Discovering the power of rumen microbes
The symbiotic relationship of microbes and the ruminant animal is truly a thing of beauty. These microorganisms, predominantly bacteria, protozoa and anaerobic fungi, depend on the ruminant to provide the physiological conditions necessary for their existence. In turn, these microorganisms are essential for digestion and fermentation of the large amount of fibrous feeds that the ruminant consumes, but otherwise cannot utilize. By providing a suitable habitat for these microorganisms, the ruminant is able to utilize the end products of microbial fermentation and microbial cells to meet its own nutritional needs for energy and protein.
The rumen is an open and continuous eco-system, which is an ideal environment for maintaining a stable microbial population that has evolved through millions of years of selection. A constant supply of substrates is provided by feed consumed by the animal, and the large holding capacity of the rumen provides the necessary volume and retention time for carbohydrates to be fermented. Additional factors that make the rumen an ideal place for microbial fermentation include: warmth, moisture, free of ultraviolet radiation, and finally, waste products of microbial fermentation that are quickly absorbed through the rumen wall. Waste products of bacterial fermentation are fuel for the cow for maintenance, growth and milk production.
Let's take a look at these important microbes and learn a little about their function.
Photo by: M. Wanapat, Khon Kaen University, Thailand
Figure 1. Rumen bacteria attached to plant fiber.
Bacteria are the key workhorses of the rumen and number in the billions of microbes per gram of rumen fluid. These bacteria are classified into a diverse community that digests fiber, starch, protein, fat and sugar, and those that produce ammonia and methane. During fermentation of feed, bacteria produce waste products volatile fatty acids (acetate, propionate and butyrate), microbial protein (microbial cells themselves), and B-vitamins. Acetate provides 60%, propionate provides 30%, and butyrate provides 10% of the total energy coming from the rumen that the cow uses to produce milk. Acetate and butyrate are used to make milk fat and propionate is converted into milk sugar or lactose.
Photo from Biology blog.com
Figure 2. Rumen protazoa.
Protozoa are also important, but their functional role in the rumen is not as completely understood. There are about half as many protozoa in the rumen as there are bacteria. Protozoa act to digest fiber, control the rate of starch fermentation and modify the rumen bacteria.
Photo by: M. Wanapat, Khon Kaen University, Thailand
Figure 3. Rumen fungi.
Fungi are the smallest component of rumen microbes. Fungi appear to be important in aiding in the digestion of poor quality forage.
In early September, I attended an American Dairy Science Association Discovery Conference titled "New developments in rumen microbiology and their potential to improve animal performance". Below are some key take-home messages from some of the world's leading rumen microbiologists.
Stuart Denman – Rumen Microbiologist, CSIRO Livestock Industries
Researchers have long studied rumen bacteria that are responsible for fiber digestion, starch digestion, and protein digestion, yet these probably only accounts for 1% of the species diversity found within the rumen.
Mary Beth Hall – U.S. Dairy Forage Research Center, USDA-Agricultural Research Service
The activity of ruminal microbes plays a crucial role in determining responses of ruminants to diets, through impact on nutrient supply, disappearance of substrate from the rumen, and production of compounds and conditions that may be deleterious to the animal.
Alex Hristov – The Pennsylvania State University
A number of compounds have shown consistent effects on rumen function (including ionophoric antibiotics, dietary oils, and plant-derived bioactive compounds) but in some conditions may have negative impacts on feed intake, nutrient digestibility, and animal productivity.
Tim McAllister – Agriculture and Agri-Food Canada, Lethbridge Research Center
The anaerobic fungi are key players in the rumen ecosystem accounting for 8 to 20% of the microbial biomass in the rumen. They have a complex lifecycle consisting of both a free-living zoospore stage and an attached phase where they send out roots similar to that of a tree. These fungal roots invade fiber particles and physically disrupt and penetrate the plant cell wall.
T.G. Nagaraja – Kansas State University
Biotic agents to manipulate ruminal microbial function include probiotics or direct-fed microbials (DFM). As applicable to ruminants, DFM's are a microbial supplement that includes live cells, products, extracts, or spent growth medium, which beneficially affect the host by improving ruminal and or intestinal microbial balance.
Gregory Penner – University of Saskatchewan
Recent studies have demonstrated that while diet can certainly affect the microbial community, there is considerable variation in the response to diets among animals. Furthermore, the host seems able to regulate the microbial community, and populations of the microbiome appear to be relatively stable.
Published in Dairy Star October 13, 2012