Synergistic Effects of Mannan Oligosaccharides and Onion Peels on In Vitro Batch Culture Fermentation of High Concentrate and Forage Diets

The current study evaluated the effect of combining mannan oligosaccharide (MOS) and onion peel (OP) on ruminal in vitro total gas (GP), greenhouse gas emissions, dry matter and fiber fraction digestibility, partitioning factor (PF₂₄; mg degradable DM per mL gas), microbial mass, and volatile fatty acids using two dietary substrates: high forage (HF) and high concentrate (HC) diets. The study was arranged as a 2 × 2 × 6 factorial design with two dietary substrates, two time points (6 and 24 h), and six treatments. The treatments included a control group with no MOS or OP administration and groups administered with 2% of a mixture containing MOS and OP in the following ratios: 1:0 (MOS), 0:1 (OP), 1:1 (MOS:OP), 1:2 (MOS:2OP), and 1:3 (MOS:3OP). No significant diet × treatment interactions were observed for any of the measured parameters. However, treatments decreased (<i>p</i> < 0.05) the undegraded portion of HC, and treatment × substrate interactions were significant (<i>p</i> < 0.05) for PF₂₄ and microbial mass. The treatments in the HC diet produced higher GP (<i>p</i> < 0.001) at 6 h compared to the treatments in the HF diet. Administration of MOS:2OP to the HC diet increased GP at 24 h of incubation, while the lowest GP was observed with the OP in the HF diet. The administration of MOS, OP, and MOS:2OP to the HC diet decreased methane production at 24 h of incubation. Additionally, MOS:2OP and MOS:3OP increased (<i>p</i> < 0.001) degradable acid detergent fiber (<i>d</i>ADF) in the HC diet at 6 h of incubation. Both OP and MOS:3OP decreased the degradability of acid detergent lignin in the HC diet (<i>p</i> < 0.001). The OP also resulted in the lowest DM disappearance (<i>p</i> < 0.001) at 24 h of incubation in the HF diet, while the MOS:3OP had the highest <i>d</i>ADF. At the end of incubation, the highest productions of total volatile fatty acids and acetate were observed (<i>p</i> = 0.002) with the MOS:OP administration in the HC diet, whereas the lowest values were observed with MOS and OP administration to the HF diet. The inclusion of mannan oligosaccharide and onion peel combinations as additives improved substrate (HC and HF) fermentation, leading to higher GP and volatile fatty production, and modulated fiber degradability by improving the breakdown of acid detergent fiber and acid detergent lignin.