Biofuel co-products as livestock feed - Opportunities and challenges

144Biofuel co-products as livestock feed – Opportunities and challengesand can be produced worldwide. While research oncellulosic ethanol has been ongoing for several decades, itscommercial viability has only been demonstrated recently.Fibre and storage carbohydrates within grasses can beconverted to alcohol by yeast after enzymatic hydrolysis,but the protein cannot be utilized for ethanol production.Therefore, the use of grass to produce ethanol, especiallyspecies that contain appreciable amounts of protein, createsnitrogenous waste for bio-refineries. However, extraction ofprotein prior to enzymatic hydrolysis and concentrated asleaf protein can be utilized by livestock, thereby reducingprotein costs and offsetting the land required for animalproduction (Dale et al., 2009). Forage crops (e.g. reedcanary grass, timothy and alfalfa, as well as barley, triticale,pearl millet and sweet sorghum hays) and crop residues (e.g.maize stover and bagasse, as well as wheat, barley, triticaleand rice straws) have been identified as potential sourcesof ligno cellulose for bio-ethanol production (Michaud,Bélanger and Surprenant, 1997).Information concerning the feeding value of co-productsfrom cellulosic ethanol or isobutanol production is currentlyquite limited. Isobutanol has potential because it can be producedin a similar manner to ethanol, but it can be directlyblended with oil-based fuels, and efficiency of fermentationis identical to ethanol production. While the potential isgreat to develop a whole new series of possible feeds foranimals, especially for ruminants, there remain a number ofunknowns. To date, data available includes primarily in vitroor in situ data, with little animal performance data.Treatment of fibrous materials is necessary to convertcellulose and other carbohydrates to forms that can befermented to ethanol or to isobutanol. However, whencellulose-rich biomasses are used as an alternative to starchrichmaize grain as a source of sugars for ethanol production,large quantities of co-products need to be disposedof, preferably in a value-added process, possibly as animalfeed. Fortunately, these cellulosic ethanol co-products areusually high in CP.Ammonia fibre expansion (AFEX) is a pre-treatment processfor cellulosic ethanol and may also be used to improveruminant digestibility of feedstuffs not traditionally usedas forages. During AFEX, concentrated aqueous ammoniais contacted with biomass under moderate temperatures(80–150 °C) and pressure (200–400 psi). After a short(5–30 minute) dwell time, the pressure is explosively released.This process has several physical and chemical effects on thelignocellulosic material that improve its digestibility. AFEXresults in cellulose depolymerization and partial solubilizationof hemicellulose. Solubilized hemicellulose and lignincomponents appear to be moved to the exterior of the cellwalls during the process, opening up the structure to facilitateaccess to cellulose by ruminal microbes and enzymes.These changes dramatically increase the rate and extent ofboth glucan and xylan release during enzymatic hydrolysiscompared with untreated material. For cellulosic ethanolproduction, AFEX treatment can increase ethanol productionfrom high fibre sources. For livestock producers, the importantconsideration is the feeding value of the remaining coproducts,although AFEX treatment may also be a means ofimproving digestibility of high-fibre feed sources.Bals et al. (2010) extracted 11 forages – including traditionalforages, agricultural residues and dedicated energycrops (e.g. switchgrass) – using the AFEX process anddigested in vitro with rumen inoculum. AFEX treatmentimproved 48-hour NDF digestion for several moderatelyindigestible forages compared with untreated samples, butshowed no improvement for highly digestible samples suchas alfalfa and maize silage. Of particular interest were maizestover and late-harvest switchgrass, as AFEX treatmentimproved digestibility by 52 percent and 128 percent overuntreated material, whereas the improvement was 74 percentand 70 percent over conventional ammonia treatment,respectively. Weimer et al. (2003) included AFEX-treatedrice straw at modest levels (70 g/kg DM) in a cattle dietand found improved milk yields and intake compared withuntreated straw. An unknown at this time is whether feedingsuch products could support the high milk productionneeded to feed the world’s future human population.The CP content of all treated samples increased tomore than 100 g/kg dry forage in the experiments by Balset al. (2010). Scott et al. (2011) showed that AFEX + enzymatichydrolysis of cellulose and hemicelluloses increasedthe N content and disappearance of plant constituents,but decreased the content of the major structural carbohydrates(ADF and NDF). The AFEX + enzymatic hydrolysistreatedforages could therefore be considered for use asa non-protein N supplement in combination with highenergydiets low in ruminally degradable protein.A practical consideration may be to extract much ofthe leaf protein prior to AFEX or other treatments forcellulosic ethanol production (Dale et al., 2009). Leafprotein properly processed to concentrate it and removeanti-nutritional factors will probably be at least as valuablein livestock diets as soybean meal protein. Leaf proteinproduced as a co-product of cellulosic ethanol productioncan be utilized by livestock (Kammes et al., 2011).The effects of conservation method on protein extractionefficiency from orchardgrass (OG) and switchgrass (SG)were evaluated by Kammes et al. (2011). Two maturitiesof OG and SG were harvested with CP concentrations of171 and 44 g/kg DM (immature) and 131 and 24 g/kgDM (mature) for OG and SG, respectively. Leaf juice wasextracted with a screw press from fresh, stored or wiltedchopped grasses. The liquid obtained was pH adjustedwith HCl, treated with or without zinc salts (chloride),with or without heat, and then centrifuged to precipi-