Sorghum Syrup

D. J. Undersander1, W. E. Lueschen2, L. H. Smith3, A. R. Kaminski1, J. D. Doll1, K. A. Kelling1, and E. S. Oplinger1

1Departments of Agronomy and Soil Science, College of Agricultural and Life Sciences and Cooperative Extension Service, University of Wisconsin-Madison, WI 53706.
2Southern Experiment Station, University of Minnesota, Waseca, MN 56093.
3Department of Agronomy and Plant Genetics. University of Minnesota, St. Paul, MN 55108.
November, 1990.

I. History:

Sorghum (Sorghum bicolor L. Moench) is native to Africa, and many of today's varieties originated there. 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 from France. Since then many other varieties have been introduced from other countries or developed domestically.

Sorghum was grown primarily 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. Interest in sorghum syrup is renewed whenever a shortage of sugar results in higher sugar prices.

Sorghum production is concentrated in areas where the rainfall is insufficient and the temperatures are too high for profitable corn production. 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 and Minnesota.

II. Uses:

Sorghum syrup is extracted from the plant by crushing the stalk. The syrup is produced primarily in the United States and is used by the food industry as a substitute for sugar. Recent research has evaluated sorghum as a feed stock for production of ethanol. The grain from sorghum can be used as feed for poultry and livestock.

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. Syrup varieties grow to be 6 to 9 ft tall. In many respects, the structure, growth, and general appearance is 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 that 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 some varieties and types of sorghum 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, the plants will be at different stages of maturity at the optimum time for harvest for syrup.

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, as can a juicy-stalked sorghum. Sugar content of the juice can range from 2 to 20%.

Sorghum leaves tend to fold rather than roll, as do corn leaves when subject to moisture stress. A heavy white wax (called "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 tip of the culm with about 95% of the flowers being self-pollinated. The inflorescence ranges from loose and open to a dense closed panicle.

Seed color varies among the sorghum varieties from white to dark brown. The endosperm is white, and the sorghums have the same vitamin A deficiency as white corn. Seed size varies considerably among the varieties, typically ranging from 1,000 to 2,000 seeds/lb.

IV. Environment Requirements:

A. Climate:

Sorghums are fast-growing, warm weather annuals. They are best suited to warm, fertile soils, as cool, wet soils limit their growth. Therefore, their production in the Upper Midwest may be limited to the warmer regions and soils of Wisconsin and Minnesota. The crop tolerates drought relatively well, although it responds to adequate fertility and soil moisture with faster growth. It also tolerates short periods of flooding better than corn. The plant will stop growing in the absence of adequate water, but it won't wilt. It will start growing again when conditions improve.

B. Soil:

Sorghums need a warm, fertile soil for optimum growth. They are suitable for planting on a wide range of soil types. Because sorghum tolerates droughty conditions, it is suited for planting on coarser textured soils if fertility requirements of the plant are met. Sorghum also grows well on loam and clay loam soils.

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. Once the seed germinates growth is normally slow for two to three weeks until the plants become established.

V. Cultural Practices:

A. Seedbed Preparation:

A firm, well-prepared seedbed is essential for a full stand. Moldboard plowing in the fall or just before planting is recommended where a heavy residue crop such as corn precedes the sorghum. If soybeans or another low residue crop precedes sorghum, fall or spring chisel plowing or disking will prepare an adequate seedbed. Tillage used should be based on soil type and erosion potential.

B. Seeding Date:

Sorghums are generally sown between May 20 and June 5. However, research at the University of Minnesota found that early planting resulted in excellent yields during 1987 and 1988 when temperatures were above normal and rainfall was below normal. The top four inches of the soil should be warm (65 to 70oF) at planting. This gives quick germination and promotes early growth. Rapid early growth is essential since weeds may severely compete with small sorghum plants if growth is slowed by cool weather.

C. Method and Rate of Seeding:

The seeding rate and method depend on the use for the crop and the equipment available. Minnesota research shows little response to planting rates of 20,000 to 80,000 plants/acre for sugar yield. Sorghum for syrup can be planted with a corn planter or with a grain drill at a rate of 10 to 15 lb/acre. Seeding depth should be 1 in. in medium or heavy soil or 1 in. in sandy soil.

D. Fertility and Lime Requirements:

Soil fertility requirements are somewhat similar to those of corn, although sorghums are usually more efficient in their use of soil phosphorus and potassium. Balanced fertilization, with ample amounts of nitrogen and adequate phosphorus and potassium, is essential to get high yields. A 5 to 7 ton/acre sorghum crop will remove about 100 lb N, 40 lb P2O5 and 180 lb K2O/acre. Follow soil test recommendations to determine phosphorous and potassium nutrient requirements. Under dryland conditions, 60 to 120 lb N/acre is recommended, with soils higher in organic matter requiring the smaller amounts. Recent Minnesota research indicates no response to N on soils with 5 to 6% organic matter with corn treated with 50 lb N/acre as the previous crop. On sandy soils apply about 20% of the nitrogen at planting (not seed placed) 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. Therefore, for row planting place fertilizer 2 in. to the side and at or slightly below seed depth. For broadcast stands, work fertilizer well into the soil before sowing. A soil pH of 6.0 or higher is adequate for sorghum production.

E. Variety Selection:

Rox Orange, or Waconia, is a medium-early maturing variety that was developed for syrup production by the Wisconsin Agricultural Experiment Station. (It has also been grown for silage in the Upper Midwest.)

Leoti Red is about the same in maturity as Rox Orange and can be grown for syrup, but it is prone to lodging.

Other varieties have been tested at the University of Minnesota, Southern Experiment Station. Yield data are shown for different varieties in Tables 1 and 2. The primary purpose of this research was to evaluate sweet and semisweet sorghum as a feed stock for ethanol production. Keller, Dale and M81E competed favorably with corn in theoretical ethanol production based on amount of sugar produced. However, these varieties lodged severely both years, which would have made mechanical harvest difficult.

Table 1. Varietal differences in total dry matter yields, percent stalk, stalk moisture, Brix, fermentable carbohydrate yield, ethanol yield and stalk lodging of sorghum and corn grown at Waseca, MN, 1987 and 1988.
Varieties Total dry matter
Percent stalk1
Stalk moisture
Brix2 Fermentable carbohydrate yield3
Calculated ethanol yield4
Stalk lodging5
  ------------------------- Average of 2 years -------------------------
Dekalb 524 8.2 -- 46 -- -- 332 15
Northrup King 301 9.4 52 68 13.2 1.81 247 47
Rox Orange 10.6 46 75 10.3 1.84 250 33
Northrup King 405 11.4 60 70 7.3 1.25 170 70
Keller 10.1 70 72 13.4 2.96 403 97
Dale 10.0 70 74 12.3 3.00 408 98
Northrup King 8361A 11.7 67 70 8.9 1.85 252 72
M81E 10.4 66 73 12.7 2.83 385 96
Northrup King 8361 13.1 68 73 7.4 1.96 267 93
LSD (.05) 1.5 3 3 1.7 0.49 66 18
  -------------------- Sorghum varieties grown only in 1988 --------------------
Cargill Mor-Cane 8.6 40 76 8.9 1.19 161 13
DeKalb FS 5 9.2 41 73 10.0 1.25 171 8
NC +940 9.2 51 70 13.2 1.83 248 15
LSD (.05) 2.0 4 3 1.9 0.56 74 17
1Dry matter basis.
Brix is approximately equivalent to percent sugar.
3Brix multiplied times stalk sap yield.
4Assuming 2.5 gal of ethanol/bu of corn grain and 14.7 lb fermentable carbohydrate/gal of ethanol.
5Stalk lodging: percent of plants lodged at least 45o or more at harvest. Lodging was much more severe in 1987 than 1988 due to two wind storms. Source: Putnam, Lueschen, Kanne and Harverstad. University of Minnesota. A Comparison of Sweet Sorghum Cultivar and Corn for Ethanol Production. Submitted for publication in the Journal of Production Agriculture.

Table 2. Effect of planting date and variety on dry matter yield, percent stalk, stalk moisture, Brix, fermentable carbohydrate yield and ethanol yield of sorghum harvested at Waseca, MN in mid-October 1987 and 1988.
  Total dry matter Percent stalk1 Stalk moisture Brix Fermentable carbohydrate yield2 Calculated ethanol yield3
  ton/a % % ton/a gal/a
Date of Planting Means
April 26 9.3 66 68 11.3 1.71 232
May 5 8.6 66 69 10.8 1.60 217
May 15 8.4 65 69 10.4 1.46 199
May 25 8.2 66 70 10.4 1.52 206
LSD (.05) 1.0 2 1 0.6 0.18 24
Variety Means
8361A 9.2 70 70 9.1 1.49 101
405 8.7 69 67 9.2 1.26 86
301 8.2 53 68 11.4 1.22 83
Keller 8.4 71 70 13.2 2.32 158
LSD(.05) 0.8 2 1 0.6 0.18 12
1Dry matter basis.
2Brix multiplied times stalk sap yield.
3Assuming 2.5 gal of ethanol/bu of corn grain and 14.7 lb fermentable carbohydrate/gal of ethanol.

F. Weed Control:

1. Mechanical: Sorghums planted in a well-prepared, warm seedbed germinate and grow rapidly and can compete well with most annual weeds. Weeds can usually be controlled with one or more rotary hoeings and cultivations between rows.

2. Chemical: If weed problems are anticipated, it may be necessary to use herbicides to control them until a full leaf canopy is formed. The herbicides registered for use in grain or forage sorghums may not be approved for use in syrup sorghums. Check with your local extension office, crop consultant or the herbicide manufacturer if information on herbicides is needed.

G. Diseases and Their Control:

A seed treatment, such as Captan, should be used to control seed rots and seedling blights. Leaf diseases, including northern corn leaf blight, maize dwarf mosaic (MDM) and anthracnose, can be problems in areas with high rainfall and humidity. Charcoal rot, which develops under hot, dry conditions after the plants have bloomed, occasionally causes serious lodging problems. 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:

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.

Greenbugs are probably the most damaging sorghum insect pest. Colonies of these aphids feed on the underside of leaves and inject toxins that destroy leaf tissue. Some sorghum varieties are resistant to greenbugs, except when attacked by large numbers of the pests while still in the seedling stage.

In the Upper Midwest the most serious pest problem for sorghum may be bird damage. Birds will eat the grain, but have less effect on syrup production. Planting larger fields in one block and locating these away from urban areas or farm buildings may help reduce the problem.

I. Harvesting:

Sorghum grown for syrup should be harvested when the seeds are fully in the dough stage. Harvesting should be done before a killing frost if possible; if not, the crop should be harvested immediately after the freeze. Strip off the leaves before the freeze to lessen the damage.

The leaves should be stripped by hand while the stalks are still standing. Stalks should be tied into bundles for easier handling. The heads can be removed either before cutting the stalks or after they have been bundled and before they are taken to the mill.

The grain of sweet sorghum can be harvested, cured and used as feed for poultry and livestock.

J. Drying and Storage:

Syrup production will be reduced if stalks are stored longer than 7 to 10 days or subjected to freezing temperatures.

VI. Yield Potential and Performance Results:

In general it takes 8 gal of juice to produce 1 gal of molasses syrup. A realistic syrup yield of 75 to 80 gal/acre can be expected from a good sorghum crop.

VII. Economics of Production and Markets:

The marketability of sorghum syrup depends on the availability of other sugars and syrups and the development of niche markets. It is advisable to identify a sorghum mill and secure a contract, if possible, before planting sorghum for syrup.

VIII. Information Sources:

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

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

Culture of Sorgo for Sirup Production. 1957. I.E. Stokes, O.H. Coleman, and J.L. Dean. U.S.D.A. Bulletin No. 2100. U.S.D.A., Washington, D.C.

Sorghum in Wisconsin. 1942. A.H. Wright. Stencil Circular 229, University of Wisconsin-Extension.

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