A Multiple Regression Model Analysing Additional Sources of Dietary Fibre as a Factor Affecting the Development of the Gastrointestinal Tract in Broiler Chickens

Abstract

The objective of this study was to compare the effects of applying a 50% wheat grain diet with those of a diet with 3% additional dietary fibre from various sources on the development of broiler chickens’ gastrointestinal tract and its related organs and to model this phenomenon based on data obtained from 35-day-old chickens using multiple regression equations. The use of various structural components, including oat hull (OH), sunflower hull (SH), sugar beet pulp (SBP), and wheat bran (WB), in proportions of 3% of the diet not only affects digestive processes in broiler chickens’ gastrointestinal tract but also causes a change in the length of their intestinal sections or the weight of related organs. These effects can be taken into account when creating an experimental model, the results of which can at least be partially applicable to human studies. The use of OH and SH (3%) in the birds’ diets resulted in a significantly higher body weight (p < 0.05) compared with the use of SBP and WB. OH in the diet significantly increased (p < 0.01) the weight of the chicken’s gizzards compared with the other dietary fibre sources, apart from SH. On the other hand, the weight of the proventriculus in chickens fed the diet containing OH was significantly lower than that of the chickens fed the diet containing SBP (p < 0.05). The use of SH in the diet caused a significant decrease (p < 0.01) in the weight of the chickens’ heart. Compared with other additional sources of dietary fibre, OH in the diet also significantly increased (p < 0.05) the lengths of the small and large intestines, as well as the total length of the intestines. A correlation analysis showed a significant, average, positive relationship (p < 0.05) between the content of TDF in the diet and the weight of the gizzard and indicated a significant positive correlation between the lengths of the jejunum and the remaining sections of the intestines. Additionally, the regression equation models indicated a significant effect (p < 0.01) of all the independent variables on the jejunal, ileal, and caecal lengths and the liver weight. The application of the regression model confirmed significant changes in the small intestine and liver weight depending on the type of dietary fibre and other independent variables, which can also be taken into account when assessing diseases in people with thin intestines. However, further studies with separate models still need to be conducted using experiments including both soluble and insoluble fibre.

1. Introduction

Animal models provide valuable information about the processes occurring in living organisms, enabling their use, at least partially, in understanding processes in the human body where, for bioethical reasons, such studies are impossible to conduct [1,2,3,4]. Flores-Santin and Burggen [5] suggest that, after mice and rats, Gallus domesticus is the third most common species applied in medical research. Taking into consideration the similarity between the gastrointestinal tracts of broiler chicken and humans, G. domesticus is used in experiments to provide information on processes related to gastrointestinal tract development, especially in the case of the small intestines’ response to different types of dietary fibre and when modulating the immunology and microbial status of animals. Moreover, in recent years, they have been used as animal models to investigate the effect of light pollution in a room on intestinal injury [6,7,8]. Of course, similar experiments with soluble and insoluble fibre were conducted with rats and mice, where the effect on body weight gains and adipose tissue were assessed in the small intestine [9,10]. Therefore, animal models of G. domesticus can be a useful alternative to those of rats or mice for describing the impact of soluble and insoluble dietary fibre on the stabilisation of microflora as well as metric changes in the small intestine depending on the inclusion and type of dietary fibre. These types of animal models can also be useful for understanding nutrition in younger people with intestinal problems, where rebuilding the microbiome and structures of the intestinal mucosa as a result of nutritional disorders, gastrointestinal diseases, and the limited absorption of nutrients in people with obesity is possible [11,12]. From the nutritional point of view for broiler chickens, concentrate mixtures based on the use of post-extraction soybean meal and a high share of wheat grain may lead to a deficiency in dietary fibre (DF) necessary for the proper functioning of both the upper and lower digestive tracts [13,14,15,16]. A high amount of wheat, triticale, or barley grain in the diet, especially in young animals, may have an antinutritional effect due to the presence of arabinoxylans, which are non-starch polysaccharides (NSPs), which lower the passage rate of digesta [17,18]. These compounds are present in cereal grains in soluble and insoluble forms, but their proportion affects the physicochemical properties of the food content, such as its viscosity, solubility, water-holding capacity, fermentability, bulk, and ability to bind bile acids [19,20]. One way to limit the action of soluble fibre contained in wheat grain in the small intestine is to add components rich in structural carbohydrates, not exceeding an amount of 3% in diets for broiler chickens depending on the age of the birds [21,22,23].

Taking into account experiments with monogastric animals, a better definition of dietary fibre was stated in an analysis conducted by Asp [24], taking into account total (TDF), insoluble (IDF), and soluble (SDF) dietary fibre. By-products of the agro-food industry show different contents of soluble and insoluble fibre [25,26]. The component most rich in insoluble fibre is oat hull (OH), composed of lignocellulose, the main component; insoluble cellulose (35–45%); hemicelluloses (32–35%); and acid-insoluble lignin (17–20%) [27,28,29]. On the other hand, sunflower hull (SH), soybean hull, sugar beet pulp (SBP), and wheat bran (WB) show higher amounts of SDF [30,31]. The differences in the proportions of the components of SDF and IDF affect the production parameters of birds; the length and weight of select digestive sections, as well as their associated organs; and consequently, the health of the birds, particularly their ‘gut health’ [19,32,33].

Studies related to the effect of different fibre sources on the gastrointestinal tract of birds are not new, and comparisons have been performed earlier by many research teams [34,35,36,37]. Naeem et al. [38] considered the use of a small amount of OH in the diet as a factor that can significantly reduce nitrogen emissions to the environment. However, there are few comparisons employing multiple regressions and taking into account all the components affecting modelling due to changes in the amount of DF in the diet in the available literature [39]. Multiple regression is a type of analysis in which independent variables have an impact on a single dependent variable, and its development also allows for path analysis [40], which then allows for the construction of a regression equation that can be used to model changes in all factors important in the overall model of a given phenomenon, including the case of the production process or digestion in the digestive tract of animals, as long as they form a logical whole [41]. This type of analysis has been used in the study of amino acid digestibility or when determining the relationship between blood markers and abdominal fat in broiler chickens [8,42].

The objective of this study was to compare the effects of a diet containing 50% wheat grain with a diet containing 3% various crushed components rich in structural carbohydrates on the nutrition of broiler chickens, specifically measuring production parameters and the weight or length of select sections of the digestive tract, from the proventriculus to the large intestine; the study used the data obtained during these measurements to create regression equations describing the range of changes in these variables due to the different types of fibre added to the mixtures.

2. Materials and Methods

2.1. Animals and Diets

This experiment on broiler chickens was approved by the Faculty Team for Animal Welfare of the Faculty of Biology and Animal Nutrition at the University of Life Sciences in Wrocław, approval no. 0058 in the Register of Users of animals used in procedures, in accordance with the ordinance described in article no.3 of Dz.U.2021 position 1331, 2338 on the protection of animals used for scientific and educational purposes. In this experiment, 150 broiler chickens were kept in 30 metabolic cages during the digestibility trial. This experiment was planned for a standard broiler chicken rearing period of 35 days. The experiment was designed under the assumption of the use of a single experimental factor in the form of additional sources of DF in five diet treatments. Each treatment consisted of six replicates with five chickens each (n = 30 in each treatment). Similar average BW was taken into consideration and checked at the beginning of the experiment, allowing for changes between the treatments to be observed based on different experimental factors. The diets differed based on their structural components, with 3% being structural additives (OH, SH, SBP, and WB), except for the control group where a standard diet containing maize and soybean meal was used. Vaccinated, one-day-old Ross 308 chickens with an average body weight of 44.8 g were kept together for the first 7 days of life and fed a starter mixture based on wheat grain and soybean extraction meal (

Table 1. Ingredient composition and chemical analysis of starter diets (g/kg).

Item	Control	3% OH	3% SH	3% SBP	3% WB
Maize	50.2	11.9	7.9	11.1	10.3
Wheat	536.6	529.5	534.2	537.0	540.9
Soybean meal	320	325	327	322	318
Soy oil	53	63	61	61	61
Oat hull		30
Sunflower hull			30
Dry sugar beet pulp				30
Wheat bran					30
NaCl	2.2	2.2	2.2	2.2	2.2
Sodium bicarbonate	2.0	2.0	2.0	2.0	2.0
Monocalcium phosphate	12.5	12.6	12.6	12.5	12.6
Chalk	14.2	14.2	14.2	13.5	14.2
DL-methionine (98%)	2.39	2.47	2.49	2.47	2.43
HCl-L-lysine (78%)	1.86	1.86	1.86	1.86	1.86
Premix DKA starter 0.5% *	5.0	5.0	5.0	5.0	5.0
Chemical composition (g/kg)
Metabolisable energy, MJ	12.51	12.51	12.50	12.51	12.50
Crude protein, g	220.2	220.1	220.5	219.8	220.2
Crude fibre	30.2	38.1	44.0	35.8	33.1
Total dietary fibre	188.4	204.6	209.2	201.6	197.8
Insoluble dietary fibre	162.1	178.2	159.3	160.1	157.4
Soluble dietary fibre	26.3	26.4	49.9	41.5	40.4
Calcium	9.4	9.4	9.4	9.4	9.4
Pavailable	4.3	4.3	4.3	4.3	4.4
Sodium	1.6	1.6	1.6	1.6	1.6
L-lysine	12.00	12.02	12.06	11.95	12.02
DL-methionine	5.51	5.53	5.56	5.52	5.52

* The premix provided the following amounts of nutrients per kilogram of diet: vitamin A, 11,500 IU; vitamin D₃, 4700 IU; vitamin E, 47 mg; vitamin K, 3 mg; vitamin B1, 3 mg; vitamin B2, 8 mg; vitamin B6, 4 mg; vitamin B12, 0.02 mg; nicotinic acid, 55 mg; pantothenic acid, 17 mg; folic acid, 1.9 mg; biotin, 0.23 mg; choline chloride, 425 mg; Mn, 110 mg; Fe, 50 mg; Zn, 105 mg; Cu, 15 mg; I, 1 mg; and Se, 0.3 mg. Abbreviations: OH—oat hull; SH—sunflower hull; DSBP—dry sugar beet pulp; WB—wheat bran.

).

At 7 days of age, chicks weighing approximately 133 g were randomly assigned to the five treatments. The birds continued to be fed a starter-type diet until 21 days of age. From 21 to 35 days of age, the chicks received a grower mix with the same types and share of structural components as in the starter mixes (

Table 2. Ingredient composition and chemical analysis of grower diets (g/kg).

Item	Control	3% OH	3% SH	3% SBP	3% WB
Maize	76.4	55.6	54.8	58.1	62.4
Wheat	550	527	527	527	527
Soybean meal	267	273	275	272	268
Soy oil	66.0	73.5	72.5	73.0	72.5
Oat hull		30
Sunflower hull			30
Dry sugar beet pulp				30
Wheat bran					30
NaCl	2.2	2.2	2.2	2.2	2.2
Sodium bicarbonate	2.4	2.4	2.4	2.4	2.4
Monocalcium phosphate	11.6	11.9	11.8	11.6	11.3
Chalk	14.2	14.6	14.6	14.0	14.9
DL-methionine (98%)	2.39	2.39	2.39	2.41	2.38
HCl-L-lysine (78%)	2.82	2.95	2.81	2.89	2.82
Premix DKA grower 0.5%	5.0	5.0	5.0	5.0	5.0
Chemical composition (g/kg)
Metabolisable energy, MJ	13.03	12.96	13.00	13.02	12.99
Crude protein, g	200.0	199.9	200.2	200.0	200.3
Crude fibre	28.9	36.8	42.7	34.4	31.8
Total dietary fibre	176.82	193.71	198.34	190.93	187.30
Insoluble dietary fibre	151.65	168.87	150.14	151.14	148.78
Soluble dietary fibre	25.12	24.79	48.17	39.74	38.49
Calcium	9.2	9.2	9.2	9.2	9.2
Pavailable	4.0	4.0	4.0	4.0	4.0
Sodium	1.7	1.7	1.7	1.7	1.7
L-lysine	11.44	11.57	11.51	11.50	11.51
DL-methionine	5.24	5.20	5.20	5.21	5.23

The premix provided the following amounts of nutrients per kilogram of diet: vitamin A, 9500 IU; vitamin D₃, 4000 IU; vitamin E, 35 mg; vitamin K, 2 mg; vitamin B1, 2 mg; vitamin B2, 5 mg; vitamin B6, 3 mg; vitamin B12, 0.015 mg; nicotinic acid, 40 mg; pantothenic acid, 14 mg; folic acid, 1.6 mg; biotin, 0.18; choline chloride, 300 mg; Mn, 100 mg; Fe, 40 mg; Zn, 100 mg; Cu, 15 mg; I, 1 mg; and Se, 0.3 mg. Abbreviations: OH—oat hull; SH—sunflower hull; DSBP—dry sugar beet pulp; WB—wheat bran.

). The chicks were kept under standard rearing conditions. The temperature on the first day of the experiment was 32 °C; then, it was systematically lowered to 24 °C during the second week and to 20 °C during the last days of the chicks’ life. The lighting program included 23 h of light and 1 h of darkness in the first few days; after day 7, the duration of daylight was shortened to 20 h, while from days 14 to 35, the duration of daylight was shortened again to 18 h. The diets were offered to the chickens ad libitum, taking into account uneaten food during the recording of feed intake. Water was provided to the chickens using nipple drinkers. Body weight, feed intake, and feed conversion ration were recorded on days 21 and 35. Humidity in the room was maintained in the range of 55–60% and was reduced with the age of the birds.

2.2. Chemical Analysis

Chemical analysis was performed during the initial stage, before the start of rearing, to select the appropriate amounts of each component within the framework of the birds’ demand for nutrients in order to determine the chemical composition of individual diets. Metabolisable energy (ME) for each component of the diet was calculated based on the equations presented in the European Tables of Energy Values for Poultry [43]. After an evaluation of ME in the components, the diets were prepared with their metabolisable energy, nutrients, and minerals in accordance with The Polish Requirements of Poultry Nutrition [44]. In terms of nutrition, starter and grower diets were used. The first starter diet was used on the chickens’ 20th day of life. In each treatment, diet per 1 kg of feed was 12.5 MJ ME/kg and about 220 g/kg of crude protein (Table 1). The second grower diet was given to chickens from 21 to 35 days of life (Table 2). The use of the mixtures was deliberately changed over time to also determine whether the additional source of DF would significantly affect the growth of the birds, or whether it would have no significant effect and would affect the health of the birds.

The chemical composition of individual components and diets was determined using the Official Methods of Analysis (AOAC) [45]. During the analysis, the following constituents were determined: dry matter (DM, AOAC; 934.01), crude protein (CP, Kjeldahl method, AOAC 984.13), crude ash (CA, AOAC 942.05), ether extract (EE, Soxhlet method, AOAC, 920.39A), and crude fibre (CF, Hennenberg and Stohmann method, AOAC 978.10). On the other hand, the contents of TDF, IDF, and SDF in the feed components were determined as the average value from previous studies [19,29,46,47]. Then, the fibre fraction content in the ingredients was used to calculate the total content for individual diets (Figure 1).

2.3. Length and Weight Measurements

On day 35, 90 birds from each feeding group were sacrificed to measure the weight or length of the sections of the digestive tract. The birds were sacrificed by means of concussion following a truncheon strike and a rapid sublingual incision to bleed the birds. Then, the gastrointestinal tracts of the birds, with sections of the intestines, proventriculus, and gizzard, were prepared, and their food content was removed. In the next stage of the procedure, measurements were taken of the length of the sections of the intestines (duodenum, jejunum, ileum, cecum, and large intestine) and the weight of the selected organs (proventriculus, gizzard, heart, and liver).

2.4. Statistical Analysis

For statistical verification, the obtained results were analysed in Statistica 13.3 (Tibco Software Inc., Palo Alto, CA, USA, 2017) using a one-way analysis of variance test. The following model of one-way ANOVA was used:

y_ij = µ + α_i +ε_ij,

where

y_ij—the value of the observed dependent variable;

µ—general mean in the population (effect of common factors);

α_i—effect of the structural component;

ε_ij—effect of random factors.

The significance of differences was determined at two levels of significance: p < 0.05 and p < 0.01. Tukey’s HSD test was used to determine the differences between mean values. The normality of data distribution in the experimental groups was tested using the Shapiro–Wilk test, while the homogeneity of variance between groups was tested using Leven’s test.

A covariance analysis was used to measure the length of intestinal segments and the weight of the selected organs; no significant difference was found in the body weight of the birds during dissection.

The Spearman correlation test was used to determine the correlation between fibre types and the selected intestinal lengths or the weight of the selected organs; the null hypothesis was verified at the p < 0.05 level of significance.

The components of the multiple regression equation were calculated using the least squares method, and the significance of individual variable coefficients was verified based on Student’s t-test, while the significance of the equation describing the effect of independent variables on the tested dependent variable was verified using the F test. The verification of hypotheses using both tests was performed at the p < 0.05 level of significance, with the R² coefficient of the fit of the data to the equation simultaneously determined. The following equation was used:

Y = β₀ + β₁ x_{1 +} β₂ x₂ + … + β_k x_k +ε,

where

Y—the dependent variable;

x—the independent variable;

β₀—the intercept;

β₁, β₂, …, β_k—coefficients;

ε—the error term.

3. Results

3.1. Performance of Broiler Chickens

Table 3. Performance of broiler chickens during the experiment in days and periods.

Item	Body Weight, kg			Feed Intake, kg			FCR, kg/kg
	7	21	35	7–21	21–35	7–35	7–21	21–35	7–35
ASC *
Control	0.132	0.65 A	1.77 Aa	0.96 bc	1.78 ab	2.75	1.87 A	1.59 AB	1.68 Aa
Oat hull	0.134	0.56 C	1.73 a	1.01 ab	1.71 b	2.78	2.35 C	1.53 A	1.72 Aab
Sunflower hull	0.132	0.64 AB	1.73 a	0.99 abc	1.84 a	2.80	1.95 A	1.69 BC	1.75 ab
Sugar beet pulp	0.132	0.61 B	1.66 B	1.03 Aa	1.77 ab	2.74	2.16 B	1.69 BC	1.80 bc
Wheat bran	0.134	0.61 B	1.64 Bb	0.94 Bc	1.81 a	2.78	2.00 AB	1.77 C	1.86 Bc
SEM	0.001	0.007	0.017	0.012	0.021	0.022	0.042	0.025	0.023
p-value	0.235	0.000	0.033	0.026	0.040	0.586	0.000	0.000	0.007

* ASC—additional structural component. The letters A, B, and C in superscript describe significant differences between the treatments at p < 0.01, and a, b, and c denote significant differences at p < 0.05.

presents the performance of broiler chickens during the entire experiment. In the case of body weight (BW) at day 7 when the chickens were allocated to the groups, no significant differences were found (p > 0.05). On day 21, the application of 3% of OH (0.56 kg) significantly decreased (p < 0.01) the BW of broiler chickens in comparison with that in the control group (0.65 kg).

The application of SH (0.64 kg) during this period led to results similar to those in the control group (0.65 kg). On day 35, no significant difference was found (p > 0.05) for oat and SH (in both cases 1.73 kg), but the chickens in the control group (1.77 kg) were significantly heavier (p < 0.01) than those in the group fed with additional sources of soluble fibre: SBP and WB (1.66 and 1.64 kg, respectively).

Between days 7 and 21, the highest feed intake (p < 0.01) was noted in the chickens fed a diet with 3% of SBP (1.03 kg), and the lowest was noted in the chickens fed with WB (0.94 kg). Between days 21 and 35, the highest feed intake (p < 0.05) was noted in the case of chickens fed the diets with SH and WB (1.84 and 1.81 kg per bird, respectively), and the lowest was noted in the treatment of broiler chickens fed a diet with OH (1.71 kg). Taking into consideration the period between days 7 and 35, the intake of feed was not statistically significant (p > 0.05).

The lowest FCR (p < 0.01) in the period from 7 to 21 days of life was observed in the case of the control diet and SH treatment (1.87 and 1.95 kg/kg, respectively). In contrast, the highest FCR (p < 0.01) was noted in the chickens fed a diet with 3% OH (2.35 kg/kg). In the period from 21 and 35 days of life, the lowest FCR (p < 0.01) was observed in the treatment of chickens fed a 3% OH diet (1.53 kg/kg) and in the control group (1.59 kg/kg). Moreover, the highest amount of feed per kg of growth (p < 0.01) was observed in the case of chickens fed a diet with 3% WB (1.77 kg/kg). Taking into considering the FCRs from entire experiment, the lowest value (p < 0.01) was noted in the control group of chickens (1.68 kg/kg), and its difference compared with that for the 3% treatment (1.86 kg/kg) was statistically significant.

3.2. Weight of Individual Organs

During this experiment, the weight of individual organs was analysed (

Table 4. Weight of analysed organs (g) at day 35.

Item	Weight of
	Proventriculus	Gizzard	Heart	Liver
ASC *
Control	6.8 Bb	26.1 C	11.2 ab	39.0
Oat hull	6.7 Bb	35.4 A	12.3 Aa	40.1
Sunflower hull	7.5 a	33.8 AB	9.9 B	40.0
Dry sugar beet pulp	7.6 a	29.2 BC	11.1 ab	37.1
Wheat bran	8.0 A	29.8 BC	10.1 b	37.4
SEM	0.153	0.830	0.274	0.607
p-value	0.004	0.002	0.017	0.313

* ASC—additional structural component. The letters A, B, and C in superscript describe the significant differences between the treatments at p < 0.01 and a, b, and c, at p < 0.05.

). The heaviest proventriculus (p < 0.01) was observed in the case of WB (8.0 g), and the lightest in the case of the control group and the OH treatment (6.8 and 6.7 g, respectively).

At the p < 0.05 level of significance, chickens fed diets with WB, SH, and SBP (8.0, 7.5, and 7.6 g, respectively) have a significantly heavier proventriculus than that of the control group and the OH treatment. The heaviest gizzard (p < 0.01) was observed in the case of the 3% OH (35.4 g) and SH (33.8 g) treatments, and the lowest in the case of the control group (26.1 g). A difference was also observed in the weight of their hearts (p < 0.01), with the heaviest found in chickens fed a diet with OH (12.3 g) and the lightest in chickens fed a diet with 3% SH. With a significance level defined at p < 0.05, the hearts of broiler chickens fed a diet with OH were significantly heavier than those of chickens fed diets with WB or SH (10.1 and 9.9 g, respectively). In the case of the liver, no significant differences were found (p > 0.05).

3.3. Length of Intestines

Table 5. Length of determined segments of intestines (cm) at day 35.

Item	Duodenum	Jejunum	Ileum	Ceca (Mean)	Large Intestine	Total Length of Intestines
ASC *
Control	28.0 a	76.0	77.6 A	19.0	9.7 A	210.3 Aa
Oat hull	25.6 Bb	77.2	77.3 A	18.0	9.7 A	207.8 a
Sunflower hull	28.3 A	73.7	70.3 B	17.5	8.9 AB	198.7 ab
Dry sugar beet pulp	26.5 ab	71.7	70.2 B	16.7	8.2 B	193.3 Bb
Wheat bran	27.6 a	75.3	72.9 AB	17.7	8.8 AB	202.4 ab
SEM	0.318	0.682	0.908	0.257	0.155	1.729
p-value	0.027	0.090	0.022	0.475	0.010	0.007

* ASC—additional structural component. The letters A, B, and C in superscript describe the significant differences between the treatments at p < 0.01, and a, b, and c denote significant differences at p < 0.05.

presents the lengths of the gastrointestinal tract of the birds, from their duodenums to their large intestines. Analysing the duodenum length, the longest (p < 0.01) was measured in the SH diet chickens (28.3 cm), whereas the shortest was measured in those fed an OH diet (25.6 cm). No significant differences (p > 0.05) were observed between the OH and SBP treatments (25.6 and 26.5 cm, respectively).

No significant differences were observed for jejunum length (p = 0.09). The longest ileum (p < 0.01) was measured in the control group and OH diet chickens (77.6 and 77.3 cm, respectively), while those measured in the SH and SBP groups were 70.3 and 70.2 cm, respectively. No significant differences were observed in the length of the ceca between treatments (p > 0.05). The longest large intestine was observed in the control group and the OH treatment of broiler chickens (in both cases, 9.7 cm) (p < 0.01), whereas in the SBP treatment, the gastrointestinal tract was the shortest (8.2 cm). Taking into consideration the total length of the intestines, the longest was found in the control group (210.3 cm) in comparison with the SBP treatment group, with a length of 193.3 cm, at the p < 0.01 level of significance.

3.4. Correlation Between Dietary Fibre Types and Measurements of Intestines and Organs

Table 6. Heat map of Spearman correlation coefficients between kinds of dietary fibre, weight of individual organs, and measurements of the intestines. Abbreviations: TDF—total dietary fibre; IDF—insoluble dietary fibre; SDF—soluble dietary fibre; PW—proventriculus weight; GW—gizzard weight; DL—duodenum length; JL—jejunum length; IL—ileum length; CL—ceca length (average); LL—large intestine length; HW—heart weight; LW—liver weight.

	TDF	IDF	SDF	PW	GW	DL	JL	IL	CL	LL	HW	LW
TDF		0.09	0.64	0.25	0.69	−0.06	−0.01	−0.23	−0.20	−0.13	−0.17	0.17
IDF	0.09		−0.51	−0.47	0.16	−0.24	0.51	0.57	0.46	0.45	0.27	0.36	Correlation coefficient [+/−]
SDF	0.64	−0.51		0.43	0.26	0.16	−0.26	−0.56	−0.23	−0.51	−0.37	0.07	no	0.00–0.35
PW	0.25	−0.47	0.43		0.14	0.31	−0.06	−0.10	−0.22	−0.28	−0.03	0.15	weak	0.36–0.57
GW	0.69	0.16	0.26	0.14		−0.16	0.24	0.12	−0.14	0.09	0.14	0.25	medium	0.58–0.80
DL	−0.06	−0.24	0.16	0.31	−0.16		−0.10	−0.08	0.28	−0.01	−0.23	0.22	strong	0.81–1.00
JL	−0.01	0.51	−0.26	−0.06	0.24	−0.10		0.66	0.51	0.50	0.21	0.56
IL	−0.23	0.57	−0.56	−0.10	0.12	−0.08	0.66		0.52	0.47	0.49	0.52
CL	−0.20	0.46	−0.23	−0.22	−0.14	0.28	0.51	0.52		0.34	0.06	0.55
LL	−0.13	0.45	−0.51	−0.28	0.09	−0.01	0.50	0.47	0.34		0.24	0.10
HW	−0.17	0.27	−0.37	−0.03	0.14	−0.23	0.21	0.49	0.06	0.24		0.35
LW	0.17	0.36	0.07	0.15	0.25	0.22	0.56	0.52	0.55	0.10	0.35

presents the relationship between individual variables calculated using a Spearman correlation.

A moderate positive correlation (p < 0.05) was found between TDF and SDF (r = 0.64) in the chickens’ diets, as well as between TDF in their diet and their gizzard weight (r = 0.69). A weak negative correlation was found between IDF fibre and SDF (r = −0.51) in the digestive tract of chickens and between IDF and proventriculus weight (r = −0.47).

However, a weak positive correlation was found between IDF and the intestines, from the jejunum to the large intestine, inclusive, in the r range from 0.45 to 0.57. A weak positive correlation was also determined between IDF content and liver weight (r = 0.36). The weight of the muscular stomach was positively weakly correlated with the SDF level (r = 0.43) and negatively weakly correlated with the IDF level (r = −0.47). No significant correlations were found between the weight of the glandular stomach and the measurements of the remaining intestinal sections and organ weights (p > 0.05). The weight of the muscular stomach was moderately positively correlated with the amount of TDF in the diet (r = 0.69), and no significant correlations were found for the remaining variables (p > 0.05). In addition, no correlation was found for the length of the duodenum and the remaining variables (p > 0.05). In the case of jejunum length, a moderate positive correlation was found in relation to the level of IDF in the diet (r = 0.51). The increase in the length of this section of the digestive tract was also moderately positively correlated with the increase in the remaining sections of the intestines, with the r coefficient in the range from 0.50 to 0.56. The liver weight was also positively correlated to a similar extent (r = 0.56). In the case of the ileum, moderate positive correlations were found with respect to IDF (positive, r = 0.57) and SDF (negative, r = −0.56).

The length of the ileum was also moderately positively correlated with the length of the jejunum (r = 0.66) and weakly with the lengths of the cecum (r = 0.52) and large intestine (r = 0.47). In addition, the length of this section was weakly positively correlated with the weights of the heart and liver (r = 0.49 and r = 0.52, respectively). The length of the cecum was positively and weakly correlated with the amount of IDF in the feed (r = 0.46), and a weak correlation was also indicated for this variable and the lengths of the jejunum and ileum (r = 0.51 and r = 0.52, respectively). A similar level of correlation (r = 0.55) was observed between the length of the cecum and the weight of the liver. The length of the large intestine was weakly positively correlated with the amount of IDF (r = 0.45), weakly negatively with the amount of SDF (r = −0.51) in the diet of broiler chickens, and weakly positively with the lengths of the small intestine components (r = 0.50 and r = 0.47, respectively). In the case of heart weight, only a weak positive correlation (r = 0.49) was found with the length of the ileum. Liver weight showed a weak positive correlation with dietary IDF (r = 0.36) and heart weight (r = 0.35). Liver weight was also weakly correlated with the jejunum, ileum, and cecum lengths, with r ranging from 0.52 to 0.56.

3.5. Model of the Development of the Gastrointestinal Tract of Broiler Chickens at Day 35

Based on an analysis of the individual variables using a multiple regression model (

Table 7. Multiple regression equations taking into account the effect of dietary fibre and the measurements analysed in this experiment.

DV	Intercept	Coefficients for Independent Variables as Constituents of Equation with Sign												p-Value	R2
		TDF	IDF	SDF	PW	GW	DW	JL	IL	CL	LL	HW	LW
PW	3.855	0.000	−0.043	0.025	-	0.029	0.160	0.068	0.054	−0.173	−0.229	0.130	−0.033	0.125	0.24
GW	−45.687	0.436	−0.101	−0.186	0.787	-	−0.399	−0.065	0.000	−0.044	1.398	−0.134	0.357	0.073	0.30
DW	26.527	0.016	−0.045	0.000	0.732	−0.068	-	−0.267	0.002	0.503	0.818	−0.270	0.202	0.144	0.22
JL	31.861	−0.061	0.072	0.082	0.748	−0.026	−0.640	-	0.299	0.236	1.533 *	−0.578	0.500 *	0.002	0.59
IL	8.718	−0.084	0.138	−0.127	1.065	0.000	0.010	0.539	-	0.585	−0.643	0.638	0.108	0.002	0.58
CL	3.550	−0.097	0.082	0.079	−0.378	−0.004	0.239	0.047	0.064	-	0.066	−0.108	0.159	0.002	0.43
LL	−0.409	−0.016	0.014	−0.018	−0.232	0.053	0.181	0.141 *	−0.033	0.030	-	0.094	−0.093	0.099	0.27
HW	11.913	0.045	−0.032	−0.104	0.443	−0.017	−0.200	−0.179	0.109	−0.169	0.316	-	0.186	0.177	0.19
LW	−32.160	0.157	−0.128	−0.077	−0.335	0.135	0.448	0.461 *	0.056	0.739	−0.930	0.555	-	0.007	0.51

* Coefficients with an asterisk mean significant effects (p < 0.05) for the independent variable in the entire model. R2—fit of the model to the data. Abbreviations in the header: DV—dependent variable; TDF—total dietary fibre; IDF—insoluble dietary fibre; SDF—soluble dietary fibre; PW—proventriculus weight; GW—gizzard weight; DL—duodenum length; JL—jejunum length; IL—ileum length; CL—ceca length (average); LL—large intestine length; HW—heart weight; LW—liver weight.

), a significant effect of the independent variables on jejunal length was found (p = 0.002, R² = 0.59). The length of the large intestine and liver weight have significant effects on the value of the dependent variable.

In addition, the independent variables also had an effect (p = 0.002) on the dependent variables with respect to the ileal and caecal length (R² = 0.58 and R² = 0.43, respectively), but none of the individual variables had a more significant effect than the others (p > 0.05). A significant effect of the independent variables was also found in the case of liver weight (p = 0.007, R² = 0.51), and in this case, jejunal length was a significant component of the regression equation (p < 0.05). A similar trend was also found in the case of equations describing the parameters of the muscular stomach weight and large intestine length (0.073 and 0.099, respectively). However, no significant influence of the independent variables on the dependent variables, such as the weight of the glandular stomach, heart weight, or duodenum length, was observed (p > 0.05).

4. Discussion

4.1. Animal Models in Human Studies

Animal models help explain the nature of phenomena occurring in living organisms. In the case of Gallus domesticus, research has been carried out on adults, but some experiments have also been performed on embryos; as a result, research has been conducted on developmental biology, virology, immunology, oncology, the epigenetic regulation of gene expression, and human diseases using this model organism [4,48,49,50]. Fu and Cheng [51] studied this animal model and linked dysbiosis in the cecum with the occurrence of aggressive behaviour in poultry, the mechanism of which can also be compared with that underlying dysbiosis of the large intestine in humans, as observed in neuropsychiatric studies. The hindgut seems to be the most promising part of the digestive tract of broiler chickens where studies can be conducted on the use of dietary fibre and its effect on the development of individual sections, as well as its relationship with microflora and the stress behaviours related to fibre balance and microbial dysbiosis caused by stressors [52]. Ojo et al. [53], in their meta-analysis and systematic review, emphasised that low concentrations of dietary fibre in a high-fat-and-sugar diet often result in type 2 diabetes. Niekamp and Kim [54], on the other hand, noted an association among microbial dysbiosis in the colon, metabolite production, and an increase in the occurrence of colorectal cancer.

Of course, due to the specificity of animals’ nutrition and the structure of their digestive tracts, not all processes contributing to human nutrition can be replicated in animal models. However, processes that occur in some sections take similar courses, especially when taking into account the structure of the epithelium and its relationship with the symbiotic microorganisms in a given section, which affect the maintenance of intestinal health and cooperation with the body’s immune system [55]. However, as highlighted by Shehata et al. [56], from the point of view of gut health, in the case of poultry, Firmicutes and Bacteroidetes are the dominant groups of microorganisms, while in the case of humans, their ratios constitute a health/metabolism-associated marker. These similarities allow for the results from model organisms to be applied to human studies and, subsequently, for microflora and epithelium regeneration methods to be created. Damage to the microflora and epithelia can also be induced by stress, as observed in studies by Ji et al. [57], where an AA broiler model was used to determine improvements in the intestinal barrier using Bacillus subtilis M6; the results of that study can also be applied to human intestinal barriers. However, in the case of Gallus domesticus, the physiological structure of the upper digestive tract has practically no similarities to the structure of the human digestive tract: Their gizzards compensate for their lack of teeth, the crop helps with storing and preparing food for further digestive processes, and only the glandular stomach performs similar functions to those of the human stomach [58]. The situation is different in the intestines, where processes are comparable with those of other monogastric animals, in this case, with the specificity of digestion in humans. In the case of the cecum, their processes are similar to those occurring in the human colon and are used in research on reducing dysbiosis in this region, with a simultaneous effect on the regeneration of the epithelium during the reconstruction of the mucin layer [56]. Additionally, the digestibility and absorption of nutrients can be studied up to the end of the ileum; the results of such comparisons can be used to create models of processes occurring in the human digestive tract that result in digestive disorders, as well as methods to repair the interaction between digestive microflora and the immune system.

The objective of this study was to compare changes in the digestive tracts of broiler chickens, using Gallus domesticus as a representative laying bird, which is less typical than slow-growing birds but also subjected to genetic selection in terms of body weight gain and the most efficient use of feed [59]. However, even in the case of such a short growth period, adaptations were seen for some sections of the digestive tract to the received type of diet. For example, Tejeda and Kim [60] drew attention to the phenomenon of compensation for the growth of birds as a result of the use of a small amount of additional DF (especially IDF) in the diet, which increases the intake of nutrients and results in better use of the feed. Additionally, the parent flocks of these birds require a special balanced diet to be maintained to enable high egg production and, therefore, a large number of chicks, which can be achieved by diluting the diet to a greater extent than the diet of fast-growing birds [61]. Similarly, in laying hens, their lower energy and nutrient requirements allow for the use of up to 10% lignocellulose [62].

4.2. Diet Differentiation and Dietary Fibre

In the conducted studies, the differentiating factor was various components obtained as by-products of the agri-food industry with different shares of SDF and IDF, which affect the dynamics of digestive processes [19,39]. In addition, based on the collected empirical data, an attempt was made to fit the data to multiple regression equations in order to create a network model of connections of this type of phenomena for the entire digestive tract, from the proventriculus to the large intestine, inclusive. Individual groups were differentiated based on the amount of each structural component (3% of the diet share) rich in DF. Five different diets were prepared, which were isoenergetic and isoproteic for both rearing periods. However, they contained different proportions of IDF and SDF to modify the gastrointestinal development process and the adaptation of the tract to the consumed type of diet and to affect the rate of content flow and, consequently, the degree of nutrient absorption from the lumen of the gastrointestinal tract, which also affects the growth rate of the birds, their feed efficiency, and their health [22,63].

For OH, as a source of lignocellulose, hydrophobic cellulose constituting waterproof cellulose or nanocellulose surfaces that can be used in medicine, pharmaceuticals, agriculture, and industry can be obtained [64], as can pectin from SBP, and further converted into pectinooligosaccharides, which have prebiotic properties in stimulating probiotic bacteria in the gastrointestinal tract [65]. They also positively influence the gastrointestinal immune barrier due to the interactions between the gastrointestinal microbiome, the host intestinal epithelium, and the immune system [66].

4.3. Kind of Fibre and Its Effect on Performance of Broiler Chickens

In poultry nutrition, large amounts of NSPs in a wheat grain diet can cause problems with digestion and absorption of components from the intestinal lumen, affecting not only body weight gain but also changes in the intestinal microflora, which cause deteriorations in litter quality and favourable conditions for the development of pathogenic bacteria [67,68]. To analyse the effect of NSPs through the addition of 3% dietary fibre (soluble and insoluble) from different sources, the chickens were fed starter (1–20 days) and grower (21–35 days) diets consisting of more than 50% NSPs. During balancing, the isoenergetic and isoproteic diets were prepared. The starter diet contained 12.5 MJ of metabolic energy and 220 g of total protein, while the grower diet contained 13.0 MJ of metabolic energy and 200 g of total protein. The differentiating element in the individual diets was the content of TDF and its IDF and SDF fractions. The use of structural components increased the amount of TDF, and the individual diets were differentiated in terms of their IDF and SDF. The 3% OH group showed an increased share of IDF compared with that in the control group, in addition to a practically imperceptible increase in SDF, as this component consists mainly of insoluble fibre. In the other groups, the component (SH, SBP, and WB), used in a share of 3%, was a source of, mainly, soluble fibre, which almost doubled its content in these diets and reduced the content of insoluble fibre compared with the control group.

Table 3 presents the results of rearing chickens from 7 to 35 days of age. During the first few days, the birds were kept together, while on day 7, they were assigned to the different treatment groups, maintaining an average body weight of 133 g within each group. Feeding from 7 to 21 days of life affected the variability in body weight of the broiler chickens. The use of 3% OH significantly reduced the body weight gain (BWG) of the chickens compared with the control group. These results may be due to the birds being fed the control diet from the first to seventh days of life and too high a share of OH during this period, resulting in a change in the diet when the gastrointestinal tract was not yet adapted to the increased absorption of nutrients. Juanchich et al. [69] emphasise, in their studies, that the size of the intestine and size of the organs increase rapidly during the first few days of gastrointestinal tract development. A change in diet requires the adaptation of the chickens’ digestive tract to the new diet, which explains the slower growth of the chickens receiving OH; only later, as a result of the compensation mechanism, did the chickens’ digestive tract allow them to achieve a higher BW than that in the other groups. Similar results were obtained in the studies of Rasool et al. [70], in which birds fed a diet containing 3% OH had significantly lower body weights than those in the control group. This amount of OH during these first few days resulted in significantly lower feed utilisation by the chickens compared with that in the group receiving SH or SBP.

Analysing the studies of Gonzalez-Alvarado et al. [71], feeding OH from the first to tenth days significantly affected (p < 0.01) the increase in BWG and FCR of broiler chickens compared with the control group; during this period, SBP also increased FCR compared with the control group. Interestingly, SH did not significantly affect the BW differences compared with the control group, while in the case of SBP and wheat bran, the difference was significant (p < 0.05). Similarly, in the study by Kimiaeitalab et al. [72], no differences in ADG were found between the control group and chickens fed a diet with 3% SH from 0 to 9 days of life and between 10 and 21 days of life.

During the rearing period from 21 to 35 days, the growth of the birds in the group receiving OH in their diet was compensated for by them consuming the lowest amount of feed, but the adaptation of the gizzard during this period allowed for much more effective use of the feed than during the previous period when the gizzard was not developed; feed use was also the highest during this period, indicating the possibility of using 3% OH without any problems during this period, with the prospect of increasing the amount of OH in the next period or introducing it interchangeably with whole cereal grain. Similar results were obtained in the study by Gonzalez-Alvarado et al. [71], where the use of OH resulted in significantly higher body weight gains compared with those in the control group and the group receiving SBP. Additionally, Kakhki et al. [73] obtained the highest body weight for their chickens on days 14, 28, and 36 of rearing when using OH and slightly worse results when using WB, while the body weights of their chickens were significantly lower (p < 0.05) when SBP was used in the diet. In the conducted experiment, the data on chickens receiving dried SBP and WB were definitely worse compared with those in the other treatments, which indicates that high amounts of SBP and WB cause a deterioration in the motility of the content, reduces the absorption of nutrients, and adversely affects the production results in broiler chicken rearing. This indicates the influence of SBP in the fermentation process and its impact on gut health, or its more effective use in ducks or geese. Similar observations were published in a study by Bamedi et al. [31], who conducted an experiment on Japanese quail; they found a significant deterioration in production parameters when SBP constituted 4% of the diet, with the 2% SBP diet resulting in similar or better production compared with the control group. On the other hand, products obtained from SBP in the form of oligosaccharides have significant applications in human nutrition. Taking into account all the additives, the most stable solution seems to be SH, which practically does not differ in results from the control diet, but over the entire period, the cost of feeding was higher. The most promising, in this case, seems to be the diet containing OH, but to achieve better results, its proportions need to be reduced in the first week to 1% and then gradually increased, up to 5% in broiler chickens reared for longer.

4.4. Morphometric Analysis of the Selected Organs and Their Correlation with Food Fibre in the Diet

The use of a higher amount of TDF, and consequently, SDF, in the diet of chickens through the use SH, SBP, and WB increased the weight of the proventriculus, which produces HCl and protein-degrading enzymes [72]. This study did not determine the weight of the pancreas, but due to the higher viscosity of the feed content, the digestive system responded by increasing the production of amylase [74,75]. The proventriculus responded significantly to the increased amounts of TDF and IDF in the diet, which reduced its weight, while the increased amount of SDF caused its weight to increase. Abdel-Daim et al. [76] emphasise that medium amounts of fibre in the diet affect HCl secretion, which seems to explain the increase in the weight of the proventriculus. In addition, IDF stimulates the proventriculus to secrete more HCl, thanks to which it acts in combination with the gizzard as a barrier against pathogenic bacteria by lowering the pH and affects enzymatic activity and nutrient digestibility [70]. Changes in the weight of the gizzard were mainly caused by the share of oat hull, which allowed for compensation of the birds’ growth after the change in diet on day 7 of the birds’ life. Dixon et al. [77] observed a lack of correlation between the weights of the gizzard and other organs; an increase in the amount of OH in the diet was accompanied by an increase in its weight, while interestingly, ad libitum feeding did not cause significant differences in BW compared with chickens fed a diet with OH. Hikawczuk et al. [63] observed an increase in the weight of the gizzard with an increase in the amount of OH in the diet; additionally, the use of this component and its amount of IDF balanced the content of NSPs in the diet with 50% barley grain, also affecting the weight of the gizzard in chickens fed a 3% OH diet. The correlation analysis in the conducted experiment showed only an increase in gizzard weight when using a higher amount of TDF in the diet, without a significant effect of any of the components, in the form of IDF or SDF, which may also suggest that the proportions of individual types of DF resulting in an increase in the amount of IDF is of significant importance to the increase in gizzard weight. The share of SH, to some (lesser) extent, explains the stability of the increase in BW throughout the rearing period compared with that in the control group. Kimiaeitalab et al. [72] found a significantly higher weight of the gizzard of chickens fed a diet with OH on day 21 of life compared with the birds in the control group. SBP and WB did not have a significant effect on the increase in gizzard weight (p > 0.05). However, in the study by Shang et al. [74], a higher gizzard weight was shown on days 21 and 42 of rearing broiler chickens fed a diet with WB compared with the control group, where the basic cereal component was maize. In the study by Gonzalez-Alvarado et al. [71], when the diet consisted of broken rice, the use of 3% SBP in the diet significantly increased (p < 0.05) the gizzard weight, but a similar experiment using a diet with 3% OH showed significantly increased gizzard weights. Liver weight correlated positively with increased dietary IDF content and increased the small intestine and caecal lengths in response to increased nutrient absorption from the jejunum and ileum and SCFA from the cecum, which was accompanied by increased heart weight.

Yokhana et al. [78] found an increase in liver weight in layer-strain poultry when 1% insoluble fibre was added to the diet of the birds. In addition, Juanchich et al. [60], in their studies, also found that the jejunum is the main section of the intestines that responds first to the type of diet, which may consequently entail the development of subsequent sections and affect the liver weight.

An increase in heart weight can be associated with an increase in blood flow rate caused by the gizzard and absorption in the intestinal section, which compensate for the dilution of the diet with greater absorption of nutrients and more finely divided flowing content. However, little information on this subject exists in the available literature. Gonzalez-Ortiz et al. [79] emphasise that the application of xylanase enzyme in the diet of broiler chickens has no effect on changing heart weight. Wang et al. [80] published a prospective cohort study on the human-related presence of DF with lower LDL cholesterol levels but were unable to perform a comparison with an even larger group of patients using a regression analysis for obvious bioethical reasons of changes in the weight of the heart as a result of increasing amounts of DF in the diet. A study conducted on chickens indicates that DF in the form of IDF increases heart weight, although more studies need to be conducted. A correlation analysis showed that heart weight decreased in response to increased SDF in the diet, while it increased in the case of increased ileum length and liver weight. However, when comparing heart weight, the diet associated with SH resulted in the lowest myocardial weight (9.9 g), while slightly heavier hearts were found in the case of the WB diet (10.1 g). The highest weight was obtained with the OH diet (12.3 g), but this value was not significantly different from that of chickens fed the control diet and the diet containing SBP. Saadatmand et al. [81] also observed, in their study, a significantly higher (p < 0.05) heart weight when an insoluble fibre in the form of 3% rice husk with 110% of threonine requirement was used compared with the 3% and 110% of threonine requirement.

4.5. Morphometric Analysis of Intestinal Sections and Their Correlation with Dietary Fibre in the Diet

The use of OH at a level of 3% significantly shortened (p < 0.05) the length of the duodenum in chickens receiving this diet, compared with chickens fed the control diet. The case with SH showed an opposite trend, with the duodenum elongated and the lengths of the jejunum and ileum shortened (p = 0.09). Shortening was also observed when using SBP (p < 0.05). A correlation analysis did not indicate a significant relationship (p < 0.05) between the length of the duodenum and the remaining variables related to the type of DF and the measurements of the digestive tract. These results are in accordance with those obtained by Scholey et al. [82], who found no difference (p > 0.05) in the duodenum length even when OH constituted 9% of the diet when measured between 21 and 35 days of broiler chickens’ lives, which also emphasises the role of the gizzard in the precise grinding of digesta. Additionally, Jangiaghdam et al. [83], in study with chickens fed a diet where the main cereal was maize, noted that the addition of SH or soybean hull had no significant effects on the length of the duodenum. Saadatmand et al. [81] also found no significant changes in duodenum length compared with the control group in the case of SBP and rice hull. The control group and the group receiving OH showed elongation of the jejunum and ileum, and the use of components rich in SDF caused shortening of the jejunum and large intestine, which suggests the presence of compounds easily absorbed in the gastrointestinal tract, especially in the case of SBP. Sadeghi et al. [25] reported a significant increase in jejunum and ileum length as a result of the use of SBP in chicken diets compared with the control group and the rice hull diet group. A correlation analysis showed that the jejunum responded to the increase in dietary IDF by increasing its length, and its growth also correlated with the growth of other intestinal segments and an increase in liver weight. The ileum lengthened when the diet was richer in IDF and shortened when the diet was richer in SDF. Increasing its length also caused increases in the length of other intestinal segments, as well as in the liver and heart weight, suggesting a response of these organs to the increased transport of nutrients through the portal vein. The cecum increased in length in response to the increase in the proportion of IDF in the diet. The increase in its length was accompanied by an increase in the length of the small intestine and liver weight. Similarly, Jimenez-Moreno et al. [20] also suggest that a small addition of DF affects the morphology of the intestines of broiler chickens. Additionally, the large intestine responded to the type of fibre. SDF caused its shortening, while IDF caused its lengthening, and with the increase in its length, the length of the small intestine also increased, showing a positive correlation. Looking at the changes from the point of view of the overall hindgut, the longest intestine length was found in the control group, which suggests a correlation with the increased content of NSPs in a 50% wheat grain diet. The length of this part increased as a form of adaptation to the consumed diet. The addition of OH shortens the total length of the intestines due to the gizzard working similarly to a mill, but in order to absorb the appropriate amount of nutrients, the digestive tract adapts by lengthening the small and large intestines. Similarly, de Souza Leite et al. [84] found an increase in the length and weight of the intestinal section in response to the addition of lignin to the diet. The greatest shortening of the intestinal length occurred with the use of SBP, which indicates the presence of easily soluble carbohydrates. Wróblewska et al. [18] found a significant reduction (p < 0.05) in total intestinal length following the use of triticale at 30% of the diet compared with a diet based on barley grain.

4.6. Experimental Model Based on Morphometric Measurements and Dietary Fibre Content

In the modelling of multiple regression equations, the effect of soluble and insoluble DF and its interaction in the form of TDF on the weight or length of organs or sections of the digestive tract and its related organs during day 35 of life for broiler chickens were taken into account. This was a long enough time to detect significant differences in the variance analyses depending on the proportions of soluble and insoluble fibre in the diet of broiler chickens [60]. Additionally, Ginindza et al. [85], comparing fast-growing Ross-308 and slow-growing Venda birds, found higher nutrient intake in slow-growing native birds at amounts of 5.7–5.8% of the diet. Modelling the entire process, in this case, took into account the data from all groups, as well as the significance of the tested equation and its fit to the data, in contrast to a previously planned experimental design.

The obtained equations also draw attention to the need to increase the amount of data or to develop a method to better equalise the data at the beginning of the experiment during replications. However, despite the availability of such data, the effects of DF use, including different shares of additional cellulose or lignocellulose in the diet, on the changes in the intestine, ceca, and liver weight and their possible fit to the multiple regression equation have been outlined. Rybicka et al. [86] noted a shortening of the small intestine length (p < 0.05) in chickens fed almond shell, compared with groups receiving lignocellulose or grounded straw. Additionally, Sadeghi et al. [25] noted an increase in liver weight when rice husk and SBP were added to the animals’ diets, compared with SBP alone, but did not find any significant differences in heart weight between their treatments (control, RH, SBP, and RH/SBP). No significant effect of the independent variables analysed in the experiment was observed on the weight of the glandular and muscular stomachs and the length of the duodenum (p > 0.05), which suggests that taking all groups into account and preparing the model did not have a quantitative effect on the final values of these parameters. Changes in the jejunum were significantly associated with changes in the large intestine length and liver weight. Similarly, the constructed regression equation model using all components also showed a significance effect (p < 0.05) on subsequent sections, such as the ileum and cecum, and fit the data better. The significance of the effect of the independent variables on the dependent variable was also indicated in the case of liver weight; among all the components of the equation, the changes in the length of the jejunum were significant. In turn, Oliveira et al. [87] noted a significant reduction in the weights of the proventriculus and gizzard following the use of SDF compared with those following the use of insoluble fibre at 35 days of age for broiler chickens. The use of soluble fibre by the researchers significantly increased the lengths of the small and large intestines.

Analyzing entire model, DF have significant effect on the description of some of changes in the lower part of the digestive tract but does not have a significant effect, in the experiment used, on changes in values in its upper section. An interesting element for the future could be to check the effect of changes in the modelling of regression equations in the case of using one component, e.g., OH, on changes in the digestive tract of chickens, also taking into account individual weeks of the birds’ life.

5. Conclusions

In the conducted experiment, individual diets containing SBP and WB resulted in lower BW and FCR. Diets containing OH and SH resulted in the body weights and FCRs being statistically similar to those in the control group on the final day of rearing. However, regarding morphometric measurements, the diet containing OH rich in insoluble fibre had a positive effect on the increase in the weight of the muscular gizzard compared with the control, SBP, and BW diets. In the case of the OH group, a higher heart weight was also observed compared with the SH group. The chickens fed the diets containing soluble fibre, SH and SBP, were characterised by shortened ileums compared with those in the control group of chickens and those receiving OH in the diet; consequently, the shortest total intestinal length was found in the case of chickens fed SBP. Modelling the process of digestive tract development based on different diets indicates a lack of significant influence of the independent variables on the upper gastrointestinal tract. Under the influence of different sources of DF, significant changes were found in the intestinal sections and the metabolically related liver, which also suggests similar model effects from the use of dietary fibre in these parts of the human alimentary tract. In the case of the multiple regression analysis, no significant effect was found on the heart weight of chickens when taking into consideration all types of structural components. Further studies are required to more precisely create models describing the separate influences of soluble and insoluble fibre on changes in the digestive tract of chickens.