*Corresponding author: Mary Jasmin Ang Department of Basic Veterinary Sciences, College of Veterinary Medicine, University of the Philippines, Los Baños, Laguna 4031, Philippines E-mail: Introduction
As of the first quarter of 2023, the Philippine broiler inventory reached 65.48 million chickens, totaling 470.21 thousand metric tons in liveweight [1]. This represents an increase from 189.71 million chickens and 455.03 thousand metric tons liveweight recorded during the same quarter in 2022 [1]. Despite this increase, the country would still need to import at least 520 thousand metric tons of chicken liveweight annually to meet local demand for poultry meat [2]. Further importation may be influenced by disease outbreaks, supply chain disruptions, and production cost volatility [3]. As a result, increasing interest has centered on Philippine indigenous chicken breeds, particularly in enhancing their use as alternative broiler stocks. This focus is driven by their ready availability, robustness, adaptability to local environmental conditions and prevalent avian diseases, and relatively low maintenance requirements [4,5].
The Darag native chicken, one of the 8 indigenous chicken breeds in the Philippines, is primarily found in Western Visayas and is recognized as the most widely raised native strain in the region [5]. Phenotypic characteristics include wheaten-colored plumage, white earlobes, a medium to large single comb, gray to black shanks, white skin, and orange irises. Darag remains an important meat source, contributing substantially to local food security and gastronomy tourism. The meat is lean and flavorful, high in protein, low in fat, and enriched with unsaturated fatty acids and free amino acids [4-6]. Additionally, Darag eggs reportedly contain lower cholesterol levels compared with many commercial alternatives [4]. However, despite these advantages, Darag and other native breeds continue to be surpassed within the commercial meat supply chain by modern broiler breeds that exhibit substantially faster growth [5]. Their slow growth rate poses a major limitation for efficient poultry meat production and restricts their utility in meeting national demand, which in turn discourages large-scale producers from adopting them as commercial stock. Furthermore, Darag production is commonly undertaken through free-range or backyard systems, resulting in considerable variability in growth performance and mortality rates [7]. Consequently, their overall marketable meat output remains limited, which may contribute to their reduced commercial popularity.
Currently, global poultry markets are experiencing heightened public demand for improved welfare standards in broiler production [8]. European countries have recently shown increased interest in adopting slower-growing commercial hybrid broilers, such as the Redbro, as an alternative to conventional fast-growing hybrids. In a two-year field and pen trial conducted in Europe, Redbro broilers demonstrated competitive growth rates, enhanced feed conversion efficiency, and increased breast meat yield, while maintaining favorable health and welfare indicators [9]. Health-related benefits included stronger leg structure, superior feather quality, and fewer cases of contact dermatitis compared with intensively reared fast-growing broiler strains [10]. In terms of welfare, Redbro broilers exhibited higher levels of physical activity, more frequent perch use, increased social and play behaviors, and quicker responses to environmental stimuli and observers [10]. These characteristics collectively contributed to reduced mortality rates and a lower incidence of carcass downgrade [10].
The Darag breed, also characterized by a slower growth trajectory, may hold comparable potential to Redbro in serving as a competitive meat-producing strain relative to fast-growing broiler hybrids. However, to date, no studies have examined the morphological characteristics of the gastrointestinal tract in the Philippine Darag native chicken, despite the potential implications of intestinal structure for growth performance. Therefore, this study analyzed the histomorphometric characteristics of the small intestine in Philippine Darag native chickens and Redbro broilers.
Materials and Methods
Ethics statements
All procedures conducted in this study were approved by the University of the Philippines Los Baños Institutional Animal Care and Use Committee (UPLB-IACUC; Approval No. UPLB-2023-036).
Animals
Two chicken breeds, the Philippine Darag native chicken and the Redbro commercial hybrid broiler-type chicken, were obtained through the GWAS Project of the Institute of Animal Science, College of Agriculture and Food Sciences, University of the Philippines Los Baños. All birds were reared at the Institute of Animal Science Experimental Poultry Farm and housed in a complete confinement system using elevated cages with pen dimensions measuring 3 ft (W) × 4 ft (L) × 2.5 ft (H) for a total of 16 weeks under controlled experimental conditions. During the initial 21 days, birds were maintained under a 24-hour light cycle at 31℃, followed by a 12-hour light cycle at 28℃ thereafter. Water was provided ad libitum. Birds received a booster mash during the brooding stage (0-21 days), a starter diet during the early growing stage (22-56 days), and a grower diet during the later growing period (57-112 days). Vaccination for Newcastle Disease was administered on day 7 (B1B1) and day 14 (La Sota).
Experimental design
The experiment consisted of 2 groups: the slow-growing Philippine Darag native chicken and the slow-growing Redbro commercial hybrid broiler. One-hundred-day-old male chicks from each group were randomly housed in 10 pens per breed (n = 10 birds/pen) and reared under the conditions described above. Sampling occurred during weeks 1, 2, 4, 6, and 8. Subsequently, 3 birds from each group were randomly chosen and euthanized to obtain the organs, including the small intestinal segments used in this study [11].
Sample collection
Birds were sacrificed at each sampling point via decapitation. A ventral coelomic incision was made to access and excise the gastrointestinal tract from the crop to the rectum. All adhering structures, including mesentery, blood vessels, and fat, were removed. Each intestinal segment was sectioned, weighed, measured for total length, cut into smaller pieces, and fixed in neutral buffered formalin for at least 72 hours [11].
Tissue processing and histological examination
Fixed tissues were washed under running tap water, dehydrated through ascending ethanol concentrations (ChemSupply, Australia), cleared in xylene (RCI Labscan Limited, Thailand), and processed using standard paraffin-embedding techniques. Serial sections of 5 μm thickness were prepared using a microtome (Thermo Scientific, Germany). For every 4 slides made from consecutive serial tissue sections, one was randomly chosen and stained with hematoxylin and eosin according to standard procedures. Histological evaluation of the duodenum, jejunum, and ileum was performed using a research microscope (Amscope T120-5M, China). Morphological features were assessed across the 8-week growth period using ImageJ software (National Institutes of Health, USA).
Histomorphometric evaluation
Histomorphometric analysis was performed using Fiji (ImageJ; National Institutes of Health), following the methodology described by Alshamy et al. [12] with minor modifications. All histomorphometric measurements were performed by a single evaluator who was provided with coded slides and therefore unaware of group assignment. Selection of well-oriented tissue structures followed standard morphological criteria. Within each section, only villi with identifiable villus tips, crypt-to-villus junctions and lamina propria, and an intact epithelial lining were chosen for measurement. For muscular layers, only regions where the inner circular and outer longitudinal layers were distinctly separable and uninterrupted were measured. When consecutive intact villi were not available, the 5 best-preserved villi within the section were selected. Around 5 to 10 measurements per parameter were obtained per tissue section for each animal. All histomorphometric procedures outlined above were performed consistently by the same evaluator to minimize variations and to enhance the reproducibility of the obtained data.
The following parameters were evaluated (1) villus height (VH), (2) crypt depth (CD), (3) villus-to-crypt ratio (VCR), (4) epithelial thickness (ET), (5) mucosal thickness (MT), and (6) tunica muscularis thickness (TMT). MT was measured from the mucosal surface at the villous tip of a correctly oriented epithelium down to the lamina muscularis mucosae. VH was measured from the base of the crypt entrance to the distal tip of full, finger-shaped villi. CD was measured from the base of the crypt to the base of the villus. VCR was calculated by dividing VH by CD. ET was determined by measuring from the basement membrane of the epithelial cells to the apical cytoplasmic border of the enterocytes. TMT was measured from the inner circular layer to the outer longitudinal layer of the tunica muscularis, or muscularis externa. Additionally, the zigzag villus pattern was measured. It is defined as the presence of at least 3 villi within the tissue section displaying ≥ 2 directional changes, resulting in an alternating right-left course along the villus axis. Whereas, the villi in parallel orientation were classified as non-zigzag.
To evaluate potential associations between small intestinal morphology and production traits, previously reported data on body weight, relative organ weight, and total feed consumption for Darag and Redbro chickens were collected at weeks 1, 2, 4, 6, and 8, as described by Nerida et al. [13]. The birds utilized in the present study were part of the experimental birds used in the study of Nerida et al. [13]. The growth performance parameters reported herein were matched at the cohort/week level.
Statistical analysis
All data were analyzed using Prism (GraphPad Software, USA) and are presented as mean ± standard deviation (SD). A mixed-effects model was used to assess the effects of age, strain, and their interaction on duodenal, jejunal, and ileal parameters. To control for potential false-positive findings associated with the mixed-effects model, Sidak’s correction was applied as the post hoc test. Two-tailed Pearson’s correlation coefficients were initially calculated to determine the relationships between histomorphometric parameters and body weight, relative organ weight, and total feed consumption in chickens. Multiple linear regression with the week set as a fixed factor was then performed following Pearson’s correlation to address the potential spurious correlations caused by age and accurately determine the relationships between histomorphometric parameters and these growth performance indices. Because the relationships between histomorphometric parameters and growth performance indices may differ by breed, regression analyses were conducted separately for Darag and Redbro in Jamovi (The Jamovi Project, Australia); hence, breed was not included as an additional covariate. Binomial logistic regression analysis was carried out to determine whether the zigzag villus pattern (1, presence; 0, absence) was associated with age, strain, and their interaction. Separate models were fitted for each intestinal segment (duodenum, jejunum, and ileum). The model included age (in weeks), strain (Darag vs. Redbro), and the age × strain interaction as predictors. The unit of analysis was an individual bird per intestinal segment. Each bird contributed one observation for each segment (duodenum, jejunum, and ileum), and zigzag presence was scored independently for every bird-segment pair. For each breed-week combination, 3 birds were sampled, each providing 3 segment observations. All values with p < 0.05 were considered as significant, p < 0.01 were considered as highly significant, and p < 0.001 were considered as very significant.
Results
Across both Darag and Redbro chickens, regardless of age, duodenal villi exhibited a broad, blunt morphology and were the longest among the small intestinal villi. Jejunal villi were the second longest and characterized by an elongated, leaf-like structure, whereas ileal villi displayed a broad apex and narrower base.
Effects of age, strain, and the age-by-strain interaction
Age exerted a significant or highly significant effect on the variability of multiple histomorphometric parameters throughout the small intestine. Specifically, in the duodenum, age significantly influenced CD, ET, and TMT, and had a highly significant effect on VH, VCR, and MT. In the jejunum, age significantly affected CD and ET and had a highly significant effect on VH, MT, and TMT. In the ileum, age significantly affected ET and MT and had a highly significant effect on VH, CD, and TMT (Table 1).
Strain also demonstrated significant to highly significant effects on multiple parameters. In the duodenum, strain significantly affected CD and ET, had a highly significant effect on TMT, and significantly affected MT. In the jejunum, strain had a highly significant effect on VH and ET and a significant effect on CD and MT. In the ileum, strain significantly affected CD and TMT and had a highly significant effect on VH and ET (Table 1). In contrast, the interaction between age and strain did not exert a statistically significant effect on any measured parameter (Table 1).
Villus height
Regardless of strain, duodenal VH (Fig. A1, Supplementary Fig. S1A) showed a consistent age-related increase (Fig. 1B). In Darag chickens, both jejunal and ileal VH increased with age, except for a slight decline in ileal VH at week 8. In Redbro, the jejunal VH increased from week 1 to week 4, decreased at week 6, and increased again at week 8, while the ileal VH initially decreased at week 2 before approximately doubling from week 4 to week 8 (Fig. 1B). Mean VH across intestinal segments was greater in Redbro than in Darag; however, the differences between strains were only statistically significant in the jejunum at week 1 (Fig. 1B).
Crypt depth
CD (Fig. 1A, Supplementary Fig. S1B) exhibited a positive association with age in both strains across all small intestinal segments (Fig. 1C). Redbro consistently showed greater CD compared with Darag; however, statistically significant differences were identified only in the duodenum at week 2 and the ileum at week 1 (Fig. 1C).
Villus crypt ratio
The VCR (Fig. 1D) demonstrated a general decline with increasing age in both strains across all intestinal segments, except for week 1 measurements in the jejunum and ileum of Darag chickens (Fig. 1D). Although duodenal and ileal VCR values were higher in Darag than in Redbro, these differences did not reach statistical significance (Fig. 1D).
Epithelial thickness
Duodenal ET (Fig. 2A, Supplementary Fig. S2A) demonstrated a consistent age-related increase in both strains (Fig. 2B). Values were consistently higher in Redbro than in Darag, with Sidak’s correction indicating statistically significant differences at weeks 2 and 4. Age-dependent increases in jejunal and ileal ET were also observed in both strains, with significantly higher mean ET recorded in Redbro at weeks 1 and 2 in the jejunum and at weeks 1 and 4 in the ileum (Fig. 2B).
Mucosal thickness
Duodenal MT (Fig. 2A, Supplementary Fig. S2B) exhibited a continuous upward trend with age in both strains, with consistently greater measurements in Redbro compared with Darag (Fig. 2C). A significant strain difference was detected at week 2. Jejunal MT increased until week 4, decreased at week 6, and subsequently increased again at week 8 in both strains. Although Redbro consistently exhibited higher mean MT than Darag, no significant differences were detected using Sidak’s correction. Ileal MT increased progressively in both strains, with greater values in Redbro but without statistically significant differences at any age (Fig. 2C).
Tunica muscularis thickness
In Darag, duodenal and jejunal TMT (Fig. 3A, Supplementary Fig. S3) increased from week 1 to week 4, declined at week 6, and increased again at week 8 (Fig. 3B). In contrast, Redbro exhibited a continuous increase throughout the study period. Redbro consistently demonstrated greater TMT compared with Darag, with significant differences observed in duodenal TMT at weeks 1 and 6 based on Sidak’s correction. Ileal TMT increased with age in both strains, and although values remained higher in Redbro, no statistically significant differences were noted (Fig. 3B).
Zigzag pattern
The earliest expression of the zigzag villi arrangement (Fig. 4A) was observed in Redbro, appearing in the jejunum as early as week 1 and becoming evident across all intestinal segments by week 4 (Fig. 4B). In Darag, the pattern was detected at week 1 in the duodenum but was not observed until week 6 in the jejunum and week 8 in the ileum (Fig. 4B). Age, strain, and their interaction did not exert a statistically significant effect on the presence of a zigzag villi arrangement in any intestinal segment. Across the duodenum, jejunum, and ileum, the overall models and the individual predictors were statistically insignificant, indicating the appearance of zigzag villi arrangement was unassociated with these factors (Supplementary Table S1).
Correlation between body weight and parameters
In the duodenum, highly significant strong positive Pearson’s correlations were identified between body weight and VH, CD, ET, and MT in both strains. TMT also demonstrated a strong positive correlation with body weight, ranging from significant in Darag to highly significant in Redbro. In both strains, VCR exhibited a negative correlation with body weight; however, this association did not reach statistical significance (Supplementary Table S2).
In the jejunum, body weight showed highly significant strong positive Pearson’s correlations with ET and CD in both strains and a significant positive correlation between body weight and VH in Darag only. A strong positive correlation between body weight and TMT was detected exclusively in Redbro. Additionally, a notable negative correlation between body weight and VCR was observed in Redbro (Supplementary Table S2).
In the ileum, a significant strong positive Pearson’s correlation was noted between body weight and MT, while a highly significant strong positive correlation was determined between body weight and ET and CD for both strains. A significant strong positive correlation between VH and body weight was observed only in Darag. In the case of TMT, like those observed in the duodenum, a significant to highly significant strong positive correlation was identified between this parameter and body weight in Darag and Redbro, respectively. In contrast, increases in body weight were associated with significant negative correlations with ileal VCR in both strains (Supplementary Table S2).
Age-controlled analysis using multiple linear regression, however, revealed that only ileal ET shows a statistically significant positive correlation with body weight for both strains. Additionally, an increase in the jejunal VCR in the Redbro strain exerts a statistically significant positive correlation on this parameter. Interestingly, duodenal VH in the Redbro strain and ileal MT and VH in the Darag strain showed a statistically significant negative correlation with the body weight (Supplementary Table S3).
Correlation between relative organ weight and parameters
In the duodenum, VCR significantly increased with the relative organ weight in both strains. Conversely, the relative organ weight demonstrated significant to highly significant strong negative correlations with all other parameters, except TMT, in both strains. In the jejunum and ileum, a higher relative organ weight was significantly correlated with increased VCR in Redbro, but this association was not observed in Darag. Additionally, the ileal MT and both jejunal and ileal ET, CD, and TMT were negatively correlated with the relative organ weight in Redbro only (Supplementary Table S4).
Age-controlled analysis using multiple linear regression disclosed that only the duodenal VCR has a statistically significant positive correlation with the relative organ weight in Redbro but not in the Darag strain. On the other hand, multiple linear regression unveiled that as the duodenal MT in Redbro and jejunal CD in Darag increase, the relative organ weight decreases (Supplementary Table S5).
Correlation between total feed consumption and parameters
Across all intestinal segments in both strains, total feed consumption exhibited a significant to highly significant strong positive Pearson’s correlation with VH, CD, ET, and MT. Total feed consumption was also significantly correlated with TMT in both strains, primarily in the ileum, with statistical significance additionally observed in the duodenal and jejunal segments of Redbro. Meanwhile, a significant negative correlation was established between feed consumption and VCR in the duodenum of both strains. In the jejunum and ileum, however, this significant negative correlation with feed consumption was only detected in the Redbro strain (Supplementary Table S6).
Age-controlled analysis using multiple linear regression revealed that only the ileal VH shows a statistically significant positive correlation with feed consumption in the Darag but not in the Redbro strain. The jejunal CD, on the other hand, displayed a statistically significant positive correlation with feed consumption in the Redbro strain only (Supplementary Table S7).
Discussion
This study demonstrated that both Darag and Redbro chickens exhibited segment-specific villus morphologies that were consistent across age groups. These findings corroborate previous evidence of ingesta-driven structural adaptation within the intestine [12-14]. Duodenal villi displayed a broad base with a blunt apex, a configuration that likely reflects the segment’s role as the first site to receive ingesta from the gizzard [15]. This structure provides a wide surface area that facilitates rapid initial nutrient absorption while conferring mechanical resilience to withstand the high volume and physical force of partially digested feed. Jejunal villi were long, slender, and leaf-like, a morphology that maximizes the absorptive surface area by extending into the lumen [15]. Such structure supports the jejunum’s primary function in major nutrient absorption. In contrast, ileal villi were shorter with a broader apex tapering toward the base, consistent with the ileum’s role in processing fluid and nutrient-depleted ingesta [15]. The expanded apex provides a wide surface area for reabsorption of water, minerals, and residual nutrients, while the short and sturdy body is more efficient for slower-moving ingesta. Collectively, these variations confirm functional specialization along the intestinal axis. Importantly, the conserved villus architecture observed across both breeds indicates shared fundamental intestinal structure despite underlying genetic differences.
The observed increases in VH and CD across all small intestinal segments in both strains suggest a metabolic adaptation to the progressively heightened nutritional demands associated with growth and maturation [16]. Similar developmental trends have been reported in multiple chicken strains by Alshamy et al. [12], Wang and Peng [16], Marchewka et al. [17], and Hiżewska et al. [18]. The concurrent decline in VCR, likely driven by crypt deepening, may reflect an increased turnover of absorptive enterocytes that supports VH and enhances nutrient uptake, or alternatively, a shift toward a higher proportion of secretory cells, which could compromise absorptive efficiency [19]. However, the maintained VH, absence of intestinal lesions, and overall normal histological appearance observed in this study indicate that crypt deepening was more likely associated with improved absorptive capacity rather than a pathological or protective response.
ET and MT significantly increased with age across all small intestinal segments in both strains, reflecting immunophysiological adaptation to mechanical and chemical stress associated with increased feed intake and greater pathogen exposure during growth [20,21]. These findings align with previous studies reporting age-dependent increases in epithelial and mucosal turnover in the proximal intestine [17,22,23]. The gradual proximal-to-distal reduction in ET and MT is consistent with the distinct physiological demands of the duodenum, jejunum, and ileum. As the primary site of entry for partially digested feed along with bile, pancreatic enzymes, and gastric secretions, the duodenum requires reinforced epithelial and mucosal layers for both digestion and protection [15].
The age-dependent increases in TMT observed throughout the intestine in both strains suggest progressive enhancement of mechanical digestive capacity and intestinal motility, supporting increased nutrient requirements and feed intake during growth [12,18]. Similar developmental trends have been described in other broiler strains [12,16,18]. Notably, TMT was consistently greater in Redbro compared with Darag, with significant differences particularly evident in the duodenum. This suggests that Redbro may possess superior propulsive and motor control capabilities that enhance mixing and regulated transit in region critical for the initiation of digestion. The earlier and more pronounced thickening of duodenal TMT in Redbro as early as week 1 may indicate a genetically driven advantage, promoting earlier functional maturity of the proximal intestine, consistent with observations in genetically enhanced commercial broiler lines [12].
Redbro exhibited the greatest TMT in the ileum, a pattern consistent with previous reports [12,18], although the difference was not statistically significant. This feature may indicate prolonged transit within the ileum, resulting in tunica muscularis hypertrophy [15,18]. Such adaptation may enhance mechanical propulsion and nutrient absorption during the final stages of digestion, as similarly emphasized by Alshamy et al. [12]. In contrast, Darag exhibited the greatest TMT primarily in the jejunum. This pattern may represent a compensatory adaptation to its reduced growth rate and lower feed intake, promoting stronger segmental contractions in the primary site of nutrient absorption to improve nutrient uptake efficiency. Alshamy et al. [12] also proposed this mechanism in slower-growing dual-purpose breeds. Alternatively, given that the ileum is generally associated with the slowest intestinal transit time [15], the presence of a thicker jejunal TMT relative to the ileum in Darag suggests more rapid proximal intestinal propulsion. In this context, hypertrophy of the jejunal tunica muscularis may reflect increased propulsion rather than enhanced motility control. Although this would accelerate ingesta movement through the principal absorptive region, it may concurrently reduce nutrient exposure time, thereby limiting absorption efficiency and contributing to the comparatively slower growth rate observed in Darag.
While TMT itself does not directly participate in nutrient absorption, its thickness may indirectly influence absorptive efficiency through regulating intestinal transit. Thicker TMT could either enhance propulsion, accelerating ingesta movement, thereby reducing mucosa-nutrient contact time, or it could sustain segmental contractions, retaining ingesta and prolonging mucosa-nutrient contact time. Since intestinal contractions are not linear (simultaneous holding, mixing, and forward propulsion of ingesta), thicker TMT may retain, mix, and propel ingesta. Faster propulsion may be beneficial when it promotes faster delivery to the jejunum, the main site of absorption, whereas slower transit may improve nutrient absorption, especially useful for the ileum with residual nutrients. The differences between breeds may represent distinct adaptive strategies among breeds [12,18].
In all intestinal segments, the zigzag villi pattern emerged earlier in Redbro than in Darag, except for the duodenum, where the first appearance occurred in Darag at week 1, followed by Redbro at week 4. The earlier development observed in Redbro likely reflects genetically driven enhancements in intestinal adaptation. Yamauchi [14] previously associated such morphological features with improved feed intake efficiency and rapid growth in genetically enhanced broiler strains. The early presence of the zigzag pattern in the duodenum of Darag at week 1 may indicate a critical functional phase of early absorptive activity. Parallel villi allow ingesta to move in a straight feed flow, whereby ingesta passes relatively faster along the intestinal lumen [14]. The zigzag arrangement of the villi, on the other hand, makes the path more complicated, which slows down the movement of ingesta and increases the time that nutrients are in contact with the mucosa. This could improve nutrient absorption or uptake [14]. The zigzag structure is known to increase the surface area of the villi and slow down the movement of ingesta to improve nutrient absorption [14,21]. However, keeping ingesta in the duodenum for a long time may delay the transfer to the jejunum, which is the main part of the body that absorbs nutrients. This situation could disadvantage Darag by reducing nutrient delivery efficiency, potentially contributing to its comparatively slower growth. In contrast, the zigzag pattern developed in the jejunum as early as week 1 in Redbro, but not until week 6 in Darag. Given the jejunum’s central role in nutrient assimilation [15], the earlier development of this morphology in Redbro may have provided a significant absorptive advantage during the early growth period, facilitating its more rapid weight gain. While Darag exhibited the zigzag configuration early in the duodenum, the earlier enhancement in Redbro’s jejunum appears to be more consequential for growth performance. Redbro also achieved a fully consistent zigzag pattern across all intestinal segments by week 4, whereas Darag did not reach the same level of consistency until week 8. This suggests an accelerated timeline of intestinal morphological optimization in Redbro, likely reflecting underlying genetic improvements, consistent with findings reported by Yamauchi [14] and Pelicano et al. [21] in genetically enhanced broiler breeds.
To date, no published studies have directly examined the relationship between various histomorphometric parameters like VH, CD, CDR, ET, MT, or TMT and body weight in chickens. In the present study, Redbro exhibited significantly higher VH and CD than Darag throughout all small intestinal segments. Because both strains were raised under identical environmental and nutritional conditions, these differences are likely attributable to genetic factors. Despite having lower VH and CD values, Darag demonstrated statistically significant positive Pearson’s correlations between VH and body weight across all intestinal segments, whereas in Redbro, this association was limited to the duodenum. These findings suggest that, although Redbro achieves greater VH and CD due to inherent genetic advantages, Darag may benefit from more consistently distributed absorptive efficiency throughout the intestine. Such adaptive capacity may allow Darag to more effectively utilize nutrients from lower-quality or nutrient-deficient diets, promoting steadier growth under constrained production systems. In contrast, Redbro’s strong reliance on duodenal absorptive performance indicates that any compromise in proximal intestinal function or nutrient availability early in the digestive tract may disproportionately hinder its growth compared with Darag. This distinction underscores the different physiological strategies employed by these 2 strains for nutrient assimilation and growth support.
In this study, the mixed effects model also revealed statistically significant positive correlations between body weight and both ET and MT across the small intestine in both strains. However, multiple linear regression unveiled that only ileal ET shows a statistically significant positive correlation with body weight in both strains. From a physiological standpoint, thicker epithelial and mucosal layers require additional energy and nutrient allocation to support continuous renewal and functionality [24]. Darag may therefore exhibit greater long-term metabolic efficiency by maintaining comparatively thinner intestinal layers while channeling nutrients toward other growth-supporting physiological processes. Because ET and MT also function as crucial immunologic barriers, the comparatively thinner ET and MT in Darag may likely represent a balance between structural sufficiency and energy conservation, which may suggest potential advantages under low-input and cost-sensitive production conditions.
Likewise, there is currently no documented evidence that determines the statistical correlation between histomorphometric parameters, like ET and MT, with total feed consumption in chickens. This study demonstrated strong, statistically significant positive correlations between total feed consumption and ET and MT in all intestinal segments for both strains. Although Darag displayed thinner epithelial and mucosal structures than Redbro, the strong statistical correlations observed between TFC and both ET and MT in the breed suggest that Darag’s comparatively thinner ET and MT may remain functionally responsive to variations in feed intake. This responsiveness may suggest potential robustness under fluctuations in diet quality or environmental stress. It may also indicate potential suitability for free-range or minimally managed systems, where birds are more frequently exposed to variable feed sources and lower biosecurity standards compared with intensive commercial operations [25].
The statistically significant positive correlations observed between body weight and total feed consumption suggest that targeted interventions, such as nutritional supplements, probiotics, or prebiotics, may enhance epithelial development in Darag and potentially improve growth performance [26]. Given that Darag typically exhibits a slower growth rate compared with Redbro, improvements in intestinal structure and function that support nutrient absorption may yield proportionally greater gains in productivity [26]. These findings also support the potential utilization of Darag in breeding programs aimed at producing dual-purpose hybrids that combine the intestinal resilience and metabolic adaptability of Darag with the rapid growth characteristics of commercial broiler strains, resulting in birds that are robust and performance efficient. Interestingly, jejunal TMT in Darag showed a positive but non-significant correlation with body weight, whereas all other intestinal segments demonstrated statistically significant positive correlations in both strains. This suggests that jejunal TMT may exert a comparatively smaller influence on body weight in Darag relative to duodenal and ileal TMT. Although no published studies have directly linked TMT to body weight, these observations raise important questions regarding potential differences in regional intestinal transit patterns between strains. Native breeds such as Darag may rely on distinct segmental motility adaptations to compensate for slower growth. In this context, increased jejunal TMT in Darag may indicate enhanced propulsion rather than improved motility control, which could reduce mucosal contact time and limit absorptive efficiency, thereby contributing to slower growth performance.
Additionally, age-controlled analysis utilizing multiple linear regression indicated that only ileal ET and ileal VH exhibit statistically significant positive correlations with body weight and total feed consumption, respectively, for both strains. Future research should involve larger sample sizes and additional time points, ideally encompassing the 16-week production period, to validate these findings and elucidate the potential relationship between intestinal histomorphometry and growth performance indices.
In conclusion, this study identified distinct age- and breed-dependent developmental patterns in the small intestine of slow-growing broiler strains. Redbro demonstrated accelerated morphological maturation aligned with rapid growth, whereas Darag exhibited more gradual but comprehensive intestinal development, supporting its suitability for low-input production environments. The strong correlations observed between VH, CD, ET, and MT and growth performance highlight the complex and integrated roles of intestinal morphology, motility, and absorptive capacity in optimizing nutrient utilization.
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