Sweet sorghum fermentables as influenced by cultivar and planting and harvest dates

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Virginia Polytechnic Institute and State University


Several crops are being touted as a renewable energy source. Sweet sorghum is adapted to many areas, grows relatively rapidly, has moderate water and nutrient requirements, and develops high levels of fermentables. A long term supply of fermentables is needed to make ethanol production economically feasible. Short term availability of fermentables results in unfavorable returns from investments in equipment and crop production.

Two planting dates (15 May and 1 June) and eight harvest dates (30 Sep. to 15 Mar.) were used in efforts to to extend the availability of sweet sorghum fermentables. Respiration of sweet sorghum tissue was measured with an infrared gas analyzer before and after freezing to quantify loss of fermentables associated with delayed harvests. Vital stains were used before and after freezing to estimate cell mortality.

No significant difference was found in the level of fermentables in sorghum from the two planting dates. Delay-ing harvest caused fermentables to decline significantly (30%) by mid-November and to continue to drop through March.

Respiration dropped 90% after exposure to -8 C, but resumed the same rate after 1 week. Vital stai!1ing showed cells die upon freezing. Relatively high respiration rates in the dead tissue suggests saprophytic microbes are responsible for the large decline in fermentables after freezing.

Eleven cultivars and lines were tested for yield of fermentables. Wray was superior in yield of fermentables, while Sumac and Sugardrip were inferior.