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When selecting sorghum products, consider university or state yield trial results and seed company data. Choose those products that have performed well in yield trials over multiple years and in multiple locations. The most important characteristics to consider in a sorghum product are yield potential, maturity, standability, and pest and disease resistance. Full-season sorghum can offer several advantages over short-season sorghum, including greater yield potential.
For maximum yield potential, choose a product with the latest maturing date suitable for the location’s average growing season, the duration of moisture availability, and cropping sequence. Sorghum requires between 2,673 and 3,360 growing degree units (GDU) to reach maturity. Aim for the product to reach physiological maturity (black layer) a week or two prior to the average date of the first killing frost. Consider planting two or more products that differ slightly in maturity to reduce the risk of yield loss due to adverse environmental conditions.
Seeding rate recommendations can vary considerably and should be determined based on the amount of predicted precipitation, the ability to irrigate, growing conditions, and seedling emergence rates. Final plant populations can range from 20,000 plants per acre in dryland conditions to over 120,000 plants per acre for irrigated fields (Table 1). Estimates of seedling emergence rates also vary and are assumed to be between 65 and 70%.3
The optimum seeding depth for sorghum is around 1 inch. Seeding deeper than 2 inches may reduce stands or delay emergence. Most grain sorghum is planted in 30-inch rows because the same equipment for corn and soybeans is often used. Narrower row spacings have not consistently improved yield potential over conventional 30-inch rows; however, narrower row spacing may be recommended for late-planted sorghum as narrow spacing promotes quicker canopy closure for better moisture retention and weed control.
Early-season weed management is critical for maximizing yield potential. Even light weed infestations early in sorghum development can significantly reduce yields. A heavy infestation at this time may cause up to a 20% yield reduction.1 Fields should be free of weeds before planting. To avoid seedling injury, follow herbicide labels for sorghum preplant application rates and restrictions and use seed that has been properly treated with a seed protectant or safener to avoid herbicide injury. Pre-emergence, residual herbicides, like Degree Xtra® or Warrant® Herbicide, can provide annual grass and small-seeded broadleaf weed control and are the foundation of weed control in sorghum. Tank mixing with appropriate herbicides can help to achieve the greatest possible residual weed control. There are limited post-emergence herbicide options for grass control in sorghum. Johnsongrass and shattercane are related to sorghum; therefore, target herbicides will also harm the sorghum crop.
Nitrogen (N), phosphorous, and potassium requirements for grain sorghum are similar to that of corn. A good guideline for sorghum following soybean is 1 pound of actual N per bushel of expected yield. For sorghum following corn or wheat, use 1.2 pounds of actual N per bushel of expected yield.2 Sorghum can be sensitive to fertilizer burn; therefore, a broadcast application should be thoroughly worked into the soil. For a row planting system, fertilizer should be placed 2 inches to the side and at or slightly below seed depth.
Insect pests of grain sorghum and the severity of infestations may differ throughout the growing season. Cutworm and wireworm are common pests that can be problematic early in sorghum development, from planting through seedling emergence. These pests can usually be controlled with seed treatments or an insecticide application at planting. Greenbugs, sugarcane aphids, and other aphids, chinch bugs, fall armyworm, and corn earworm may appear after seedling emergence. Late-season pests, such as sorghum webworm, sorghum midge, spider mites, and stink bugs, often appear around flowering and can remain through grain maturity. Insecticides applied during outbreaks can help to control all of these mid- to late-season pests; however, insecticides applied to kill these pests will also kill off the beneficial insects that help to control sugarcane aphids and other aphids, which can lead to a spike in the aphid populations. Sugarcane aphids can be a serious problem in sorghum. The use of tolerant hybrids, insecticidal seed treatments, scouting for aphid thresholds, and insecticide applications when thresholds are reached are important management tactics for preventing yield loss caused by this pest. Some insect pests, such as billbugs and other beetles, are difficult to control once an outbreak has occurred. Preventative, at-planting insecticide applications should be considered if these insects have been a problem in the past.
Many types of fungi and some bacteria and viruses cause diseases of sorghum seedlings, roots, leaves, stalks, and grain. Diseases are difficult to control after they develop. Therefore, the most inexpensive, efficient, and effective disease control procedures are preventive. If a disease has been a problem in the past, consider planting products with genetic resistance when available. Plant into fertile soils using high-quality seed with traits for good stalk strength. Avoid planting into cool, wet soils.
Regular scouting is an important element of insect and disease management. When an infestation is confirmed, use economic thresholds to determine if and when to use chemical control. If the field has had a history of infestation of a particular insect pest or disease, plant resistant products, if available, and use crop rotation to reduce pest densities and disease inoculum.
Grain sorghum can be slow to dry after reaching maturity. This may be problematic when sorghum is used as a late-planted rescue crop or when fall conditions delay harvest. Several chemical desiccants are available for pre-harvest use on grain sorghum. If applied when the grain is physiologically mature, at 25 to 35% moisture, sorghum yields are not adversely affected.3 Grain is at the correct moisture level when kernels at the bottom of the head (the last to mature) show an abscission layer (black layer) at the tip of the kernel.
Combine adjustments are critical to minimize harvest losses. Harvest losses can be compounded by the fact that grain sorghum often ripens unevenly. The Grain Sorghum Production Handbook published by Kansas State University is a good resource for setting up the combine for grain sorghum harvest and recommendations for drying and storage.3
Using sorghum in a double-cropping system along with an early-maturing winter crop, such as wheat or triticale, can help to maximize annual dry matter production while simultaneously providing environmental benefits such as reduced erosion. In northern climates with a shorter growing season, sorghum can be double-cropped for biomass production and harvested early prior to seed maturity. In southern climates with a longer growing season, sorghum can be double-cropped for grain production. The suitability of sorghum for a double-cropping system depends largely on the genotype. While full-season products tend to have greater yield potential in a single-cropping system, early maturing products that can maximize dry matter production earlier in the growing season are best suited for double-cropping.