Addition of Corn Silage Enables Effective Methane Inhibition and Chain Elongation to Produce Caproate in Cofermentation with Beef Cattle Manure

Converting beef cattle manure into bioproducts, such as medium-chain carboxylic acids (MCCAs), can help achieve a circular bioeconomy in animal agriculture. The objective of this study is to produce MCCAs from cattle manure (CM) via cofermentation with corn silage (CS), without extra measures to inhibit methanogenesis or addition of pure chemicals for electron donors. In Stage 1, mixing CM and CS at a ratio of 9:3 resulted in an acidic pH at ∼5.0, which completely inhibited methanogenesis. Acetate and butyrate were the dominant acidogenic products at 22.04 ± 0.16 and 5.52 ± 0.49 g/L on Day 36, together accounting for 86.8% of all the carboxylic acids produced. In Stage 2, acetate and butyrate were further converted to caproate following a second dosage of CS, which resulted in CM:CS ratios of 9:9 and 9:18. The ratio of 9:18 resulted in a max caproate concentration of 7.13 ± 0.99 g/L on Day 57 and a max production rate of 0.69 g/L/day. Lactobacillus and Caproiciproducens were simultaneously enriched in Stage 2, suggesting the collaboration of these two genera in caproate production. These results demonstrate our technology as a feasible way to obtain high-titer caproate from cattle manure without exogenous inputs of pure chemicals.