The path from dietary crude protein to the supply of protein available to the cow is far from direct. As much as 80% of the protein and NPN consumed by the animal will be broken down to ammonia and other compounds in the rumen, where the resident microbes have the opportunity to assimilate these nutrients into microbial cell protein (MCP). Unused ammonia may move to the liver, be converted to urea, and then recycle through the blood or saliva back to the rumen, where it rejoins the ammonia pool. Some of the bacteria and protozoa simply recycle within the rumen, as their MCP becomes "dinner" for other microbes. But the majority of these cells, either after they die or when the feed particle they are attached to is sufficiently degraded to flow out of the rumen, move on through the cow's digestive system. Once they reach the small intestine, their MCP serves as a fundamental "natural," or amino acid, protein source. In fact, microbial protein will account for 40-80% of a ruminant animal's metabolizable protein supply.This range is due to a number of factors, primarily diet-related. If the feeds a cow consumes do a good job of meeting the needs of the rumen bugs -- primarily supplying degradable crude protein, and an energy source that can be utilized by fiber-digesting bacteria -- microbial numbers and activity will be enhanced. That leads to:
Ø More fermentation activity, with
o More Volatile Fatty Acids (VFA) produced, resulting in
o Increased energy supply to the animal;
Ø More feed processed, via increased feed intake and digestion, resulting in
o Increased supply of dietary protein and energy substrates; and,
Ø More microbial cells flowing through the digestive system, resulting in
o More high-quality MCP available for absorption at the small intestine.
The Rumen as a "Protein Factory" We can think of the rumen as a manufacturing facility for natural protein, where we can manage the supply of raw materials and the working conditions to optimize protein output. For beef cows, the primary input is going to be the forage that makes up the majority of the diet. Not only does this serve as the main substrate for fermentation activity, but intake of roughages also stimulates saliva production. Saliva contributes moisture, urea, and buffering capacity to the rumen environment, which can all have a positive impact on microbial activity.
The nutrient content of the diet obviously has a major impact on the yield of nutrients from ruminal fermentation. In general, a crude protein level of 11-12% is optimal for MCP production. But it is critical that enough of the protein consumed be degradable in the rumen. And since fiber-digesting bacteria have a primary need for ammonia, it can be beneficial to supply a portion of the supplemental crude protein as an efficient NH3 source such as urea. The following data (Griswold, et al., 2003) compares diets that supplied equal amounts of crude protein, but in different forms.
Like any living organism, the rumen microbes require a balanced diet, which means an energy (carbon chain) source as well as protein / nitrogen. Different bacterial species utilize different carbohydrates; breakdown of forages can only be enhanced by supplying supplemental energy in a form that the fiber-digesting bacteria can utilize. This is best done with soluble fiber (corn gluten, wheat midds, soy hulls) or sugar (molasses). Adequate sulfur is also necessary for maximum bacterial growth and activity.
Diet formulation and delivery should also function to enhance the rumen "work" environment for those microbes that most efficiently degrade high-roughage rations. Considerations include:
An Even Nutrient Supply. In a review by Huntington and Archibeque (1999), they stated that "ruminal ammonia concentrations fluctuate by a factor of 5 to 8 in animals consuming meals, with less to essentially no variation as frequency of daily meals increases." Dehority and Tirabasso (2001) showed that in sheep, feeding frequency significantly impacted the number of bacteria present in the rumen:
Having pasture or hay available at all times, with a free-choice supplement, offers the best opportunity for even intake of nutrients, and thus the presence of the largest possible "work force."
Minimal Starch. On a forage-based diet, it is important to avoid competition between starch- and fiberdigesting bacteria, as well as the pH reductions that come with breakdown of significant levels of starch. Work with steers eating a low-quality prairie hay (Hannah et al., 1991) clearly illustrates this.
Limited Fat. If dietary fat levels exceed 5%, fiber digestion is inhibited.
Type and delivery method of supplementation obviously have a major impact on the net flow of nutrients. In forage-based diets, supplemental soluble nitrogen and readily-available sugar energy, offered continuously in a self-feeder, can effectively maximize ruminal production and outflow of microbial cell protein -- and the amount of true protein available to meet the cow's needs
Source: Dr. Cathy Bandyk, Quality Liquid Feeds