The Impact of the Dairy Cow Diet on Anaerobic Digestion of Manure

ABSTRACT. The present work is part of a research project focused on the evaluation of the effect of diverse dairy cow diets on manure composition and on the emissions of GHG from dairy farms, including the effect in terms of biogas/biomethane potential in the case of exploitation of manure in an anaerobic digestion (AD) process. In fact, anaerobic digestion determines the production of biogas (CH4 and CO2) in a confined environment, preventing uncontrolled emissions of GHG related to poor manure management practices. The project aims to study the evolution of the microbial community from the digestive system of the animals (in manure), and subsequently in an anaerobic digestion process and in the soil after the land spreading phase. This in relation with the performance of the AD process and with the environmental impact of the spreading, as gas emissions. Two dairy farms (Farm A and C) and one fattening farm (Farm B) were selected. The animal feed was represented by a total mixed ration of hay, silage and concentrates, with higher concentration of TS, NDF, ADF and ADL for Farm A, higher starch concentration for Farm C and lowest TS, while intermediate characteristics were reported for Farm B. Manures from the three farms presented different TS concentration (highest for Farm B). The anaerobic digestion process of manures from the three farms presented very different developments and dynamics: manure C presented a rapid increase of biogas production and methane concentration, with a decrease of daily production in the proximity of the 50th day of process, while manures from Farm A and B, showed a critical initial phase characterized by low methane production and low pH. Full methanogenic conditions were reached only after about 50 days from the start. This difference seems to be related mainly to the organic load of digesters treating manure A and B, leading to an excess of volatile fatty acids and partial inhibition of methanogenic activity and to the lack of inoculum, not used to avoid external alteration of microbial populations.