The Effect of Increasing Neutral Detergent Fiber Level through Different Fiber Feed Ingredients throughout the Gestation of Sows

Simple Summary To reduce feed costs, the exploration of feed resources is currently the main research direction. In the past, fiber feed was generally regarded as anti-nutritional factors. Recently, fiber has received special attention due to its many beneficial effects. Therefore, this study selected five common household and production wastes as fiber sources and added them to the diet of pregnant sows to explore the impact on sows and piglets. The results of this study indicated that adding corn gluten feed (CG) significantly improved the digestibility of sows and body weight (BW) of piglets, which provide evidence and insight for the application of corn gluten feed in gestation sow diets. Abstract The purpose of this study was to determine the effect of increasing dietary neutral detergent fiber (NDF) levels on pregnant sows, and to select the best feed ingredients based on reproductive performance, plasma biochemistry parameters, colostrum and milk composition, and nutrient digestibility. Seventy-two multiparous sows were randomly allotted to six dietary treatment groups (n = 12). The feeding of chicory meal (CM), wheat bran (WB), corn gluten, and rice bran meal (RBM) increased the average weaning weight of piglets compared with the control (CON) group (p < 0.05). Supplementation with CG diet increased the sow BW, weight gain, and back fat thickness compared with WB and RBM on day 107 of gestation (p < 0.05). Furthermore, Supplementation with CG diet resulted in lower plasma urea nitrogen (PUN) and higher total protein (TP) concentrations in plasma (p < 0.05). Feeding CM diet and soybean curd residue (SCR) diet reduced the total protein and globulin, and supplementation with CM diet significantly increased the PUN (p < 0.05). The apparent total tract digestibility (ATTD) of crude protein (CP), crude fat (EE), calcium (Ca), phosphorus (P), neutral detergent fiber (NDF), and acid detergent fiber (ADF) were decreased following the addition of CM, WB, or SCR to the diets (p < 0.05). The ATTD of NDF and ADF were significantly increased in the CG group (p < 0.05). In conclusion, the feeding of CG diet to sows have an excellent effect.


Introduction
Diets with high neutral detergent fiber (NDF) levels are rarely used in growing and finishing pigs, which may reduce dietary energy density and digestibility. However, NDF contributes to intestinal peristalsis and maintenance of body health, which may be needed by sows [1]. According to animal welfare legislation in the European Union (council directive 2001/88/EC), all pregnant, dry sows and gilts have to be provided with sufficient quantities of bulky or high-fiber feed to alleviate feeding frustration during gestation and to prepare females for ad libitum feed supply during lactation [2]. The influence of high NDF level diets on sow has been increasingly addressed. NDF is considered to be a dietary ingredient resistant to mammalian enzyme degradation and cannot be hydrolyzed and (CM, n = 12), (4) basal diet with 17.6% soybean curd residue (SCR, n = 12), (5) basal diet with 27% corn gluten (CG, n = 12), and (6) basal diet with 46.5% rice bran meal (RBM, n = 12). The CON diet contained 8.4% NDF, and the experimental diets, which contained 16% NDF, were fed to sows from day 1 to day 107 during gestation. Sows were freely fed the CON diet from day 108 until weaning (day 21) during lactation. Sows were fed according to their daily nutrition requirements, which were based on the criterion of the NRC 2012 model [21]. The ingredients are shown in Table 1.  ------25  Wheat germ  ------100  Soybean oil  ------10  Paddy  ------80  Extruded soy flour  ------90  Limestone  10  11  10  9  10  10  15  Dicalcium phosphate  13  11  13  13  13  11 11 Choline chloride (50%) 3

Housing, Feeding, and Management
The diet was provided for sows twice a day. The sows were checked daily to judge whether they had returned to estrus using a mature boar. Pregnant sows were identified ultrasonically on the 20th day after insemination. Sows were eliminated from the experiment if they were not pregnant. Finally, a total of 72 pregnant sows were used. These pregnant sows were randomly divided into six groups corresponding to the six diets. Sows were rebred under the same conditions. Afterwards, the sows were transferred to the farrowing house. The farrowing room was environmentally controlled, and the average ambient daily temperature was approximately 18-20 • C. The sows were fed 1.8 kg/day of the gestation diet from day 1 to 30, 2.4 kg/d from day 31 to 90, and 3.0 kg/d from day 91 to the day of the expected date of confinement. On the day of farrowing, the sows were not fed. The feed allowance of the sows was raised to 3.0 kg on day 1. Thereafter, this amount was increased daily by 1.0 kg until the maximum was reached. The sows were fed three times a day, and the feed refusals were weighed daily; the actual feed intake for each sow was recorded.
On the third day after birth, the piglets received an iron injection (Iron Dextran, Jiangxi Chuangdao Animal Health Co., Ltd., Nanchang, China). Commercial creep feed (15.8 MJ of metabolizable energy/kg, 210.0 g CP/kg, and 15.6 g lysine/kg) was offered to the piglets 7 days after birth. The intake of the creep feed was not recorded.

Diet Collection and Analyses
Samples of the feed were obtained from each dietary treatment. The diets were analyzed for crude protein (CP) (Kjeldahl method), NDF (Van Soest method), acid detergent fiber (ADF) (Van Soest method), crude fiber (CF) (Van Soest method), crude fat (EE) (Soxhlet extraction), Ca (titration method), P (colorimetry method) [22]. The composition analysis of fiber sources in the diet are shown in Table 2.

Sow and Litter Performance
The sows were weighed three times: on the day of insemination, on day 107 of gestation and on day 21 of lactation (weaning). Within the first 12 h after birth, the numbers of total piglets, born alive and dead, were recorded, and the piglets were tattooed for identification. The piglets were individually weighed at farrowing and weaning. The number of piglets that died during lactation was recorded. The survival rate at birth per treatment was analyzed after calculating the survival rate at birth per litter. The survival rate at weaning per treatment was analyzed after calculating the survival rate at weaning per litter.

Milk Collection and Analyses
Milk was collected on days 0 and 14 from each of the sows post-farrowing. Milk (30 mL) was collected with a 50 mL threaded pipe and hands from all of the functional mammary glands of the sows. Colostrum was collected within 24 h of when the piglets were born. Milk was collected 1 h before the sows were fed, and the sows were injected with 1 mL of oxytocin to stimulate milk release before collection on day 14 of parturition. Milk was divided into vacutainer tubes that were frozen at −20 • C until analysis.
The colostrum and milk samples were analyzed for lactose, protein, fat, and total solids with a fully automatic milk analyzer (Milko ScanTM FT + Analyzer, Foss).

Fecal Samples
Fecal samples were collected one time daily from day 100 to 103 of gestation and day 14 to 16 of lactation to determine the fecal nutrient digestibility in 12 sows per treatment group. Individual fecal samples from each sow at these days were mixed together, homogenized with 10 mL HCl (purity of 10%) per 100 g fecal sample, and then stored at −20 • C, dried at 60 • C and analyzed. Fecal dried samples were frozen at −20 • C until analysis.
The fecal samples were analyzed for NDF, ADF, EE, Ca, and P to calculate the apparent total tract digestibility (ATTD) of the NDF, ADF, EE, Ca, and P. The ATTD of the nutrients was calculated according to the methods of Gerritsen et al. [23].

Blood Sample Collection and Analyses
Heparin tubes were used to obtain blood samples (10 mL) from an ear vein of 12 sows per treatment on day 100 of gestation and day 14 of lactation. After the blood samples were centrifuged at 3000× g for 10 min, the plasma was partially transferred to a 1.5-mL Eppendorf (EP) tube and immediately stored at −20 • C until analysis.

Statistical Analyses
Statistical analyses were performed using SPSS 20.0 (IBM-SPSS Inc., Chicago, IL, USA). Data was subject to a variance equality test of Levenne method. Then, one-way analysis of variance (ANOVA) was performed, and multiple comparisons were carried out using the Tukey HSD. Each pig was considered to be a statistical unit, results are presented as mean values. The data were expressed as the means ± SD (standard deviation), and a value of p < 0.05 was considered statistically significant. The null hypothesis of the experiment is that the diets of the CON, WB, CM, SCR, CG, and RBM groups have similar effects on pregnant sows and offspring, and the alternative hypothesis was that dietary effects of CON, WB, CM, SCR, CG, and RBM groups on pregnant sows and their offspring were different. When p < 0.05, the null hypothesis was rejected, when p > 0.05, the null hypothesis could fail to be rejected.

Sow Performance
As shown in Table 3, compared with WB, SCR and RBM groups, treatment with CG group significantly increased sow BW on day 107 of gestation (p < 0.05), and no significant difference compared with the CON and CM groups was observed (p > 0.05). The CG diet supplement increased weight gain and back fat thickness compared with WB and RBM on day 107 of gestation (p < 0.05), with no significant differences among the CON, CM, and SCR groups (p > 0.05). Furthermore, the group fed with WB of sow BW on day 21 of weaning, sow gestation BW change, back fat thickness on day 107 of gestation and back fat gain in gestation was lower compared with the CON groups (p < 0.05). Changes in sow weight and back fat thickness during lactation did not differ among the treatments (p > 0.05).

Litter Performance
As shown in Table 4, Feeding CM, WB, CG, and RBM diets during gestation increased the piglet average weaning weight compared with the CON groups (p < 0.05), but no difference was observed in litter performance among the treatments (p > 0.05), including the total piglets born, piglets born alive, survival rate at birth, piglets at weaning, survival rate at weaning, average birth weight, and average daily gain (ADG).

Serum Biochemical Indexes
As shown in Table 5, compared with those in the CON, CM, SCR, and WB groups, sows supplemented with CG diet had a lower PUN concentration in plasma on day 100 of gestation (p < 0.05). In addition, supplementation with CM diet produced significantly higher concentrations of PUN in plasma compared with those in the CON, CG, and RBM groups (p < 0.05). The concentrations of TP in plasma on day 107 of gestation were significantly increased by the treatment supplemented with CG diet (p < 0.01) compared with those in the CON, CM, and SCR diets. Feeding CM and SCR diets resulted in a lower TP and globulin concentration in plasma compared with the CON groups (p < 0.01). Sows that were fed WB diet had higher concentrations of globulin on day 100 of gestation compared with the sows in the CON, CM, and SCR groups (p < 0.01). Supplementation with CG and WB resulted in higher concentrations of TP in plasma than that with SCR on day 14 of lactation (p < 0.01). Furthermore, supplementation with CM and SCR diets led to lower concentrations of globulin than in those in the CON diet, WB, and CG groups (p < 0.01). In addition, no difference was observed in the concentrations of GLU, TG, ALT, AST, AKP, and ALB during gestation and the concentrations of GLU, PUN, TG, ALT, AST, AKP, and ALB during lactation among the treatments (p > 0.05).

Composition of Colostrum and Milk
The effects of diet on the colostrum and milk of sows are shown in Table 6. There were no prominent changes in the colostrum and milk composition of the sows (p > 0.05).

Diet Nutrient Digestibility
As shown in Table 7, the ATTD of CP was increased when CON, SCR, and CG was added to the diets compared with that in the CM, WB, and RBM groups during lactation (p < 0.01). In addition, the ATTD of EE, Ca, and P was increased when CON and SCR was added to the diets compared with the other groups (p < 0.01). Moreover, the ATTD of the CM group EE and RBM group CP, Ca was lower than the other groups (p < 0.01). The ATTD of NDF, ADF, and CF was increased (p < 0.01) in the CG groups compared with CON groups (p < 0.01). For NDF and CF, the sows that were fed CM and RBM diets had lower ATTD compared with CON group during lactation (p < 0.01), and supplementation with CM diet significantly decreased the ATTD of ADF compared with the CON group (p < 0.01).

Sow Performance
The study by Peet-Schwering et al. [25] showed that the increase in BW and back fat during gestation in the high-fiber diet group was significantly lower than in the control group, and weight and back fat loss during lactation were increased compared to that in the control group, which is consistent with the results of this experiment. High-fiber diets provide less net energy than control diets, which encourages sows to accumulate less back fat during gestation. The addition of soybean husks to the diet to increase crude fiber levels results in lower average back fat thickness [26]. In an analysis using inulin as the source of crude fiber, there was no difference in the weight of sows between the test group and control group, and the addition of inulin significantly reduced back fat thickness [27]. Our research results show that supplemented with CM and CG diet have a certain inhibitory effect on sow weight loss during gestation and back fat loss during lactation compared with supplemented high-fiber diets. Studies have shown that sow back fat thickness could reflect nutrient retention and potential reproductive efficiency [28]. However, the degree of fiber utilization by sows is also related to the physical and chemical properties of dietary fiber, the degree of lignification, solubility, time through the intestine, and degree of fermentation, among which solubility is an important factor affecting the utilization of dietary fiber in monogastric animals. The physical and chemical properties of dietary fiber will change the viscosity of intestinal chyme, the speed of intestinal passage, and the absorption of nutrients, thereby reducing the efficiency of nutrient absorption [29]. This may provide an explanation for the lower performance of sows in the WB and RBM groups.

Litter Performance
No significant differences were observed in the total piglets born, piglets born alive and weaning, survival rate at birth and weaning, average birth weight and ADG of piglets. The results of this experiment suggested that there were no negative effects of CM, SCR, WB, CG, and RBM diets supplementation during gestation on litter performance. Quesnel et al. [2] reported that sows fed with different fiber levels, while maintaining the same nutritional level, showed no difference in piglet birth weight. In other studies, high fiber intake during gestation had no effect on the number of live births and birth weight [19,30]. Under the same conditions of dietary energy intake, increasing the level of dietary fiber during gestation did not improve sow litter performance [31][32][33]. The above results are similar to our findings. Supplementation of high-fiber diets in the sow during gestation does not affect litter performance.
Some studies found that sows fed high-fiber diets during gestation had an increase average weaning weight of piglets [34,35]. This result is consistent with our experimental findings, in which fiber supplementation during gestation significantly increased the average weaned piglet weight compared with the CON group. In sow studies, prolactin is a key hormone for the initiation and maintenance of milk production, and a high-fiber intake tends to increase prolactin concentrations during gestation [36]. Studies have shown that diets with high fiber levels have been very successfully used to regulate sow milk production [37][38][39]. Collectively, data from the above references suggest that the level and source of fiber affect the prolactin response in pregnant sows. Because the growth of the suckling pigs was almost entirely dependent on the milk of the sows, in this study, the greater average weaning weight gain of piglets may be related to the treatment effect of sows fed a fibrous diet. Feeding high-fiber diets for sows may indirectly affect the weaning weight of piglets by regulating milk secretion. A greater number of sows and litters are needed to explore whether feeding sows a high-fiber diet during gestation can improve colostrum production or composition in further research.

Serum Biochemical Indexes
Measuring serum biochemical parameters of farm animals can provide important information regarding health and metabolism. The change in serum PUN concentration can reflect the whole body status of protein and amino acid metabolism and utilization in animals, when the amino acid metabolism in the body is strong, the serum urea nitrogen concentration will decrease [40,41]. It is known that serum globulin is synthesized and secreted by the immune organs of the animal, which is closely related to the body's immunity. Changes in total protein and albumin content can reflect the metabolism and absorption of proteins by the animal body, of which ALB is the most important indicator of protein metabolism [42]. The content of TP and globulin in the CM group and SCR groups was significantly lower than in other groups, and the PUN content was significantly increased. This result indicates that the addition of CM and SCR diets during gestation will reduce protein metabolism and reduce protein utilization efficiency, and it may reduce animal immunity. In addition, our experimental results showing a decreased PUN level and increased TP content in the CG group, indicating that CG diet increases nitrogen deposition in sows and is beneficial to the utilization of protein and amino acids in the diet. The different effects of different fiber sources on TP, PUN, and ALB in the experiment may be related to the different sources of dietary fiber affecting the nitrogen utilization efficiency of the organism.
When fiber reaches the hindgut, it is used by gut microbes as a substrate for fermentation [43]. The short-chain fatty acids produced by fermentation are absorbed into the blood through the intestinal epithelium, and acetic acid can enter the liver with the blood circulation to participate in the body's metabolism; propionic acid participates in gluconeogenesis, and butyric acid is mainly directly used by intestinal epithelial cells [44]. When short-chain fatty acids are absorbed by the intestine, glucose absorption in the intestine is reduced, and glucose levels in the blood can be stabilized [45]. The body will maintain a stable blood glucose concentration for a long time after the animal consumes dietary fiber [46]. This shows that the body of the animal has a stronger ability to regulate GLU, which may be the reason why GLU supplemented with fiber feed during gestation and lactation has not changed.
Serum AST and ALT have been proposed as indicators of depressed liver function. The increased activity of AST and ALT in serum suggests liver cell damage and leaching of these enzymes into the blood [47], and an excessive accumulation of these enzymes in the serum often prefigures liver injury [48,49]. Under normal conditions, AST and ALT levels are maintained at their highest levels in cardiomyocytes and liver, while blood levels are low. Once the myocardium and liver of the body are damaged, a large amount of AST and ALT in the liver are released into the blood, resulting in an increase in the activity of both enzymes in this compartment [50]. In this study, no changes in ALT and AST levels were observed in the group supplemented with the five high-fiber diets during gestation and lactation. We speculate that supplementation with fiber feed during gestation will not cause free radical damage to the liver.

Composition of Colostrum and Milk
The quality of milk is mainly reflected in the growth performance of piglets. Colostrum can increase piglet immunity and can also provide piglets with a variety of other nutrients, which is beneficial for their development. Thus, it is very important for piglets to consume more milk as early as possible. The development of the mammary gland during gestation directly affects milk production during lactation. Some research results have shown that high energy intake affects the development of sow mammary glands; in particular, increased intake of energy after 75 days of gestation is not conducive to mammary gland development [51]. Therefore, high-fiber diets supplementation during late gestation will not affect colostrum production, as there is a sufficient energy reserve in the sow to satisfy milk production. Studies have found that reducing energy intake during gestation can increase the fat content of colostrum [52]. The addition of fiber to the diet can reduce the energy concentration in the feed and avoid the high energy intake that affects milk secretion. In the current study, no significant differences were observed in the colostrum and milk fat, lactose, protein, and total solid content in the six treatment groups. However, colostrum milk fat showed a tendency to increase compared with that in the control group. Kirchgessner et al. [53] found that high-fiber diets promote milk fat content in sow colostrum and regular milk. However, some studies have shown that a high-fiber diet increases milk production compared with the control group, with no significant difference in milk composition [2,54], milk protein content generally is not affected by diet [55]. It was also reported that dietary fiber level during gestation did not affect total solids and lactose content of colostrum or milk [56]. These inconsistent results may be explained by different sources of fiber feed and different physical and chemical properties. The effects of different sources of fiber on sow milk composition have been less reported, necessitating further experiments for confirmation.

Diet Nutrient Digestibility
There are many reports on the effects of dietary fiber on animal nutrient digestibility, but most studies have suggested that dietary fiber reduces dietary nutrient digestibility. In this study, supplementation with CG diet significantly increased the ATTD of the NDF and ADF compared with those in the other groups. The ATTD of CP, EE, Ca, and P of RBM, CM, and WB groups were significantly reduced compared with those in the control group. Dietary fiber can promote the flow of dry matter and reduce energy utilization in the ileum and feces and the digestibility of starch, crude protein, lipids, and energy [57][58][59], With increases in dietary fiber of diet, the apparent digestibility of crude protein, crude fat and energy in feces is reduced [60]. Many studies have shown that dietary fiber reduces the residence time of chyme in the digestive tract and increases the speed of circulation, thereby reducing the digestibility of almost all nutrients [61,62].
The research by Girard et al. [63] showed that sow utilization of calcium, phosphorus, copper, and zinc decreased after feeding with a high-fiber diets (wheat bran/corn cob). Due to the strong cation exchange capacity of dietary fiber, it can absorb mineral elements. Some people think that this is the main reason for the reduced utilization of mineral elements. In addition, when chyme passes through the small intestine, the hydration of soluble fiber and water increases its viscosity, which promotes the formation of an immobile water layer, hinders contact between the chyme and digestive enzymes, and reduces the digestibility of nutrients [64]. In addition, no differences in the ATTD of the nutrients were observed between treatments during lactation because the lactating sows were fed the same diet. The results of this experiment revealed no negative effects on nutrient digestibility in gestating sows fed diets supplemented with CG and SCR, which represent a good source of fiber in sow feed.

Conclusions
This study has demonstrated that compared with the 8.43% NDF level in the control group, with the exception of the WB diet, there were no significant differences in sow performance in the experimental groups increased the NDF level to 16%. The use of CM, WB, CG, and RBM as a fiber source in feed had the potential to improve the average weaning weight of piglets. Furthermore, sows supplemented with CG diet had a lower PUN and a higher TP plasma concentration on day 100 of gestation, and the digestibility of NDF and ADF was higher in the CG diet.