OREGON STATE UNIVERSITY
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Sorghum is an important cereal crop in semiarid regions of the world, most often grown in areas that
are too dry for corn. Several types are commonly grown for forage, including forage sorghum,
sudangrass, and sorghum x sudangrass hybrids. Forage sorghum grows from 5 to 15 feet (1.5-4.5 m)
tall, depending on the cultivar. Hybrids can be fertile and produce grain yields comparable to grain
sorghum, or they can be sterile and produce no grain.
Forage sorghum usually does not regrow following harvest, unlike sorghum-sudangrass or
sudangrass, so forage sorghum is best adapted to a single-cut harvest for silage. Forage sorghum
silage is slightly lower in energy than corn silage and similar in protein. Yields of forage sorghums are
comparable to corn and can range from 15 to 30 ton per acre (37-74 MT/ha) depending on the soil,
weather, and the hybrid. Both grain sorghum and forage sorghum have more resistance to deer
damage than corn. Consequently, they are suitable for planting in fields where deer damage makes
corn production unprofitable.
Forage sorghum has higher temperature requirements than corn, with a minimum temperature for growth of 60°F (15.5 °C) and highest yields occur when the mean temperatures during the growing season are between 75 and 80°F (24-27 °C). For high production, a medium-to late maturing sorghum cultivar (maturity within 110 to 130 days) requires approximately 18 to 26 inches (450-650 mm) of water during a growing season. Seasonal response to water requirements are: early in the growing season, average daily water use is low. Approximately 1 to 2.5 mm/day could be enough to avoid water stress. This period is roughly the first 25 to 30 days (up to approximately the 7-leaf stage). The water requirement then increases to around 7 to 10 mm/day until the boot stage. Maximum daily water use occurs from the boot stage until after anthesis. The daily water requirement then decreases gradually during grain fill as the crop begins to senesce leaves and mature
Suited to excessively drained to moderately well drained soils (wet soil intolerant; ED-MWD). Tolerates moderately acid soils (pH 5.6–7.3). Intolerant of Aluminum (persistence reduced at 0.5 ppm Al3+ and pH 4.2). Moderately tolerant of salinity, 3–6 dS/m (millimhos/cm).
Forage sorghums typically have slightly lower energy values than corn silage, but are similar in protein. When compared to sorghum-sudangrass, forage sorghum silage is higher in energy and lower in protein. Because of the lower quality of forage sorghum crops compared to corn, they are best used in situations where forage sorghum yields are higher than corn or where livestock energy requirements are less than what is supplied by corn silage.
Prussic acid and nitrates: There is potential for prussic acid poisoning or nitrate toxicity with forage sorghum. These problems are not normally a concern when sorghum is harvested for silage. Prussic acid occurs mostly in young plants or plants harvested shortly after a frost or drought-ending rain. The regrowth from plants killed by a frost is particularly high in prussic acid content. Because prussic acid is volatile, it dissipates during the harvesting and ensiling process and is rarely a problem in sorghum silage. Wait four days following a killing frost to harvest forage sorghum. This allows any prussic acid formed to dissipate before harvesting. After four weeks of ensiling, any prussic acid problem has been alleviated. Questionable forage can be tested for prussic acid (HCN) at a qualified laboratory. The concern of prussic acid and nitrate poisoning is greatest when sorghum is grazed less that 24 to 30 inches (61-76 cm) in height, immediately after a killing frost, or on regrowth of sorghum killed by an early frost. Nitrates can be a problem with forage sorghum as they are with corn silage, but this is rare and usually associated with harvesting a heavily manured or fertilized crop during or immediately following a severe stress such as a drought. The ensiling process reduces nitrate content in the silage. When high nitrate levels are suspected, leave 10 to 12 inches (25-31 cm) of stalk in the field, because nitrates tend to accumulate in this region of the plant.
