Percentage bypass protein values of some common feeds are shown in the table given below;

Bypass Protein (% of undegraded protein ) in common feeds

Ruminant bacteria can use various sources of nitrogen, energy (derived from fermentation), and minerals for growth. Lack of any of these factors can limit bacterial growth because requirements are interrelated. It is estimated that of the bacterial species normally present in the rumen, 80% can use ammonia as the sole source of nitrogen for growth, 26% require ammonia absolutely, and 55% can use either amino acids or ammonia. Ammonia deficiency in the rumen reduces the efficiency of bacterial growth and may reduce the rate and extent of digestion of feed in the rumen., which, in turn, may reduce feed intake.

In addition to the microbial protein synthesised in the rumen, the animal has available another source of amino acids via the true protein in the feed which escaped breakdown in the rumen and arrived at the small intestine. On the avg. approx. 40% of the true protein escapes degradation in the rumen. In some cases like rapidly growing, young ruminants and ruminants at peak production, in which microbial protein plus the normal dietary protein escaping ruminal breakdown may not be adequate to provide for optimal growth and feed efficiency. Present technology is being geared to manipulate the quantity of dietary protein rumen fermentation or breakdown, thereby increasing the supply of protein (amino acids) to the small intestine. In addition to increasing the amount of bypass protein that escapes degradation in the rumen, two other requisites are important;

1. the ruminant must be able to digest the bypass protein in the small intestine, and
2. the bypass protein must provide the amino acids that the animal needs.

The following definitions are pertinent to the discussion that follows;

1. Degradability of protein refers to the rate and extent to which a feed protein may be broken down in the rumen. This is important for two reasons; (1) Breakdown in the rumen supplies the microbes with peptides, amino acids, and ammonia; and (2) the nondegraded portion is swept out the rumen to the abomasum and the small intestine for possible breakdown there and absorption as peptides and amino acids. The degradability of protein is affected by a number of factors, including;

• Solubility of the various nitrogen fractions in the feed.
• the particle size of the ingested material, and
• the level of feeding.

Also the ionophores are very effective in decreasing the breakdown of natural protein  by rumen bacteria (enhancing rumen bypass).  Urea a source of NPN is 100% degraded. Casein, a very high quality protein, is 90% degraded. 

2. Solubility of protein is an estimate of the protein that breaks down rapidly in the rumen. If a protein is extremely insoluble, it is less likely to degraded, For example zein (corn protein), is extremely insoluble, with 40-60% escaping ruminal degradation. On the other hand, casein (milk protein) is highly soluble and is almost totally degraded in the rumen.

3. Microbial protein is protein produced by the microbes in the rumen. The common recommendation in the use of NPN (source of ammonia for microbial growth) is that it can provide upto one-third of the total protein.

Normally, the amount of protein that escapes ruminal degradation is dependent largely on the length of time the protein remains in the rumen. One can speed the rate of flow through the rumen by increasing dietary intake or frequency of feeding, or by reducing the feed particle size. Also, if large quantities of salt are fed, there is an increase in water intake resulting in a faster rate of flow through the rumen. Thus, speeding feeds through the rumen ultimately increases the amount of dietary protein that reaches the lower gut.