Several ruminant animals rely almost exclusively on the complex polysaccharide matrix from the plant cell wall (CW) as their primary energy source via volatile fatty acids produced through ruminal and some hindgut fermentation processes. The CW contains different types and proportions of polysaccharides, proteins, phenolic compounds, and minerals in their macromolecular structure that influence the rate and extent of fiber digestion and selective retention of particulate matter due to its physical characteristics (buoyancy and comminuting) in the reticulorumen. The biosynthetic formation of the CW dictates possible manipulation mechanisms (targeted plant and microbes selection) and processing methods (physical, chemical, microbial, and enzymatic treatments and the use of genetically engineered bacteria) to increase its digestibility, leading to better utilization of the CW by the ruminant animal and hopefully lower the contribution of ruminants' greenhouse gas emissions. Early studies on lignin biosynthesis have led to more advanced studies focusing on replacing traditional monolignols with homopolymers that are easier to deconstruct or degrade. Concurrently, laboratory methods must be developed, evaluated, and modified to accurately reflect the digestibility and nutritive value of CW brought about by modern manipulation mechanisms or processing methods. However, the laboratory methods must also be reliable, precise, feasible, trivial, easy to implement, and cost-effective, but at the same time environmentally friendly and aware. For instance, although the acid detergent lignin has been demonstrated to behave uniformly as a nutritional entity, its chemical determination and association with carbohydrates still lack consensus. Spectroscopy (near-infrared and Raman) and in vitro gas production techniques have been adopted to assess plant chemical composition and nutritive value, but an incomplete understanding of the impacts caused by disrupting the CW for sample processing still exists. Different variations of multicompartmental and time- and age-dependent mathematical models have been proposed to determine the ruminal rates of degradation and passage of fiber. However, low-quality and incomplete data due to inconsistent marker results used to determine passage rates and transit time of fiber in the gastrointestinal tract have hindered advancements and adoptions of the next generation of computer models to understand ruminal fiber degradation.The underlying principles of forage cell wall utilization by ruminants have been known for over 50 years, but a significant amount of knowledge of the structure and synthesis of critical components of the plant cell wall, mechanisms and methods to alter its digestibility, and assessment techniques to quantify its components as well as their fermentability has been accumulated in the last 30 years. Such knowledge has even allowed us to make recommendations about the importance of fiber in the diet to improve animal performance and welfare. For instance, some industries (especially the paper mill and biofuels) have attained significant advancements toward modifying plant lignin (a critical component of the cell wall that reduces fermentability) and lignin-degrading microorganisms that could assist the animal nutrition community in increasing the digestibility of forage cell wall without further pretreatment. There are many techniques and technologies available to increase cell wall digestibility and, consequently, animal productivity. However, each has potential and limitations, and when used alone, it may not yield the best outcome. From a ruminant nutrition perspective, combining such techniques and technologies with the next generation of mathematical models seems more likely to yield significant improvements in forage cell wall digestibility. 

https://academic.oup.com/jas/advance-article-pdf/doi/10.1093/jas/skad009/48546136/skad009.pdf

Revisiting Mechanisms, Methods, and Models for Altering Forage Cell Wall Utilization for Ruminants

Abstract To decrease the time and cost of experiments as well as the use of animals in nutrition research, in vitro methodologies have become more commonplace in the field of ruminant nutrition. Therefore, the objectives of this review are 1) to describe the development of different in vitro methodologies, 2) to discuss the application, utilization, and advantages of in vitro methodologies, 3) to discuss shortcomings of in vitro methodologies, and 4) to describe the potential developments that may be able to improve in vitro methods. Having been used for decades, some in vitro methodologies such as pure, batch, and continuous cultures have been very well documented and utilized to investigate a wide array of different aspects of nutrition, including the effects of different dietary compositions, individual fermentation end products, and impacts on the microbiome of the rumen. However, both batch and pure cultures can result in a build-up of end products that may inhibit fermentation, as they culture ruminal contents or defined strains of bacteria, respectfully. Continuous culture; however, allows for the removal of end products but, similar to pure and batch cultures, is applicable only to ruminal fermentation and cannot provide information regarding intestinal digestion and bioavailability. This information for in vitro can only be provided using an assay designed for total tract digestibility, which is the three-step procedure (TSP). The TSP may be improved by coupling it with cell culture to investigate the absorption of nutrients in both the ruminal and intestinal phases of the methodology; however, the TSP needs further development to investigate all nutrients and the methodologies available for cell culture are still relatively new to ruminant nutrition. Therefore, while in vitro methodologies provide useful data in the field of ruminant nutrition without the continuous use of animals, there is still much work to be done to improve the methodologies to further apply them.

/pdf/unraveling-the-pros-and-cons-of-various-in-vitro-1gpvu24g.pdf

Unraveling the pros and cons of various in vitro methodologies for ruminant nutrition: a review

Gnotobiotic experimentation helps define symbiogenesis in vertebrate evolution

The human gastrointestinal (GI) tract holds a complex and dynamic population of microbial communities, which exerts a marked influence on the host physiology during homeostasis and disease conditions. Diet is considered one of the main factors in structuring the gut microbiota across a lifespan. Intestinal microbial communities play a vital role in sustaining immune and metabolic homeostasis as well as protecting against pathogens. The negatively altered gut bacterial composition has related to many inflammatory diseases and infections. β-glucans are a heterogeneous assemblage of glucose polymers with a typical structure comprising a leading chain of β-(1,4) and/or β-(1,3)-glucopyranosyl units with various branches and lengths as a side chain. β-glucans bind to specific receptors on immune cells and initiate immune responses. However, β-glucans from different sources differ in their structures, conformation, physical properties, and binding affinity to receptors. How these properties modulate biological functions in terms of molecular mechanisms is not known in many examples. This review provides a critical understanding of the structures of β-glucans and their functions for modulating the gut microbiota and immune system.

/pdf/b-glucans-a-potential-source-for-maintaining-gut-microbiota-179mf2f7.pdf

β-glucans: a potential source for maintaining gut microbiota and the immune system

This study assessed the impact of dry (DR) vs. temper rolled (TR) and low (13%, LP) vs. high protein (18%, HP) wheat on ruminal fermentation, growth, and liver abscesses in feedlot cattle. Crossbred Angus steers (302 ± 34 kg; n=160; 24 ruminally cannulated) were used in a backgrounding (BG) to finishing (FN) trial. The BG diet consisted of 60% barley silage, 35% wheat, and 5% supplement, and the FN diet contained 10% barley silage, 85% wheat, and 5% supplement (DM basis). Four transition (TN) diets were used to adapt cattle to the FN diet. A numerical increase in large particles and reduction in small particles occurred when both HP and LP wheat were TR, with this response being greater for HP wheat. Steers experienced lower (P≤0.03) ruminal pH with HP-DR and LP-TR than HP-TR wheat during TN. Steers fed HP wheat BG diets tended to exhibit greater (P ≤ 0.09) G:F and NEg than steers fed LP wheat. Greater (P=0.01) ADG were exhibited by FN steers fed LP wheat. Liver abscesses were more (P<0.001) severe with HP wheat. While HP wheat improved the growth of BG cattle, it increased the severity of liver abscesses during FN.

Effect of dry or temper rolling of high or low protein wheat and its impact on rumen parameters, growth performance, and liver abscesses in feedlot cattle.

There is a knowledge gap regarding the factors that impede the ruminal digestion of plant cell walls or if rumen microbiota possess the functional activities to overcome these constraints. Innovative experimental methods were adopted to provide a high-resolution understanding of plant cell wall chemistries, identify higher-order structures that resist microbial digestion, and determine how they interact with the functional activities of the rumen microbiota. We characterized the total tract indigestible residue (TTIR) from cattle fed a low-quality straw diet using two comparative glycomic approaches: ELISA-based glycome profiling and total cell wall glycosidic linkage analysis. We successfully detected numerous and diverse cell wall glycan epitopes in barley straw (BS) and TTIR and determined their relative abundance pre- and post-total tract digestion. Of these, xyloglucans and heteroxylans were of higher abundance in TTIR. To determine if the rumen microbiota can further saccharify the residual plant polysaccharides within TTIR, rumen microbiota from cattle fed a diet containing BS were incubated with BS and TTIR ex vivo in batch cultures. Transcripts coding for carbohydrate-active enzymes (CAZymes) were identified and characterized for their contribution to cell wall digestion based on glycomic analyses, comparative gene expression profiles, and associated CAZyme families. High-resolution phylogenetic fingerprinting of these sequences encoded CAZymes with activities predicted to cleave the primary linkages within heteroxylan and arabinan. This experimental platform provides unprecedented precision in the understanding of forage structure and digestibility, which can be extended to other feed-host systems and inform next-generation solutions to improve the performance of ruminants fed low-quality forages. 

Mechanistic insights into the digestion of complex dietary fibre by the rumen microbiota using combinatorial high-resolution glycomics and transcriptomic analyses

Zhang Zheng

Papers

Mechanistic insights into the digestion of complex dietary fibre by the rumen microbiota using combinatorial high-resolution glycomics and transcriptomic analyses

Characterization of various wheat types and processing methods using in vitro ruminal batch cultures