ABSTRACT
Summary There is commonly a conflict between output/unit of area of land and output/unit of support-energy (SE) input. The possibility of using a proportion of the total land area for biomass energy production suggests an approach for calculating grassland output /ha unconfounded with differences in SE input. Output can be calculated for an area of land that is devoted to grassland and biomass energy crops in proportions that result in a net demand for SE of zero. Outputs of utilized metabolizable energy (UME) are related to inputs of SE for (1) grazed grass with high application of nitrogen (N) fertilizer and (2) the conservation of grass by different methods. SE inputs in fuel, fertilizers, technical chemicals, and manufacture and maintenance of machinery and buildings are calculated. The biomass crop considered is a short-rotation forest (SRF) giving a yield of 12.5 t (1,000 kg) dry matter (DM)/ha and a net SE output of 195.5 GJ/ha. For grass with N and grass-clover, the annual SE inputs are calculated as 33 and 2 GJ/ha respectively, giving UME outputs of 102 and 78 GJ/ha and UME output: SE input ratios of 3:1 and 37:1, respectively. The areas of SRF required would be 0.17 and 0.01 ha/ha grassland, respectively. The UME outputs/ha total land area (grassland + SRF) then become 87 and 77 for grass with high N and grass-clover, respectively. A single cut of grass of 5 t DM/ha is considered to be conserved as silage, Held-cured hay, or barn-dried hay. The energy inputs for production and conservation are 17, 12, and 25 GJ/ha, respectively, to give UME outputs of 40, 30, and 39 GJ/ha and UME output: SE input ratios of 2:3, 2:6, and 1:6, respectively. However, when adjusted for the required area of SRF, the UME outputs become 37, 28, and 35 GJ/ha for silage, Held-cured hay, and barn-dried hay, respectively. These calculations suggest that the adoption of grassland systems with a high ratio for UME output: SE input may not improve land use when the requirements both for animal feed and for SE are considered. If levels of UME output/ha are low, the area of land required for animal-feed production will be high. More intensive production systems using a smaller land area coupled with biomass energy production on the remaining area may be the most appropriate strategy.
