What’s Wrong With My Herd? Part 9 – Rumen Fermentation
Animals do not really have a requirement for dietary protein, but rather a requirement for dietary amino acids. The ‘ideal protein’ would provide the cow’s digestive system with her requirement for essential amino acids (the building blocks of proteins). The protein that reaches the intestine is a combination of amino acids from the growth of micro-organisms in the rumen and protein that originates in the feed and has remained undigested. Microbial protein has the correct ratio of essential amino acids, so maximizing microbial protein is a very efficient method of meeting the cow's protein requirements. The two most limiting amino acids are methionine and lysine, as they are in short supply, and a market has developed to provide rumen-protected sources of both these amino acids. Corn grains are also a good source of bypass methionine and certain ‘treated’ soybean meals can be a source of lysine.
The microbial population in the rumen can also take advantage of non-protein nitrogen (NPN) sources like urea and convert them into microbial protein. However, in order to fully utilize NPN, there must be a readily available source of energy that encourages microbial growth and also provides the carbon that is required for amino acid production. Multiple computer modeling systems are available to help the nutritionist maximize microbial protein production.
Fiber is converted into milk by a symbiotic relationship between the microbial population and the rumen.
Energy from forage
Mammalian enzymes cannot digest fiber, but rumen microbes can, producing VFAs as by-products, and these can be used for energy by the cow. Acetic acid (acetate) and propionic acid (propionate) are the two most prevalent and important VFAs.
Acetic acid is a two-carbon sugar closely associated with milk fat production. Acetate production is lower in diets with limited fiber, or fiber that is poorly utilized. The cow eructates and re-chews her feed as part of the rumination process, introducing additional buffering capacity with her saliva.
This pH balance is essential for healthy rumen fermentation and subsequent VFA production, and it is one of the reasons that poor milk fat is associated with diets that either lack total fiber or contain effective fiber that is the wrong size to stimulate re-chewing of the feed. Rumen bacteria utilize starch, so the cow does not need to be fed enough digestible carbohydrate to meet her needs. The liver is responsible for the production of glucose from non-carbohydrate sources and one such source is the deamination and use of carbon from certain amino acids. Fortunately, propionate can also be used to produce glucose, and feed additives may be used in jurisdictions that allow them.
The ionophore monensin can shift the ratio of VFAs in the rumen, increasing propionate production to support the production of glucose, which is used for energy and lactose synthesis. Levels of lactose in the milk are more constant than levels of milk fat or protein. A shortage of glucose can manifest in reduced milk production as lactose helps drive milk volume by regulating the amount of water drawn into the secretory cells in the mammary gland.
Protein reaching the intestine is a combination of amino acids from the growth of micro-organisms in the rumen and protein in the original feed.
Feeding grain can result in over-production of VFAs and lactic acid production, which can cause subclinical or clinical acidosis, which can reduce the integrity of the rumen wall and cause liver abscesses. Some of the effects of grain feeding can be compensated for by various buffing compounds, which help to increase pH.
Live yeast, autolyzed yeast and yeast cultures can be used to improve rumen fermentation, increasing the digestibility of the fiber and organic matter in the diet. These yeast products may work in different ways, whether by improving fermentation by creating a more anaerobic environment or supplying the nutrients required to stimulate bacterial growth.
Toxin mitigation products
Rumen bacteria can break down many plant and fungal toxins, but some toxins can reduce microbial production, and subsequent animal production. Producers should avoid feeding contaminated feedstuffs to high-yielding cows, which are susceptible to more disorders, and consider mitigation programs to minimize the effects of these toxins. An effective rumen allows the cow to utilize forage and produce food for human consumption, and feeding programs that maximize rumen fermentation by creating a healthy environment for microbial growth will help maximize animal production.
This article originally appeared in International Dairy Topics.