Corn stover is one of the most common crop residues, with an annual global yield of approximately 1.15 billion tons (FAOSTAT, 2021). Although it is commonly discarded as waste, using it efficiently would be extremely beneficial for biofuel/livestock production. This would also ameliorate the land and air pollution risks currently associated with its disposal. However, the high lignocellulose content and high degree of lignification in mature corn stover hinder its efficient utilization (Wang et al., 2020). Therefore, disrupting the recalcitrant structure of corn stover and developing an economical storage method are essential steps to ensure its year-round supply and efficient use.

Steam explosion technology has emerged as a promising method for pretreating food processing byproducts because of its high efficiency, environmental friendliness, and low cost (Nie et al., 2021). Although it was originally applied in fields such as pulping and wood processing (Martin-Sampedro et al., 2014), animal feed processing (Xie et al., 2024), and bioenergy production (Hoang et al., 2023), recent studies have also suggested that steam explosion could reduce the antigenicity, β-conglycinin, and phytic acid content in soybean meal (Kong et al., 2022), and change the ruminal fermentation of corn stover based on in vitro findings (Wang et al., 2020). Steam explosion can degrade hemicellulose into soluble monosaccharides or oligosaccharides (Zhao et al., 2018). Under the process of Steam explosion, the content of neutral detergent fiber and hemicellulose can be reduced from 73.2 and 34.7 to 49.7 and 6.7, respectively (Shi et al., 2019). Meanwhile, the morphological structure is broken, the accessibility of cellulose is increased (2.44, 2.83, 4.08-4.33 mg/g) and the content of acid detergent lignin is decreased (17.52%) (Nie et al., 2021). This process effectively disrupts the cell walls of plant-based materials, enhancing the extraction and biological activity of their active components, which makes it a promising method for corn stover pretreatment.

However, existing research has primarily focused on the structural changes of straw induced by steam explosion technology, but the specific feed effects of steam-exploded straw remain unclear. There is also a need to further explore this technology’s acceptance by animals, its palatability, and its impact on the rumen, which is the primary site of digestion in ruminants. Specifically, although steam explosion changes the structure of lignocellulosic materials, their subsequent digestibility and utilization by animals remain unclear. This study was established to investigate the effects of consuming corn straw pretreated with steam explosion and fermentation on the growth performance of fattening sheep, such as their growth performance and serum parameters, along with rumen metagenomic sequencing results. We hope that the assessment of these factors will provide valuable insights into sustainable livestock feeding practices and promote the utilization of agricultural byproducts in animal nutrition.

This study was conducted to determine the impact of steam explosion and fermentation pretreatment of corn stover on the rumen microbial community and the growth performance of sheep that subsequently consumed it. A previous study demonstrated that the treatment of plant material with a combination of steam explosion and microbial degradation can overcome the drawbacks of conventional microbial pretreatment technology, and can facilitate the subsequent high-value conversion of lignocellulose (Zhang W. et al., 2023). However, the results of our study indicated that compared with the CON group, the sheep in the SEM group had lower growth performance (Table 2). Thus, our results do not support our hypothesis that the combination of steam explosion processing and fermentation treatment would promote the utilization of corn stover as sheep feed.

Many studies have demonstrated that steam explosion processing of corn stover leads to significant structural changes, such as the shortening of cellulose polymers, increased porosity of lignocellulose, and enhanced saccharification efficiency through enzymatic hydrolysis (Yang and Wang, 2019). However, in the present study (Tables 2, 3), pretreating corn stover with steam explosion and microbial degradation unexpectedly reduced the DDMI and growth performance of sheep, while also decreasing the TP and ALB concentrations in their serum. A potential explanation for these findings is that steam explosion treatment may alter the structural properties of bioactive components in plant-based materials, which may, in turn, affect their functions. Specifically, steam explosion can reduce particle size (Wang et al., 2022), increase surface area, and create larger cavities and gaps between plant cells (Cheng et al., 2020) These structural changes may influence the bioavailability and digestibility of proteins, potentially reducing their nutritional value. Moreover, excessive steam explosion reduced the biological activity of the active components in plant materials. For instance, Wenyu Cui et al. (2023) found that excessive steam explosion reduced the oil-holding capacity of grape pomace. In our study, sheep fed corn stover pretreated with steam explosion and fermentation had lower serum levels of TP and ALB, which are generally considered to be indicators of protein synthesis and metabolism in ruminants (Liu et al., 2018). These lower levels suggest that protein synthesis and absorption may have been impaired in this group (Kim et al., 2017). Thus, it is suggested that steam explosion processing would lower the nutritional value of protein in lignocellulosic biomass, consistent with the findings of a previous study on typical crop byproducts (He et al., 2022). This reduction in protein quality could be attributed to protein degradation and the Maillard reaction, both of which are induced by excessive steam explosion, leading to a decrease in crude protein content and a loss of nitrogenous nutrients (He et al., 2022). Thus, steam explosion can lower the utilization of nitrogen content by animals, which would ultimately reduce their growth performance. Before the application of steam explosion technology, more animal studies should be conducted to evaluate the effectiveness of steam-exploded corn stover on animals.

Rumen microbiota has been demonstrated to be critical to host health by providing metabolic products, maintaining metabolic function, ensuring proper development of the immune system, and defending against pathogens (Lozupone et al., 2012; Yin et al., 2023). Microbial diversity is vital for the maintenance of microbial functions in the host (Li et al., 2018); high microbial diversity stabilizes the community structure and promotes its resistance to fluctuations in the environment (Li H. et al., 2019). This explains the significant associations of rumen microorganisms with host performance in the current study. This is supported by previous studies showing a difference in microbial diversity between steers with efficient or inefficient use of feed (higher alpha diversity was observed in the efficient group; (Li F. et al., 2019), and showing that a more diverse rumen microbial community can promote the use of high-fiber herbage in yaks (Fan et al., 2020). Research has also revealed that the diversity of human intestinal microbiota improves the efficiency of dietary fiber fermentation (Tap et al., 2015), and reflects better health and strong metabolic capacity (Clarke et al., 2014). Meanwhile, in our study, the rumen microbial diversity in the SEC group was lower than that in the CON group. This may be explained by various toxins being generated during steam explosion processing, which may impede microbial growth or enzymatic hydrolysis (Nie et al., 2021). Thus, we speculate that a failure to accurately control the strength and uniformity of steam explosion treatment may produce compounds that adversely affect microbiota composition, abundance, and reproduction.

Diet is considered to be the most important factor determining microbial composition (O’Hara et al., 2020). The microbiome plays a critical role in providing nutrition to host ruminants, thereby influencing their performance (Huang et al., 2021). Previous research has shown that the genus Eubacterium, as symbiotic bacteria, may play a vital role in feed digestion (Mao et al., 2012) and residual feed intake (Hernandez-Sanabria et al., 2012) in cattle, with it being able to regulate the micro-ecological balance by producing short-chain fatty acids (Dadi et al., 2020). Meanwhile, both Eubacterium and Methanosphaera have been negatively associated with methane production by depriving methanogens of hydrogen for methanogenesis, resulting in less CH₄ production per unit of carbon (Cunha et al., 2019). In the present study (Figure 2A), the genera Eubacterium and Methanosphaera were distinct signature taxa in the rumen of the CON group, suggesting that they contribute to CH₄ reduction, thereby reducing the loss of energy in the activity of the microbial community, and in turn, improving the utilization of feed. Meanwhile, the genus Clostridium, which is associated with cellulose degradation (Huws et al., 2018) and nitrogen fixation (Noguchi et al., 2006), was one of the distinct signature taxa in the rumen of the CON group. In contrast, several genera belonging to the phylum Ciliophora (Ichthyphthirius, Stentor, Stylonychia, Tetrahymena, Pseudocohnilembus, and Paramecium) and the genus Neocallimastix, which showed positive correlations with methane were significantly more abundant in the SEC group (Saborío-Montero et al., 2020). Moreover, a previous study demonstrated that, in cows with inefficient feed utilization, rumen metabolism shifted toward waste products such as methane, as evidenced by the specific enrichment of methane-related species and the increase in methane production (Shabat et al., 2016). Ruminants emit enteric CH₄ as a byproduct of feed degradation and fermentation processes, which is mainly exhaled/eructated and not retained by the host. The loss of energy associated with the production of methane, which is estimated to vary between 2 and 12% of gross energy consumption, is metabolically undesirable as this energy could otherwise be used in beneficial ways (Saborío-Montero et al., 2020). An in vitro study also demonstrated that, because the improvement of fermentation efficiency produced more fermentation products, more methane was produced to remove metabolic hydrogen and keep the H₂ partial pressure low (Wang et al., 2020). Thus, we speculated that the application of steam explosion and fermentation pretreatment may make corn stover more easily degradable, but it may also change the ruminal microbial community, lower the population and reproduction of several microbial species, and increase methane-related species, thus reducing the efficiency of feed utilization. Our results suggest that the effects of this pretreatment combination on the nutritional composition, functional components, and physicochemical properties of plant-based raw materials for ruminant consumption need to be further investigated before it can be widely applied. To address the negative effects observed in the SEC group, further research should explore potential strategies to mitigate these effects. For instance, adjusting the fermentation process by increasing the pressure, as demonstrated by Kong et al. (2022) and Zhang W. et al. (2023), may be a beneficial strategy to improve the utilization efficiency of the feed (Kong et al., 2022). Additionally, optimizing the proportion of steam-exploded corn stover in the diet, combined with the use of alternative microbiota, such as cellulase and lactic acid bacteria, has been shown to enhance the degradation of lignocellulosic material and improve fermentation efficiency (Nie et al., 2021; Zhang H. et al., 2023) could be considered potential solutions to enhance the utilization efficiency of the feed. These strategies may help improve microbial activity and fermentation efficiency, ultimately leading to better feed conversion and growth performance in ruminants. Future studies will focus on evaluating these approaches to determine their effectiveness in improving the nutritional value of steam-exploded corn stover and mitigating the observed negative effects.

In addition, the results of the KEGG pathway analysis indicated that the different types of corn stover significantly impacted the gene functional profiles in the sheep rumen microbiome (Figure 3). Notably, in the rumen microbiome of sheep fed the untreated corn stover, ABC transporters were the most expressed pathway in membrane transport, which directly participate in ATP production. These transporters also act as a barrier to protect ruminants from the invasion of toxic substances into the stomach (Hamana et al., 2012). The lower expression of ABC transporters in the SEC group indicates that less ATP may be produced in this group. The decrease in the relative abundance of the major microbial phyla and the lower Chao1 index (indicating lower microbial diversity) in the SEC group could explain the decrease in microbial activities in the rumen. Although we identified several specific taxa and KEGG pathways associated with the consumption of corn stover after its processing by steam explosion and fermentation in combination, the mechanisms behind these changes and their impact on rumen activity remain unclear (Figure 4). Although our findings provide important insights, the application of steam explosion and fermentation pretreatment to corn stover requires the need for further study such as, for example, a focus on animal utilization, before this approach can be widely applied.

Our results demonstrated that feeding sheep corn stover pretreated by steam explosion and fermentation can disrupt the balance of their rumen microbiota. The diversity and relative abundance of taxa associated with lower methane production were significantly reduced in the SEC group. We hypothesized that, compared to the CON group, sheep in the SEC group may have experienced greater energy loss due to increased methane production and less active microbial populations, leading to inefficient feed utilization. Methane production represents a loss of energy that could otherwise be used for growth and metabolic processes. Furthermore, the reduced microbial diversity observed in the SEC group likely contributed to decreased fermentation efficiency, further reducing energy availability. These factors could explain the reduced feed intake and lower microbial diversity, ultimately resulting in poorer growth performance in the SEC group. However, these hypotheses are speculative and require further validation. Future studies should focus on evaluating the digestibility and bioavailability of both untreated and steam-exploded corn stover to gain a deeper understanding of its nutritional impact. Additionally, exploring rumen fermentation parameters will offer valuable insight into the broader implications of steam explosion processing, especially in terms of energy loss and nitrogen utilization in ruminants. In addition, further optimization of the production process, with particular attention paid to the proportion of corn stover in the diet, is needed before the widespread application of steam explosion to corn stover.