5 ft tall for easier combining.

2) Forage sorghum which grows 6 to 12 ft tall, produces more dry matter tonnage than grain sorghum, is coarse stemmed and used for silage.

3) Sudangrass, a fine stemmed, short season sorghum grown to furnish pasture or green feed during mid-summer when perennial grasses are dormant.

4) Sorghum-sudangrass hybrids are a cross between the two forage types that have intermediate yield potential and can be used for pasture, hay or silage.

5) Sorghum-almum, also called Columbusgrass, sorghumgrass, sorgo negro or sudan negro.

Sorghum production is concentrated in areas where corn production is limited because the rainfall is insufficient or unfavorably distributed and the temperatures are too high. Thus most of the domestic sorghum acreage is in the southern Great Plains states, with Texas, Kansas and Nebraska the leading producers. However, some sweet sorghum has been grown for syrup or silage in Wisconsin since the state was settled.

Forage sorghum production has been limited in the Upper Midwest because the crop matures late and, except on droughty soils, does not generally produce as many total digestible nutrients per acre as well-adapted, high yielding corn hybrids. Recently, there has been renewed interest in the crop during seasons of high temperatures and drought.

II. Uses:

Forage sorghums are used primarily as silage for livestock. They are sometimes grown and harvested with soybeans to improve the protein content of the silage. Sudangrasses and sorghum-sudangrass hybrids are grazed by livestock or fed as green chop or hay.

Sorghum harvested at the soft dough stage of development and stored as silage contains 52 to 65% dry matter digestibility, 8 to 12% crude protein, 60 to 75% neutral detergent fiber, and 34 to 40% acid detergent fiber. The higher the grain content, the higher the digestibility. Ensiled grain has a digestibility of about 90%.

Forage sorghum usually produces as much silage per acre as corn. However, sorghum silage contains less grain and is higher in fiber than corn silage. Though the protein content of sorghum silage is similar to or slightly higher than that of corn, it is less digestible. Animal consumption of sorghum silage is also generally somewhat less than that of corn.

To obtain the optimum rate of gain for most livestock, sorghum silage must be supplemented with protein, minerals and vitamins. It is generally suggested that sorghum silage constitute not more than 50% of the forage in dairy cow rations but may be adequate alone for other categories of animals.

Sorghum plants, particularly young plants, contain an alkaloid which releases hydrocyanic, or prussic acid, when hydrolized. This can be toxic to livestock. Young plants, branches in the leaf axils of injured plants and new shoots from the crown at the soil surface contain more than twice as much acid as the mature leaves of normal plants. When the crop is cut and field-cured, or is ensiled, and the hydrocyanic acid degrades (2 to 3 weeks after ensiling), and the danger is greatly reduced. Sudangrass contains less than half as much hydrocyanic acid as most sorghums. A low-acid Sudangrass variety (Piper) was released by researchers in Wisconsin.

During periods of drought or other plant stress, sorghums tend to accumulate nitrates, which can poison livestock. If retarded crop growth is observed, analyze the forage for excessive nitrates before feeding it. In the case of high nitrate levels, the forage should be ensiled or combined with other feeds low in nitrate to reduce daily nitrate intake.

III. Growth Habits:

Sorghum is a coarse grass that grows as an annual in the Upper Midwest. Stems are erect and solid and reach a height of 2 to 12 ft. In many respects, the structure, growth, and general appearance of forage sorghums are similar to corn: stalks have a groove on one side between the nodes; grooved internodes alternate from side to side; a leaf is borne at each node on the grooved side, with the leaf sheath and blade arrangement also much like that of corn.

The buds which form at the nodes often develop into branches. Buds that form near the crown develop into grain-producing tillers. The tillers develop their own roots but remain attached to the old crown. The culms or stalks of forage sorghums are juicy. If the pith is not juicy, the midrib of the leaf is white in color because of the air spaces in the tissues; when the air spaces are filled with juice, the color is more neutral. Because of this difference in moisture content, juicy and non-juicy stalked varieties will be at different stages of maturity at the optimum time for silage. Otherwise, there is no difference between juicy and non-juicy stalked hybrids.

Another variation between varieties is the sweetness of the juice within the stalk. Sweetness is not related to juiciness; a dry-stalked sorghum can be either sweet or non-sweet, just as a juicy stalked sorghum can. A sweet forage sorghum is preferred by livestock and likely to be consumed in greater quantity if it is used as green chop, hay or bundle feed. Stalk sweetness appears to be of no concern if the crop is to be ensiled because most of the soluble plant sugars are converted to organic acids in the fermentation process.

Under drought conditions, sorghum leaves tend to fold rather than roll, as do corn leaves. A heavy white wax (bloom) usually covers sorghum leaf blades and sheaths, protecting them against water loss under hot, dry conditions.

In contrast to corn, both the male and female flowers of sorghums are in a panicle at the end of the culm. The panicle may be loose and open. About 95% of the flowers are self-pollinated, although this varies with the variety grown.

Seeds vary in color among the sorghum varieties, from white to dark brown. The endosperm is white, and the sorghums have a deficiency of Vitamin A, as does white corn. Though seed size varies considerably among the sorghums, it ranges from approximately 1,000 to 2,000 seeds/oz.

IV. Environment Requirements:

A. Climate:

Sorghums are fast-growing, warm weather annuals that will provide plenty of feed in mid-summer when many other forages slow down. Sorghums are best suited to warm, fertile soils; cool, wet soils limit their growth. Therefore, their production in the Upper Midwest may be limited. The crop tolerates drought relatively well, though adequate fertility and soil moisture maximize sorghum yields. The plant becomes dormant in the absence of adequate water, but it does not wilt readily. Growth resumes when moisture conditions improve.

Sudangrass and sorghum-sudangrass hybrids can be successfully grown where "95 RM" or later maturity corn hybrids can be produced for grain. However, early maturing varieties can be grown farther north on sandy soils which warm up quickly in the spring. Where moisture is limited, sorghums and sorghum-sudangrass hybrids generally produce more silage than corn. Forage sorghums, which are generally later in maturity than sudangrass, may not mature enough for silage even when grown in southern Wisconsin and Minnesota.

B. Soil:

Sorghums need a warm, fertile soil.

C. Seed Preparation and Germination:

Seed should be treated with a fungicide, such as Captan, to control seed rots and seedling blights. The effectiveness of the seed treatment will be reduced if germination and emergence are delayed due to cold, wet soil conditions. Soil temperature should be 60oF for germination; rapid germination and emergence occur when soil temperature is 70oF.

V. Cultural Practices:

A. Seedbed Preparation:

Seedbed preparation is similar to that for corn. A firm, well-prepared seedbed is essential for a full stand. Plow in the fall or just before planting; fall plowing will provide a firmer seedbed. For sandy soils, do not disk or harrow after plowing because of wind erosion hazards.

B. Seeding Date:

Sorghums are generally sown between May 20 and June 5. The soil should be warm (65 to 70oF) at 4 in. Sorghum seedlings are slow growing, especially in cool soils. Consequently weed control may be a problem under these conditions.

C. Method and Rate of Seeding:

The seeding rate and method depends on the use for the crop and the equipment available. Sow sudangrass or sorghum-sudangrass hybrids grown for pasture or green chop at 20 to 30 lb/acre with a grain drill or a broadcast seeder. If a broadcast seeder is used, cover the seed with about 1 in. of medium or heavy soil or 1 in. of sandy soil. For sudangrass pasture, plant 1/3 to  acre/cow. Delaying the seeding on one-half the area for 2 weeks will spread production over a longer grazing season. Surplus forage can be harvested and stored as silage or as hay.

For silage production, the usual procedure is to plant forage sorghum in rows at a seeding rate of 5 to 10 lb/acre. Plants grown in 30 to 40 in. rows usually yield as much as those grown in solid stands, and the lower leaves are retained on the plant longer. Planting in rows reduces lodging and permits harvesting with conventional silage equipment.

D. Fertility and Lime Requirements:

Soil fertility requirements are somewhat similar to those of corn at the same yield goals, although sorghums are usually more efficient in their use of phosphorus and potassium. Follow soil test recommendations to determine nutrient needs and fertilizer recommendations. A 5 to 7 ton forage sorghum crop will remove about 40 lb/acre P2O5 and 180 lb/acre K2O. Follow soil test recommendations to determine nutrient requirements. Under dryland conditions, 60 to 120 lb/acre N is recommended, with soils higher in organic matter requiring the smaller amounts. On sandy soils apply half the nitrogen before planting and the remainder within 30 days after emergence. Where the sorghum is planted in rows, the nitrogen may be sidedressed when the crop is 8 to 16 in. tall.

Sorghum seed is sensitive to fertilizer burn. Therefore, for row planting place fertilizer 2 in. to the side and at or slightly below seed depth. For broadcast stands, work fertilizer into the soil thoroughly before sowing.

A soil pH of 6.0 is adequate for sorghum production.

E. Variety Selection:

Types of sorghum include sudangrass, sorghum-sudangrass hybrids, forage sorghums and Sorghum almum. Sorghum almum is not recommended because tests show no superiority as a silage or pasture crop in Wisconsin or Minnesota. Prussic acid content ranges from 4 to 10 times higher in Sorghum almum than in Piper sudangrass. Volunteer plants may also become weed problems in succeeding crops.

Sudangrass can be harvested as pasture, green chop or silage, but is superior in forage yields to other sorghums only when used for pasture. It provides abundant pasture in mid to late summer when perennial cool season forages, such as alfalfa, timothy, and bromegrass are generally dormant. Yields of 3 to 4 tons/acre of dry matter or 10 to 12 tons/acre of green feed or silage are possible. Sudangrass can be pastured 5 to 6 weeks after planting. The pasture may be cut or grazed when regrowth reaches 18 to 20 in., which may take 3 to 4 weeks under favorable weather conditions. In the Upper Midwest three or more cuttings per season are common. Hay as green chop should be made at or before the boot stage, approximately 7 to 8 weeks after planting. (45 to 60 days are required for regrowth before a second cutting.) Piper is an early maturing sudangrass variety that is preferred for pasture. Piper usually yields as much as any other variety for pasture, has a low prussic acid content and has more disease resistance than other sudangrass varieties.

Sorghum-sudangrass hybrids are taller, have larger stems and can be higher yielding than sudangrass. In Wisconsin, however, they have generally yielded no more than Piper sudangrass when harvested three times at pasture growth stages. At Rosemont, MN 'Sudan' sorghum sudangrass hybrid averaged about 1 ton/acre more than Piper sudangrass when cut twice. The hybrids seem more vigorous under less frequent cutting, so they may yield more than Piper. Sorghum-sudangrass hybrids have higher stalks than sudangrass and are normally harvested for green chop or silage but may be used for pasture or hay if chilled at a high seeding rate and harvested at immature stages (18 to 24 in. tall) when harvested as green chop or silage. The sorghum-sudangrass hybrids usually yield less than forage sorghums.

Forage sorghums are best harvested as silage. The feed value (TDN) of sorghum silage per acre is about 90% that of corn silage. On more productive soils, with favorable moisture and fertility, corn is a better silage crop, producing comparable yields and higher feed value than forage sorghums and sorghum-sudangrass hybrids, (Table 1). On light droughty soils, however, forage sorghums and sorghum-sudangrass hybrids are superior to corn in some years. Most forage sorghums and forage sorghum hybrids are medium to late maturing.

Table 1. Silage dry matter yield and quality of annual forages at Rosemount, Minnesota.1,2,3
    Forage quality
Species Variety Yield (T/acre) CP (%) DDM (%) NDF (%)
Dent corn Pioneer 3732 9.4 6.7 67.6 41.3
Sorghum Sweetreat 9.5 5.2 64.7 44.7
  Sorgo 10 7.6 7.1 58.9 52.1
  Pioneer 956 8.8 5.9 54.7 61.5
  Pioneer 931 10.1 6.1 47.6 69.3
Popcorn Purdue 405 6.4 6.9 65.2 57.1
Sweet corn Jubilee 5.6 9.3 73.6 35.4
1Forage seeded 7 May and fertilized with 150 lb N/A.
2Plant population: Corn-28,000 ppa; Sorghuns-150,000 ppa.
3Harvest stage and data: All corn harvested at black layer, sorghums at hard dough; Pioneer 931 did not set seed.

F. Weed Control:

1. Mechanical: Sudangrass and sorghum-sudangrass hybrids planted in a well-prepared, warm seedbed germinate and grow rapidly and can compete well with most annual weeds. Weeds can be controlled with cultivation if the crop is planted in rows 20 in. or more wide.

2. Chemical: If weeds become a problem, it may be necessary to use herbicides to control weeds until a full leaf canopy is formed. Few of the herbicides registered for use in grain sorghum are specifically approved for use in forage sorghums. Check with your local extension office or crop consultant if you need information on herbicides.

G. Diseases and Their Control:

A seed treatment, such as Captan or Vitavax, should be used to control seed rots and seedling blights.

Stalk and root rots are not often problems in forage sorghum because harvest for silage generally takes place before the disease causes much damage. Charcoal rot, which develops under hot, dry conditions after the plants have bloomed, occasionally causes serious lodging problems. Early harvest may be necessary in the most severe cases.

Foliar diseases are rarely a problem when sorghum is to be harvested for silage, because they also generally appear at later stages of plant growth. Northern corn leaf blight has been a problem in the Upper Midwest. Maize dwarf mosaic (MDM), a virus carried from overwintering johnsongrass and other weeds by several aphid species, can be controlled by choosing resistant or tolerant hybrids. There are also strains resistant to anthracnose, which can attack leaves and/or stalks. Genetic resistance has not yet been developed for sorghum downy mildew, a soil-borne fungus disease which can be a serious problem for forage sorghum production. However, this disease has not occurred in the Upper Midwest.

Losses due to disease can be minimized by selecting resistant hybrids, planting disease-free seed, providing optimum growing conditions (soil fertility and pH), rotating with other crops and removing infested debris.

H. Insects and Other Predators and Their Control:

Forage sorghums are attacked by wireworms, seed beetles, cutworms, aphids (especially greenbugs), sorghum midge, chinch bugs, spider mites, armyworms and earworms. Some of these pests can be controlled with insecticide seed treatments in the planter box. However most of these insects do not normally occur in sufficient populations to warrant control in the Midwest.

I. Harvesting:

Sorghum and sorghum-sudangrass leaves and stems are coarse and high in moisture at time of harvest. Sweet sorghum and sorghum-sudangrass often cannot be stored as hay because of the difficulty in drying the forage to a safe storage moisture content of 25% or less.

Sudangrass should be harvested for hay whenever it reaches the boot stage.

Harvesting solid stands that have been allowed to reach 4 ft or more in height may be difficult with conventional equipment. Forage cut at earlier stages of growth is easier to handle and higher in feeding value. Cut at a 6 in. height to allow more rapid regrowth.

1. For pasture -- Sudangrass and sorghum-sudangrass hybrids are usually ready for grazing 5 to 6 weeks after planting. Since prussic acid is highest in the immature plant parts, new shoots may be dangerous until they reach a height of at least 18 in. Rotational grazing will provide maximum production at a nutritious, yet safe stage of growth. Allow time for 18 in. or more of regrowth before regrazing. Livestock may selectively graze and trample the crop if the plants are too tall (over 40 in.).

There is increased danger of prussic acid poisoning when frost kills the top-growth, leaving the base of the plant from which new shoots may emerge. Cattle will frequently avoid the tall frosted growth and graze the young shoots, which may contain toxic levels of prussic acid. Harvesting the crop for silage may be the best way to handle frost injured sudangrass or sorghum-sudangrass hybrids.

Under adequate soil moisture conditions, productive stands of sudangrass may be expected to carry 2 to 3 head of mature livestock per acre from early July until frost.

2. For hay -- Harvesting sudangrass and sorghum-sudangrass hybrids when the seed is in the soft to dough stage gives highest yields, but curing is difficult at this stage. A more practical plan is to harvest when the forage is about 30 in. high. This method results in a better quality hay that is easier to cure, and one or two additional crops may be produced.

Sudangrass and sorghum-sudangrass for haylage should be crimped and windrowed, wilted to 40 to 50% moisture, and then chopped as fine as possible.

3. For silage -- Harvest sudangrass, sorghum-sudangrass hybrids and forage sorghums in the medium dough stage when total plant moisture is 65 to 70%. For beef cattle, the crop can be harvested a few days later.

4. For green chop -- Sudangrass and sorghum-sudangrass hybrids can be used for green chop to bolster summer feed supplies. The forage is nutritious over a wide range of plant heights and can produce large yields. To be sure of a second crop, harvest the first cut by the heading stage, leaving 6 to 8 in. of stubble. Do not allow green chop to sit in the wagon long enough to heat; this will increase prussic acid levels.

5. For green manure -- Sorghums are generally not recommended for green manure. They do not supply nitrogen to the soil, as do the legume crops. However, a crop of sudangrass or a sorghum-sudangrass hybrid can supply up to 5 tons of dry matter per acre as organic material. Apply nitrogen fertilizer or barnyard manure for a balance of nutrients.

VI. Yield Potential and Performance Results:

Tests conducted in Wisconsin in the 1960s indicate that yields of dry matter were highest under the silage management program, followed by the green chop and then the pasture management program. Silage yields ranged from 4 to 12 tons of dry matter per acre.

Several of the sorghum-sudangrass hybrids and the forage sorghum varieties produced higher yields than the sudangrass varieties under silage management. Sudangrass and sorghum-sudangrass hybrids are generally earlier in maturity than forage sorghums, and thus provide a better chance of obtaining a crop at the desired stage of development before frost. There were no differences among varieties and hybrids when used as pasture.

VII. Economics of Production and Markets:

On-farm utilization as feed may be the best alternative for forage sorghum growers in the Upper Midwest. Otherwise, it is advisable to secure a market for the crop before planting.

VIII. Information Sources:

* Rohweder, D.A., J.M. Scholl, P.N. Drolsom and M.D. Groskopp. 1965. Sorghums for Forage in Wisconsin. Circular 638, University of Wisconsin-Extension.

* Hughes, H.D., and D.S. Metcalfe. 1972. Crop Production. Third Ed. The Macmillan Company, New York.

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