Annual production of grass silage for biogas: effects of fibrolytic enzyme additives on ensilage efficiency and specific methane yields
Keywords:grass yield, silage, fibre, enzymes, effluent, CH4 yield per unit land area
The aims of this study were to quantify the effects of fibrolytic enzyme additives applied to each of four consecutive cuts of unwilted grass at ensiling on ensilage characteristics and specific CH4 yields (SMY) per unit mass and per unit land area. In light of the importance of the primary growth yield, the effects of the timing of Cut 1 were also investigated. Furthermore, the mass-SMY and area-SMY effect of any effluent produced during ensilage was determined.
At each of four cuts that comprised annual growth, samples from four replicate plots of Lolium perenne and of Phleum pratense were untreated (control) or were treated with either of two fibrolytic enzymes (ENZ 1 and ENZ 2) prior to ensiling for 120 days. The mass-SMY of silages and effluents were determined using an in vitro batch anaerobic digestion test. Total annual CH4 yield per ha of grassland was quantified. The effects of altering the timing of Cut 1 were also assessed.
On average, ENZ 1 and ENZ 2 reduced neutral detergent fibre by 9% and 15%, respectively, compared to the control silages. The fibrolytic effects of added enzymes were successful at aiding silage preservation under some but not all of the challenges to ensilage provided in this study. Furthermore, ENZ 1 and ENZ 2 increased effluent outflow by 46% and 96%, respectively.
The mass-SMYs for silages from each cut or either grass species were not significantly enhanced by enzyme treatments. The area-SMYs of silages were numerically negatively affected (P>0.05) by added enzymes (i.e. 4143, 4058, 3944 m3 CH4 ha-1 a-1 for control, ENZ 1 and ENZ 2 treatments, respectively). However, when the effluent was collected and utilised as a feedstock the 6, 10 and 17% increase in annual area-SMY for the control, ENZ 1 and ENZ 2 treatments, respectively, therefore resulted in total area-SMY values for the ENZ 1 and ENZ 2 treatments that were 101 and 105% of the control treatment, respectively.