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Protein, carbohydrate interactions in rumen: The balancing act

Mary Beth Hall Published on 06 February 2014

The dairy industry has made great strides in reducing the amount of protein we feed while improving performance. Not only has this had a positive impact on a farm’s bottom line but also on reducing nitrogen flowing into the environment.

But there’s a new wrinkle in how protein can influence rumen fermentation that may have us adjusting how we formulate to meet requirements – not feeding more, but maybe feeding differently.



We talk about improving feed efficiency for cows, but what about improving feed efficiency of rumen microbes? Research from the ’80s and ’90s showed that the more protein you gave to rumen microbes, the more microbial protein was produced from the same amount of carbohydrate.

Basically, giving microbes more feed protein at the same time that carbohydrate was available increased the efficiency of growing microbes. Timing protein and carbohydrate so they are available to the microbes in the right amounts at the same time is called “synchrony.”

Drs. Will Hoover and Sandy Stokes-Goff of West Virginia University recommended a ratio of approximately two-to-one for nonfiber carbohydrates and rumen-degradable protein to optimize microbial protein yield.

Because rumen microbes are a valuable source of protein to the cow, many studies investigated synchrony in an effort to improve animal performance, but the results were mixed. Finding out why microbes make more microbial protein when given more feed protein might tell us what we need to do to make synchrony work.

We usually think about rumen microbes fermenting carbohydrates to produce organic acids, gases and more microbes, but both bacteria and protozoa can also convert carbohydrates in feed into glycogen. Glycogen is a carbohydrate with the same basic structure as starch that the microbes store inside their cells.


When microbes have more feed carbohydrate available than they need to meet their energy needs, they may store it as glycogen and ferment it later. The glucose, fructose, sucrose, fructans and starch in feeds can all be converted to glycogen by both protozoa and bacteria. Protozoa do not convert lactose to glycogen, but bacteria have been reported to do so.

There are pros and cons to glycogen. Storing feed carbohydrate as glycogen has the advantage of slowing the fermentation rate, production of acid and pH reduction in the rumen. If cells containing glycogen flow out of the rumen, the glycogen can be digested in the small intestine – an advantage if the animal needs glucose.

Disadvantages are that the feed carbohydrate that has been converted to glycogen will have a slower rate of fermentation, microbial yield will likely be reduced, and it takes energy to make glycogen. It costs one adenosine triphosphate (ATP) for each sugar molecule added onto glycogen.

That’s roughly one-fourth to one-half of all the ATP energy that microbes get from fermentation of a sugar molecule. That cost of storage represents feed energy that was not available for microbial growth and which reduces microbial efficiency. Also, if the cells with glycogen flow out of the rumen, the glycogen was never fermented and never supported production of microbial protein.

Making more rumen-degradable protein available at the same time microbes are taking up carbohydrates reduces storage of glycogen by rumen microbes. In this case, more feed carbohydrate is immediately fermented, less is stored as glycogen, and less energy is spent on glycogen storage.

Why would protein have this effect? Think of protein as a limiting nutrient for microbial growth: If you give microbes more protein, they grow more cells. Growing cells requires a lot of energy to synthesize all the complex molecules needed to build new cells.


If microbes have a use for the energy, like growing more cells, they give that function priority and reduce other non-essential processes like glycogen storage or energy spilling that uses energy.

Reduced glycogen production makes more feed energy available for cell growth, more cells are produced, and efficiency of microbial growth per amount of carbohydrate fermented increases. If it is delivered to the small intestine where the cow can use it, more microbial protein provides more high-quality protein and a better amino acid balance to the cow.

So what else do we need to consider for making this work on the farm? To be honest, this is a work in progress. The nuts and bolts for diet formulation and management still need to be worked out. But here are some factors to consider:

  • What types of degradable protein have this effect? In the lab, degradable true protein seems to have greater effect than nonprotein nitrogen sources like urea, but we still need to see if this varies by carbohydrate type.
  • What feed sources of proteins work? In our research, increasing rumen-degradable protein using soybean meal (no, we have not tried canola) seemed to have the desired effect with dry-ground or high-moisture corn as starch sources.
  • Alfalfa silage can be an excellent source of amino acids, peptides and non-protein nitrogen in the soluble protein fraction that might be tried with sugars. We really need to explore which sources to combine to get the desired results. This is a matter of having the right amounts of degrading protein available at the right time, not just feeding more protein.
  • Are there times you may not want to speed up fermentation? If acid production in the rumen is a concern, increasing the rate of fermentation would further decrease rumen pH and possibly reduce fiber fermentation or increase the risk of ruminal acidosis.
  • Protein is not the only factor involved (make sure enough fiber or effective fiber is being fed; be careful on starch levels), but reducing the amount of degradable protein may slow down fermentation.
  • Do passage rates matter for the protein effect to change cow performance? For microbes produced in the rumen to provide nutrients to the cow, they need to pass to the abomasum and small intestine, where they can be digested.

If microbes die and recycle in the rumen, their amino acids won’t be useful to the cow. For the synchrony between protein and carbohydrate to yield more nutrients to the cow, the liquid passage rate needs to harvest the microbes and deliver them post-ruminally.

Can we do this using buffers or salts to increase liquid passage? Do we need to stick to using more undegradable protein to meet cow requirements? More work needs to be done in this area to sort out the right balance for getting feedstuffs to ferment in the rumen and getting them to pass.

At the end of the day, will this protein effect matter to commercial herds? Maybe. Nutritional synchrony is an old idea that made sense, but making it work was a challenge.

We still need to sort out the practical balance of types and amounts of fermentable carbohydrate and protein, and how passage needs to be manipulated to get feeds digested in the rumen, but harvest microbes. Done right, improving the efficiency of microbes would add to the efficiency of the dairy system. It’s something we need to explore. PD

Mary Beth Hall has a Ph.D. in animal science from Cornell University and 30 years of experience in dairy cattle nutrition.

Mary Beth Hall
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