H.P. "Sonny" Viator, Montgomery "Wink" Alison, Kenneth Gravois, Kun Jun Han, Dustin Harrell, Allen Hogan, W. "Buddy" Pittman, Michael Salassi and Jerry Whatley

The federal mandate for energy security through the development of sustainable biofuels has revived interest in sweet sorghum as a renewable energy crop. Sweet sorghum’s advantages as a biofuel crop are that it contains large amounts of sugar for fermentation to ethanol and fiber for cellulosic conversion. Also, it has the attributes necessary to grow on all types of soil, even in relatively droughty conditions and on marginal land.

The LSU AgCenter has a long history of evaluating sweet sorghum varieties for the production of syrup, sugar and ethanol. The sugarcane industry through the American Sugar Cane League provided a grant to evaluate management practices, to identify geographic areas and soil types within the state best suited for production and to conduct economic analyses of different production scenarios. Specific objectives were to:

• Evaluate variety performance. 
• Establish optimum nitrogen fertilizer rates.
• Assess the influence of plant density on yield. 
• Determine the feasibility of growing a second (ratoon) crop from a single spring planting.

Plots were harvested at all sites at the hard dough stage of seed development, and stalk samples were taken to the Sugar Research Station in St. Gabriel, La., for assessment. Juice was extracted with a three-roller mill and then filtered before determining the percentage of soluble sugars. Sugar content was based on 90 percent juice purity. Resulting information is from only one growing season, and interpretations and recommendations made from some of the data are not conclusive.

Researchers are confident, however, that a sufficient number of production environments were sampled in 2008 to make a valid comparison of variety performance. Four varieties – M81E, Topper, Theis and Dale – were grown at the Hill Farm Research Station in Homer, Rice Research Station in Crowley and Southeast Research Station in Franklinton, and all but Dale were grown at the Iberia Research Station in Jeanerette. Both production and fertilizer management were not consistent across the four locations. Applied nitrogen fertilizer ranged from 60 pounds per acre plus previously applied manure to 120 pounds per acre, and plant density ranged from approximately 30,000 to more than 40,000 plants per acre.

In spite of these differences, the variety M81E surfaced as the highest-yielding variety under all growing conditions. Averaged over the test sites at Hill Farm, the Rice Station and Southeast Station, M81E produced more than 2,000 pounds more fermentable sugar per acre than the other varieties. At the Iberia station, it yielded equivalent to Topper. Theis showed problems with seedling emergence at several sites, which may have been a reflection of poor seed quality. Topper and Dale produced similar average yields between those of M81E and Theis. M81E also produced the highest yields on fields planted near Lacassine and Lake Charles to demonstrate sweet sorghum production practices to potential growers and processors.

Information gleaned from the other studies conducted during 2008 is considered preliminary and requires additional research before the data can be used. Certain observations, however, can be made. Ratoon yield at the Rice Station was about half of that of the first crop. Ratoon crops require fewer inputs than primary crops and may be economical at times when fall-planted crops like sugarcane do not immediately follow sweet sorghum in a rotation.

Pinpointing the fertilizer nitrogen requirements to optimize sorghum yield is a difficult and lengthy process because it relies on evaluating the response to applied rates across multiple locations and soil types rather than on tools that predict crop needs. The 2008 tests revealed that fermentable sugar yield tended to optimize at intermediate application rates in keeping with previous findings. High fermentable sugar yields achieved with high plant densities are encouraging and with further investigation may lead to a production system not currently practiced.

Production of sweet sorghum as a biofuel feedstock in Louisiana could be easily incorporated into an existing sugarcane production system. Sugarcane farmers could use the same harvest and transportation equipment. Sweet sorghum could be planted on sugarcane rows in the spring following the plowing out of the previous sugarcane stubble crop and preparation of the seed bed in a manner similar for sugarcane production.

Variable production costs of establishing a sweet sorghum crop are estimated to be approximately $193 per acre. This cost includes plowing out the previous sugarcane stubble and planting sweet sorghum and includes specific expenses for seed, fertilizer, herbicides, fuel and labor. At maturity, the sweet sorghum could be harvested with a single-row sugarcane combine harvester along with tractor/wagon units for transporting harvested sweet sorghum stalks from field to trucks for delivery to a processing facility. Estimated variable harvest costs are $145 per acre, using a diesel fuel price of $2.20 per gallon.

Sweet sorghum also could be produced on narrower rows, similar to corn or soybeans. Crop establishment costs would be somewhat lower for this scenario because fewer trips over the field would be required to prepare the seedbed for planting. The economic feasibility of this type of sweet sorghum production system would require the identification of a practical and cost-effective harvesting system.

Best management practices designed to protect soil and water resources have yet to be developed for sweet sorghum under Louisiana conditions. Because sweet sorghum uses water efficiently and requires relatively low nitrogen fertilizer rates, it should be considered to be a conservation-friendly crop. Moreover, growing sweet sorghum during the sugarcane fallow period is a best management practice that provides soil protection at a time the soil is vulnerable to erosion.

Sweet sorghum appears to be a promising biofuel crop, but the production knowledge base needed for commercialization does not compare favorably to staple crops like sugarcane, rice and cotton. The tools necessary to optimize production, such as weed control chemicals and fertilization practices, have not been completely identified at this point. Nevertheless, interest in producing ethanol from sweet sorghum is growing, with commercial production and processing rapidly becoming a reality in the state. 

 
H.P. "Sonny" Viator, Professor and Coordinator, Iberia Research Station, Jeanerette; Montgomery "Wink" Alison, Coordinator, Macon Ridge Research Station, Winnsboro; Kenneth Gravois, Professor and Coordinator, Sugar Research Station, St. Gabriel; Kun Jun Han, Assistant Professor, Southeast Research Station, Franklinton; Dustin Harrell, Assistant Professor, Rice Research Station, Crowley; Allen Hogan, Agent, Jefferson Davis Parish, Jennings; W. "Buddy" Pitman, Professor, Hill Farm Research Station, Homer; Michael E. Salassi, Fairbanks Endowed Professor, Department Of Agricultural Economics & Agribusiness, LSU AgCenter, Baton Rouge; Jerry Whatley, Agent, Calcasieu Parish, Lake Charles, La.

(This article was published in the fall 2009 issue of Louisiana Agriculture.)