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.