Biogas potential from cow manure : influence of diet

Abstract: Cow manure is an excellent substrate for biogas production in anaerobic digesters though the gas yield from a single substrate is not high. However, mixing cow manure with other kind of waste materials in co-digestion can optimize the production of biogas. In this thesis work the biogas potential from cow manure as a single substrate was investigated. The questions to be resolved were if 1) the biogas potential wasaffected by the feeding strategy of the cows and 2) there is correlation between manure methane potential and enteric methane emission from the same feed. Six fistulated Swedish red breed dairy cows were offered three different types of feed mixtures A(high starch and low fibre), B (medium starch and fibre), and C (low starch and high fibre) during three experimental periods (1, 2, and 3). The complete diet was composed of forage and concentrate. The forage was high quality grass silage, and the concentrate consisted of barley, oat, peas, and rapeseed cake. Each cow received only one type of feed mixture during each experiment period. During the last 5 sampling days of each experiment period, the cows manure was collected and frozen at -20 °C. A batch type reactor was then operated at 37 °C to investigate the methane potential of the manures. The result showed that enteric methane emission of the cows was weakly positively correlated with methane potential of their manure (R=0.2). A better fit was found between starch content in the cow diet and methane potential of the manure though it was not significant alone (P=0.19). The result of the present work was against the hypothesis "less enteric methane of the cow will give high gas potential of the manure."