1. Introduction
Extrusion has been widely used in feed processing for over 70 years [
1]. It can change the physio-chemical properties of feed ingredients by applying constant moisture, pressure, and high temperature with the combination of shear force. Thus, extruded ingredients have greater nutrient utilization and reduced anti-nutritional factors [
2].
Full-fat rice bran (FFRB), an important by-product of the rice milling industry, is commonly used as an alternative to energy feed material in animal diets because it contains 15–22% oil. However, the presence of anti-nutritional factors (ANFs) such as phytic acid hinders its commercial promotion and widespread application [
3,
4]. Corn distillers dried grains with solubles (DDGS) is a by-product of the fuel ethanol industry that can provide energy and protein for animals. However, the effects of extrusion on nutritive values of FFRB and DDGS in growing pigs is still poorly characterized. DDGS and FFRB are increasingly used to replace SBM in pig diets. There are many processing technologies to produce DDGS and FFRB which contain varying chemical components. Therefore, it is important to evaluate energy contents and amino acid digestibility of DDGS and FFRB produced in China.
Extrusion increased the ATTD of starch and energy of faba bean diets and decreased the feed wastage, possibly due to the starch gelatinization after extrusion [
5,
6,
7,
8]. In addition, pigs fed extruded peas showed greater digestibility of starch and amino acids (AA) [
9] despite the existence of various anti-nutritional factors (ANFs) in peas [
10]. Extrusion decreased the total glucosinolate content and fiber fractions (NDF, ADF) in canola meal with screw speeds of 350 rpm and 450 rpm, respectively. Increasing the screw speed of the extruder to 350 rpm seems to reduce the total content of glucosinolates in canola meal (CM), and by increasing the screw speed to 450 rpm, the NDF and ADF contents decreased in CM [
11]. Moreover, extrusion could improve the storability of feedstuffs [
12]. When using extruded FFRB in broiler diets, significantly increased fat digestibility was observed compared with non-extruded FFRB [
13].
Therefore, the objective of this study was to determine energy contents, the ATTD of energy and other nutrients, and the standardized ileal digestibility (SID) of amino acid (AA) in pigs fed either corn DDGS or FFRB with or without extrusion.
4. Discussion
Body weight was randomly selected. In addition, many publications have reported that evaluated precision of energy values and amino acid digestibility would improve as inclusion levels of tested ingredients increased; however, pig performance would be suppressed if dietary inclusion levels of FFRB and DDGS are too high because FFRB and DDGS contain high levels of fiber components. In addition, all diets were semi-pure diets in Exp. 2; dietary fiber and protein contents were provided by FFRB and DDGS. Therefore, the inclusion level of 40% FFRB and DDGS in Exp. 2, not 29.06%, was used in order to ensure reasonable nutrient levels for pigs. Of course, all results in this study are shown under conditions of specific pig body weight and inclusion levels of tested ingredients.
FFRB and DDGS are important by-products of the rice and corn industry, and have the potential to be used as alternatives to energy feed ingredients in animal diets [
3,
4]. Different digestibility of energy and nutrients may be caused by different chemical compositions and processing technologies [
17,
18]. In the extrusion process, the utilization rate of nutrients is improved through high temperature and pressure, which leads to the rupture and expansion of intact cells in the material in response to the sudden drop in pressure and temperature when leaving the extruder [
19]. Extrusion has no effects on the content of nutrients and GE of both FFRB and DDGS, as previously reported by Huang (2021) [
20]. The chemical compositions of DDGS and FFRB herein were consistent with previous literature results [
20,
21].
Extrusion increased starch digestibility and energy utilization by the pigs, which is one of the reasons for the increased ME [
9]. Ether extract was the best predictor and positively correlated with the DE and ME. Higher EE content in FFRB resulted in higher DE content [
22]. Similarly, extrusion significantly increased the DE and ME of DDGS and FFRB in our study. In pig feed formula, energy content is the largest cost [
23]. In the present study, extrusion increased energy digestibility, and the DE content increased from 13.84 to 15.86 MJ/kg and ME from 13.36 to 15.24 MJ/kg. This observation was consistent with the published data that extrusion leads to the increase in ATTD of GE in field peas [
24]. The increased ATTD of GE observed for extrusion diets was a result of increased AID of AA and starch [
25]. The increased nutrient digestibility is probably caused by the cleavage of non-starch polysaccharides into smaller fragments, thereby substantially reducing their anti-nutritive effects [
26]. Extrusion could also improve the digestibility of specific nutrients in DDGS, such as protein, by changing the structure, function, or chemical properties of DDGS [
27]. The improvement of fiber digestibility may be that extrusion contributes to the redistribution of insoluble and soluble fiber components. In our study, extrusion improved DE and ME content and ATTD of GE, DM, and OM; moreover, extrusion tended to increase ATTD of CP and fiber fractions.
In previous studies, the results for the effects of extrusion on CP digestibility were not consistent in swine nutrition. Extrusion did not affect the AID of CP in FFRB fed to pigs [
28]. Similarly, extrusion did not affect the ATTD and AID of CP and indispensable AA in corn fed to 20 kg pigs [
29]. In our study, the extruded diet increased the AID and SID of most AA compared with the non-extruded diet, but it had no significant effects on AID or SID of CP, Lys, Trp, and Cys. The increase in dietary EE content delayed gastric emptying [
30], and slower gastric emptying may lead to slower evacuation passage, thus providing a longer time for peptide and AA digestion and absorption [
31]. Therefore, these results showed that extrusion is more suitable for FFRB processing than DDGS in pig diets. The suggested explanation is that FFRB has more fat and therefore a longer passage time, which made possible a better digestibility of amino acids. In this experiment, the AID levels of the essential AA of FFRB were slightly lower than those reported by Huang [
27]. This difference may be due to different sources of FFRB or different extrusion conditions. At present, there are few studies focusing on the effects of extrusion on FFRB, which has less AID of AA compared with defatted rice bran [
20]. This phenomenon has been improved after extrusion. The AID levels of CP in DDGS and extruded DDGS were approximately 55% and 60%, similar to previous reports [
27].
The addition of oil in the diet of growing pigs not only increased the AID of AA, but also increased SID [
2]. Extrusion technology can improve the effect of high fiber by-product diets on the growth performance of pigs [
9]. Perhaps this result was caused by greater SID of Lys and most of AA in the FFRB after extrusion in the present study. It is reported that under low moisture content, Lys may be lost due to high temperature extrusion or shear stress [
32]. Extrusion significantly increased the AID of nutritional components in DDGS diets of corn and wheat [
17,
33] in pigs, especially SID of AA. Denaturing of proteins by heat temperature may explain greater AID and SID of AA in the present study. Depending on the AA source, heat denatures proteins at a temperature range of 25–100 °C [
34]. In addition, extrusion treatment will break the fat wall of raw materials and degrade the fat to form glycerol and free fatty acids, and free fatty acids will combine with starch and protein to form a complex to reduce the digestibility of nutrients. As the extrusion temperature rises, the synthesis of this complex will increase. When the temperature of the inner wall of the extrusion chamber exceeds 100 °C, the number of composites decreases significantly [
35]. Studies have shown that the improvement of nutrient mass digestibility of raw materials by single screw extrusion is better than that by twin screw extrusion [
24]. The greater shear force of the single screw extrusion process can destroy the level and structure of fat [
36], indicating that it is important to know the optimal expansion equipment and conditions to achieve the maximum digestibility of fat. In this experiment, extrusion led to the decrease in digestibility of some amino acids, perhaps due to the Maillard reaction between amino acids and reducing sugar, which renders some amino acids invalid [
37]. In addition, more fibers combine with lysine, which hinders the opportunity of binding with protease and reduces the digestibility of amino acids [
31]. Lewis found that with the increase in water content, the in vitro digestibility of CP first decreased and then increased. The main factors affecting the CP digestibility of extruded feed were water content and extrusion temperature, followed by screw speed. The CP digestibility could be significantly increased by increasing feeding speed in corn germ extrusion [
38]. However, the results from the present study indicate that extrusion effectively enhances energy and nutrient digestibility, and is beneficial for improving AID and SID of AA.
In conclusion, extrusion of FFRB and DDGS improved the digestibility of GE, OM, CP, and some AA, and thereby increased the content of DE and ME. Extrusion technologies should be considered to optimize feed utilization in pigs.