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An estimated 25 million tons of animal manure is produced globally every year, causing considerable impact to the environment. These impacts can be managed through the use of anaerobic digestion (AD) This process achieves waste degradation through enzymatic activity, the efficiency of the AD process is directly related to microorganisms that produce these enzymes. Biomethane potential (BMP) assays remain the standard theoretical framework to pre-determine biogas yield and have been used to determine the feasibility of substrates or their combination for biogas production. However, an integrated approach that combines substrate choice and co-digestion would provide an improvement to the current predictive models. PICRUSt (Phylogenetic Investigation of Communities by Reconstruction of Unobserved States) addresses the limitations of assays in this regard. In this paper, the biochemical functions of horse, cow, and pig manures are predicted. A total of 135 predicted KEGG Orthologies (KOs) showed amino acids, carbohydrate, energy, lipid, and xenobiotic metabolisms in all the samples. Linear discriminant analysis (LDA) combined with the effect size measurements (LEfSe), showed that fructose, mannose, amino acid and nucleotide sugar, phosphotransferase (PST) as well as starch and sucrose metabolisms were significantly higher in horse manure samples. 36 of the KOs were related to the acidogenesis and/or acetogenesis AD stages. Extended bar plots showed that 11 significant predictions were observed for horse-cow, while 5 were predicted for horse-pig and for cow-pig manures. Based on these predictions, the AD process can be enhanced through co-digestion strategies that takes into account the predicted metabolic contributions of the manure samples. The results supported the BMP calculations for the samples in this study. Biogas yields can be improved if this combined approach is employed in routine analysis before co-digesting different substrates.

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Anaerobic digestion (AD) of organic waste is considered a sustainable solution to energy shortage and waste management challenges. The process is facilitated by complex communities of micro-organisms, yet most wastes do not have these and thus need microbial inoculation using animal manures to initiate the process. However, the degradation efficiency and methane yield achieved in using different inocula vary due to their different microbial diversities. This study used metagenomics tools to compare the autochthonous microbial composition of cow, pig, chicken, and horse manures commonly used for biogas production. Cows exhibited the highest carbon utilisation (>30%) and showed a carbon to nitrogen ratio (C/N) favourable for microbial growth. Pigs showed the least nitrogen utilisation (<3%) which explains their low C/N whilst horses showed the highest nitrogen utilisation (>40%), which explains its high C/N above the optimal range of 20–30 for efficient AD. Manures from animals with similar gastrointestinal tract (GIT) physiologies were observed to largely harbour similar microbial communities. Conversely, some samples from animals with different GITs also shared common microbial communities plausibly because of similar diets and rearing conditions. Insights from this study will lay a foundation upon which in-depth studies of AD metabolic pathways and strategies to boost methane production through efficient catalysis can be derived.

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