Ask The Equine Nutritionist

QUESTION

ANSWER 

*What is lignin?

Okay lecture alert, I'll try to keep this simple and short, so it may be over simplified!!

As grasses (and other plants grow) the plant uses the energy of sunlight to produce simple sugars from water and CO2 in the air, the sugars are then bound together into chains called starches.
The starches accumulate and the plants binds them into longer chains called hemi-celluloses.

In order to build the cell walls on the new cells as the plant grows it uses the hemi-celluloses and binds them with a special linkage called a Beta-glycans into branching chains called cellulose.  As the plant matures the walls need to strengthened and stiffened and so the plant starts to bind into the cellulose, nitrogen (also from the air or the roots) the nitrogen is bound irreversibly to the cellulose to form the plant component lignin.

In order of digestibility it goes: sugar-/ starch-/ hemi-cellulose-/ cellulose-/ lignin.

Thus as the plant ages you get a gradual accumulation of progressively less and less digestible material.
Starches and sugars are what we call simple carbohydrates, they can be digested by all mammals, and are broken down to the component sugars which cross the GI tract wall and raise the blood sugar and cause the release of insulin. The term -ose means sugar. EG glucose, mannose, lactose etc etc.

The more complex hemi-celluloses and cellulose cannot be digested by mammalian enzymes. (its those Beta-glycan links that we can't break), but certain bacteria can break them. SO herbivores all posses huge fermentation tanks (rumen or caecum and colons) where the cell wall material can be mixed with bacteria and the bacteria break it down, this is known as bacterial fermentation. In the process of breaking down the cell walls the bacteria excrete waste products we call fatty acids.

These have names like acetate, butyrate and proprionate. The abbreviation for them is FFA's which stands for Free Fatty Acids. The mammal (horse in this case) absorbs the FFA's from the tank, and passes them to the liver where they enter into fat metabolism. (They are very simple fats). Except the proprionate, but we won't go there, that’s another lecture for another day!

Humans do not posses a big enough fermentation tank to extract all the energy in the cell walls of plants, we can do some but not much, so to us cellulose is a non-digestible fiber and passes thru, that’s why we are omnivores not herbivores, (sorry all vegetarians). To the horse and other herbivores the cellulose is a valuable source of energy, they can get their GI bacteria to break it down, and the resulting gasses pass into their fat metabolism and not much into carbohydrate metabolism.

The only animal that can breakdown the nitrogen-carbon bonds of the lignin are termites. SO to all mammalian. even the herbivores, lignin is a non-digestible fiber.

How digestible any given plant material is depends on the percentage of lignin it contains. Lignin is produced as the plant goes through its growth cycle, so the amount present depends on the age or stage of development of the plant at the time it was either eaten or harvested. The older the plant the higher the cellulose and lignin content.

Thus spring grasses are high in sugars, (early stage of growth) and dangerous to horses who are unable to handle much sugar and who then founder. AS they mature the grasses lay down more and more cellulose and subsequently lignin, thus are less dangerous. When the have bloomed and seeded the grasses die back leaving only the lignified skeleton, which almost totally indigestible.
Cereal plants (wheat, barley rye etc) are kinds of grasses, and follow the same pattern. By the time they are harvested for the seeds (grains) their stalks are almost entirely lignin. Even though when they are cut for hay they are cut much earlier in their growth cycle they still tend to have a lot of lignin in their strong stalks. Hence the tendency for the cereal hays (oats hay, barley hay) to be too indigestible for a baby. An adult with its much bigger GI tract can handle them but most young babies <2 yrs can't.


Sources of cellulose, that are relatively low in starch and sugar:

Grass hays, cut and baled at the milk-ripe stage, eg BEFORE they go to seed.
Legume hays (esp if cut and baled young),
Sugar beet pulp.

Fermented grain products (distillers grains, etc) The fermentation process removes most of the soluble (simple) sugars and starches leaving behind the cell walls and the bacterial protein.

Apple pulp
Citrus pulp
Paper (I know its wood pulp but it is mostly cellulose)

Hope this sheds some light!!




Dr. Melyni Worth Ph.D. - 2004 - 2007(c)

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