There is some concern that bioenergy crops are more trouble than they're worth. They may produce a lot of biomass, but doesn't that come at a cost of lots of added fertilizer and water? Are fertile, midwestern climates the only suitable locations for growing crops, for food or fuel? For some plants, that may be true. But different plants are well-adapted to arid & semi-arid landscapes.
One group of plants with a fairly long history of cultivation includes various Agave species. Agave plants are succulents with a special type of photosynthesis (a process plants use to convert sunlight into energy) called CAM (Crassulacean acid metabolism) that increases water-use efficiency relative to most other plants. Traditionally, Agave varieties have many different uses: they have been used for production of fibers like sisal, for honey/sugar substitutes (agave nectar), and the flowers and stalks are edible. The most well-known use for agave is the fermentation & distillation of the sap to make mezcal, one type of which is tequila.
In addition to these uses, the leaves are also good sources of biomass that can be used as feedstock for conversion into biofuels. Although Agave farms may not produce as much biomass (in terms of tons per hectare) as Miscanthus or Sugarcane (see table below), Agave plants can be productive with as little as 12% of the water needed by these large grasses.
Table: Estimated productivity, rainfall, and nitrogen requirements of current or potential bioenergy crops (see article by Somerville et al. 2010. Science 329:790-792 for references).
| Crop | Average productivity (MT ha–1 year–1) | Ethanol yield (liter ha–1) | Seasonal water requirements (cm year–1) | Tolerance to drought | Nitrogen requirements (kg ha–1 year–1) |
| Corn | 3800 (total) | 50–80 | low | 90–120 | |
| Grain | 7 | 2900 | |||
| Stover | 3 | 900 | |||
| Sugarcane | 80 (wet) | 9950 (total) | 150–250 | moderate | 0–100 |
| Sugar | 11 | 6900 | |||
| Bagasse | 10 | 3000 | |||
| Miscanthus | 15–40 | 4600–12,400 | 75–120 | low | 0–15 |
| Poplar | 5–11 | 1500–3400 | 70–105 | moderate | 0–50 |
| Agave spp. | 10–34 | 3000–10,500 | 30–80 | high | 0–12 |
There is a lot of interest in producing energy sustainably in this country. Water is a particularly valuable resource, particularly in the western U.S. Using excessive irrigation or groundwater to grow crops in arid regions is not a responsible use of this resource. However, many plants, such as Agave, are naturally adapted to climates that experience little and/or erratic rainfall. By planting and harvesting climatically-appropriate energy crops, even the more extreme regions of this diverse country could become good producers of bioenergy.
But doesn't agave grow too slow to be an efficient producer over time? Seems like it would take too long to be productive' And if the answer is to plant more acreage to accommodate production, is there an issue of the effect of such a monoculture? (I am for it as long as there is no effect on the tequila production!)
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