Soy hulls is a by-product of producing dehulled or defatted soybean meal and consists of the outer covering of the soybean. Soy hulls contain about 13% crude protein (CP), 35% crude fiber (CF), abundant mineral (Ca, Fe, and P), and group B vitamin content than in other barley feeds . Chee et al.  reported that the CF content of soy hulls (32.5%) was significantly higher than that of wheat bran (8.7%). In addition, soy hulls are a great feedstuff because they have a relatively high digestibility due to their low lignin content compared to other grain by-products or plant-based feed ingredients .
Fibrous feedstuffs, such as soy hulls are used at appropriate levels for weaning and finishing pigs [4–6]. Fibrous diets have been reported to help improve digestibility by altering the shape of the digestive tract in weaning piglets . Many studies have been conducted in the pig finishing period to analyze the growth performance, nutrient digestibility, and meat quality produced by feeding fiber feed to reduce the cost of feed [8,9]. However, there is an insufficient number of studies on the effects of soy hulls supplementation on nutrient digestibility in growing pigs. In addition, many previous studies on fibrous feed using barley silage, poorly soluble dietary fiber, and straw exist [8–11] and there is a lack of research on soy hulls in pigs. Therefore, we conducted this study to determine the effect of replacing corn with soy hulls on nutrient digestibility of growing pigs.
MATERIALS AND METHODS
The protocol for the experiment was approved by the Institutional Animal Care and Use Committee of Chungbuk National University, Cheongju, Korea.
A total of three crossbred (Duroc × Landrace × Yorkshire) barrows were allotted to a 3 × 3 Latin square design. The pigs (average initial body weight of 36.9 ± 4.0 kg) were individually placed in 1.2 m × 0.7 m × 0.96 m stainless steel metabolism cages in an environmental controlled room (24 ± 2°C).
Table 1 shows the nutrient contents of the soy hulls used in this experiment. The dietary treatments were the control (CON) basal diet (corn-soybean meal based diet), whereas in the test diets, soy hulls were included at 5% (H1), 10% (H2), gradually replacing corn (Table 2). The daily feed allowance was adjusted to 2.7 times the maintenance requirement for digestible energy (DE) (2.7 × 110 kcal of DE/kg BW0.75) . The allowance was allotted into two equal parts and fed at 08:30 and 17:30 h. The diets were mixed with water in a ratio of 1:1 (Wt/Wt) before feeding. Water was available ad libitum through a drinking nipple. The pigs were individually weighed at the beginning of each period and the amount of feed supplied and any feed residual quantity for each period was recorded. Each experimental period consisted of a 4-day adaptation period followed by a 3-day collection period to collect feces and urine. The fecal and urine were collected by total collection method. Feces were immediately collected when the feces appeared in the metabolism cages, kept in plastic bags and stored in a freezer at −20°C until analyzed.
|Digestible energy (MJ/kg)||1.47|
1) Provided per kg of diet: 12.5 mg manganese, 179 mg zinc, 140 mg copper, 0.5 mg iodine, and 0.4 mg selenium.
2) Provided per kg of diet: 20,000 IU of vitamin A; 4,000 IU of vitamin D3; 80 IU of vitamin E; 16 mg of vitamin K3; 4 mg of thiamine; 20 mg of riboflavin; 6 mg of pyridoxine; 0.08 mg of vitamin B12; 120 mg of niacin; 50 mg of calcium pantothenate; 2 mg of folic acid; and 0.08 mg of biotin.
Urine was collected into 18.9L urine collection plastic buckets that were placed under the metabolism cages and collected once a day into 50 mL of 6 mol/L H2SO4 was added. The collected urine was weighed and stored at −20°C. The collection of feces and urine were conducted according to the methods described by Song et al. . Fecal samples were dried at 70°C for 72 h in a forced air oven and ground through a 1-mm screen, and thoroughly mixed before a subsample was collected for chemical analysis. Diets and feces were analyzed for dry matter (DM), CP, CF . The gross energy of diets, feces, and urine were analyzed using an adiabatic oxygen bomb calorimeter (Parr Instruments, Moline, IL). The content of nitrogen in the urine was also analyzed .
RESULTS AND DISCUSSION
In daily balance of nutrient, the intake of crude fiber was different among treatments (p < 0.05; Table 3). Pigs fed with H2 diet excreted higher concentrations of DM, and CF than pigs fed with H1 diets and basal diet (p < 0.05). Pigs fed with H1 and H2 diets excreted higher concentrations of CP than pigs fed with CON diet (p < 0.05).
The ATTD of crude fiber from H1 and H2 diets were higher than CON diets (p < 0.05; Table 4). Pigs fed with H2 diet were significantly lower (p < 0.05) the ATTD of CP than the pigs fed with CON and H1 diets.
Feeding fibrous diets to pigs has produced a variety of effects. The most prominent effect of dietary fiber supplementation for pigs is changes of the gastrointestinal environment . The addition of fibrous feed to pig diets increases the secretion of swine intestinal fluid . It also reduces the gastric emptying time and increases the digestive viscosity [17,18]. Fibrous feed increases the amount of digestion in the stomach, decreases the rate of digestive movement, and increases satiety . Stanogias & Pearce  reported that supplementation of soy hulls and wheat bran to swine diets increased the amount of digested products compared to the basal diet, which resulted in a decrease in the digestive movement speed and a feeling of satiety for better feed efficiency and digestibility. Due to the effects of reduced gastric emptying time, increased digestion viscosity, and reduced digestion rate with fibrous feed supplementation, digestion becomes easier and digestibility increases.
In the present study, crude fiber digestibility of growing pigs increased by increasing soy hulls content in diets, but, CP digestibility decreased. In other studies, the inclusion of graded levels of soy hulls in animal diets decreased DM and GE digestibility by increasing dietary levels of soy hulls . Rainbird  and Rainbird & Low  reported that dietary supplementation of CF increases the amount of digestion time in the gastrointestinal tract, resulting in increased DM digestibility of growing pigs. Varel & Yen  reported that the CF digestibility was increased by increasing contents of CF in swine diets. However, they explained that it also adversely affects digestibility of nutrients. The increase in crude fiber digestibility was accompanied by a corresponding decrease in the digestibility of dry matter, organic matter, crude protein, and nitrogen free extract.
Digestive condition of the stomach and small intestine can be affected by dietary fiber before digestive products reach the large intestine. Lenis et al.  reported that dietary fiber in growing pigs diets may decrease in nutrient digestibility due to increased endogenous excretion and/or nutrient absorption. The amount of endogenous nitrogen loss knowledge to distinguish the apparent digestibility of protein and amino acid. Regardless of digestive conditions, true digestibility is an advantage over apparent digestibility in that it is a basic characteristic of feed . Many studies have reported that endogenous protein loss increased by increasing dietary fiber content [27–28, 30]. Mosenthin et al.  and Dilger et al.  suggested that the protein loss was due to a reduced resorption rate of endogenous nitrogen, not just an increase in nitrogen loss in the small intestine.
Results from the present study indicate that replacing corn with soy hulls in corn-soybean meal based diets can increase crude fiber digestibility of growing pigs. Moreover, there is no difference in CP digestibility between basal diet and 5% replacing corn with soy hulls in diet. It is concluded that 5% replacing corn with soy hulls may be a usable alternative to corn in growing pig diets.