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. 2023 Jan 3:101:skad009.
doi: 10.1093/jas/skad009.

Forages and Pastures Symposium: revisiting mechanisms, methods, and models for altering forage cell wall utilization for ruminants

Luis O Tedeschi  1 , Jordan M Adams  1 , Ricardo A M Vieira  2
Affiliations

Forages and Pastures Symposium: revisiting mechanisms, methods, and models for altering forage cell wall utilization for ruminants

Luis O Tedeschi et al. J Anim Sci. .

Abstract

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.

Keywords: assessment; composition; fiber; model; nutritive value; prediction.

Plain language summary

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.

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Conflict of interest statement

The authors declare no perceived conflict of interest.

Figures

Figure 1.
Figure 1.
Structural model of the middle lamella (ML) and interactions with molecular components of the primary cell wall (PW). Pectin-pectin interaction (Type 1), pectin-hemicellulose/cellulose interaction (Type 2), pectin-phenolic-hemicellulose interaction (Type 3), protein-protein-pectin interaction (Type 4), rhamnogalacturonan-I (RG-I), and rhamnogalacturonan-II (RG-II). Reproduced with permission from Zamil and Geitmann (2017).
Figure 2.
Figure 2.
Examples of carbohydrates found in plant cell walls are represented by ball-and-stick Pyran and Furan structures. White, red, and grey balls are hydrogen (H), oxygen (O), and carbon (C) atoms (1–6), respectively. The four-asymmetric-carbon glucose (C6H12O6) has 16 isomers. Some pentoses (C5H10O5) and hexoses or hexose derivatives (e.g., C6H12O5) occur in the cell walls of forage plants of both the Fabaceae and Poaceae families. Pyrans (α−L− Rhamnose, α−D− Xylose, and β−D− Mannose), Furans (α−D− Fructose and α−L− Arabinose), and Epimers (α−D− Fructose, β−D− Mannose, and α−D− Galactose) are natural occurrences that confer variability in monosaccharides, disaccharides, oligomers, and derived polymers. Available from https://pubchem.ncbi.nlm.nih.gov/ (Accessed in May 2022).
Figure 3.
Figure 3.
Structural model of a primary cell wall synthesis. Reproduced with permission from Cosgrove (2005).
Figure 4.
Figure 4.
Model of xylan-cellulose interaction in secondary cell walls showing the xylan docking on less-ordered amorphous cellulose. Reproduced with permission from Gao et al. (2020).
Figure 5.
Figure 5.
Examples of phenolic compounds found in forage plants and cell walls are represented by ball-and-stick aromatic structures. White, red, and grey balls as hydrogen, oxygen, and carbon atoms (1–6), respectively. Available from https://pubchem.ncbi.nlm.nih.gov/ (Accessed on May 2022).
Figure 6.
Figure 6.
Model of possible covalent cross-links between polysaccharides and lignin in cell walls. Adapted from Iiyama et al. (1994), Krause et al. (2003), and Li (2021).
Figure 7.
Figure 7.
Example of an ester linkage to arabinoxylan. A = arabinose (blue); X = xylose (orange); P = p-coumaroyl (green) or F = feruloyl (green) substitutes in Poaceae. The acronym PAXX stands for O-[5-O-((E)-p-coumaroyl)-α-L-arabinofuranosyl]-(1→3)-O-β-D-xylopyranosyl-(1→4)-D-xylopyranose, whereas FAXX stands for O-[5-O-((E)-feruloyl)-α-L-arabinofuranosyl]-(1→3)-O-β-D-xylopyranosyl-(1→4)-D-xylopyranose. R can be either hydrogen (H) if the aromatic ring belongs to p-coumaric acid or an ether linkage to oxygen (O) and a methyl group (-CH3) if the ring belongs to ferulic acid. Adapted from Hartley et al. (1990).
Figure 8.
Figure 8.
A graphical representation of escapable and inescapable pools to determine the fractional passage rate of solids from the rumen. Based on the model proposed by Seo et al. (2009) and adapted with permission from Tedeschi and Fox (2020).
See this image and copyright information in PMC

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