As work continues on future biofuels based on cellulose sources, researchers are working to answer a question that many crop farmers ask today: "Where will all that cellulose come from?" Of course, corn and soybean farmers aren't likely to turn over profitable ground to raising switchgrass and other sources. One potential source land in the Conservation Reserve Program.
But turning to CRP ground for the cellulose raises another question: "What is the cellulose production capacity for that ground?" That's what researchers have been working to answer in a multi-year study looking at biomass harvests on CRP ground in northwest Oklahoma.
The program was a cooperative effort between USDA's Agricultural Research Service Grazinglands Research Labe in El Reno, Okla., in cooperation with the Natural Resources Conservation Service District Conservationist stationed in Buffalo, Okla. They identified six CRP sites in the region for multi-year biomass harvests. Results of the study were published in the March-April 2010 issue of Crop Science.
Three of the sites had native grasses and three were introduced old world bluestem, another potential cellulosic source for biofuels. The sites were harvested over three years and consisted of three harvest date treatments: early August, early October and post frost. Harvest was timed to avoid bird nesting seasons.
When averaged across all years, locations and harvest treatments, old world bluestem with the productivity winner with nearly twice the dry biomass feedstock per acre than the native mixed grasses. October harvests produced the maximum yield for both types of fields.
In the multi-year study, biomass production fell for all sites and all treatments, but the rate of decline was faster for the bluestem ground suggesting that mixed native grasses may be more sustainable long term, but the researchers say more long-term study is required.
Some of the decline in output was attributed to low rainfall in the third year. And in the first year, output coud have been higher due to accumulated leaf litter. Although burning to remove that litter can boost grassland productivity, the study demonstrated that harvesting does not yield the same results. Nutrient removal was small, but cumulative, so without nutrient inputs, the researchers say it's unlikely either type of grass would be sustainable.
Finally, the researchers note that given the low yields observed in the study, use of CRP land for biomass feedstock production in low rainfall regions may not be economical. If harvests were restricted to every third year, that would require more land to meet the needs of the new industry. Finally, the yield advantage of the bluestem raises serious questions about the use of introduced grass species to maximize biomass yield, and whether that's compatible with native perennial species.
More work is obviously needed, but the multi-year study does offer insight to the challenges ahead for the switch to cellulosic biofuels.