The rumen isn’t a democracy. When starch-loving microbes get more resources, the fiber-degrading community doesn’t necessarily get a proportional share.
That helps explain a nutritional paradox highlighted by Ohio State University professor Jeff Firkins during a recent webinar in Balchem’s Real Science Lecture Series: Increasing starch availability can increase microbial protein production, but not necessarily by as much as we might expect. The reason is that the rumen is an ecosystem of competing and cooperating microbes, and pushing one population can change the resources available to another.
Understanding those interactions offers another way to think through unexpected responses to dietary changes.
Rumen Microbes Are Sprinters and Marathoners
Firkins uses a useful analogy for the microbial competition happening inside the rumen.
Amylolytic bacteria are the sprinters. They can quickly take advantage of readily available starch.
Cellulolytic bacteria are the marathoners. They specialize in breaking down fiber, a slower process that requires a different set of resources.
The two groups don’t operate independently. They share nutrients and other resources, so shifting the diet toward more rapidly available starch can change the environment for fiber-digesting microbes.
That creates an important tradeoff. Fiber digestion provides energy for microbial growth, so increasing one source of carbohydrate doesn’t necessarily translate into a proportional increase in microbial protein.
More Starch Can Come With a Fiber Cost
Recent research illustrates this tradeoff. A 2013 meta-analysis found that each 1 percentage-unit increase in dietary starch was associated with a 0.48 percentage-unit decrease in total-tract NDF digestibility and a 0.61 percentage-unit decrease in ruminal NDF digestibility. A separate 2018 analysis found a 0.59 percentage-unit decrease in NDF digestibility for each 1 percentage-unit increase in dietary starch.
That doesn’t mean increasing starch is inherently bad. Higher starch availability can increase microbial protein production. The point is that the response doesn’t happen in isolation.
In some studies, improving starch availability increases microbial protein. In others, the increase is smaller than expected or microbial protein efficiency declines. Firkins points to the interaction between starch and fiber as one reason.
“Most of the problem is when we put starch and fiber together, we cause a negative associative effect,” he explains.
That distinction is useful when evaluating a ration change. A change that looks beneficial from an energy standpoint may also alter fiber digestion and feed intake, potentially changing how cattle perform on the diet.
Give Rumen Microbes What They Need
So what does the microbial community need to keep doing its job?
Firkins points to a few key factors:
- Balance starch with effective fiber. More starch isn’t necessarily more microbial protein if fiber digestion takes a hit.
- Provide adequate rumen-degradable protein (RDP). About 10% is a useful target. Too little can leave microbes without enough nitrogen to grow.
- Think beyond ammonia. Microbes can also use preformed amino acids and branched-chain volatile fatty acids. Having those building blocks available can save microbes some of the energy they’d otherwise spend making them.
- Look at the whole ration when making changes. Starch, fiber, protein and fat all interact, so improving one part of the diet can sometimes create a tradeoff somewhere else.
The bottom line: microbes need the right mix of nutrients to grow.
Protein for the Microbes
Not all dietary protein follows the same path through the cow. Some is broken down in the rumen, where microbes can use it to grow. That’s where rumen-degradable protein, or RDP, comes in.
Firkins cautions against thinking of RDP as simply a way to supply ammonia. Research using isotope labeling has shown that some cellulolytic rumen bacteria can incorporate nitrogen from preformed amino acids and peptides, rather than relying entirely on ammonia. Branched-chain amino acids and the volatile fatty acids made from them also can help support microbial growth.
Why does that matter? Providing some of these building blocks means microbes don’t have to spend as much energy making them themselves.
That may be especially helpful for the specialized bacteria responsible for breaking down cellulose. They’re good at their job, but they don’t have all the metabolic tools that some other rumen microbes do.
The takeaway isn’t that we need to start formulating amino acids for rumen microbes. It’s that a diet can provide plenty of nitrogen and still not give the microbial community everything it needs to grow efficiently.
As Firkins puts it, “If you don’t give microbes enough RDP, you’re going to lower the amount of microbes.”
Put the Microbial Picture Together
This is where collaboration between the veterinarian and nutritionist becomes especially useful.
Changes in feed intake, manure consistency, cud chewing, body condition or production can provide clues about how cattle are responding to a dietary change. The nutritionist can then look at those observations alongside changes in starch, fiber, RDP and other nutrients.
Together, they can ask a more useful question than simply whether the ration provides enough protein:
Is the diet creating the conditions for the rumen microbial community to work efficiently?
That perspective also can help identify unintended tradeoffs. Increasing starch may improve energy availability while reducing fiber digestion. Lowering dietary protein may improve nitrogen efficiency but go too far if RDP becomes limiting.
The goal isn’t to optimize one part of the diet in isolation. It’s to connect what’s happening in the ration with what’s happening in the animal.
The Value of Microbial Protein
Microbial protein is an exceptionally valuable source of protein for the animal. Firkins estimates it is about 82.4% true protein and 80% intestinally digestible, giving it a quality that makes it comparable with feed proteins in terms of metabolizability.
The opportunity, then, isn’t simply to produce more microbial protein. It’s to produce it without disrupting the microbial ecosystem that makes it possible.
That means looking beyond any single number on a ration sheet. Fiber digestion, starch availability, rumen-degradable protein and feed intake all provide pieces of the picture when trying to understand how cattle are responding to a diet.
A well-functioning microbial community isn’t the answer to every herd health problem, but it is part of the foundation for efficient digestion and nutrient use.
The rumen’s sprinters and marathoners don’t have to compete for the same finish line. The better strategy may be making sure both have what they need to keep running.
