The ruminant feedstock provides the growth substrate for all the micro-organisms that rely on fermentation and in the ultimate analysis also provides all the growth substrates on which other species of micro-organism depend; whether they are autotrophs or heterotrophs.

The feed components of interest in this teaching unit are plant carbohydrate polymers. The two broad divisions of these compounds are the soluble and insoluble polymers.

Soluble polymers include the starches which are nutrient stores in plants. The insoluble polymers include cellulose and the pectins and hemicelluloses which are structural components of plant material. The insoluble, structural carbohydrate polymers are called plant fibre. Plant fibre  in combination with the largely indigestible heterogeneous structural material called lignin, and the glycoprotein, extensin, make a rigid and strong structure that is insoluble in aqueous media.

Ruminants grazing mixed populations of plant pasture species consume a diet containing a high proportion of plant fibre.

The nutritional use of plant fibre.
 
 
Cats and other carnivores are almost completely unable to take nutritional advantage of plant fibre. Non-ruminants, and in particular carnivores, may eat diets with a relatively low plant fibre content but whose starch or other a-linked glucose polymer content (such as glycogen) may be high. Glucose polymers containing a-glycosidic links can be hydrolysed by digestive enzymes such as amylase synthesised in the mammalian pancreas and carnivores may obtain a high proportion of daily glucose needs from this source. 

Plant fibre in contrast can not be degraded by hydrolases elaborated by animal cells and can not be used as a nutrient but plant fibre does serve a useful function in the carnivore diet; it is a bulking agent important for stool formation.

This is Natalie's cat Togie.

Herbivores however can use plant fibre as a nutrient but different species vary in their ability to do so.  Ruminants are remarkable for their ability to utilise plant fibre very well. Even diets high in really intractable fibrous feed materials are able to be used relatively well in ruminants. If you examine faeces of different herbivore species you can glean some indication of the extent to which plant fibre is utilised as a nutrient source. Ruminants typically produce faeces which do not contain obvious structural plant material. Contrast this with horse faceces which even on good quality hay show obvious evidence of the fibre content of the diet. The reason is that ruminants have a much more agressive digestive system, specifically designed to enable them to thrive on coarse feed materials.

As we have just seen the soluble a-linked glucose polymers like starch can be digested by pancreatic amylase to release glucose for absorption in the small intestine. The main chemical difference between soluble and insoluble plant carbohydrate polymers is that the insoluble carbohydrate contains sugars joined together mostly with b-glycosidic ether links. These linkages can not be hydrolysed by enzymes synthesised in mammalian tissues. They can however be hydrolysed by many bacterial and fungal hydrolases and ruminants harbour multiple species of these microorganisms, in symbiotic association, in their greatly expanded anterior gut; the structure known as the reticulo-rumen.
 
This Brahman steer is an example of the nutritional importance of symbiotic micro-organisms in primary production.

These symbiotic micro-organisms synthesise hydrolase enzymes which they secrete into the growth medium where they degrade both a and b -linked dietary carbohydrate polymers.

These hydrolases catalyse the release of monosaccharides from starches and from cellulose, pectins and hemicellulose which are then available for use as the growth substrates for many of the rumen micro-organisms. These substrates are mostly fermented to obtain ATP for growth and the volatile fatty acids propionate, acetate and butyrate, plus lactate are generated as end-products.

These organic acids are waste products of microbial metabolism but are absorbed by the host ruminant and used as substrates in the host tissues for intermediary metabolism.

This use of alpha linked polymer by the micro-organisms precludes glucose release by amylase digestion in the upper small intestine and this means that gluconeogenesis occurs constantly in the liver of adult ruminants to compensate for the lack of glucose taken up from the upper small intestine.
 

Details of plant fibre chemistry

Cellulose

Cellulose is the most abundant organic compound in the biosphere. It is an unbranched polymer of a-D glucose joined by 1-4 b-glycosidic ether links. Digestive enzymes elaborated by mammalian tissues can not degrade cellulose and it can only be of nutritional benefit to animals that have gut microorganisms that make cellulases that release the glucose. The glucose released in this way however is the growth substrate for the microorganisms which obtain their ATP by fermenting it. The host ruminant absorbs the end products of fermentation for use as nutrients.

The diagram below shows three glucose monomers linked by 1-4 b-glycosidic ether linkages in the structure of cellulose.  Cellulose fibrils are made up of many thousands of glucose molecules linked end to end in this way.

The image below shows how the cellulose fibrils are aligned to make up sheets which are arranged in overlapping structures to make up plant cell walls.

The image below gives a closer view…

Other components of plant fibre

Hemicelluloses are short, linear polymers of hexose and pentose sugars on a 1-4 b-polyxylose core. Examples are the xylans and glucomannans. Like cellulose these polymers can only be of nutritional benefit to the host after microbial fermentation of the component monosaccharides and absorption of the end products.

Xylan

Glucomannan

Pectins are 1-4 b-galacturonic acid polymers with short branches of mixed hexose and pentose sugars. They are most abundant in soft plant tissues and their sugar content is released by depolymerising pecitinases, both hydrolases and lyases, produced by a number of species of  organisms.

Lignin is a complex, highly cross-linked heterogeneous polymer of coumaryl alcohol and its methoxy derivates sinapyl and coniferyl alcohols.  Plants synthesise these compounds from phenylalanine. Lignin exists in close association with the insoluble plant fibre materials discussed above and adds to their support functions.  Lignin is highly resistant to chemical and enzymic hydrolysis and for this reason it is not able to be digested and is of no nutritional value.  More important however is the fact that because it is closely interwoven with the carbohydrate polymer plant fibre structures it significantly reduces fibre digestibility when it is present in high abundance.  This is one of the reasons for the low nutritional value of straw.

Extensin is a glycoprotein rich in hydroxyproline residues that are attachment sites for carbohydrate side chains. It is thought to form a network complementary to cellulose with cellulose microfibrils penetrating the extensin matrix.