By Diego D. M. L. Navarro and Hans H. Stein, University of Illinois
The concentration of digestible energy and metabolizable energy in feed ingredients fed to pigs is usually determined using a single inclusion rate of a test ingredient in the diet and the difference procedure is used to calculate DE and ME in the ingredient. However, it is not clear if different inclusion rates result in comparable DE and ME values if the difference procedure is used.
Previous studies using wheat middlings or wheat bran indicated that different inclusion rates may result in variable DE and ME values. It is also not clear if there is a saturation point in the fermentation capacity in the hindgut of growing pigs, which may influence the amount of energy obtained by the pig from hindgut fermentation of high-fiber ingredients. If that is the case, it may be hypothesized that the DE and ME obtained for a high-fiber ingredient may be reduced with increasing inclusion rate.
Therefore, an experiment was conducted to test the hypothesis that increasing the inclusion rate of high-fiber ingredients in experimental diets decreases the calculated DE and ME in these ingredients because greater concentrations of fiber may overwhelm the ability of microbes to ferment fiber. It is also possible that increasing dietary fiber increases passage rate in the digestive tract and, thus, reduces the time available for fermentation. The four high-fiber ingredients used were canola meal, corn germ meal, sugar beet pulp and wheat middlings (Table 1).
A total of 20 ileal-cannulated pigs weighing approximately 31 kilograms (68 pounds) were used. A basal diet based on corn and soybean meal and a corn-SBM diet with 30% cornstarch were formulated. Six additional diets were formulated by replacing 15% or 30% cornstarch by 15% or 30% corn germ meal, sugar beet pulp or wheat middlings, and two diets were formulated by including 15% or 30% canola meal in a diet containing corn, SBM and 30% corn starch. Corn starch was used so that added fiber in experimental diets is attributed to the test ingredient. After the energy metabolism trial, a colored marker was included in the feed and the time of first appearance of this marker in ileal digesta and in the feces was recorded.
The time from marker ingestion to first appearance in the digesta at the end of the ileum was not different among pigs fed experimental diets (Table 2). In contrast, the time from marker ingestion to first appearance in the feces was linearly reduced as 15% or 30% canola meal, corn germ meal, sugar beet pulp or wheat middlings was added to the diet compared with the corn starch diet. This may explain the reduction in the digestibility of energy in diets that was observed as the fiber-rich ingredients were added, resulting in a decrease in transit time in the hindgut and therefore less time for the digesta to be fermented. This indicates that dietary fiber primarily affects passage rate of digesta in the hindgut of pigs.
The calculated values for DE and ME were not different between 15% or 30% inclusion rate within each ingredient (Table 3). The observation that concentration of DE and ME in feed ingredients were independent of inclusion rates indicates that under the conditions of this experiment, utilization of energy from the test ingredients was equally efficient in diets with 30% inclusion compared with diets with 15% inclusion. This indicates that the microbial population in the hindgut was not overwhelmed by the increased inclusion of fiber in the diet or the increased flow of nutrients into the large intestine.
In conclusion, inclusion of high-fiber ingredients may have a negative effect on the concentration of DE and ME in diets fed to pigs. However, inclusion rate does not affect calculated values for DE and ME in feed ingredients with relatively high concentration of fiber indicating that microbial capacity for fermentation of fiber in pigs is not overwhelmed by inclusion of 30% high-fiber ingredients in the diets. The time of first appearance of digesta in the feces was reduced as inclusion of fiber in the diets increased indicating reduced transit time in the hindgut of pigs fed high-fiber diets, but this had no impact on values for DE and ME and ATTD of GE in test ingredients.