Digestion Process in Cattle

Energy Digestion

In the rumen, microbial digestion of cellulose and hemicellulose (from roughages) and starch (from grains) results in the production of energy rich byproducts called volatile fatty acids (VFA’s) which are absorbed by the animal through the rumen wall. This is the major source of energy for the animal. Some starch is not digested in the rumen and is passed on to the true stomach (abomasum) and small intestine where it is broken down by the animal’s enzymes and absorbed.

Rumen microbe species are specialized in their ability to break down either starch or cellulose. When the diet is high in roughages, the cellulose (fibre) digesting microbes multiply and dominate. With a high grain diet the number of starch digesting microbes increases. Changes in the composition of a ration should be made gradually to allow time for the rumen microbe population to adapt. About 2 weeks is necessary for making major changes in ration ingredients.

Grains vary in their rate of breakdown in the rumen. This is due to the chemical nature of the starch and the physical structure of the grain. For example, dry corn is degraded in the rumen much more slowly than high moisture corn or dry wheat. This has important implications for the maintenance of rumen health when feeding high grain feedlot rations.

Protein Digestion

Crude protein includes both true protein and non-protein nitrogen (NPN). The digestion of a particular protein depends to a large extent on how easily it dissolves in rumen fluid. Highly soluble protein is more likely to be broken down by rumen microbes than is insoluble protein. Nonprotein nitrogen sources (e.g. urea, ammonia) are 100% soluble in the rumen. The rumen microbes use the nitrogen released in the rumen to form their own microbial protein. Microbes are continually being moved with digesta into the lower digestive tract, where they are digested and absorbed by the animal. Most of the protein which is not soluble in the rumen (bypass or escape protein) passes unchanged to the lower digestive tract. A portion of this protein is broken down by the animal’s enzymes and absorbed. Digestible bypass protein is efficiently utilized and is an important component in rations for fast growing beef cattle.

The activity of the rumen microbes in breaking down and reforming dietary protein has important implications for the ruminant:

1. ruminants can thrive on diets containing low quality, low cost protein (relative to monogastrics) since rumen microbes upgrade the protein quality by manufacturing limiting amino acids

2. ruminants can utilize some inexpensive non-protein nitrogen (such as urea) in their diet as a protein substitute.

For optimum performance, a balance of rumen soluble protein (and NPN) and bypass protein is required. Diets with high levels of soluble protein and/or NPN may not supply adequate amounts of protein to the small intestine. Diets with high levels of bypass protein may not supply adequate amounts of nitrogen to rumen microbes for efficient microbial growth and feed digestion. Optimum diets usually contain 30-40% available bypass protein and 60-70% rumen soluble protein. Less than 30% of total protein should be in the form of NPN.

In order for rumen microbes to utilize NPN, sufficient soluble carbohydrates (e.g. starch) must included in the diet. Without adequate available energy in the diet, the capacity of the microbes to utilize NPN would be overloaded. Excess NPN will be absorbed by the animal as ammonia, and excreted. If NPN levels are high, toxicity will occur (urea poisoning).

Ration Formulation

A properly formulated ration supplies adequate amounts of all nutrients to allow cattle to achieve a desired level of production. Accurate ration formulation requires

1. precise description of the class of cattle (sex, weight, frame size, body condition, desired rate of gain, stage of production)

2. knowledge of management practices utilized (implant usage, feed additives)

3. accurate description of the nutrient content of the available feeds

Laboratory analyses of forages is essential for accurate ration formulation. The nutrient content of forages varies greatly depending on the type, stage of maturity at cutting and how well it is preserved. For more information on lab analysis see OMAF Factsheet, “Feed Sampling and Analysis” Agdex 400/60. Nutrient content of grains is not as variable as forages, but lab analysis is recommended. Help in formulating rations is available from your OMAF county office, feed industry representatives and consultants.

A knowledge of the basic digestive system of cattle and the role of various nutrients is important to beef producers. Combined with accurate feed analysis, it allows the formulation of balanced rations which will meet production goals in an economic manner. It also enhances the management of the feeding program by providing the background information necessary to prevent or resolve problem situations.