Carbohydrates comprise approximately 60% of the total ration. Considering this, do we really understand the full impact of carbohydrate metabolism at cow-level digestion? Clearly not, and we have substantial learning ahead of us. Dissecting the carbohydrate building blocks – fiber, sugar and starch – can offer a closer look at the opportunities in our understanding of carbohydrates, and help us better learn to factor in what the cows have to say.

Carbohydrates are divided into three general components – fiber, starch and sugar. Fiber, the most slowly digested component, comprises around 30% of the total ration, while starch follows closely offering around 25% of the total ration make up, but potentially more energy than fiber. Sugar accounts for about 5% of the ration. Ration performance rides on a combination of carbohydrate content and digestion. Starch and sugars, the non-fiber carbohydrates, have historically been considered rapidly digested by the animal. Fiber on the other hand is relatively poorly degraded. So, what is the most important about each of these carb components?

Better understanding 1) feed carbohydrate measurements and lab technique accuracy and opportunities, and 2) rumen or total tract digestion approaches, can help us rationalize diet nutrition opportunities. We can further consider opportunities in the context of many, known live-animal measurements (in vivo) based on sound research.

The Neutral Detergent Fiber (NDF) assay is an indirect measurement – in the laboratory everything but the fiber is washed away and what is left over is weighed. Think of the process like dirty clothes in a washing machine; all the dirt or contaminants are washed off, and only clothing is left.

Fiber is not necessarily a consistent chemically defined compound, making measuring it very vague. Fiber is a heterogeneous feed component, partially composed of cellulose which is simply many glucoses linked together. Cellulose is easily degraded by rumen fibrolytic bacteria, however overall NDF is not easily degraded in most cases. The difficulty is posed by additional NDF components– including lignin and hemicellulose.

Sugar is similar to fiber in that it is not one single defined chemical compound– but rather a group of many different chemically defined compounds lumped into one group; we understand sugars to truly be 5 or 6 carbon rings. In the future we will continue to better define sugars.

Inversely, starch poses easier identification as a chemical compound clearly defined. Starch is a linear and/or branched glucose chain. In the laboratory, we break starch molecules, similar to breaking the linkages within a chain, resulting in free glucose that can be directly measured.

Starch is a major dietary carbohydrate component following fiber in diet quantity, and is necessary for proper rumen fermentation. Accurate measurement of this diet element is pertinent, yet an additional challenge lies in the animal: digestion is not consistent in the rumen and varies substantially.

Within a commercial laboratory there are several different approaches to assessing starch (or NDF) potential:

• in-vitro: Analysis is completed via ‘lab bench’ means. This method is very flexible and relatively quick, but furthest removed from cattle – taking place in a flask in the laboratory.

• in-situ: This method centers around incubating feed within the rumen. Feedstuffs are digested in the rumen within a porous bag. Feeds or TMRs are exposed to the real, dynamic rumen environment but are still contained in porous bags, which have been debated.

• in-vivo: These are total tract digestion measures similar to that used by leading researchers. This approach is the most accurate method for nutrient digestion, taking into account what goes into the animal, and what comes out. This approach is time consuming and expensive to complete, applies to TMRs only, and obtaining representative samples can be difficult.


In vivo analysis brings the industry closer to accurate cow-level digestion measures. And, while we continue to move toward better digestion measures, there is still much to learn in this area – with a common goal of more accurate and precise forecasting.

As ration models are continually upgraded and improved, we foresee more accurate digestion coefficients, rates and opportunity to improve passage rates.

So what is real rumen nutrient digestion? Research meta-analyses demonstrate that there is massive variability in rumen NDF and starch digestion. Table 1 is a snip of a larger table focusing on a survey of meta-analyses reporting in vivo digestion results for lactating cattle. Real rumen NDF digestion averages 42% and rumen starch digestion averages 60%. These means are valuable, but equally or more meaningful were reported ranges: NDF ± 24 units and Starch ± 31 units!

Table 1: Survey of meta-analyses reporting in vivo rumen and total tract NDF and starch digestion results for dairy cattle

The starch story: Continually evolving accuracy

Fiscal Implications

Relatively minor changes in TMR rumen NDF or starch digestibility can trigger substantial changes in microbial protein and milk production. Changing as little as 10 units in fiber digestibility can trigger a potential 11 lbs. of milk loss. Changing rumen starch digestibility by little as 10 units can mean a change of 4.5 – 5 lbs. of milk. That can mean as much as $0.60 to $0.80 per cow per day opportunity.

Several following articles will discuss options to improve forward predictions.

While current nutrition models are fantastic to improve on-farm precision, profitability and performance, opportunities remain. Forging innovative and accurate frontiers in digestion is a continual driver for new research. Dairy owners and competition within the industry continually challenge nutritionists to get closer to real, cow-level digestion and performance.

1 Published meta-analysis references cited by Goeser can be shared upon request or within 2014 4-State Dairy Nutrition and Management Conference proceedings