The chemical composition and structure of dietary fiber, as well as physicochemical characteristics such as bulk density, swelling, water binding capacity, and viscosity, may influence the digestion of energy and nutrients in feed ingredients. Having a complete account of the components of a feed ingredient, particularly the type and concentration of fiber, is necessary in order to accurately the energy value of an ingredient using prediction equations.

An experiment was conducted to obtain a detailed description of the composition of ten feed ingredients, and to compare the analyzed gross energy (GE) with the GE predicted by adding together the measured energy-containing components. A secondary objective was to measure the in vitro apparent total tract digestibility (IVATTD) and in vitro apparent ileal digestibility (IVAID) of dry matter (DM) and organic matter (OM) in the feed ingredients, and to determine the correlations between the physicochemical characteristics of the ingredients and the in vitro digestibility.

Materials and methods

Ten feed ingredients were tested in this experiment:

• two sources of grain—corn and wheat;
• two sources of oilseed meals—soybean meal and canola meal;
• four sources of high fiber co-products—distillers dried grains with solubles (DDGS), corn germ meal, copra expellers, and sugar beet pulp; and
• two synthetic sources of fiber—cellulose and pectin.

With this variety of ingredients, a wide range of IDF and SDF concentrations were tested.

The IVAID was determined using a two-step procedure which simulated gastric and small intestinal digestion. In step 1, flasks containing the samples were incubated in a water bath at 39ºC with constant shaking for 2 hours with pepsin added, representing the digestion processes in the stomach. In step 2, samples were incubated in the same manner for 4 hours with pancreatin added, simulating the digestion processes in the small intestine.

To determine IVATTD, a third step was added to simulate fermentation in the large intestine. Viscozyme, a carbohydrase mix, was added to degrade soluble fiber and the sample was incubated at 39ºC for 18 hours with constant shaking.

After the final step of each respective procedure, dry matter and ash analysis were performed on the residue from the flasks to calculate the IVAID and IVATTD of dry matter and organic matter.

The gross energy (GE) content of the ingredients was analyzed, and compared with the values for GE calculated from all of the energy-contributing components using the following equation:

GE = (AEE¹ × 9700 kcal/kg) + (Total AA¹ × 5400 kcal/kg) + [(Total starch + fructo-oligosaccharides + NSP) × 4200 kcal/kg] + [(glucose + fructose + sucrose + stachyose + raffinose) × 3700 kcal/kg] + (lignin × 6957 kcal/kg)

¹AEE = acid hydrolyzed ether extract; AA = amino acids.

Composition of test ingredients and calculated vs. analyzed GE

The analyzed composition of the test ingredients is shown in Table 1. The analyzed nutrient composition of all ingredients except synthetic cellulose added up to 100% or greater; however, only copra expellers and sugar beet pulp added up to exactly 100%. Human factors and non-homogenized samples can introduce inaccuracies in analysis. The calculation of crude protein may also introduce error because it assumes that the protein in the feed is composed of 16% nitrogen, which may not be true for all ingredients.

The analyzed GE differed from the GE calculated from the energy-contributing components by anywhere from 1 to 417 kcal/kg. The calculated GE values for DDGS, canola meal, and copra expellers were less than the analyzed GE by 232, 190, and 134 kcal/kg respectively. This indicates that some energy-contributing components may be analyzed as a different component of the feed ingredient.

Correlation between physicochemical characteristics and in vitro digestibility of OM and DM

The IVAID of DM was greatest (P < 0.05) in pectin, followed by (in order from greatest to least) soybean meal, wheat, canola meal, copra expellers and DDGS, corn and corn germ meal, sugar beet pulp, and synthetic cellulose. The results for IVAID of OM followed the same pattern except that the value for copra expellers was greater than that for DDGS. The IVATTD of both DM and OM were also greatest (P < 0.05) in pectin, followed by soybean meal, wheat, corn, sugar beet pulp, canola meal, copra expellers, corn germ meal, DDGS, and synthetic cellulose.

The in vitro digestibility of DM in sugar beet pulp increased from 26.23% AID to 81.28% ATTD, a difference of 55%. This indicates that the fiber fraction in sugar beet pulp is poorly digested in the small intestine, but highly fermentable in the hindgut. Other ingredients in which ATTD of DM was at least 20% greater than AID of DM were corn (39.57%), copra expellers (22.73%), and canola meal (20.59%).

The bulk density, swelling, water binding capacity, and viscosity of the ingredients are shown in Table 3. Insoluble dietary fiber (IDF) was positively correlated (P < 0.05) with total dietary fiber (TDF) and negatively correlated (P < 0.05) with IVAID of DM, IVAID of OM, IVATTD of DM, IVATTD of OM (Table 4). TDF was negatively correlated (P < 0.05) with IVAID of DM, IVATTD of DM, and IVATTD of OM. There was a tendency (P < 0.10 )for the concentrations of NDF and ADF to be negatively correlated with the IVAID of DM. No correlations were observed between in vitro digestibility of DM or OM and bulk density, swelling, water binding capacity, viscosity, or the concentrations of crude protein, GE, AEE, lignin, or soluble dietary fiber.

Key points

• Errors in analysis as well as inaccuracies in calculations used to determine the concentrations of energy-contributing components can lead to overestimation or underestimation of GE in feed ingredients.
• The fiber fraction in soybean meal and wheat are well digested in the stomach and small intestine relative to other ingredients commonly used in swine diets. The fiber fraction in sugar beet pulp, corn, copra expellers, and canola meal is well fermented in the hindgut.
• The concentrations of IDF and TDF are highly correlated with digestibility of DM and OM.

Table 1. Analyzed nutrient composition of corn, wheat, soybean meal, canola meal, distillers dried grains with solubles, corn germ meal, copra expellers, sugar beet pulp, synthetic cellulose, and pectin, as-fed basis

¹SBM = soybean meal; CM = canola meal; DDGS = distillers dried grains with solubles; CGM = corn germ meal; CE = copra expellers; SBP = sugar beet pulp; SF = synthetic cellulose.

²AEE = acid hydrolyzed ether extract.

³TDF = total dietary fiber; IDF = insoluble dietary fiber; SDF= soluble dietary fiber.

⁴SCT = soluble condensed tannins; ICT = insoluble condensed tannins.

⁵FOS = fructo-oligosaccharides.

⁶Summation of moisture, ash, CP, AEE, TDF, Total starch, glucose, fructose, sucrose, stachyose, raffinose, and fructo-oligosaccharides.

⁷The difference between the calculated gross energy of the components and the gross energy of the diet.

Table 2. In vitro ileal and total tract digestibility of DM and OM in corn, wheat, soybean meal, canola meal, dried distillers grains with solubles, corn germ meal, copra expellers, sugar beet pulp, synthetic cellulose, and pectin

^a-jMeans within a row lacking a common superscript letter differ (P  < 0.05).

Table 3. Bulk density, swelling, water binding capacity, and viscosity of corn, wheat, soybean meal, canola meal, dried distillers grains with solubles, corn germ meal, copra expellers, sugar beet pulp, synthetic cellulose, and pectin

^a-jMeans within a row lacking a common superscript letter differ (P  < 0.05).

¹WBC = water binding capacity.

Table 4. Correlation coefficients between fiber content, physical characteristics, and in vitro ileal and total tract digestibility of DM and OM of corn, wheat, soybean meal, canola meal, dried distillers grains with solubles, corn germ meal, copra expellers, sugar beet pulp, synthetic cellulose, and pectin

*P < 0.10; **P < 0.05; ***P < 0.01

This report is based on unpublished research by Diego Navarro, Erik Bruininx, Lineke de Jong, and H. H. Stein.