Carbohydrates are the main energy source for ruminants. Carbohydrates (CHO) can be classified into simple carbohydrates with 2 or fewer sugar units or complex carbohydrates with 3 or more sugars. Monosaccharides have a single sugar unit, can occur in nature, and can be produced from digestion. These include glucose, fructose, and galactose and are common in human food as well.
Ruminants rely on 5 major CHOs which are cellulose, hemicellulose, pectin, fructans, and starch. Pectin, cellulose, and hemicellulose are fibers, therefore can only be digested and utilized when microbes are present. There are two types of starch in ruminant diets which are amylose and amylopectin. Cereal grains have a high content of starch, ranging from 45% for oats to 72% for corn.
Beef cattle are able to utilize these fiber carbohydrates because rumen microbes are able to break down the beta linkages. Breaking these linkages causes the complex CHO molecule to be broken down into smaller molecules which can be then be absorbed into the microbes or can be directly absorbed through the rumen wall or intestinal wall. High fiber feedstuff such as silage, hay, and straw are high in cellulose and hemicellulose requiring the animal to chew more (to increase salivation) and increase rumination (chewing cud) to promote digestion. Increased chewing and rumination increase enzyme production which helps to promote particle breakdown. Smaller feed particles are more readily digested and microbes can bond to these particles easier, increasing digestion and utilization of these feed particles.
Microbes bind to feed particles and break bonds to produce volatile fatty acids and amino acids for protein synthesis in the rumen. Volatile fatty acids (VFAs) are the main byproduct of microbial CHO digestion. These VFAs can be absorbed by the microbes and used as an energy source for microbial protein synthesis. VFAs that are not absorbed by microbes can be absorbed through the rumen, omasum, abomasum, and intestinal wall. These VFAs move through the bloodstream directly to tissues or to the liver where they get converted back to glucose for energy.
Hopefully, this helps to explain the importance of the carbohydrate-protein ratio for microbial growth and protein synthesis in the rumen. Check out the protein metabolism blog to learn more on how proteins are digested in ruminant animals. Please contact me if you would like to know more details about VFAs or any other questions you may have.
Resources
My University Notes
https://www.slideshare.net/PakRose1/carbohydrate-metabolism-animal-nutrition
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