Grass for Biogas Production—Anaerobic Methane Production from Five Common Grassland Species at Sequential Stages of Maturity

Abstract

Grassland biomass represents the most significant feedstock resource in Ireland, accounting for approximately 91 % of the 4.3 million hectares of agricultural land. Grass can be an excellent energy crop and may be classified as a high yielding (up to 15 t dry matter ha^{− 1} a^-1), low input perennial crop. Consequently, grass will be a dominant feedstock for anaerobic digestion (AD) on Irish farms. This study investigated the effects of stage of maturity of five grass species on methane production using dried, milled samples in a small-scale (160 ml), high-throughput batch digestion test. Five common grass species (perennial ryegrass, Italian ryegrass, cocksfoot, timothy and tall fescue) were grown in field plots (with three replicate blocks) under a high nitrogen fertiliser input (125 kg N ha^{− 1}) and harvested at five sequential dates (fortnightly from 12 May to 7 July; n = 75 plots) in the primary growth. Of the five grass species investigated, average total CH₄ production was highest (P  <0.01) for the perennial ryegrass. On average, the rate of digestion decreased (P < 0.001) with increasing plant maturity. Although total CH₄ production decreased numerically with advancing plant maturity, this difference was not significant (P  >0.05).