Logistics for Sustainable Sweet Sorghum Biomass Production

Linda Benedict, Viator, Sonny  |  5/13/2015 9:14:44 PM

H.P. “Sonny” Viator

Sweet sorghum is considered a potential crop for biofuel production in much of the tropical and temperate regions of the world. LSU AgCenter researchers are investigating sweet sorghum, along with energycane, as potential feedstock for biorefining. Sugar refinery operations, which operate for about three months beginning in October in temperate and sub-tropical regions, could be extended several months by sequentially processing sweet sorghum and energycane for biofuel production. A competitive advantage for growing sweet sorghum in the sugarcane region is that early- and medium- maturity hybrids or varieties can be planted on traditional sugarcane rows during the year that sugarcane fields are left fallow in preparation for planting. Sugarcane, a crop that regrows after harvesting, is grown on a four- to five-year cycle before it is replanted. The sweet sorghum can then be harvested and transported using sugarcane equipment in time to plant a new sugarcane crop in the fall of the year.

Expansion to more northerly latitudes beyond the confines of the traditional sugar growing region would allow for the use of full-season hybrids and varieties, which mature too late for use during the sugarcane fallow period. Unlike energycane, which does not possess sufficient cold tolerance for production above latitude 35 degrees north, sweet sorghum can be produced in a more expansive geographic zone.

LSU AgCenter researchers are studying the logistics of sustaining sweet sorghum feedstock deliveries from midsummer to the time energycane is available for processing in October, a period of three months. First generation sweet sorghum hybrids provided by Ceres Inc. were planted monthly from early April to early June in 2012 and 2013. Selected were hybrids of early, medium and late maturity. Sweet sorghum maturity, or time from planting to flower initiation, is controlled by the length of the day (photoperiod). Some hybrids are more sensitive to the length of day than others. The 90-day hybrids, which are early-maturing, are not sensitive to day length. The 120-day, or medium-maturity, hybrids are moderately sensitive to the length of the day. The full-season hybrids won’t initiate flowering until the photoperiod is a certain length of day. Temperature also plays a role and can alter the expected response to the length of day.

Individual plots were harvested as seed panicles reached the hard dough stage of maturity, approximately 30 days after midflowering. Fifteen stalks, with panicles and leaves intact, were randomly selected from the plots for juice and fiber analyses. Biomass weight at harvest was determined using only the center row of each three-row plot to avoid border effects. Combine extractor fans were turned off to ensure the capture of maximum biomass. Total fermentable sugar yields were estimated.

Biomass weight and total fermentable sugar yield varied considerably among hybrids and across planting dates and years. This variation was expected because of the disparities in genetic yield potential of hybrids of different maturity timelines and dissimilar growing environments associated with planting dates. Feedstock was available for processing from the initial harvest of the early- maturing, April-planted hybrids in late July until final harvest of the June-planted, late-maturity hybrids in late October, a period of three months. For both years, the lowest average fermentable sugar yields occurred for the April-planted, early-maturity hybrids and the highest average fermentable sugar yields occurred for the Mayplanted, medium-maturity hybrids. Biomass and sugar yields were surprising lower for the full-season hybrids than the medium-maturity hybrids. The full-season hybrids were expected to yield the highest based on their relative performance in other regions of the world. Several of these full-season hybrids, which are photoperiod sensitive, initiated flowering very late or not at all and, therefore, were harvested prior to optimal sugar accumulation.

To ensure for continual feedstock processing, refineries will have to coordinate with growers to establish production plans. For example, a refinery capable of grinding a daily quota of 1,000 tons of sweet sorghum biomass would require approximately 90,000 tons for a three-month processing season. Based upon the 2012 and 2013 information on maturity and performance for each sweet sorghum maturity group/planting date combination, it is possible to project a harvesting schedule. As shown in the table below, approximately 3,000 acres would need to be planted, with the majority of the acreage planted in April and June because of lower yields for those planting dates relative to the May planting. Additionally, because the early-maturity hybrids are relatively low yielding, a larger planting of that maturity group is required to meet the tonnage needs for the July harvest. Also, note that the scheduling for future production shown in the table did not include projections for the early planted, late-maturity hybrids or early-maturity hybrids planted after April because higher-yielding maturity/planting date combinations were available in each two-week harvest window for meeting the biomass demands. Maturity and performance of these hybrids may differ by geographic location and prevailing growing conditions. Therefore, each area will have unique circumstances. 

H.P. “Sonny” Viator is a professor and resident coordinator at the Iberia Research Station in Jeanerette.

(This article was published in the spring 2015 issue of Louisiana Agriculture Magazine.)

 

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