Evaluation of Herbaceous Biomass Crops in the Northern Great Plains: Results and Discussion: Part 3

Chemical Composition of Annuals

Nitrogen level and cropping system (wheat vs. fallow) at all sites and years had little effect on the chemical composition of annual biomass species (data not presented). Likewise, 2 and 3-way interactions generally were nonsignificant; therefore, all subsequent data are presented at 50 kg N ha ^-1and meaned across cropping systems where included.

Chemical composition of annual biomass species as influenced by environments is presented in Table 13 as a mean of species and cropping systems. Significant year X site interactions were common for many of the chemical components (data not presented); therefore, the data were not meaned across years and sites. In addition, kochia and foxtail millet were not included in all environments.

Ash, N, and TNC concentrations were quite variable while the fibrous components (NDF, ADF, ADL, hemicellulose, and cellulose) were more consistent among environments (Table 13). Ash ranged from 58 to 259 g kg^-1, N from 6.1 to 21.3 g kg^-1, and TNC from 52 to 259 g kg^-1. In contrast, NDF ranged from 578 to 688 g kg^-1, ADF from 288 to 451 g kg^-1, ADL from 24 to 56 g kg^-1, hemicellulose from 192 to 333 g kg^-1, and cellulose from 264 to 375 g kg-^-1 among environments. However, a general trend was difficult to detect.

Chemical composition (meaned across years and cropping systems) of four annual species at 50 kg N ha^-1 and at six sites is presented in Table 14. Within a site, foxtail millet and kochia generally were higher in ash, NDF, N, and cellulose concentrations and lower in TNC concentration than the sorghums (Table 14, Fig. 5). Kochia also was higher in ADL and lower in hemicellulose concentrations than the other species. Sorghum X sudan generally was lower in ADF, ADL, and cellulose but higher in hemicellulose concentrations than forage sorghum. Chemical composition of annual species within environments has been discussed elsewhere (Meyer et al., 1990, 1992, 1993).

Yields of chemical components of annuals generally had a greater number of species comparisons significant than chemical concentrations across sites and years (data not presented). Most significant component yields were related to differences in biomass yields, but significant differences among species in chemical composition contributed at times. These results are similar to the component yields of perennial species/mixtures. Yields of chemical components as affected by species within environments was discussed elsewhere (Meyer et al., 1990, 1992, 1993).

Biomass Yields on a Moisture Gradient

Scatter diagrams of biomass yields vs. growing-season (April through September) precipitation were developed for six perennial (Fig. 6) and four annual (Fig. 7) species to evaluate biomass cropping on a moisture gradient. Biomass yields were not closely associated with growing season precipitation. Coefficient of determinations range from 0.06 for kochia to 0.41 for forage sorghum (Table 15) using environments (sites and years) for the observations. Obviously, growing-season precipitation is an important factor in determining the level of biomass production, but it accounts for less than 40% of the variation noted.

Meyer et al. (1991) in a preliminary evaluation reported that coefficient of determinations ranged from 0.33 to 0.61 for the same species when evaluated within a year with 6 to 9 observations. Therefore, increasing the observations by increasing the years evaluated reduced the association between biomass yield and growing-season precipitation.

Biomass Yields of Perennials and Annuals Compared

Biomass yields of perennials and annuals were determined in separate experiments; therefore, direct comparisons are not possible. However, the perennial and annual species generally were grown in very close proximity, so a general discussion seems appropriate.

Average biomass yields across years of perennial species/mixtures (Fig. 2) at all sites rarely exceeded biomass yields of annual species (Fig. 4). The Leonard site is an exception to this general comment in that several perennials had greater biomass yields than foxtail millet and kochia under both cropping systems (Fig. 2 and 4). The production costs of a ton of perennial biomass will be cheaper than with an annual once stands are established, but the superior yields produced by annuals will probably make annuals cheaper per ton of biomass.

Figure 5

Figure 6

Figure 7