Sorghum is known for its drought tolerance, which is why it's used along with corn in ethanol plants in the Southwest where heat and frequent dry spells hurt crops. Bill Rooney, associate professor and coordinator of TAMU's sorghum breeding, and Juerg Blumenthal, associate professor and state sorghum cropping systems specialist, see their new biomass sorghum as an ideal fit for agriculture in the Southeast, from Texas eastward. They visualize sweet sorghum will fit best where sugarcane is being raised, or was raised in the past, along the southern U.S. coast.
To the untrained eye, sorghum looks like a corn plant until the pannicle—the seed-bearing flowering head—appears. Grain sorghum, also called milo, has been bred to be shorter than forage sorghums to facilitate harvesting of the red grain. Milo is primarily used for feed, although it has food uses, and it's used in the ethanol industry where it’s interchangeable with corn. TAMU's work has two tracks—one forage sorghum and the other sweet sorghum.
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Biomass for Cellulosic Ethanol
Until three years ago, forage sorghum research concentrated on improving yield and palatability, traits that make it attractive as an animal feed. Rooney's new work is focused on developing parent lines for a hybrid biomass sorghum bred for maximum yields, tall stands, thick stalks and an extended growing season. The goal is to breed the maternal line to be shorter to aid seed harvest, while the hybrid cross produces tall plants. "The plant we're interested in is a plant that keeps growing vegetatively," Rooney says. Sorghum is a very photoperiod-sensitive crop, he explains. His new hybrids don't initiate flowering until daylight shortens to less than 12 hours per day, thereby allowing the plants to expend more energy producing biomass rather than on reproduction. "They are bred to be tall with very thick stalks so they stand up," he says. "Because they are not reproductive, they are more tolerant of drought."
The biomass sorghum can be left in the field and harvested all winter using silage choppers. The harvested biomass can then be stored in silage pits. The forage sorghums currently used in the breeding program grow 10 to 12 feet tall and yield 11 to 13 tons per acre. "Our goal is 15 to 20 dry tons per acre," he says. "I think we have the genetics to do that." The breeding program he leads has no plans to do transgenetic work with sorghum because it readily hybridizes with johnsongrass, an aggressive weedy relative, he says.
Last year, Blumenthal raised large plots of commercially available forage sorghums that produced 13 tons of dry matter per acre. That is the biomass equivalent of a 300-bushel corn crop, he says, adding that in Texas the average corn crop under irrigation yields 150 bushels. He predicts the TAMU breeders will boost the average yield for biomass sorghum to 15 to 17 tons per acre in three to four years, and 20-plus tons within a decade.
Sorghum crops grow well on land that's suitable for row crops, Blumenthal says. Potentially, it could be grown anywhere in the Corn Belt, he says, but sorghum is better suited to the South. While it's known for its drought tolerance, he and Rooney believe it can be more economically produced in the humid growing areas from East Texas to the eastern seaboard. That region receives more rainfall, which is needed for maximum biomass yields and where it will outperform corn. "The Southeast grows pretty crappy corn," Blumenthal says with a chuckle. "100-bushel-corn is common." Corn yields suffer because high temperatures during the day continue into the night, unlike the northern regions where summer evenings generally cool down. "We lose a lot [of yield] because of nighttime respiration," he explains. A second problem is that the shallower soils of the region don't hold as much moisture as the deeper soils of the Corn Belt.
Biomass sorghum can endure periods of stress much better than grain sorghum or corn, says Blumenthal. He relates his experience last year with two plots, one grown under irrigation and the other under dryland conditions. Both plots were cut in mid-July and then a severe drought set in and less than two inches of rainfall was recorded in the next three months. In late August, the dryland plots were knee-high while the irrigated plots were more than 6 feet tall. Later in the season, good rains fell and by the time the plots were cut there was no difference in height, although the irrigated plots produced more tonnage.
A Sweet Story
Sweet sorghum looks like forage sorghum, but has an entirely different history. Traditionally, it was grown where sugarcane wouldn't grow to supply settlers with sweet syrup for cooking. It is still grown in small quantities—20 acres is a large field, Rooney says. The molasses-like syrup produced by the sweet sorghum is used for barbeque sauces and regional dishes. Sweet sorghum isn't used to make crystallized sugar because it has compounds that are costly to remove. In the ethanol production process, however, there is no difference between sugar and sorghum, Rooney says.
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