Kai Yuan Ph.D. QLF Senior Research & Technical Advisor
In feed samples, the value of sugar is usually analyzed as water-soluble carbohydrates (WSC) and ethanol-soluble carbohydrates (ESC). What are WSC and ESC? Which value should you use as an estimate of sugar?
What is the difference between WSC and ESC?
Most low molecular weight simple sugars (such as sucrose, glucose and fructose etc.) are soluble in ethanol, plus some small-chain fructans. Most fructans are soluble in water but not in ethanol. So the difference between WSC and ESC is that WSC contains fructans plus simple sugars, and ESC contains only simple sugars.
What are fructans?
Fructans are polymers of fructose, mostly seen from cool-season C3 plants. Corn is considered a warm-season C4 plant and is not known to accumulate fructans. Fructans are only contained within the plant cell wall. They are not as available nor fast fermenting like supplemental sugars from QLF liquid feeds such as sucrose, glucose, and fructose. These supplemental sugars in QLF feeds are more effective at driving rumen microbial growth compared to fructans.
Why there is a big difference between WSC and ESC from the same feed sample?
If you look at forage analysis results, sometimes there can be quite a big difference between WSC and ESC. For example, a BMR corn silage sample came back from the lab at 4.8 % WSC and 1.7% ESC. Conventional corn silage tested at 4.2% WSC and 0.65% ESC. Which number do you use? First of all, as discussed above, a warm-season plant like corn is not known to accumulate fructans. The much higher number in WSC vs. ESC could be due to error in assay per se. There could be some “soluble starch” accounted as fructans. Second of all, in fermented feed like corn silage, most of the available sugars are likely to have been converted into silage acids in the fermentation process, and the residual sugars are less likely to be available for rumen microbial utilization. Therefore, in the case of fermented feed like corn silage, you should use ESC. On the other hand, if you are feeding unfermented hay or grasses, those fructans are real, and you should use the WSC number.
Sugar level in the ration?
If the majority of ingredients in the basal ration are fermented silages, the basal sugars from those feed ingredients will not behave the same as supplemental sugars in the rumen. In this case, one should not pay too much attention to those basal sugar levels in the ration, as they are not as available to the rumen and do not act like supplemental sugars such as sucrose, glucose, and fructose.
A good understanding on the assays behind sugar analysis should help us to distinguish the differences among various types of sugars, and to decide which value (WSC or ESC) to use to describe different feed ingredients. This should help prevent under- or over-estimate sugar levels in basal rations.