Effects of Modified Corn Straw Dietary Fiber on Growth Performance, Nutrient Metabolism, Serum Biochemistry, Antioxidant Capacity, and Hepatic Lipid Deposition in Broiler

Simple Summary

The functions of dietary fiber in maintaining intestinal microbial balance and enhancing immune capacity have been increasingly elucidated. Corn straw as a by-product can be transformed into dietary fiber through modification processes. The study demonstrated that 0.5–1.5% modified corn straw dietary fiber (MCDF) can be recommended in broiler diets in later stage, as it had no negative effects on growth performance, nutrient metabolism, and effectively reduced blood lipids and glucose levels, enhanced serum antioxidant capacity, and ameliorated hepatic vacuolar degeneration and lipid deposition. The findings would provide a theoretical basis and data support for the application of MCDF in poultry production systems.

Abstract

This study aimed to evaluate the effects of modified corn straw dietary fiber (MCDF) on growth performance, nutrient metabolism, serum biochemistry, antioxidant capacity, intestinal morphology and hepatic lipid deposition in broilers. A total of 200 one-day-old Arbor Acres broilers were randomly allotted to four groups: the control group was fed basal diet, while three treatment groups supplemented with MCDF at 0.5%, 1%, and 1.5% of the basal diet, respectively. The results revealed that MCDF reduced ADG (p = 0.008) and increased feed/gain (p = 0.028) in the early stage, with no significant effects on ADG and feed/gain in the later or overall rearing periods (p > 0.05). In the early stage, 1% and 1.5% MCDF reduced ether extract metabolizability (p = 0.001), and 0.5–1.5% MCDF reduced phosphorus metabolizability (p < 0.001). Compared to the control group, 0.5–1.5% MCDF had no significant effects on nutrient metabolism in later stage and slaughter performance, but it reduced the 24 h pH of breast muscle and serum levels of LDL-C, HDL-C, TG, TC, and GLU (p < 0.05). The 1% MCDF decreased L* at 15 min and 24 h of leg muscle (p < 0.05). The 0.5% and 1% MCDF increased serum T-AOC levels and 1% MCDF increased CAT (p < 0.05), whereas 1.5% MCDF decreased SOD (p < 0.05). MCDF increased the villus height-to-crypt depth ratio by reducing crypt depth (p < 0.05) and improved the histomorphology of hepatocytes, accompanied by a reduction in the number of lipid droplets in the liver. Therefore, 0.5–1.5% MCDF can be recommended in broiler diets in the later stage, as it had no negative effects on growth performance, nutrient metabolism, and effectively reduced blood lipids and glucose levels, enhanced antioxidant capacity, and ameliorated hepatic vacuolar degeneration and lipid deposition.

1. Introduction

Dietary fiber (DF) has long been regarded as an antinutritional and nutrient-diluting component in poultry diets. The fiber content in the diet is strongly negatively correlated with growth performance, as well as the metabolizability of protein and fat in poultry [1]. Therefore, in earlier poultry production practices, DF was kept to a minimum and rarely supplemented in diets. In general, dietary fiber supplementation at levels of 1.0–1.5% has been reported to exert beneficial effects on intestinal health and digestive processes in poultry [2]. In our previous studies, dietary supplementation with 0.5% and 1.5% modified corn straw fiber in laying hens during the later stage of peak egg production showed that the 0.5% inclusion level improved laying performance, enhanced intestinal morphology, and reduced hepatic lipid deposition [3]. However, the physiological roles of DF have been increasingly studied. It has been widely reported that DF is not digested by small intestinal enzymes, but rather undergoes microbial fermentation in the hindgut, producing short-chain fatty acids that help regulate the intestinal microbiota balance [4]. The incidence of intestinal diseases in poultry is increasing concomitant with the phased discontinuation of antibiotics [1]. Multiple studies have indicated that DF promotes gastrointestinal health and development by influencing villus morphology [5], altering gastrointestinal pH, length and weight, enhancing gastrointestinal motility, and reducing pathogen colonization in the gastrointestinal tract [6]. Thus, the moderate inclusion of DF in diets enhances poultry nutrition and intestinal health. However, the influence of DF on various parameters in poultry varies depending on its source, type and inclusion level [1]. Thus, it is necessary to determine the optimal type, form and concentration of DF to achieve the best performance and economic returns under commercial production conditions.

Corn straw, a by-product of maize cultivation, is rich in cellulose, hemicellulose and lignin [7]. It is produced in large quantities and is inexpensive, with an annual global output reaching approximately 1 billion tons [8], making it a high-quality source of DF. However, due to the unbalanced ratio of soluble dietary fiber (SDF) to insoluble dietary fiber (IDF) in corn straw, its physicochemical properties are relatively poor. Therefore, in previous work, corn straw was modified to increase the proportion of SDF and improve its physicochemical properties [7]. The dietary supplementation of steam-exploded straws led to an increased relative cecal length in Greylag geese, whereas their average daily feed intake (ADFI) and feed conversion ratio were both reduced [9]. Wang et al. (2021) processed corn straw into saccharified corn straw and used it to partially replace maize in broiler diets at different inclusion levels [10]. They found that replacing maize with 4% saccharified straw in the early stage and 4–12% in the later stage showed no significant negative effects on growth performance, nutrient metabolizability or serum antioxidant capacity in broilers. Therefore, this present research was conducted to apply the modified corn straw dietary fiber (MCDF) in poultry nutrition to evaluate its effects on growth performance, nutrient metabolizability, slaughter performance, serum parameters and intestinal and liver morphology in broilers. The findings would provide a theoretical basis and data support for the application of MCDF in poultry production systems.

2. Materials and Methods

2.1. Animal Ethics Statement

The Animal Ethics and Welfare Committee of Henan Agricultural University (HNND2024031205) approved all protocols, which were conducted strictly following the relevant guidelines.

2.2. MCDF Preparation

Aspergillus niger (GDMCC 3.576) was obtained from the Guangdong Microbial Culture Center (GDMCC, Guangzhou, China), and cellulase (347 FPU/g) was sourced from Xiasheng Industrial Group Co., Ltd. (Yinchuan, China). Corn straw, supplied by a farm (Shangqiu, China), was dried and crushed before use.

Corn straw was treated with 8% (w/w) NaOH solution at a solid-to-liquid ratio of 1:2 (g/mL) and subjected to autoclaving (LDZX-30-L, Shanghai Shenan Medical Instrument Factory, Shanghai, China) at 121 °C and 0.1 MPa for 40 min. After cooling to ambient temperature, 6% (v/v) hydrogen peroxide was added based on the liquid volume, and the reaction was carried out for 4 h. Subsequently, the pretreated corn straw was subjected to fermentation with cellulase (10 FPU/g) and Aspergillus niger (1 × 10⁷ CFU/g) at 30 °C for 5 d, followed by natural air drying. The pH of the fermentation system was 5.5 [7]. The composition of MCDF is listed in

Table 1. The composition of different corn straw dietary fiber (dry matter basis).

Composition	Ordinary Corn Straw Dietary Fiber	Modified Corn Straw Dietary Fiber
CP (%)	5.79	6.99
EE (%)	0.96	0.93
Ca (%)	0.56	0.78
P (%)	0.13	0.07
Cellulose (%)	35.12	23.78
Hemicellulose (%)	28.39	6.36
SDF (%)	2.64	17.15
IDF (%)	76.24	43.59
Reducing sugar (mg/g)	6.56	34.70

Note: EE: Ether extract. CP: Crude protein. Ca: Calcium. P: Phosphorus. SDF: Soluble dietary fiber. IDF: Insoluble dietary fiber.

.

2.3. Experimental Design

The broiler basal diet was prepared following the recommendations of the NRC (1994) [11]. The basal diet composition and nutritional levels are shown in

Table 2. The basal diet compositions and nutrient levels of broilers (%, air dry matter basis).

Composition	Early Stage	Later Stage
Corn	58.82	64.96
Soybean meal	32.59	27.58
Fish meal	2.00	1.00
Soybean oil	3.00	3.00
CaCO3	1.40	1.30
CMP	1.40	1.43
Methionine	0.14	0.08
Salt	0.35	0.35
Premix	0.30	0.30
Total	100	100
Nutrients
ME (MJ/Kg)	12.53	12.74
CP	22.76	20.00
Ca	1.06	0.93
TP	0.71	0.63
AP	0.45	0.40
Lys	1.18	1.00
Met	0.50	0.40
Met + Cys	0.91	0.73

Note: CMP: Calcium hydrogen phosphate. CP: Crude protein. Ca: Calcium. TP: Total phosphorus. AP: Available phosphorus. Lys: Lysine. Met: Methionine. Met + Cys: Methionine and Cystine. Nutrients were the experimental value, except for ME, Lys, Met, Met + Cys, AP. Lys, Met and Met + Cys represent their total amounts in the diets. The premix provides the following per kg of the diet: Cu 8 mg, Fe 100 mg, Zn 60 mg, Mn 80 mg, Ca 0.45 mg, Se 0.35 mg, VA 12,000 IU, VD₃ 4350 IU, VE 30 IU, VK₃ 3.9 mg, VB₁ 3 mg, VB₂ 2.5 mg, VB₆ 1.4 mg, VB₁₂ 0.03 mg, D-biotin 0.15 mg, folic acid 1.2 mg, niacinamide 33 mg, D-pantothenic acid 12 mg, 0.05% choline (50%), 0.02% phytase and 0.10% premixed carrier zeolite powder.

. Two hundred one-day-old Arbor Acres broilers with uniform initial body weight were randomly allocated to four experimental groups, with five replicates per group and ten birds in each replicate. The experiment lasted for 42 d, partitioned into two stages: the early stage (1–21 d) and the later stage (22–42 d).

Control group: Basal diet.

0.5% group: Basal diet supplemented with 0.5% MCDF.

1% group: Basal diet supplemented with 1% MCDF.

1.5% group: Basal diet supplemented with 1.5% MCDF.

2.4. Production Performance

The initial body weight of the chicks was recorded at 1 d of age. Broilers feed intake was recorded, and the associated throws were collected and recovered. Broilers fasting weight was measured at 22 d and 43 d of age. Average daily gain (ADG), ADFI and feed-to-gain ratio (F/G) were subsequently calculated for each replicate.

2.5. Nutrient Metabolizability

The total fecal collection method was employed to measure nutrient metabolizability. Feces were collected daily during the metabolic trial period and feathers, feed residues, and other impurities were removed before weighing. To preserve nitrogen content, samples were reacted with 10% sulfuric acid, followed by drying at 65 °C and grinding for later use. Crude protein and ether extract in feed and feces were determined by the Association of Official Analytical Chemists (AOAC, 1990) [12]. Calcium and phosphorus were measured using potassium permanganate ammonium molybdate method [13], respectively. The calculation formula is as follows [14]:

$$\begin{gathered} \mathrm{Nutrient} \mathrm{metabolizability} (\%) = \\ {\displaystyle \frac{\mathrm{Feed} \mathrm{intake} \left(\mathrm{g}\right)\times \mathrm{nutrient} \mathrm{in} \mathrm{feed} (\%)-\mathrm{feces} (\mathrm{g})\times \mathrm{nutrient} \mathrm{in} \mathrm{feces} (\%) }{\mathrm{Feed} \mathrm{intake} \left(\mathrm{g}\right)\times \mathrm{nutrient} \mathrm{in} \mathrm{feed} (\%) }} \end{gathered}$$

(1)

2.6. Slaughtering Performance

After blood collection, broilers were slaughtered to evaluate slaughter performance. The measured parameters included slaughter rate, half evisceration rate, full evisceration rate, breast muscle rate, leg muscle rate, bursal index, glandular stomach index, gizzard index, heart index, liver index and spleen index. Intestinal segments were removed, naturally straightened in a test tray, and the lengths of the duodenum, jejunum, ileum and cecum were measured with a soft ruler. The formulas are as follows:

$$\mathrm{Slaughter} \mathrm{rate} (\%) = \mathrm{slaughter} \mathrm{weight}/\mathrm{live} \mathrm{weight} \times 100$$

(2)

$$\mathrm{Full} \mathrm{evisceration} \mathrm{rate} (\%)=\mathrm{full} \mathrm{evisceration} \mathrm{weight}/\mathrm{live} \mathrm{weight} \times 100$$

(3)

$$\mathrm{Half} \mathrm{evisceration} \mathrm{rate} (\%)=\mathrm{half} \mathrm{evisceration} \mathrm{weight}/\mathrm{live} \mathrm{weight} \times 100$$

(4)

$$\mathrm{Leg} \mathrm{muscle} \mathrm{rate} (\%)=\mathrm{leg} \mathrm{muscle} \mathrm{weight}/\mathrm{full} \mathrm{evisceration} \mathrm{weight} \times 100$$

(5)

$$\mathrm{Breast} \mathrm{muscle} \mathrm{rate} (\%)=\mathrm{breast} \mathrm{muscle} \mathrm{weight}/\mathrm{full} \mathrm{evisceration} \mathrm{weight} \times 100$$

(6)

$$\mathrm{Organ} \mathrm{index} (\mathrm{g}/\mathrm{kg}, \mathrm{cm}/\mathrm{kg})=\mathrm{organ} \mathrm{weight} \left(\mathrm{g}\right) \mathrm{or} \mathrm{length} \left(\mathrm{cm}\right)/\mathrm{live} \mathrm{weight} \left(\mathrm{kg}\right)$$

(7)

2.7. Serum Indices

Blood samples were collected from the wing vein of broilers using disposable vacuum tubes without anticoagulant. The samples were subsequently centrifuged to separate the serum for further analyses (3000× g, 4 °C, 10 min). Serum biochemical, antioxidant, and immune parameters in broilers were analyzed with five replicates per treatment. The supernatant was collected and stored at −20 °C. Serum levels of alanine aminotransferase (ALT), alkaline phosphatase (ALP), aspartate aminotransferase (AST), triglyceride (TG), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C) and glucose (GLU) were measured using an automatic biochemical analyzer (Au5800, Beckman Coulter, California, CA, USA).

Serum antioxidants were measured using commercially available kits. Total Antioxidant Capacity (T-AOC, A015-2-1), Malondialdehyde (MDA, A003-1-2), Superoxide Dismutase (SOD, A001-3-2) and Glutathione Peroxidase (GSH-Px, A005-1-2) were determined using kits from Nanjing Jiancheng Bioengineering Institute (Nanjing, China), while Catalase (CAT, BC0205) was measured using a kit from Solarbio (Beijing, China).

The levels of serum immunoglobulin M (IgM, JL 13475-96T), immunoglobulin G (IgG, JL 10953-96T) and immunoglobulin A (IgA, JL13570-96T) were determined using enzyme-linked immunosorbent assay (ELISA) kits. The kits were purchased from Shanghai Jianglai Biotechnology Co., Ltd. (Shanghai, China), and the procedures were strictly followed according to the instructions provided with the kits. Absorbance was measured using a full-wavelength microplate reader (Epoch, BioTek Instruments, Vermont, VT, USA).

2.8. Meat Quality

Samples of the left breast and leg muscles were collected from broilers for evaluation of meat quality. Meat color lightness (L*), redness (a*) and yellowness (b*) and pH were measured at 15 min and 24 h after slaughter using an NS800 spectrophotometer and a PHBJ-260 portable pH meter (INESA Scientific Instrument Co., Ltd., Shanghai, China), respectively.

Two pieces (3 g each) of breast and leg muscles were collected after slaughter and weighed (W1). The samples were then stored in drip loss tubes at 4 °C for a duration of 24 h. After storage, the surface moisture was removed and the samples were reweighed (W2).

$$\mathrm{Drip} \mathrm{loss} (\%)=(\mathrm{W}1 - \mathrm{W}2)/\mathrm{W}1 \times 100$$

(8)

Breast and leg muscles were stored at 4 °C for 24 h and weighed (W1). The samples were subsequently boiled in 100 °C water for 30 min, suspended at room temperature for 20 min, and surface moisture was removed using filter paper before reweighing (W2).

$$\mathrm{Cooking} \mathrm{loss} (\%)=\mathrm{W}2/\mathrm{W}1 \times 100$$

(9)

A strip of breast and leg muscle (3 × 1 × 1 cm) was cut along the direction of muscle fibers and stored at 4 °C for 48 h. The sample was cooked in an 85 °C water for 30 min. A texture analyzer (C-LM3B, Tenovo International Co., Ltd., Beijing, China) was employed to measure the mechanical resistance of the samples to shear. Following a standardized protocol, each strip was subjected to three independent measurements where the blade traversed perpendicular to the muscle fiber direction. The final shear force was expressed as the average (N) of these replicates.

2.9. Histological Analysis of the Intestine and Liver

Samples from the mid-jejunum (2 cm) and the left lobe of the liver were uniformly collected and fixed in 4% paraformaldehyde. Following fixation with 4% formaldehyde, intestinal and hepatic tissue sections were subjected to hematoxylin and eosin (H&E) staining for histological analysis.

Oil Red O staining was performed according to the method described by Zhao et al. [15], and liver samples were consistently collected from the same site of the left lobe. For Oil Red O staining, fresh liver samples were immediately frozen and sectioned using a cryostat (CM1950, Leica Instruments Shanghai Co., Ltd., Shanghai, China). The frozen sections were mounted on glass slides and stained with Oil Red O to visualize neutral lipid accumulation. Hematoxylin was used as a nuclear counterstain. Villous height (VH, the tip to the villus-crypt junction) and crypt depth (CD, the base to the villus-crypt junction) were determined using CaseViewer software 2.4 (3DHISTECH Ltd., Budapest, Hungary). Oil Red O stained sections were quantitatively analyzed using ImageJ software 1.54 (National Institutes of Health, Bethesda, MD, USA) under identical threshold settings.

2.10. Statistical Analysis

The experimental data were analyzed using SPSS (Version 26, SPSS Inc., Chicago, IL, USA). Data were tested for normality using the Shapiro–Wilk test and for homogeneity of variance using Levene’s test. All data met the assumptions for parametric analysis. Differences among groups were analyzed by one-way ANOVA, followed by Tukey’s HSD test for multiple comparisons. Results are presented as mean with standard error of the mean (SEM). Statistical significance was set at p < 0.05.

3. Results

3.1. Growth Performance

As shown in

Table 3. Effects of different doses of modified corn straw dietary fiber on the growth performance of broilers.

Items	CON	0.5%	1%	1.5%	SEM	p Value
Early stage (1–21 d)
Initial weight (g/bird)	41.20	41.10	41.10	41.10	0.071	0.956
Final weight (g/bird)	655.20 a	628.00 ab	609.60 b	624.40 ab	5.302	0.009
ADG (g/d/bird)	29.24 a	27.95 ab	27.07 b	27.78 ab	0.251	0.008
ADFI (g/d/bird)	41.15 a	40.09 bc	39.72 c	40.77 ab	0.156	<0.001
Feed/gain	1.41 b	1.44 ab	1.47 a	1.47 a	0.009	0.028
Later stage (22–42 d)
Final weight (g/bird)	1585.60	1598.90	1568.22	1640.00	26.644	0.830
ADG (g/d/bird)	44.30	44.06	44.46	48.36	1.19	0.559
ADFI (g/d/bird)	89.15 b	88.86 b	89.05 b	93.15 a	0.446	<0.001
Feed/gain	2.03	2.07	2.01	1.94	0.543	0.879
1–42 d
ADG (g/d/bird)	36.77	37.09	36.36	38.07	0.635	0.830
ADFI (g/d/bird)	65.15	66.48	64.82	66.96	0.412	0.199
Feed/gain	1.78	1.81	1.79	1.76	0.316	0.965

Note: CON: Basal diet. 0.5%: Basal diet + 0.5% MCDF. 1%: Basal diet + 1% MCDF. 1.5%: Basal diet + 1.5% MCDF. ADG: Average daily gain. ADFI: Average daily feed intake. ^a,b,c Within a row, values with no common superscripts differ significantly (p < 0.05). SEM: Standard error of the mean. n = 5.

, 1% MCDF significantly reduced final body weight and ADG of broilers during early stage (p < 0.05). Additionally, 1% and 1.5% MCDF significantly increased the F/G (p < 0.05). The 0.5% and 1% MCDF significantly reduced ADFI (p < 0.05). However, no significant differences were observed in the final body weight, ADG and F/G during later stage and overall experimental period (p > 0.05). The ADFI was significantly increased by 1.5% MCDF during the later stage (p < 0.05).

3.2. Nutrient Metabolizability

Table 4. The effect of different doses of modified corn straw dietary fiber on nutrient metabolic rate of broilers (%).

Items	CON	0.5%	1%	1.5%	SEM	p Value
Early stage (1–21 d)
EE	59.23 a	56.86 a	49.78 b	49.14 b	1.297	0.001
CP	59.65	58.16	56.31	53.13	1.170	0.236
Ca	37.40	37.40	35.91	31.76	1.137	0.208
P	51.85 a	38.35 b	40.05 b	39.44 b	1.375	<0.001
Later stage (22–42 d)
EE	70.01	63.67	57.59	56.14	2.377	0.088
CP	54.07 ab	61.49 a	52.18 ab	45.97 b	1.911	0.007
Ca	26.77 ab	32.40 a	25.65 ab	21.06 b	1.543	0.026
P	29.37 ab	31.15 a	29.11 ab	24.66 b	0.923	0.022

Note: CON: Basal diet. 0.5%: Basal diet + 0.5% MCDF. 1%: Basal diet + 1% MCDF. 1.5%: Basal diet + 1.5% MCDF. EE: Ether extract. CP: Crude protein. Ca: Calcium. P: Phosphorus. ^a,b Within a row, values with no common superscripts differ significantly (p < 0.05). SEM, standard error of the mean. n = 5.

presents the impact of varying MCDF inclusion levels on nutrient metabolizability in broilers. The EE metabolizability during both the early and later stage decreased progressively with the increasing levels of MCDF (1–21 d: p < 0.001; 22–42 d: p = 0.088). In the later stage, the metabolizability of CP (ANOVA, p = 0.007), Ca (ANOVA, p = 0.026), and P (ANOVA, p = 0.022) was significantly reduced in the 1.5% MCDF group compared with the 0.5% MCDF group. Nevertheless, no significant differences were detected between the MCDF-supplemented groups (0.5–1.5%) and the control group (p > 0.05). However, 0.5–1.5% MCDF significantly reduced P metabolizability in early stage (p < 0.001).

3.3. Slaughter Performance and Organ Index

The effects of different levels of MCDF on slaughter performance and organ index of broilers are shown in

Table 5. The effects of different doses of modified corn straw dietary fiber on slaughter performance and organ index of broilers.

Items	CON	0.5%	1%	1.5%	SEM	p Value
Slaughter rate (%)	92.57	93.21	92.51	92.78	0.160	0.415
Half evisceration rate (%)	81.64	81.24	82.46	82.21	0.224	0.228
Full evisceration rate (%)	74.31	74.38	75.30	75.00	0.224	0.330
Breast muscle rate (%)	11.49	11.56	10.73	12.03	0.177	0.058
Leg muscle rate (%)	10.83	10.41	10.73	10.31	0.170	0.676
Bursal index (g/kg)	1.29	1.00	1.12	1.41	0.101	0.514
Glandular stomach index (g/kg)	3.33	3.09	3.38	3.58	0.076	0.154
Gizzard index (g/kg)	12.83	11.27	10.76	12.01	0.297	0.067
Heart index (g/kg)	4.99 a	3.22 b	3.17 b	4.36 a	0.183	<0.001
Liver index (g/kg)	15.90 b	19.05 a	17.02 ab	16.64 ab	0.432	0.048
Spleen index (g/kg)	0.85	1.01	0.84	0.79	0.055	0.550
Pancreas index (g/kg)	1.80	2.12	1.93	1.86	0.051	0.144
Duodenum index (cm/kg)	16.93	14.95	16.17	15.75	0.376	0.314
Jejunum index (cm/kg)	32.93	34.04	32.87	28.14	0.850	0.059
Ileum index (cm/kg)	34.92 a	31.63 ab	33.78 a	28.97 b	0.732	0.015
Cecum index (cm/kg)	8.98	9.06	9.61	8.36	0.193	0.152

Note: CON: Basal diet. 0.5%: Basal diet + 0.5% MCDF. 1%: Basal diet + 1% MCDF. 1.5%: Basal diet + 1.5% MCDF. ^a,b Within a row, values with no common superscripts differ significantly (p < 0.05). SEM, standard error of the mean. n = 8.

. The heart index was significantly reduced by 0.5% and 1% MCDF, while 0.5% MCDF significantly increased the liver index (p < 0.05). The breast muscle rate (ANOVA, p = 0.058) and gizzard index (ANOVA, p = 0.067) showed a certain trend among the different treatments, but post hoc pairwise comparisons using Tukey’s HSD did not reveal any significant differences between the groups. Relative to the control group, 1.5% MCDF significantly reduced ileum index of broilers (p < 0.05).

3.4. Meat Quality

Table 6. Effect of different doses of modified corn straw dietary fiber on meat quality of broilers.

Items	CON	0.5%	1%	1.5%	SEM	p Value
Breast muscle
L* 15 min	49.63	47.99	46.00	47.96	0.523	0.104
a* 15 min	3.79	4.29	4.32	3.82	0.177	0.602
b* 15 min	7.94 ab	8.91 a	7.02 b	8.78 a	0.264	0.032
L* 24 h	56.46	59.67	56.40	58.30	0.809	0.426
a* 24 h	4.87	6.57	6.15	5.89	0.281	0.141
b* 24 h	8.19	9.46	9.66	10.15	0.327	0.161
Drip loss (%)	11.56	8.75	4.44	9.53	1.072	0.119
Cooked meat rate (%)	56.26	57.66	51.44	53.78	0.897	0.104
Shear force (N)	14.28	10.46	10.03	10.67	0.784	0.222
pH15 min	6.12	5.90	6.04	6.13	0.038	0.091
pH24 h	5.89 a	5.69 b	5.75 b	5.78 b	0.020	0.001
Leg muscle
L* 15 min	58.51 a	59.67 a	54.68 b	58.62 a	0.488	<0.001
a* 15 min	10.08	9.55	10.40	9.31	0.342	0.685
b* 15 min	11.29	12.19	10.66	11.91	0.292	0.262
L* 24 h	62.86 a	62.61 ab	59.81 b	64.08 a	0.535	0.040
a* 24 h	7.91	8.62	8.42	7.59	0.369	0.754
b* 24 h	9.49	10.38	9.88	10.29	0.275	0.658
Drip loss (%)	2.91	3.98	3.26	3.69	0.275	0.564
Cooked meat rate (%)	55.50	56.09	54.61	55.83	0.545	0.817
Shear force (N)	7.43	6.71	7.02	7.31	0.407	0.936
pH15 min	6.31	6.23	6.35	6.23	0.271	0.319
pH24 h	6.31	6.26	6.28	6.29	0.200	0.876

Note: CON: Basal diet. 0.5%: Basal diet + 0.5% MCDF. 1%: Basal diet + 1% MCDF. 1.5%: Basal diet + 1.5% MCDF. L*: Lightness. a*: Redness; b*: Yellowness. ^a,b Within a row, values with no common superscripts differ significantly (p < 0.05). SEM, standard error of the mean. n = 8.

shows the effects of different levels of MCDF on broilers meat quality. Different levels of MCDF significantly decreased the pH of broiler breast muscle at 24 h (p < 0.05). Relative to the control group, 1% MCDF significantly reduced L* of leg muscle at 15 min and 24 h (p < 0.05). The pH measured at 15 min in breast meat showed a tendency toward a difference among treatments (ANOVA, p = 0.091), although pairwise comparisons using Tukey’s HSD did not reveal significant differences between groups.

3.5. Serum Indices

Table 7. The effects of different doses of modified corn straw dietary fiber on serum biochemical, antioxidant and immunological indices of broilers.

Items	CON	0.5%	1%	1.5%	SEM	p Value
Serum biochemistry indices
ALT (U/L)	6.67	6.30	6.70	5.20	0.365	0.542
AST (U/L)	256.20	270.07	222.55	221.20	7.761	0.055
ALP (U/L)	1234.00 a	753.00 b	1096.67 a	818.00 b	57.377	<0.001
LDL-C (mmol/L)	0.71 a	0.42 b	0.41 b	0.45 b	0.035	0.001
HDL-C (mmol/L)	1.49 a	0.88 b	0.80 b	0.92 b	0.072	<0.001
TG (mmol/L)	0.72 a	0.42 b	0.44 b	0.47 b	0.131	0.001
TC (mmol/L)	1.70 a	0.95 c	1.29 b	1.06 bc	0.080	0.002
GLU (mmol/L)	8.14 a	5.86 b	5.92 b	6.09 b	0.252	<0.001
Antioxidant indices
CAT (U/mL)	5.73 b	7.65 b	15.29 a	7.65 b	4.113	<0.001
T-AOC (mmol/mL)	0.59 b	0.74 a	0.75 a	0.71 ab	0.228	0.048
MDA (mmol/mL)	3.71	3.82	4.93	4.54	0.200	0.072
SOD (U/mL)	17.88 a	16.94 a	18.87 a	5.53 b	1.689	<0.001
GSH-PX (U/mL)	1105.56	1382.52	1207.77	1395.15	45.170	0.034
Immune indices
IgA (μg/mL)	392.26	373.50	396.48	403.91	5.094	0.183
IgG (μg/mL)	2759.32	2773.22	2675.28	2559.73	49.988	0.510
IgM (μg/mL)	933.13	904.10	922.09	927.32	10.054	0.794

Note: CON: Basal diet. 0.5%: Basal diet + 0.5% MCDF. 1%: Basal diet + 1% MCDF. 1.5%: Basal diet + 1.5% MCDF. ALT: Alanine aminotransferase. AST: Aspartate aminotransferase. ALP: Alkaline phosphatase. LDL-C: Low-density lipoprotein cholesterol. HDL-C: High-density lipoprotein cholesterol. TG: Triglyceride. TC: Total cholesterol. GLU: Glucose. CAT: Catalase. T-AOC: Total antioxidant capacity. MDA: Malondialdehyde. SOD: Superoxide dismutase. GSH-PX: Glutathione peroxidase. IgA: Immunoglobulin A. IgG: Immunoglobulin G. IgM: Immunoglobulin M. ^a,b,c Within a row, values with no common superscripts differ significantly (p < 0.05). SEM, standard error of the mean. n = 5.

summarizes the effects of MCDF on serum biochemical profiles, as well as antioxidant and immune indices in broilers. Serum ALP levels (p < 0.05) were significantly decreased by 0.5% and 1.5% MCDF. Compared to the control group, 0.5–1.5% MCDF significantly reduced the serum levels of TG, TC, LDL-C, HDL-C and GLU (p < 0.05). Meanwhile, 0.5% and 1% MCDF significantly increased T-AOC (p < 0.05). AST (ANOVA, p = 0.055) and MDA (ANOVA, p = 0.072) showed a tendency toward differences among treatments, GSH-PX activity was significantly affected by treatment (ANOVA, p = 0.034); however, Tukey’s HSD multiple comparisons indicated that none of these three parameters differed significantly between individual groups. CAT was significantly increased by 1% MCDF (p < 0.05), while SOD was significantly decreased by 1.5% MCDF (p < 0.05). The supplementation with MCDF did not exert a significant effect on the measured serum immune indices (p > 0.05).

3.6. Intestinal and Liver Tissue Morphology

As shown in Figure 1A, the effects of different levels of MCDF on jejunum villus morphology in broilers were presented. In all groups (control group and 0.5–1.5%), the villi exhibited a finger-like shape. Figure 1B illustrates the effects of MCDF on VH, CD and villus height to crypt depth ratio (VH/CD) in the jejunum of broilers. The CD was significantly decreased in 0.5–1.5% MCDF groups, compared to the control group (p < 0.05). The VH/CD ratio in the control group was significantly lower than that in the 0.5–1.5% MCDF groups (p < 0.05).

As shown in the HE staining images (Figure 2), in the control group, hepatocytes exhibited irregular sizes and disorganized arrangement. Most hepatocytes exhibited a dense distribution of abundant lipid vacuoles of varying sizes. The nuclei were deformed and displaced toward the cell periphery. Some hepatocytes were enlarged, and hepatic sinusoids were absent, indicating diffuse hepatic steatosis. In contrast, in the 0.5–1.5% MCDF groups, the hepatic lobules and sinusoids were clearly visible, hepatocyte size was uniform, and the arrangement was orderly, with a marked reduction in vacuole structures. Relative to the control group, the 1.5% MCDF group showed a reduction in vacuoles, although more than in the 0.5% and 1% MCDF groups. In the 0.5% and 1% MCDF groups, only a few scattered lipid vacuoles were observed.

According to the Oil Red O staining images (Figure 2), the control group exhibited large areas of intensely stained red lipid droplets of varying sizes densely distributed throughout the liver tissue. The nuclei were deformed and displaced to the cell periphery, indicative of hepatic steatosis. Compared with the control group, the relative area of red lipid droplets was significantly reduced in the 0.5–1.5% MCDF groups (p < 0.05).

4. Discussion

4.1. Effects of Different Levels of MCDF on Growth Performance of Broilers

Previous studies have shown that modification of corn straw increases the SDF content, and the antioxidant capacity, water-holding capacity, cholesterol adsorption, antibacterial activity, and fermentability of both modified IDF and SDF are significantly enhanced [7]. Research has indicated that inclusion of IDF in the diet can stimulate the activity of enzymes, such as protease and pancreatic enzymes, and modulate the size of the gastrointestinal tract in poultry, thereby improving digesta retention and feed efficiency. SDF can influence intestinal health and nutrient metabolizability by affecting small intestinal transit rate and hindgut fermentation. However, some studies have also reported that SDF may increase digesta viscosity and hinder enzyme diffusion, leading to reduced nutrient metabolism. When used as a functional nutrient, DF should be considered in terms of particle size, SDF/IDF ratio, and structure to define thresholds for excessive intake [16].

In livestock and poultry production, the efficient yield and quality of meat protein have become key concerns for both farmers and consumers [17]. Therefore, ADFI, ADG, and F/G are of great significance for assessing broiler production performance and economic efficiency. The inclusion of appropriate levels of DF in broiler diets has been shown to enhance nutrient metabolizability and improve intestinal health and growth performance [18]. Within this study, MCDF negatively affected broiler growth in the early stage, with no significant impact in the later stage and across the full rearing period. This may be because the digestive system of young broilers is not yet fully developed, requiring some adaptation mechanisms to maintain growth rate [19]. Previous research also indicated that adding 30 g/kg of beet pulp or rice hulls to basal broiler diets negatively affected early-stage growth performance [20]. Kheravii et al. (2018) found that dietary supplementation with oat hulls reduced feed intake and feed efficiency at 16 d of age, while having no significant impact on performance at 24 and 35 d [21]. Okrathok and Khempaka (2020) observed that supplementation with 0.5–1.5% modified cassava pulp dietary fiber had no significant effect on growth performance at either stage [22]. Other studies have indicated that increasing crude fiber levels in the diet reduced the growth rate and body weight of male ROSS 308 broilers [23]. Although high lignocellulose content has been shown to exert limited influence on growth during the fattening phase [19], some studies suggested that certain types of DF may promote broiler growth during this stage [24,25]. The inconsistent results regarding the effects of DF on broiler performance during different growth stages may be attributed to differences in rearing conditions, broiler strains and the chemical composition, particle size and inclusion level of the DF used. Broilers may compensate for the reduced nutrient density caused by fiber inclusion by increasing their feed intake [26], which may explain the increased ADFI observed in the 1.5% MCDF group. It has been shown that the water holding capacity and adhesiveness of dietary fiber can, to some extent, increase endogenous secretions in the gut, which may lead to endogenous losses and could negatively affect nutrient metabolizability at the 1.5% MCDF level [27]. Furthermore, due to the superior fermentability of MCDF, the improved growth performance observed at 1.5% MCDF may also be related to SCFAs produced during MCDF fermentation, which can directly provide energy for intestinal cells and modulate skeletal muscle energy metabolism and protein synthesis pathways [28].

4.2. Effects of Different Levels of MCDF on Nutrient Metabolizability of Broilers

Nutrient metabolizability is a critical indicator reflecting the digestive efficiency and growth performance of animals and improving it can enhance production efficiency [18]. DF, by escaping digestion and absorption in the gastrointestinal tract, provides opportunities to modulate intestinal morphology, interact with nutrients and gut microbiota, and regulate gastrointestinal motility, thereby influencing nutrient metabolism and growth performance [29]. Previous studies have shown that poultry diets should contain low to moderate levels of DF, as high levels may reduce nutrient metabolizability and absorption, ultimately impairing animal performance [16]. EE metabolizability was reduced by 1% and 1.5% MCDF in both the early and later stage, which may be related to the stronger cholesterol-binding capacity of MCDF, promoting the excretion of fat. In this study, 0.5% MCDF increased the metabolizability of CP, Ca, and P, which may be associated with the improved water-holding capacity of the modified corn straw, providing a favorable microenvironment for digestive enzymes surrounding the feed and thereby enhancing nutrient metabolizability. However, 1.5% MCDF significantly decreased the metabolizability of CP and P in the later stage, compared with 0.5% MCDF, possibly because DF levels can influence inevitable endogenous amino acid losses at the distal ileum [30], and phytate in high-fiber diets can bind minerals, increasing their excretion [31]. The development of the gastrointestinal tract strongly influences the metabolizability of nutrients. Within this study, the jejunum index and ileum index of the 1.5% MCDF group were lower than those of control group. Similarly, dietary supplementation with 30 g/kg unmodified soybean hulls significantly reduced the apparent ileal metabolizability of Ca in broilers, possibly due to the strong calcium-binding capacity of soybean hulls, leading to the formation of highly insoluble complexes that impair Ca metabolizability [32]. Conversely, previous research has shown that inulin can enhance Ca metabolizability in laying hens and contribute to improved eggshell thickness [18].

4.3. Effects of Different Levels of MCDF on Slaughter Performance and Organ Index of Broilers

In this study, different levels of MCDF had no significant effect on the slaughter performance of broilers. This was consistent with the findings indicating that mannan oligosaccharides did not exert a significant influence on the carcass rate, leg muscle rate, or breast muscle rate in broilers [33]. Insoluble fiber has been shown to significantly increase the relative weight of the liver in both chicks and laying hens [34], which aligns with the increased liver index observed in broilers fed 0.5% MCDF in this study. Previous studies have indicated that the presence of large amounts of DF in the gastrointestinal tract can increase the size of gastrointestinal organs, an effect attributed to the increased volume of digesta passing through the gut [35]. In this study, the duodenum index, ileum index and gizzard index in the MCDF group were lower than those of the control group to varying degrees, which is inconsistent with previous studies. This may be because the modification of MCDF increased the SDF content, and a higher inclusion level of MCDF could increase small intestinal viscosity, thereby affecting transit rate and subsequently organ development [29]. Changes in organ indices may be associated with increased maintenance requirements due to enhanced tissue synthesis and protein turnover, thereby diverting a greater proportion of nutrients toward the maintenance of organ tissues, which may consequently impair muscle protein deposition and overall growth performance [35].

4.4. Effects of Different Levels of MCDF on Meat Quality of Broilers

Meat quality directly influences consumer preferences and the economic returns for producers. It is typically evaluated based on parameters such as pH, meat color, drip loss, cooking loss and shear force [36]. After slaughter, glycolysis occurs in the muscle tissue, producing lactic acid and thereby lowering the muscle pH [37]. The typical pH range of chicken meat is between 5.8 and 5.9; pH higher than 6.1 is associated with dark, firm and dry (DFD) meat, whereas pH lower than 5.7 indicates pale, soft and exudative (PSE) meat [38]. Therefore, pH has a direct impact on meat quality. In this study, MCDF significantly reduced the 24 h pH value of broiler breast muscle, though the underlying mechanisms require further investigation. L*, a*, and b* values are employed to assess meat color, which serves as a key colorimetric indicator of meat quality. A higher L* indicates greater water exudation from the muscle, which is associated with an increased risk of PSE meat [37]. Generally, higher a* and lower b* values are associated with an improvement in the sensory attributes of meat [39]. In the present study, 1% MCDF reduced the b* of breast muscle at 15 min, as well as the L* of leg muscle at both 15 min and 24 h, suggesting a potential improvement in meat color to some extent.

4.5. Effects of Different Levels of MCDF on Serum Indices of Broilers

Serum biochemical parameters act as sensitive biomarkers that reflect the physiological status of chickens. These parameters provide indirect insights into organ function and nutritional metabolism [40]. ALT and AST are among the most important serum markers for evaluating liver damage [41]. Hepatobiliary disorders are often associated with increased ALP activity [42]. The addition of 0.5% and 1.5% MCDF to the diet decreased serum ALP levels, suggesting a possible improvement in hepatobiliary function in broilers. Serum levels of TG, TC, HDL-C and LDL-C are important parameters for evaluating lipid metabolism and deposition in poultry [43]. MCDF significantly reduced the levels of LDL-C, TG, TC, and GLU, which may be related to its stronger cholesterol-binding capacity. This property could promote the excretion of cholesterol, thereby reducing the absorption of lipids and cholesterol in the intestine and further lowering serum lipid and cholesterol levels. By clearing excess cholesterol from peripheral tissues to the liver for catabolism, HDL-C contributes to a lowered risk of atherosclerosis [44]. In contrast, elevated LDL-C levels increase the risk of cardiovascular diseases [45]. The inclusion of inulin in the diet has been demonstrated to lower the cholesterol content of eggs [46]. In agreement with previous reports indicating that elevated dietary lignin decreases serum HDL and LDL levels in broilers [16], the present study observed that MCDF similarly reduced HDL-C and LDL-C concentrations. This effect may be related to the release of phenolic compounds present in MCDF, which can enhance the activity of lecithin–cholesterol acyltransferase on the surface of HDL particles, thereby promoting the transport of extrahepatic cholesterol to the liver, followed by the degradation of HDL-C and its associated cholesterol [16]. Other studies have also demonstrated that inulin can linearly reduce serum triglycerides and VLDL-C concentrations without affecting TC, HDL-C or LDL-C levels [47]. In terms of antioxidant indices, 1% MCDF increased serum CAT and T-AOC in broilers, which may be related to the antioxidant properties of MCDF [7]. The decrease in SOD activity at 1.5% MCDF may be related to the antioxidant properties of both IDF and SDF in MCDF [7], as the endogenous antioxidant defense system is dose-dependent with respect to antioxidant supplementation [48]. However, the exact mechanism remains unclear. In laying hens, the activities of SOD, CAT and GSH-PX in serum were found to increase quadratically with inulin supplementation, while MDA levels decreased in a quadratic manner [46]. As shown in this study, MCDF had no significant effect on the serum immune parameters of broilers. Similarly, Okrathok et al. (2023) reported that 0.5–1.5% modified dietary fiber from cassava pulp had no significant effect on serum immunoglobulin levels in broilers [49]. These findings suggested that MCDF can reduce blood glucose and lipid levels and enhance antioxidant capacity in broilers but has no significant impact on serum immune function.

4.6. Effects of Different Levels of MCDF on Intestinal and Liver Tissue Morphology in Broilers

Absorptive mucosal epithelial cells are in a dynamic state, undergoing regular apoptosis and shedding, and are rapidly replenished by newly generated cells from the crypts. VH, CD, and the VH/CD can serve as indicators of absorptive efficiency and intestinal health [31]. A reduction in CD suggests enhanced cellular maturation and secretory function and may also indicate reduced stress [18]. MCDF has improved fermentability and can be fermented by gut microbiota to produce SCFAs, which further promote intestinal epithelial cell maturation and reduce excessive proliferation, thereby decreasing CD [31]. The decline in nutrient metabolizability observed in the 1.5% MCDF group was inconsistent with the trend in the VH/CD, which may be attributed to excessive SDF. It can increase intestinal viscosity and hinder enzyme diffusion, ultimately leading to reduced nutrient metabolizability [29]. Based on previous research, the poultry intestine responds rapidly and relatively consistently to DF, often resulting in increased VH and improved arrangement of intestinal epithelial cells [29]. For instance, supplementation with 1% or 1.5% micronized wheat fiber significantly increased jejunum VH, VH/CD ratio and villus thickness in quails. Conversely, broilers in control group showed increased crypt depth, suggesting enhanced epithelial renewal and elevated intestinal index, which might reflect higher energy consumption within the gut [50].

In poultry, approximately 90% of de novo lipogenesis occurs in the liver, which serves as the primary site of lipid metabolism [51]. Dysregulation of hepatic lipid metabolism leads to excessive triglyceride deposition, resulting in fatty liver. Hepatic injury further induces a decline in growth performance and disease resistance, as well as an increase in sudden death rate in broilers [52]. Therefore, alleviating hepatic fat accumulation is crucial for sustainable poultry production. Previous studies have demonstrated that DF can modulate lipid metabolism in poultry [3,53]. In the study, MCDF reduced hepatic vacuolar degeneration and lipid accumulation in broilers. In our earlier research, it was observed that dietary supplementation with 0.5% modified corn straw significantly reduced hepatic fat, TC, TG and lipid droplet levels [3]. Related studies have shown that DF can markedly alleviate hepatic steatosis [54]. SDF from pear pomace was reported to reduce hepatic lipid vacuolation in high-fat-diet-fed mice in a dose-dependent manner [55]. MCDF was found to elevate the VH/CD ratio in the jejunum and regulate lipid metabolism in the liver, thereby enhancing the overall well-being of broilers.

5. Conclusions

In summary, MCDF had a negative impact on the early growth performance and nutrient metabolism of broilers. However, 0.5–1.5% MCDF showed no effects on growth performance and nutrient metabolism during the later feeding periods, slaughter performance, organ indices and meat quality, and it increased antioxidant capacity. The 0.5–1.5% MCDF effectively reduced serum lipid and glucose levels and alleviated hepatic vacuolar degeneration and lipid accumulation. The results demonstrated that supplementing broiler diets with 0.5–1.5% MCDF in later stage was both feasible and effective.