Sweet Sorghum Agronomy Guide
Native to Africa, sweet sorghum is a grass plant that has been available to U.S. growers since 1853. Several types of sorghum exist: grain sorghum grown as a cereal crop; forage sorghum grown for livestock silage; fiber sorghum grown for its cellulose and hemicellulose content; broom sorghum grown for its long, elastic stalks used in broom-making; and sweet sorghum grown for its stalks’ sugar content. Relative to other sorghums, sweet sorghum uses more resources to produce soluble sugar in the stalk juices.
In temperate U.S. climates, sweet sorghum may be a viable crop. It may work well as a rotational crop. For example, it may rotate well with corn and soybeans as companion crops. The crop may grow as tall as 14 feet, and its fresh-weight biomass yield may range from 20 tons to 50 tons per acre in good conditions. During somewhat dry years with good late-season sun, sweet sorghum sugar levels typically are higher. The crop has potential as a bio-energy feedstock, and bio-energy co-products generated during the biofuel production process also have uses.
Depending on an area’s climate, sweet sorghum could be a double crop because its production process takes about 100 days. As a possible ratoon crop, a single sweet sorghum planting could yield more than one harvest. If not used for biofuel, a second cutting could be directed toward haying or grazing uses. In some research, ratoon production hasn’t consistently indicated that it would be a viable sweet sorghum production model.
Sweet sorghum performs best in loam and sandy loam soils. If adequately fertilized, then well-drained silt loams may also work for sweet sorghum production. The crop tends to perform more poorly in soils with too much clay or too much organic matter. Soil pH levels should be at least 5.8. In fields that lack the fertility needed for soybean or corn production, sweet sorghum may be an alternative. Selecting a site should also consider proximity to market. Research from the Mississippi Delta region suggests that sweet sorghum fields should be near biofuel facilities.
Planting date depends on soil temperatures. For the best seed emergence, plant sweet sorghum when soil temperatures warm to at least 65 degrees F. Sweet sorghum can handle drought and hot weather, but it doesn’t respond well to cool weather. With regard to its moisture needs, sweet sorghum plants can temporarily postpone growth and metabolic processes during low-moisture periods and continue growing when water is available.
When planting seed, the seedbed should have a 4- to 6-inch prepared depth. Alternatively, sweet sorghum growers may also consider a minimum-till or no-till model. Seed planting depth on average should range from 0.5 inches to 1 inch. Lighter soils require deeper seed planting, and heavier soils need shallower planting depths.
To maximize sweet sorghum stalk size, reduce lodging and boost stalk sugar content, plant population shouldn’t exceed 60,000 plants per acre to 100,000 plants per acre. Plant populations that are too high may lead to reduced stalk diameter and an increased likelihood that lodging becomes a problem. Before selecting the preferred row spacing (i.e., 7.5-inch drill or 30-inch row), growers should consider the type of harvesting equipment that will be used to ensure that they match.
When choosing sorghum varieties for biofuel production, the best options have straight, strong stalks. Sugar content should also be high. A sterile male hybrid introduced by the University of Kentucky increases juice yields by 25 percent. Because the male cultivar produces no seed, the crop is less susceptible to lodging. Non-flowering hybrids also don’t produce grain. Variety selection may also enable producers to choose cultivars that better manage cold weather or resist disease or pests. By staggering planting dates and choosing multiple maturities, producers can extend the harvest season and manage the production influx needed to process soon after harvest.
Unlike perennial bioenergy crops such as switchgrass and miscanthus, sweet sorghum is an annual that doesn’t demand a long-term obligation. From a soil fertility perspective, sweet sorghum efficiently uses nitrogen. In some tests, Missouri research found that sweet sorghum biomass yields could reach high levels after the crop followed soybeans and had no nitrogen. If applying nitrogen, 60 pounds per acre is a guide. Some studies have indicated that applying nitrogen doesn’t appreciably alter sugar production, but the nitrogen applications would influence biomass production. Nitrogen response will vary by soil type, but applications that exceed 60 pounds per acre to 90 pounds per acre usually don’t produce significant gains in biomass production. If producers remove sweet sorghum biomass from fields, then they’ll also be removing phosphorus and potassium.
If the selected sweet sorghum varieties produce seed, then removing the seed heads may preserve high sugar levels in the stems and prevent carbohydrates from moving into the seed heads. Research has indicated varied success with deheading sweet sorghum, and deheading may reduce the ability to extract sugar from sweet sorghum. The choice to dehead sweet sorghum may differ by variety.
Weeds, Pests and Diseases
Weed management is a top challenge when growing sweet sorghum. One good weed control strategy involves addressing weeds before they become established. To reduce competition for sweet sorghum, avoid planting it in fields that have a Johnson grass or bermuda grass problem. Because few registered herbicides are available for sweet sorghum, producers commonly manage weeds using cultivation. Missouri research indicates that atrazine and metolachor may control grass and broadleaf weeds in sweet sorghum. Treating seed with a fluxofenin herbicide safener is also an option to reduce sweet sorghum damage.
From an insect perspective, chinch bugs and worms present challenges for sweet sorghum. The chinch bugs tend to create more problems during drought. Sweet sorghum fields planted late tend to be most vulnerable to worms. Any pest that presents problems in grain sorghum is a potential challenge for sweet sorghum.
Like with pests, any disease prone to infecting grain sorghum may also influence sweet sorghum. Diseases that affect sweet sorghum include leaf anthracnose, red stalk rot and maize dwarf mosaic virus. To avoid disease infecting sweet sorghum seedlings, consider applying fungicide seed treatments. Rotating sweet sorghum and non-grass crops may also decrease disease outbreak potential.
Harvest and Storage
Sweet sorghum harvest must occur before a hard freeze. Preferably, producers harvest sweet sorghum when sugar content and fresh yield reach their highest levels. If the sweet sorghum variety produces seed, then monitoring seed for the hard dough phase will indicate when the plant has matured to the desirable harvest stage. Alternatively, when determining whether to harvest sweet sorghum, producers may gauge the sugar content of sweet sorghum juice using a refractometer that measures the Brix level. Ideally, producers should wait until the Brix level ranges from 14 percent to 20 percent. Near infrared spectroscopy is an alternative method to assess sweet sorghum feedstock quality.
Mechanical harvest is preferred for large production areas. However, producers may lack commercial harvesting equipment designed to handle significant production volumes.
Raw sweet sorghum juice has a short shelf life. As such, it’s not well-suited for storage. Ideally, processing facilities will operate at least within a few miles of sweet sorghum production areas. Harvesting innovation has included in-field technology that harvests the sweet sorghum, extracts juice, ferments the juice and distills it to create a purified ethanol. In-field systems would allow for fairly immediate processing and address storage, transportation and processing challenges involved in sweet sorghum biofuel value chains.
If forced to store sweet sorghum, then create storage conditions that discourage juice quality deterioration. Juice may be refrigerated for a few weeks. Bagasse is the sweet sorghum stalk by-product that results after juice extraction. When bagasse won’t be used immediately, then it will need to be ensiled to prevent spoilage. To ensile bagasse, pack the chopped material tightly in an area, or wrap the material in plastic. Both create a low-oxygen, anaerobic environment and help with preserving bagasse.
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