Considerable research has been conducted to screen and select herbaceous plants as potential biomass candidates that are mainly unexplored in the Continental United States. Other research has concentrated on cash crops such as sugarcane and sweet sorghum, and still other research has emphasized tropical grasses. In the late 1970s, a comprehensive screening study of plants grown in the United States generated a list of 280 promising candidates from which up to 20 species were recommended for field experiments in each region of the country (Saterson and Luppold, 1979). The four highest-yielding species recommended for further tests in each region are listed in Table 4.16. Since many of the plants in the original list of 280 species had not been grown for commercial use, the production costs were estimated as shown in Table 4.17 for the various classes of herbaceous species. The results were used in conjunc­tion with yield and other data to develop the recommendations in Table 4.16.

A large number of research projects directed to small-scale field tests of potential herbaceous energy crops have since been carried out. The productivity ranges for some of the promising species for the U. S. Midwest and Southeast are shown in Table 4.11. The results of this research helped to establish a strategy that herbaceous biomass energy crops should be primarily grasses and legumes produced by use of management systems similar to those used for conventional forage crops. It was concluded that the ideal selection of herba­ceous energy crops for these areas would consist of at least one annual species, one warm-season perennial species, one cool-season perennial species, and one legume. Production rates, cost estimates, and environmental considerations indicate that perennial species are preferred to annual species on many sites, but annuals may be more important in crop rotations.

In greenhouse, small-plot, and field-scale research tests conducted to screen tropical grasses as energy crops, three categories emerged, based on the time required to maximize dry-matter yields: short-rotation species (2 to 3 months), intermediate-rotation species (4 to 6 months), and long-rotation species (12 to 18 months) (Alexander, 1991). A sorghum-sudan grass hybrid (Sordan 70A), the forage grass napier grass, and sugarcane were outstanding candidates in these categories. Minimum-tillage grasses that produced moderate yields with little attention were wild Saccharum clones and Johnson grass in a fourth category. The maximum yield observed was 61.6 dry t/ha-year for sugarcane propagated at narrow row centers over 12 months. The estimated maximum yield is of the order of 112 dry t/ha-year using new generations of sugarcane and the propagation of ratoon (regrowth) plants for several years after a given crop is planted.

Overall, the research that has been completed in the United States on the development of herbaceous biomass energy crops shows that a wide range of suitable species exist from which good candidates can be chosen for each area.

aSatterson and Luppold (1979).

bAs defined by U. S. Dept, of Agriculture (1972); excludes Alaska and Hawaii.