So, what’s in a cow’s rumen anyway? Well, a whole lot.

The rumen of a typical lactating dairy cow in mid-lactation is a 25-gallon stomach holding a 190-pound bubbling mixture of liquids and solids. This 190-pound mix is an actively fermenting mass, changing and releasing the nutrients within as the ruminal microbes live, grow and die.

If you took the total 190 pounds out of the rumen and dried it down, you would drive off 165 pounds of water. This water is either the water moving freely around the active fermentation (probably 80 percent of 165 pounds) or the water trapped inside the fermenting feed or ruminal microbes that do the fermentation. The dry matter would weigh 25 pounds and is made up of the fermenting feed (the majority) and the dried-out bodies of ruminal microbes. The fermenting feed in the rumen might be equal in weight to about half of the daily intake, but that varies depending on the timing and size of the cow’s last meal.

The microbes that do the work of fermentation in the rumen are numerous. The healthy rumen has among the highest microbial population concentrations found anywhere on the planet. A lactating dairy cow can have microbial concentrations of 10,000,000,000 microbes per milliliter or cubic centimeter (cc). So, a 10-cc syringe of ruminal fluid would contain 100,000,000,000 microbes and the rumen of our typical dairy cow would be home to 1,000,000,000,000,000 ruminal microbes (That’s one quadrillion or million billions.). Needless to say, they are very, very tiny.

The job of the ruminal microbes is to ferment the feed eaten by the cow, especially plant fiber. Fermentation converts the plant fiber to nutrients the cow can access. To oversimplify, the ruminal fermentation serves to release glucose chains from different configurations (sugar, starch, plant fiber) found in the feeds a cow ingests. Chemically, glucose is a ring of six carbon atoms with hydrogens and hydroxides sticking off it and is the building block of most feed a dairy cow will eat.


For simplicity (and possibly silliness), think of these glucose molecules as donuts (another ring structure). The feed the cow ingests contains billions and billions of these tiny, tiny donuts and these shipments of donuts into the rumen are packaged many different ways.

The microbes that live in the rumen get the shipments first and their job is to release the donuts from their packaging which they do in return for the food and shelter of the rumen. Some of the donuts are not packaged (i.e. simple sugars) and microbes enjoy them quickly and easily. Others are heavily packaged (i.e. glucose chains that form plant fiber) and the microbes must use an array of tools that would make Batman jealous to free these donuts.

The donuts move in steadily and microbes tend to go for easy ones first then get to as many of the heavily packaged donuts as they can. Once the donuts are free from the packaging, the microbes can take one, and only one, bite from each donut; that is the house rule the microbes must live by when they live under the protective roof of the cow’s rumen. And by this rule, hungry microbes are forced to release lots and lots of donuts that the cow can use.

Continuing with the silly metaphor, cows can use only loose donuts and don’t really mind the missing bites as long as they have lots of donuts provided by the microbes. Since cows don’t have thumbs or tools to open the packaging, they couldn’t get at all the donuts without the help of the hard-working ruminal microbes. Scientifically, the relationship between ruminal microbes and the cow is called symbiosis, a relationship where both parties gain benefits by the association. PD

—Excerpts from Miner Institute Farm Report, October 2008

Steve Mooney
William H. Miner Agricultural