Selection of an empirical detection method for determination of water-soluble carbohydrates in feedstuffs for application in ruminant nutrition

Authors: Hall, Mary Beth

Submitted to: Animal Feed Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/27/2014
Publication Date: 12/1/2014
Publication URL: https://handle.nal.usda.gov/10113/61994
Citation: Hall, M. 2014. Selection of an empirical detection method for determination of water-soluble carbohydrates in feedstuffs for application in ruminant nutrition. Animal Feed Science and Technology. 198:28-37.

Interpretive Summary: Water-soluble carbohydrates are a very important source of digestible nutrients in ruminant livestock diets. These carbohydrates include simple sugars, sucrose, oligosaccharides, and fructans that are rapidly fermented in the rumen. Because of their feeding value, it is important to be able to measure water-soluble carbohydrates for use in balancing diets in order to accurately determine whether carbohydrates in the feeds are adequate to meet animal needs, or if supplements need to be purchased. Two commonly used methods for measuring water-soluble carbohydrates were evaluated: a reducing sugar assay and the phenol-sulfuric acid assay. The two methods gave relatively similar results, but the reducing sugar assay gave much higher water-soluble carbohydrate values for cool season grasses. The reason for the inflated values could not be determined. The phenol-sulfuric acid assay gave a high value for solubilized carbohydrate in distillers grains. This was likely due to the response of that assay to measuring carbohydrates composed mostly of glucose. In view of all results, the phenol-sulfuric acid assay is preferred over the reducing sugar assay for detection of water-soluble carbohydrates. Selection of carbohydrate standards to more closely reflect water-soluble carbohydrate composition could further improve accuracy. Having one agreed-upon assay for water-soluble carbohydrates will improve the accuracy and consistency of values that nutritionists use for diet formulation. This will allow more accurate evaluation of feeds and formulation of diets to meet animal needs and provide a sound base for arriving at more refined feeding recommendations for water-soluble carbohydrates.

Technical Abstract: Water-soluble carbohydrates (WSC) are commonly measured in ruminant feedstuffs for use in diet formulation. However, we lack information as to which empirical detection assay most correctly measures WSC. The objective of this study was to determine which commonly used empirical assay was most appropriate for measurement of WSC based on equivalency to results from high performance ion chromatography with pulsed amperometric detection plus starch analysis of the water extract (HPIC). Empirical analyses used were a reducing sugar assay (RSA) that uses p-hydroxybenzoic acid hydrazide-based reagent and 50:50 glucose:fructose solutions as standards and the phenol-sulfuric acid assay (PSA) using sucrose solutions as standards. Twenty samples including cool season grasses (CSG), legume forages, nonforage feedstuffs, silages, or warm season grasses were used with a 0.2-g air-dried sample extracted in 35 mL of deionized water for 1 h at 40°C with continuous shaking. Water extracts for HPIC and RSA analysis were hydrolyzed with 0.037 M H2SO4 incubated at 80°C for 70 min. Theoretically, RSA should give the essentially same results as HPIC, excepting that RSA can also detect reducing ends of unhydrolyzed molecules. PSA detects all solubilized carbohydrates. On average, RSA and PSA values were greater than those found for HPIC by 76.2 g WSC/kg dry matter (DM). The two classes of feeds that showed differences between PSA and RSA were CSG and silages. For CSG, empirical assays for WSC were 54.1 and 20.6 g/kg DM greater than HPIC for RSA and PSA, respectively; for silages differences were smaller at 8.8 and 15.9 g WSC/kg DM. Cool season grasses contain fructans, however, the elevated RSA values were in excess of predicted differences due to the inflated recovery value for fructose (106.5% of actual) found for this RSA. Elevated RSA values obtained for CSG suggest that interference is affecting these grasses to a greater degree than other samples. Distillers grains showed an elevated value for PSA (69.1 g WSC/kg DM greater than HPIC); this is largely explained by the high recovery value for glucose (128.2% of actual) noted for this method when sucrose is used as the standard. Neither PSA nor RSA perfectly reflected HPIC values, however PSA gave more similar values. Gross differences between RSA and HPIC for CSG are an issue, particularly without clear, resolvable basis for the discrepancy. Accordingly, PSA is preferred over RSA for detection of WSC. Selection of standards to more closely reflect WSC composition could further improve accuracy.