# Metabolomics Study Revealed the Effects of CaO-Treated Maize Straw on the Rumen Metabolites

## TL;DR

This study shows that treating maize straw with calcium oxide (CaO) improves its digestibility for ruminants by altering rumen metabolism and enhancing cellulose and amino acid metabolism.

## Contribution

The study reveals how CaO treatment at higher concentrations disrupts lignin-cellulose bonds and boosts rumen metabolism through non-targeted metabolomics.

## Key findings

- High-efficiency CaO treatment (5% and 7%) disrupts lignin-cellulose networks, enhancing cellulose metabolism and amino acid synthesis.
- Dihydro-3-coumaric acid positively correlates with Prevotella and key fermentation indicators like acetate and propionate.
- Metabolites from the HE group are enriched in amino acid and tryptophan metabolism pathways.

## Abstract

Untreated maize straw has a low feed digestibility because of its high cellulose and lignin content, which restricts its use in animal feeding. A previous study by our team has demonstrated that calcium oxide (CaO), an affordable and eco-friendly alkaline reagent, may enhance the rumen degradation rate and fermentation effect of maize straw, while enriching the abundance of cellulose-degrading microorganisms. This study further explored the effects of different levels of CaO treatment on corn straw on rumen metabolism by liquid chromatography–mass spectrometry non-targeted metabolomics. Results showed that CaO in the HE group (High-Efficiency group: 5% and 7% levels) significantly broke down the network structure formed by lignin and cell wall polysaccharides in corn stalks, and improved cellulose metabolism and amino acid synthesis and metabolism. This study provides data for the efficient utilization of crop by-product maize straw as feed, contributing to the sustainable development of agriculture.

As an important limiting factor, lignin hinders the utilization rate of maize straw in ruminants. CaO treatment increases the feed digestibility of maize straw by disrupting the ester bonds between hemicellulose, cellulose, and lignin in maize straw. Our previous research found that CaO treatment of corn straw may increase its feed digestibility by altering the rumen microbes’ abundance. This study further investigated the molecular mechanism of CaO treatment to enhance feed utilization and also examined its ongoing effects on rumen metabolites. Rumen fluid was collected to analyze microbial metabolites using liquid chromatography–mass spectrometry (LC–MS) non-targeted metabolomics. Maize straw (moisture content of 60%) treated with four levels of CaO (0%, 3%, 5%, and 7%) was used as the fermentation substrate for a 6 h in vitro culture. Based on the effect of CaO-treated maize straw on the rumen microbial diversity, no significant differences were observed in microbial composition between the 0% and 3% treatment groups or between the 5% and 7% treatment groups. However, the microbial structure of the 0% and 3% treatment groups differed from that of the 5% and 7% groups. Therefore, the four levels were divided into a low-efficiency group (LE group: 0% and 3% levels) and a high-efficiency group (HE group: 5% and 7% levels) for principal component analysis (PCA) and partial least squares discriminant analysis (PLS-DA). Compared with the LE group, most of the ruminal metabolites that showed increased levels in the HE group were products of lignin degradation. Among these differential metabolites, Dihydro-3-coumaric acid had a significant positive correlation with Prevotella and fermentation indicators like acetate, propionate, and butyrate. KEGG analysis showed differential metabolites were primarily enriched in the amino acid metabolism, tryptophan metabolism, phenylalanine, tyrosine, and tryptophan biosynthesis, and cyanoamino acid metabolism. The higher CaO concentration in the HE group effectively disrupted most covalent bonds with lignin, significantly enhancing cellulose degradation and ultimately supporting improved rumen metabolism.