Date Full Report Received


Date Abstract Report Received



Primary Investigator:

Fiber is the carbohydrate portion of the diet which cannot be digested by enzymes secreted by the intestinal tract of the pig. For example, starch is a carbohydrate, but is not considered to be fiber because the pig produces enzymes which breaks down starch [However, to be exact, a small portion of the starch in corn and other grains is resistant to these enzymes, is called “resistant starch” and therefore is considered to be part of the fiber in the diet]. On the other hand, cellulose is considered to be fiber, because the pig cannot digest it. Therefore, fiber is not digested but at least parts of it can be fermented by the microbes that live in the gut of the pig – primarily in the caecum and large intestine. Fiber has been characterized in many ways. One method of characterization is soluble versus insoluble; soluble fiber is that portion of the total fiber which is fermented fairly well in the pig, while insoluble fiber is that portion which is poorly fermented. While fiber is viewed as one, albeit poor source, of energy in the diet, it has other impacts on the pig. Insoluble fiber can affect the gastrointestinal physiology of the pig, especially the digestibility of the diet. In this project two experiments were conducted: In experiment 1, The impact of insoluble dietary fiber (IDF) on diet digestibility was studied under two diet formulation conditions: constant nutrient, meaning as fiber was added to the diet, nutrients were kept constant, or nutrients were allowed to float, meaning that as fiber was added to the diet, nutrient levels changed and ingredients were kept constant. Both approaches are used in research, but there is almost no information on which is the best approach to evaluate the impact of fiber on energy and nutrient digestibility. It is obviously very important to study the effect of fiber on diet digestibility in the best possible manner, e.g., the one that gives us results that can be best used in practical diet formulation. Twenty-one ileal-cannulated growing gilts (33 kg BW) were assigned to 1 of 7 dietary treatments. Diets consisted of a basal corn-soy diet with 0% corn DDGS, or diets containing 15, 30 or 45% of DDGS as a source of IDF. Diets were formulated using either constant nutrient or constant ingredients approaches, as described above. In experiment. 2, the effect of an IDF source on the ability of the phytase enzyme to do its job was evaluated. A total of 480 pigs (6.80 kg BW), housed in 48 pens were fed one of 8 dietary treatments. Diets corresponded to a series of 4 corn-soybean-meal-based diets with 4 levels of added phytase (0, 109, 218 and 327 FTU/kg) to increase the standardized total tract digestible phosphorus (STTD P) and a second series of 4 diets, based on corn, soybean meal and 20% bran (source of IDF) with the same 4 levels of added phytase. All diets were deficient in available phosphorus content. Growth performance, bone ash and total tract digestibility of DM, NDF, and ADF were determined. Results from experiment 1 showed that the addition of IDF (in the form of added DDGS) decreased ileal digestibility of dry matter, energy, starch and the fiber components (except for ADF). Ileal digestibility of fat decreased with the addition of IDF when nutrients were balanced, but increased when nutrients were allowed to float. The addition of IDF decreased total tract digestibility of DM, GE, and the fiber components except for SDF that increased and ADF that was not affected. As for ileal digestibility, total tract digestibility of fat decreased with the addition of IDF when nutrients were balanced, but increased when nutrients were allowed to float. Estimates of ileal digestibility of IDF, TDF and NDF were lower when nutrients were maintained constant compared to when nutrients were allowed to float. Estimates of total tract digestibility of DM, IDF, TDF and NDF were lower when nutrients were maintained constant compared with when nutrients were allowed to float. Results from experiment 2 showed that as expected, phytase increased rate and efficiency of gain as well as the degree of bone mineralization; it did not influence feed intake, or the digestibility of dry matter, neutral detergent fiber or acid detergent fiber. On the other hand, the addition of insoluble fiber (in the form of 20% corn bran) decreased rate and efficiency of gain, the latter in a modest amount. Insoluble fiber did not affect feed intake or bone mineralization. In conclusion, the addition of insoluble dietary fiber decreased the use of most of the diet components along the intestinal tract. However, adding insoluble dietary fiber increased the fermentation of soluble dietary fiber in the large intestine. The effect of IDF on the digestibility of fiber is greater when the constant ingredient approach is used compared to the constant nutrient approach. Additionally, the constant ingredient approach is an inconvenient method when fat digestibility is evaluated since fat level confounded the response to adding IDF. The addition of insoluble fiber did not affect the ability of phytase to improve growth performance and bone mineralization. Therefore, phytase is equally effective at releasing phytate-bound phosphorus in low or higher fiber diets.
Key findings:

  • Insoluble dietary fiber, such as that found in corn and corn co-products, decreases the digestibility of other dietary components.
  • However, in addition to the decrease in digestibility mentioned above, insoluble dietary fiber promoted fermentation of soluble dietary fiber in the large intestine. Thus, the effects of insoluble dietary fiber are not all bad.
  • Balancing the level of fat in digestibility studies is recommended; otherwise, fat levels may increase or decrease and this may result in incorrect conclusions about the digestibility of fat.
  • Phytase can be used with confidence in both high and low fiber diets, as it works effectively in both cases.