The development and current status of perennial rhizomatous grasses as energy crops in the US and Europe

Abstract

Perennial grasses display many beneficial attributes as energy crops, and there has been increasing interest in their use in the US and Europe since the mid-1980s. In the US, the Herbaceous Energy Crops Research Program (HECP), funded by the US Department of Energy (DOE), was established in 1984. After evaluating 35 potential herbaceous crops of which 18 were perennial grasses it was concluded that switchgrass (Panicum virgatum) was the native perennial grass which showed the greatest potential. In 1991, the DOE's Bioenergy Feedstock Development Program (BFDP), which evolved from the HECP, decided to focus research on a “model” crop system and to concentrate research resources on switchgrass, in order to rapidly attain its maximal output as a biomass crop. In Europe, about 20 perennial grasses have been tested and four perennial rhizomatous grasses (PRG), namely miscanthus (Miscanthus spp.), reed canarygrass (Phalaris arundinacea), giant reed (Arundo donax) and switchgrass (Panicum virgatum) were chosen for more extensive research programs. Reed canarygrass and giant reed are grasses with the C₃ photosynthetic pathway, and are native to Europe. Miscanthus, which originated in Southeast Asia, and switchgrass, native to North America, are both C₄ grasses. These four grasses differ in their ecological/climatic demands, their yield potentials, biomass characteristics and crop management requirements. Efficient production of bioenergy from such perennial grasses requires the choice of the most appropriate grass species for the given ecological/climatic conditions. In temperate and warm regions, C₄ grasses outyield C₃ grasses due to their more efficient photosynthetic pathway. However, the further north perennial grasses are planted, the more likely cool season grasses are to yield more than warm season grasses. Low winter temperatures and short vegetation periods are major limits to the growth of C₄ grasses in northern Europe. With increasing temperatures towards central and southern Europe, the productivity of C₄ grasses and therefore their biomass yields and competitiveness increase.

Since breeding of and research on perennial rhizomatous grasses (PRG) is comparatively recent, there is still a significant need for further development. Some of the given limitations, like insufficient biomass quality or the need for adaption to certain ecological/climatic zones, may be overcome by breeding varieties especially for biomass production. Furthermore, sure and cost-effective establishment methods for some of the grasses, and effective crop production and harvest methods, have yet to be developed.

This review summarizes the experience with selecting perennial grasses for bioenergy production in both the US and Europe, and gives an overview of the characteristics and requirements of the four most investigated perennial rhizomatous grasses; switchgrass, miscanthus, reed canarygrass and giant reed.

Introduction

Perennial grasses have been widely used as fodder crops for centuries, often contributing significantly to energy supply on farms through the use of draft animals. For example, as late as 1920 in the United States, 27 million animals provided traction power on farms and in cities, fuelled by some 35–40 million hectares of grasslands [1]. In the 21st century, perennial grasses may be set for a comeback through a number of different energy conversion pathways.

There has been increasing interest in the use of perennial grasses as energy crops in the US and Europe since the mid-1980s. The characteristics which make perennial grasses attractive for biomass production are their high yield potential, the high contents of lignin and cellulose of their biomass and their generally anticipated positive environmental impact.

Energy crops are crops which are produced with the express purpose of using their biomass energetically. High contents of lignin and cellulose in their biomass are desirable, especially when they are used as solid biofuels, for two main reasons. First they have a high heating value due to the high content of carbon in lignin (about 64%). Secondly strongly lignified crops can stand upright at low water contents. Therefore their biomass has lower water contents, the biomass can dry ‘on the stem’ and a late harvest for improved biomass quality is possible [2]. The biomass of perennial grasses has higher lignin and cellulose contents than the biomass of annual crops.

There are many ecological benefits expected from the production and use of perennial grasses. The substitution of fossil fuels or of raw materials based on fossil fuels by biomass is an important contribution to reduce anthropogenic CO₂ emissions. Compared to other biomass sources, like woody crops and other C3 crops, C4 grasses may be able to provide more than twice the annual biomass yield in warm and temperate regions because of their more efficient photosynthetic pathway [3].

Unlike annual crops, the need for soil tillage in perennial grasses is limited to the year in which the crops are established. The ecological advantages of the long periods without tilling are reduced risk of soil erosion and a likely increase in soil carbon content [4], [5]. Furthermore, due to the recycling of nutrients by their rhizome systems, perennial grasses have a low demand for nutrient inputs [6]. Since they have few natural pests, they may also be produced with little or no pesticide use [7].

Studies of fauna show that due to long-term lack of soil disturbance, the late harvest of the grasses in winter to early spring and the insecticide-free production, an increase of abundance and activity of different species, especially birds, mammals and insects, occurs in stands of perennial grasses [8], [9]. Perennial grasses can therefore contribute to ecological values in agricultural production. They can also function as elements in landscape management and as habitat for different animals.

In both the US and in Europe, there are various candidate perennial grasses available which differ considerably in their potential productivity, chemical and physical properties of their biomass, environmental demands and crop management requirements. The initial question for biomass research with perennial grasses was to identify those grasses that best fulfill the demands of bioenergy production, namely high biomass yields and appropriate biomass characteristics.

The aim of this review is to summarize previous experience with selecting perennial grasses for bioenergy (i.e. energy from biomasss) production in both the US and Europe, and to give an overview of the characteristics and requirements of the four most investigated perennial rhizomatous grasses (switchgrass: Panicum virgatum, miscanthus: Miscanthus spp., reed canary grass: Phalaris arundinacea and giant reed: Arundo donax).