Cattle: A Potential Market for Post-Extraction Algal Residue
High lipid content, ability to thrive on otherwise non-arable land, and high productivity per unit of land mass make algal biomass an attractive second-generation biofuel. Currently, algae contain less than 50 percent lipid, suggesting the coproduct, post-extraction algal residue (PEAR), will be produced in excess of the biofuel. Consequently, capturing value from this coproduct stream impacts the economic feasibility and environmental sustainability of biofuel production from algal biomass.
Approximately one-third of the corn biomass used in ethanol production exits the system as distillers grains, a coproduct currently trading for $225 dollars per ton (dried) or 77 cents per gallon of ethanol produced. Distillers grains have largely been marketed to and utilized by ruminant systems (i.e., beef and dairy cattle) because the ruminant digestive tract is designed to be forgiving and tolerant of the nutritive variance inherent in coproducts. Dependence on a vast microbial metapopulation in the foregut allows ruminants to convert coproducts and other low-quality sources of nutrients into palatable, high-quality meat and dairy products. The capacity of ruminants to utilize coproducts coupled with significant cattle populations (10.1 million head of feedlot cattle, 30.9 million beef cows, and 9.2 million head of dairy cattle) suggests the ability to utilize more than 12 million tons of PEAR per year. The U.S. DOE estimates 4.3 million tons of PEAR is produced per billion gallons of fuel; therefore, cattle are a potential market for PEAR from 3 billion gallons of algal biofuel.
Collection of data to support the development of a feed market for PEAR and the subsequent translation of that data into realistic estimates of market value and use of PEAR have been major components of our research. Initial and ongoing research focuses on describing the nutritive value of unique sources of PEAR in terms and units familiar to agricultural industries. The primary drivers of PEAR value and utilization are protein and residual lipid contents with value discounts occurring for high ash content, mineral imbalances, and mineral concentrations that limit PEAR incorporation into the diets. Processes favoring retention of omega-3 fatty acids in PEAR or as a separate coproduct will add significant value and increase the number of available markets. Upstream processes (methods of biomass cultivation, harvesting, drying, and extracting) and algae species are the major drivers of nutritive value and protocols should be developed to optimize lipid quantity and quality with consideration to how these impact PEAR nutritive value.
Secondly, we have evaluated the palatability of protein supplements containing divergent levels of PEAR from a single source. Our results demonstrated that supplements containing as much as 60 percent PEAR in combination with dried distillers grains or cottonseed meal were readily consumed; however, when a 100 percent PEAR was fed, palatability was reduced. Blending with other feed ingredients clearly improves palatability; however, upstream processes and species of algae are likely significant drivers of palatability that need to be evaluated as additional sources of PEAR become available in sufficient quantities for this type of research.
Our final area of ongoing research is determining the impact of PEAR on digestion and absorption of dietary nutrients in cattle. Ultimately, this data will be coupled with palatability data to create optimal vehicles for delivering PEAR in protein supplements for cattle. One challenge/limitation with our feeding research is the limitation to one source of PEAR. In contrast to ethanol production, which was born from a mature crop (corn) and established processes, algal biofuel production is an emerging industry and is, at this stage, largely characterized by discovery. As algal biofuel production matures into a viable industry, we anticipate significant research on the utilization of PEAR in cattle feed. Continued PEAR research in the developmental stage of algal biofuel is required to steer the industry toward production streams that are optimized for both biofuel production and coproduct value and utilization.
Authors: M. L. Drewery, T. A. Wickersham
Undergraduate Student, Animal Scientist; Texas A&M