New information on grasses as renewable energy crops

There has been considerable attention directed toward the use perennial warm-season grasses as renewable energy crops.

However, information on yield optimum nitrogen requirements for these grasses managed as energy crops has been limited in the central United States.

To address this need, Iowa State University researchers recently reported on the effect of nitrogen fertilization allocation between the above- and belowground plant components of four warm-season grasses.

The allocation of plant nutrients to roots before crop harvest is a desirable trait for energy crops as it improves biomass quality and nutrients in the roots can be recycled by the crop for future growth.

In the two-year study (2006-2007), established stands of big bluestem, switchgrass, indiangrass, and eastern gamagrass were fertilized with 0, 58, 125, or 196 pounds of nitrogen per acre in the spring and harvested following frost in the fall.

The optimum yield after two years was 6 tons per acre at 125 pounds of nitrogen per acre for all grasses except eastern gamagrass, which demonstrated lower yield and a consistent linear response to nitrogen.

For big bluestem and switchgrass, 125 pounds of nitrogen per acre maximized root biomass and favored the allocation of nutrients to roots over shoots (above ground growth).  In contrast, for indiangrass and eastern gamagrass, root biomass and root nutrient allocation were adversely affected by nitrogen.

For all four grasses, 196 pounds of nitrogen per acre shifted allocation of nutrients to shoots over roots.

The researchers concluded that identification of crops and management practices that optimize yield, and resource partitioning to roots at low to intermediate nitrogen rates will promote the development of productive and efficient bioenergy systems by furnishing large quantities of high quality, low cost feedstocks.