Estimation of biogas yield from manure of cattle on natural grassland

The use of high-fibre manure for biogas generation is essential as that increases the efficiency of nutrient use from grassland. Grass is the main source of energy for ruminants. Digestible nutrients are converted to animal production, whereas undigested polysaccharides (starch and fibre), lignin and anaerobic microbes are excreted in the manure. These undigested nutrients can be converted to energy for electricity generation. Therefore, manure is a high potential substrate for anaerobic biogas digesters. Cows on balanced diets with low starch content tend to yield manure with the high specific methane yield (166.3 Nl CH4 (kg VS)-1). However, because of the low efficiency in forage fibre digestion, manure fibre of grazed cattle is high as well as the manure microbial content due to hind gut fermentation. This enhances manure methane yield. Manure enrichment and co-digestion with highly fermentable crop residues is essential to increase biogas yield and optimize nutrient utilization from pasture. The aim of the study is to assess nutrient quality in manure from forage-fed small dairy breeds and assess the effects of manure enrichment with crop residues on in vitro gas production in co-digesters. Forage nutrient quality and concentration of methanogens and anaerobic gaseous yield in both rectal manure and manure composited samples with fermentable crop residues will be assessed. Chemical composition will be assessed to determine content of starch and structural carbohydrates. Rate and extent of fermentation gas production will be determined in vitro and fitting the gas production data to the dual pool logistic equation. Methanogens bacteria will be identified using 16S RNA gene sequencing on ion PGM system. Data obtained will be analysed using PROC GLM for means and variances and statistical significance will be determined by analysis of variance. Correlations and regression analysis will be done to determine the correlations between manure non-structural and structural carbohydrates, gaseous yield and bacterial populations. The utilisation of methane from manure of grazed ruminants compensates for the low nutrient utilization efficiency of extensively managed large ruminants.