Alfalfa
Alfalfa (Medicago sativa) is a perennial flowering plant cultivated as an important forage crop in many countries around the world. The plant is also known as lucerne in some parts of the world, and grows to a height of up to 1 meter (3 ft), and has a deep root system, sometimes stretching more than 15 meters (49 ft) making it very resilient, especially to droughts. Alfalfa is already cultivated in some parts of the world as forage for cattle, and is most often harvested as hay, but can also be made into silage, grazed, or fed as greenchop. Alfalfa usually has the highest feeding value of all common hay crops. When grown on soils where it is well-adapted, alfalfa is often the highest yielding forage plant, but its primary benefit is the combination of high yield per hectare and high nutritional quality. Then there are a number characteristics that would recommend it as a bioenergy crop [182]. Furthermore, this perennial with a deep root system enhances soil quality and carbon content. It is a legume and requires no nitrogen fertilizer input. When alfalfa is used in rotation with a row crop, such as corn, alfalfa eliminates the need for nitrogen input during the following production year and is associated with a 5-15% boost in corn yield. Finally, alfalfa leaves are high in protein content (26-30%). As such, alfalfa is unique among bioenergy crops in having a readily available coproduct, whereby the leaves can be marketed as animal feed and the stems converted to biofuels. One disadvantage of alfalfa is that
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Table 3.18 Chemical composition of wild-type, late-harvested
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its biomass production yield is lower than those cited for other bioenergy crops, most of which are warm season grasses. However, in the past alfalfa has been managed for forage quality to the detriment of biomass yield. More recent studies have started to focus on improving biomass yield. In one study that emphasized biomass production, the yield per hectare was doubled, while maintaining production of the protein-rich leaf biomass [182].
The potential of alfalfa as a biomass resource has been reviewed [183], and chemical composition analysis shows higher protein content compared to other lignocellulosic biomass forms proposed for cellulosic ethanol production; the chemical composition of alfalfa stems are shown in Table 3.18. Some of the common pretreatment methods like dilute acid [184], hot water [185, 186], and per acids [187] have been tested on alfalfa.
Dien et al. have recently evaluated alfalfa stems for biochemical conversion into ethanol using dilute acid and ammonia pretreatments [188]. In this study, two alfalfa lines were compared; a reduced S-lignin transgenic cultivar generated through down-regulation of the caffeic acid O-methyltransferase gene and a wild-type control. Both were harvested at two maturities. All the samples had similar carbohydrate contents. In this study, Dien and coworkers found that ethanol yields for alfalfa stems pretreated with dilute acid were significantly impacted by harvest maturity and lignin composition, whereas when pretreated with dilute ammonia, yield was solely affected by lignin composition. Furthermore, use of a recombinant xylose fermenting Saccharomyces strain during the fermentation of ammonia pretreated alfalfa samples further increased ethanol yields. Interestingly, ethanol yields obtained with xylose- fermenting yeast were 232-278 l/ton and are a significant enhancement for the reduced S-lignin cultivars [188].