By Pedro E Urriola, Jerry Shurson, Zhikai Zeng and Jae Cheol Jang, University of Minnesota
One of the challenges of improving the nutritional value of corn distillers dried grains with solubles for pigs is to find ways of degrading the relatively high amount of fiber it contains. Corn DDGS contains 23% more gross energy (GE; 4,849 kcal/kg) than corn (3,933 kcal/kg), but the proportion of net energy (NE; 2,384 kcal/kg) in DDGS is only 49% of GE, whereas 70% of the GE in corn is NE (2,672 kcal/kg). Therefore, about half of the potential NE in DDGS is wasted, resulting in reduced caloric and nutritional efficiency when used in swine diets. The reason for low conversion of GE into NE is because corn DDGS contains a greater proportion of GE from dietary fiber than corn, and this fiber is resistant to degradation in the gastrointestinal tract of pigs.
Numerous studies have been conducted to evaluate the addition of various feed enzymes (carbohydrases and proteases) for improving energy, fiber and nutrient digestibility, but responses have been inconsistent. In studies where enzyme addition has improved digestibility, the magnitude of improvement has been modest, and often does not result in improved growth performance. In addition, our previous studies have shown than the apparent total tract digestibility (ATTD) of total dietary fiber (TDF) varies from 23 to 57% among DDGS sources (Urriola et al., 2010). This suggests that there are unique differences in the chemical and/or physical characteristics of fiber in DDGS that determine the extent of fiber digestibility.
To explore the relationship between the physical structure of corn DDGS relative to fiber digestibility in the small intestine and fermentability in the large intestine, we conducted a study (funded by the National Pork Board) where we incubated 15 sources of corn DDGS with fecal inocula for 8, 12 and 72 hours to measure rapid, intermediate and slow fermentable neutral detergent fiber (NDF) using a modified in vitro assay developed in our laboratory (Huang et al., 2017). Then, we compared in vitro values with in vivo fiber digestibility values (Kerr et al., 2013), and observed that a significant portion of NDF (21.6%) was degraded during the first 8 hours of incubation. Interestingly, this value of NDF digestibility was of similar magnitude (21.5%) to the apparent ileal digestibility value observed in corn DDGS (Urriola et al., 2010; Jaworski et al., 2015; Huang et al., 2017).
This observation suggests that in vitro degradation of NDF at 8-hour incubation represents the portion of NDF in corn DDGS that is readily digestible in the small intestine of pigs. After 72-hours incubation, about 68.6% of NDF disappeared, and resulted in ATTD of NDF (59.3%) greater than previously reported. Altogether, these results suggest that fiber in corn DDGS is composed of fractions that are rapidly degraded in the small intestine, while other fractions are more slowly degraded and undergo microbial fermentation in the large intestine. It appears that the low digestibility of this slowly degradable fraction is what makes corn DDGS fiber resistant to degradation by feed enzymes.
We subsequently used X-ray diffraction to investigate what makes the fiber in corn DDGS slowly degradable. We compared the relationship between the crystalline configuration of fiber structure in two sources of DDGS with different apparent total tract digestibility (ATTD) of NDF (Figure 1). We observed that DDGS with low (44.5%) ATTD of NDF had lower crystallinity index (9.2 vs. 12.2%) than DDGS with high ATTD of NDF (57.3%). These observations suggest that the differences in digestibility of fiber among sources of DDGS were related to the crystalline configuration of the fiber. This crystalline structure of feed is highly ordered linkage and thereby more resistant to degradation in the large intestine of pigs.
Therefore, we used ammonia fiber expansion (AFEX) to decrystallize the ordered fiber matrix and degrade the unfermentable portion of the fiber in DDGS. This process used pressure (300 psi), heat (100°), and hydroxide groups (ammonia). In vitro digestibility of energy during fermentation was determined for the non-treated and AFEX treated DDGS with and without fiber degrading enzymes. Pre-treatment of DDGS using AFEX increased the digestibility of gross energy and thereby increased the estimated digestible energy content of corn DDGS by about 700 kcal/kg (Table 1). This is the first time that energy digestibility in corn DDGS has been shown to improve with pre-treatment using AFEX.
In summary, dietary fiber in corn DDGS is composed of a heterogeneous group of carbohydrates with various rates of fermentation. The slowly fermentable portion of corn DDGS is resistant to digestion in the small and in the large intestine due to the complexity of the carbohydrates. The use of AFEX appears to be effective for increasing the digestibility of fiber and subsequently energy value of corn DDGS. However, it is necessary to study methods to apply the pressure, temperature and chemical treatment in AFEX under practical conditions.
*This project was funded by the National Pork Board and key findings of this research discussed in this article were obtained from Zeng et al. (2018).
Huang, Z., P. E. Urriola, I. J. Salfer, M. D. Stern, and G. C. Shurson. 2017. Differences in in vitro hydrolysis and fermentation among and within high-fiber ingredients using a modified three-step procedure in growing pigs. J. Anim. Sci. 95:5497–5506. doi:10.2527/jas2017.1907. Available from: https://www.animalsciencepublications.org/publications/jas/abstracts/0/0/jas2017.1907
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￼Zeng, Z.K., J.L. Zhu, G.C. Shurson, C. Chen, and P.E. Urriola. 2018. Improvement of in vitro ileal dry matter digestibility by non-starch polysaccharide degrading enzymes and phytase is associated with decreased hindgut fermentation. Anim. Feed Sci. technol. 246:52-61.