Ensiling Cyanide Residue and In Vitro Rumen Fermentation of Cassava Root Silage Treated with Cyanide-Utilizing Bacteria and Cellulase

Cyanide is a strong toxin in many tropical forage plants that can negatively affect ruminants. The aim of this study is to determine the cyanide removal efficiency, silage quality, and in vitro rumen fermentation of fresh cassava roots ensiled without an additive (control) and with Acremonium cellulase (AC), two cyanide-utilizing bacterial inoculants (Enterococcus feacium KKU-BF7 (BF7) and E. gallinarum KKU-BC10 (BC10)), and their combinations (BF7 + BC10, AC + BF7, AC + BC10 and AC + BF7 + BC10). A completely randomized design was used with eight treatments × four small-scale silo replicates. Additionally, extra silage samples (seven silos/treatment for individually opening after 0, 1, 3, 5, 7, 15, and 30 days of ensiling) were added to observe the changes in the total cyanide concentration and pH value. The fresh cassava root contained an optimal number of lactic acid bacteria (105 colony forming units/g fresh matter), and the contents of dry matter (DM) and total cyanides were 30.1% and 1304 mg/kg DM, respectively. After 30 days of ensiling, all silages demonstrated a low pH (<3.95; p < 0.01). Cyanide content ranged from 638 to 790 mg/kg DM and was highest in the control (p < 0.01). The addition of BF7 + BC10 increased the crude protein (CP) content (p < 0.01). The addition of AC decreased the fibrous contents (p < 0.01). The control had less acetic acid and propionic acid contents (p < 0.01) and a greater butyric acid content (p < 0.01). However, the degrees of in vitro DM digestibility (IVDMD) and gas production were similar among treatments. Methane production ranged between 29.2 and 33.3 L/kg IVDMD (p < 0.05), which were observed in the AC + BC10 and BF7 + BC10 treatments, respectively. Overall, our results suggested that the cyanide removal efficiency after 30 days of ensiling with good-quality cassava-root silage was approximately 39% of the initial value. The enterococci inoculants and/or AC could improve the ensiling process and cyanide removal efficiency (increasing it to between 47 and 51% of the initial value). The novel enterococci inoculants (BF7 + BC10) were associated with a decreased cyanide content and an increased CP content. They appeared to promote the methanogenesis potential of the cassava root silage. More research is required to validate the use of cyanide-utilizing bacterial inoculants in cyanogenetic plants, bioenergy fermentation, and livestock.