Feeding a dairy cow’s rumen is as complicated and complex as feeding the world’s billions of people in a sustainable, efficient and environmentally friendly way.

Inside the rumen, cows are digesting corn silage to produce milk in the most efficient way possible, and it all starts with what you’re feeding – and the delicate balance between fiber and starch.

Breaking down rumen function

The rumen acts as a big fermentation vat as part of the cow’s digestive system. The rumen is filled with up to 40 gallons of fluid with billions of rumen microbes. These microbes are made up of billions of bacteria, millions of protozoa and thousands of fungi. Rumen microbes are the true workers in the cow’s digestive system.

These organisms break down and digest the feedstuffs fed to the cow. Proper and good rumen function is dependent upon the activity, viability and delicate balance of these rumen microbes.

Keep in mind: The rumen microbes must be able to ferment the ingredients that make up the ration. A limited amount of starch from grain is a good source of energy and can be fermented by the rumen microbes. However, if too much starch is fed, the end product of fermenting starch will be acids and cause the rumen pH to drop.


The goal should be to hold close to a pH of 6.5 to maintain microbe viability. High-starch, high-grain and grain byproduct diets will drop the rumen pH to 5.5 or below. If rumen pH drops this low, the cow risks acidosis by killing off sensitive rumen microbes and impairing rumen function. Correcting acidosis is a matter of having enough forage and digestible fiber.

Build a ration for increased digestibility

Fiber is key to keeping the delicate balance among the differing groups of rumen microbes. Many of the species in the rumen microbe pool are designed to digest and ferment fiber. Rations need to include adequate fiber to maintain this population of rumen microbes and to form a mat within the rumen.

The mat is helpful in retaining and controlling rate of passage of feed ingredients through the rumen, giving the rumen microbes time to ferment the ingredients completely. With too much fiber, dry matter intake can be restricted from delays of fermentation activity in the rumen.

Fiber digestibility is measured through sample testing to evaluate levels of undigested neutral detergent fiber (uNDF) expressed as a result of a 240-hour rumen incubation time. Rations with a high level of uNDF will have a slower rate of passage.

Silage quality comparison

For example, in comparing Hybrid A and Hybrid B, if you feed 25 pounds of dry matter from corn silage, and we can lower the uNDF as a percent of dry matter from Hybrid B’s 12.4 percent uNDF 240-hour value to Hybrid A’s value of 6.3 percent uNDF 240-hour value, we create the potential for an additional 1.52 pounds of dry matter intake.

Each incremental increase of 1 pound of dry matter intake equates into 2 to 3 pounds additional milk. Therefore, the difference between these two hybrid levels of uNDF has the potential to generate an additional 3 to 4.5 pounds of milk.

A high-producing dairy cow has a very large demand for metabolizable energy. To obtain the metabolizable energy needed, producers will increase grain levels as high as possible. What is often overlooked is: Fiber is also a carbohydrate or a metabolizable energy source.

To extract the maximum amount of energy from fermentable fiber, producers need to seek out sources of fiber that offer both effectiveness in forming the rumen mat and high levels of fermentability to supply metabolizable energy. Producers can review their ration’s fiber digestibility through results of laboratory testing.

The industry standard for fiber digestibility is listed as neutral detergent fiber digestibility (NDFd) and is measured after a 30-hour incubation time period. The higher your NDFd numbers, the more forage your herd can consume.

At a minimum, dual-purpose corn silage should test at 50 percent NDFd in a 30-hour test. For BMR and other enhanced silage hybrids, your results should be at or above 65 percent in a 30-hour in-vitro test.

With these two hybrids, if we are able to improve upon Hybrid B’s 57.8 percent NDFd 30-hour value and raise it to a value similar to Hybrid A’s 69.6 percent NDFd 30-hour value, the combination of increased digestibility and rumen function will support increased milk production.

When applied in advanced dynamic ration modeling systems like the Cornell Net Carbohydrate and Protein System (CNCPS), this increase in NDFd offers the potential to gain 4 pounds of milk per head per day.

BMR corn hybrids have more NDFd because they contain less lignin. Less lignin in the plant provides more room for digestible fiber to be available in the rumen. With more fiber and less space wasted, cows are able to eat more for increased dry matter intake. This results in increased milk production and feed efficiency.

Consult with your nutritionist and seed adviser to evaluate your hybrid options to meet these goals of digestibility and performance.

Even with grain corn kernels, fermentability and digestibility are important. Kernel digestibility will be listed as starch digestibility (starch-d). Similar to NDFd from fiber, starch-d measures the potential of the grain to supply metabolizable energy to the cow. Starch-d or fermentability can also be applied to corn silage.

Vigorous rumen microbes can fully break down highly fermentable ration ingredients. The substrates of the fermentation process in the rumen can be absorbed through the rumen lining or escape the rumen to be absorbed in the intestinal tract.

Additionally, a healthy rumen will supply many rumen microbes to the intestinal tract for further digestion and absorption as they escape the rumen with the passage of rumen fluids and the other substrates. These rumen microbes then become both a metabolizable protein and energy source to be used as fuel for each animal.

The bottom line

Source highly digestible and fermentable fiber and carbohydrate feed ingredients to promote good rumen function and maximize metabolizable energy potential. Corn silage is an excellent source of fermentable carbohydrates, with BMR hybrids leading the pack in digestibility.

These silage options, along with a completely balanced diet that incorporates ingredients of high fermentability, will aid in healthy rumens and happy cows.  end mark

PHOTO: When evaluating the role of silage, rations can’t consider starch digestibility alone but must balance it with fiber digestibility to meet the needs of rumen function. Photo courtesy of Mycogen.

Bill Sutliff

Evaluating your herd’s rumen

How can you tell if your cows have healthy rumens? Use these steps to detect issues with the herd or opportunities to improve your ration’s efficiency.

1. Monitor herd activity. If cows have a bright appearance, a curious attitude and cud-chewing activity, these are the right signs your cows are feeling good.

2. Observe your cows’ manure. Good signs of a healthy rumen are manure that is creamy in texture, with a dimple in the middle of the pile and no obvious signs of feed passing through the digestive tract. Appearance of mucus in fresh manure can give you an indication of rumen acidosis or subclinical acidosis. If you are concerned, consult your nutritionist to use a manure screener to dissect the pile more fully.

3. Review results. Check your milk butterfat and milk protein tests to see if they are appropriate. Strive for a minimum of 3.5 percent butterfat for a Holstein herd with a fat-to-protein ratio of approximately 1.25-to-1. Your veterinarian and nutritionist can help you assess whether fat and protein levels are in line.

4. Monitor milk urea nitrogen (MUN) level. This level shows the signs of an effective balance of ingredients with similar rates of digestion. Typical ranges should be from 8 to 12 and are farm-dependent. This also is a very environmentally friendly way to watch for wasted protein.