Vegetable oils are produced from numerous oil seed crops. Some of these oils have been evaluated in other laboratories as substitutes for diesel fuel. While all vegetable oils have a high energy content, most require some processing to assure safe use in internal combustion engines.
The simplest form of agricultural biomass energy use involves direct combustion of cellulosic crops or residues, such as hay, straw, or corn fodder, to heat space or produce steam. Such fuels are useful for heating farm buildings and small commercial buildings in rural areas and for drying crops. Ideally, energy crops should be produced on land not needed for food production. This use should not increase the erosion hazard or cause other environmental damage.
All crops were planted on clean seedbeds which had been prepared by moldboard plowing and two or more diskings. Nitrogen and potassium fertilizers were broadcast prior to the final disking. Several preplant or preemergence herbicides were used according to the manufacturers' recommendations for specific crops. One or two post-emergence cultivations and some hand-hosing were used to assure complete weed control. Fungicide treatments were used on potatoes and sugar and fodder beets. Bird netting was used to protect sunflowers.
In addition to a high-yield capability, sweet potatoes have several other advantages. They can be planted and harvested mechanically and have good storage life so that an alcohol production facility could be supplied with feedstock over a period of several months. Many of the highest-yielding cultivars of sweet potatoes, including 'Jewel' are marketed for food and are also readily eaten by cattle. Thus, there could be considerable flexibility in marketing a sweet potato crop and in using those which might not find a ready sale. Dry weather at transplanting may hinder stand establishment unless irrigation is available. Harvesting is difficult unless sweet potatoes are grown on sandy or loamy soils.
Jerusalem artichokes (sunchokes) have been widely promoted as an energy crop and had the second highest alcohol production potential in this series of experiments. However, the species seems to have several disadvantages which may well outweigh its high-yielding potential. The crop was relatively easy to grow but difficult to harvest and store. The tubers are tightly attached to the crown of the plant and, when dug by machine, come up as a mass of tubers, crown tissue, and soil which does not separate well like potato or sweet potato. Thus, much hand labor was needed for harvest. In addition to that, the tubers do not store well after digging and must be utilized quickly to prevent spoilage and loss. However, the tubers do keep well in the soil and presumably could be stored in the field and dug as needed whenever the soil was not too wet. Another problem is that they may become weedy.
Sweet sorghum has been grown in the South for making syrup for many years and several widely adapted, high-yielding cultivars with high sugar concentrations are available. Sorghum production and harvesting can be completely mechanized with currently available equipment, and the juice can be fermented directly without enzyme treatment or cooking. Thus, the economics of production and processing could offset the lower alcohol yielding potential. An additional advantage to sorghum is that the crushed stalks make good quality silage which could be used to winter cattle. The primary disadvantage of sorghum is that juice cannot be stored without refrigeration; therefore, use of sorghum as a feedstock is probably limited to a 2- or 3-month harvest period in the late summer and fall.
Potatoes are somewhat lower in total carbohydrates than sweet potatoes; thus, alcohol yields would be less even if total yields were similar. Potatoes have all the handling advantages of sweet potatoes and are easier to establish, since seed pieces rather than transplants are used. The technology for making alcohol from potatoes is well developed and this crop is worthy of consideration in areas where high yields are possible.
Sugar beets and fodder beets were found to be completely unadapted for growth in the Southeast. While early growth was satisfactory, foliage diseases caused severe defoliation in late July and August. Regrowth of leaves was made at the expense of stored sugars; thus, yield and sugar concentrations in the roots at harvest was only about half of what is observed in the major beet growing areas.
Summary results from the highest-yielding cultivars of several oil seed crops are shown in Table 2. In addition to oil, all of these crops produce a high protein oil seed meal which is a valuable feed for livestock and a human food supplement.
Soybeans are the most commonly grown oil seed crop in the Southeast and the easiest to grow and harvest. Yields are quite dependent on rainfall and various management factors including crop rotation and double-cropping with wheat. It is not particularly difficult to produce in excess of 3000 kg/ha in a good year with an adapted cultivar. While soybeans produced less oil than several of the other crops that were tested, the high protein content of the oil seed meal makes it a valuable by-product. The crop also has the advantage of being a legume and thus requires no N fertilizer.
Sunflowers are widely adapted in the United States but are most commonly grown in the western and northern Corn Belt. Sunflowers have been grown in localized areas of the Southeast for 15 or 20 years primarily where there are specialty markets for the seed. Some cultivars of sunflowers have a short growing season, and the crop has potential in double-cropping scenarios either as the first or second crop.
It was difficult to measure the yield of sunflowers in our research plots because of bird damage. Mechanical harvesting requires the use of specialized combine headers.
Okra is a crop with considerable potential as an oil seed, but the growth habits of most cultivars currently grown make harvesting for seed impractical. Okra ripens one pod at a time on each stalk over a period of 2 or 3 months. When ripe, the pods of most cultivars dehisce and most of the seeds either fall out or are rain damaged. The data shown here which were collected by hand-harvesting mature pods at frequent intervals show the potential for oil and oil seed meal production by okra. The yields of combine-harvested okra seed were only about 25% as great. Research by nutritionists has shown that okra meal may be useful as a human food source.
Sesame grew very well in these experiments but was difficult to harvest because of seed shattering. It is commonly grown in areas of low summer rainfall where it can be allowed to dry in a windrow before combining. There are non-shattering varieties of sesame, but they are very low yielding.
Sallower was found to be very low yielding at this location.
Of the cellulose-producing crops shown in Table 3, only kenaf was grown in this series of experiments, but the others have all been grown at this location, and their yield potential is well documented. Kenaf was difficult to harvest and process because it contains many long, stringy fibers which wrap on shafts and otherwise foul machinery.
Each crop listed in Table 3 was chopped, used for boiler fuel, and the heat output measured. Coastal bermudagrass produced the least heat per kilogram but produced the greatest amount per hectare because of its high yield. Bermudagrass contains a lot of silica which formed chunks of glass in the furnace. Corn stalks produced the most heat per unit. Except for kenaf, the crops shown here could be produced as perennial sods or no-till planted, in the case of maize, and could be used to produce energy on erosion prone land.
Data collected on starch and sugar crops indicate that sweet potato and sweet sorghum have the best potential for alcohol production in the Southeast. While sweet potato had the greatest calculated alcohol yield per acre in these studies, sweet sorghum likely could have equaled it if a more efficient juice extraction process could have been used. It is easier to completely mechanize planting, harvesting, and processing of sorghum and the crop is more amenable to no-till planting on rolling land than is sweet potato. Sweet sorghum juice does not require cooking or enzymatic treatment before fermentation. However, sweet potato has a clear-cut storage advantage since sorghum juice must be fermented within a few hours unless it is refrigerated or frozen.
Soybeans appear to have the greatest immediate potential for fuel oil production in the Southeast because the equipment and facilities needed for planting, harvesting, and processing are readily available throughout the region. Sunflower production requires special attachments on combines for harvesting, but, even with that constraint, the crop could be readily grown and processed in the region. While okra appears to offer considerable long-term potential for oil and meal production, these characteristics have not been important in past cultivar development and much plant breeding work remains to be done to make okra a viable fuel crop.
Over the last half century, most American farmers have moved toward fewer and fewer enterprises. It seems likely that, if agricultural crops are grown for fuel, they will be grown on large areas for centralized processing. It is unlikely that many farmers will develop small enterprises to produce fuel for their own use.
| Crop | Cultivars | N rate (kg/ha) | Yield (MT/ha) | Calculated alcohol yield (l/ha) |
| Sweet potato [Ipomoea batatas (L.) Lam.] | 'Jewel' | 120 | 42.6 | 5821 |
| Jerusalem artichoke (Helianthus tuberosus L.) | 'Mammouth French White' | 56 | 30.7 | 4169 |
| Sweet sorghum [Sorghum bicolor (L.) Moench.] | 'Meridian 71-1' | 112 | 72.8 | 2196 |
| Potato (Solanum tuberosum L.) | 'Pontiac' | 112 | 22.6 | 1830 |
| Sugar beet (Beta vulgaris L.) | 'USH 20' | 168 | 31.4 | 1640 |
| Fodder beet (Beta vulgaris L.) | 'Mono Rosa' | 168 | 32.3 | 1309 |
| Yield (kg/ha) | ||||
| Crop | Cultivar | Seed | Protein | Oil |
| Sunflower (Helianthus annuus Mill.) | 'Interstate S-7101' | 2240 | 426 | 801 |
| Okra (Hibiscus esulentus L.) | 'White velvet' | 4677 | 1169 | 794 |
| Soybean (Glycine max L.) | 'Essex' | 3360 | 1008 | 604 |
| Sesame (Sesamum indicum L.) | 'Paloma' | 1599 | 671 | 639 |
| Sallower (Carthamus tinctorius L.) | 'S-108' | 555 | 222 | 211 |
| Crop | Dry matter yield (metric T/ha) | Heat captured (joule/hg-dry matter) | Total energy (joule/ha) |
| Coastal bermudagrass (Cynodon dactylon L.) | 17.9 | 426 x 103 | 7625 x 106 |
| Kenaf (Hibiscus cannabinius L.) | 11.2 | 620 x 103 | 6944 x 106 |
| Corn stalks (Zea mays L.) | 6.72 | 701 x 103 | 4711 x 106 |
| Tall fescue (Festuca arundinacea Schreb.) | 7.8 | 557 x 103 | 4345 x 106 |
| Wood chips | Variable | 586 x 103 | Variable |