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Birds have an anatomically and physiologically flexible gastrointestinal tract (GIT), which allows them to better adapt to several feeding circumstances (Mtei et al. 2019). Diets rich in fiber cause changes in intestinal morphology, so, depending on the source and inclusion level, the length and weight of digestive organs can be altered, as well as the number of intestinal villi and cell proliferation rate (Jiménez-Moreno et al. 2019).
Palm kernel, fruit of royal palm (Roystonea regia (Kunth) O.F. Cook), is one of the main alternative and energy foods available in Cuba during most of the year. Its chemical composition is acceptable for feeding monogastric animals. It stands out, among other foods, for its high concentration of ether extract (32.25%) and crude fiber (33.95%) (Caro et al. 2015). Studies carried out by Rodríguez et al. (2020) demonstrated the possibility of including up to 15% of palm kernel meal in the ration for broilers. However, in the references, no results were found related to its effect on digestive morphometry of these animals. The objective of this study was to determine GIT morphometric indicators of broilers that consume palm kernel meal in the ration.
An amount of 40 male broilers, 8 d old (HE21), housed in individual metal cages, were used. They were subjected to experimental diets until 42 d, which were formulated according to the requirements established by NRC (1994) for this category of birds. The control consisted of a conventional diet (soybean and corn), and the other three treatments included 5, 10 and 15% of palm meal, so that rations remained isoprotein and isoenergetic for the periods of start, growth and finish. These diets were described by Rodríguez et al. (2020), as well as the procedure for the elaboration of the ingredient meal. During the entire experimentation period, broilers had free access to water and food.
At 42 d, ten broilers were weighed and slaughtered per treatment, according to the jugular vein bleeding method, following traditional procedures (stunning, bleeding and evisceration), exactly two hours and thirty minutes after food ingestion. The method of stunning with electric shock was used before the method of exsanguination by jugular puncture (Sánchez 1990). Subsequently, gizzard, caecum and accessory organs (liver and pancreas) were weighed on a technical scale (SARTORIUS, Germany). Weights were expressed as relative to live weight (LW, g.kg-1).
A completely randomized design was used, with four treatments and ten repetitions, in which the animal represented the experimental unit. For the analysis of results, the statistical package Infostat (Di Rienzo et al. 2012) was used and, when necessary, mean values were compared according to Duncan (1955).
No differences were observed among treatments for the different filled sections of the gastrointestinal tract, so the presence of food, at the time of determination, did not influence the results, since being continuous feeding, the morphometric effect appeared when consuming food in time, product of the digestive process. In relation to control, there was an increase in the size of the gizzard and of empty caeca in 10 and 15% respectively, from the inclusion of palm kernel meal in the ration (table 1).
Table 1 Relative weights of digestive and accessory organs of the GIT of broilers fed palm kernel meal in the ration
| Indicators, g.kg-1 | Palm kernel inclusion levels, % | SE ± P-value | ||||
|---|---|---|---|---|---|---|
| 0 | 5 | 10 | 15 | |||
| Full gizzard | 28.70 | 34.69 | 38.60 | 37.54 | 3.62 | 0.2301 |
| Empty gizzard | 16.89a | 23.28ab | 26.55b | 25.68b | 2.26 | 0.0197 |
| Full caeca | 6.93 | 7.05 | 7.96 | 9.29 | 0.96 | 0.2930 |
| Empty caeca | 3.57a | 4.42ab | 4.31ab | 5.38b | 0.46 | 0.0360 |
| Liver | 21.56 | 20.80 | 20.90 | 23.31 | 1.91 | 0.7763 |
| Pancreas | 2.07 | 1.91 | 2.20 | 2.00 | 0.20 | 0.7767 |
a,b Values with different letters in the same line show differences at P < 0.05 (Duncan 1955)
Results indicate a normal physiological adjustment, with regard to the use of the fibrous component of the diet, in order to make a more efficient use of food. Caro et al. (2015) reported high bulkiness, as well as low solubility in palm kernels, so that the increase of relative weight of the empty gizzard could be due to the increase of the time of permanence of the food in this organ. According to Mtei et al. (2019), a well-developed gizzard reduces its pH with the reinforcement of antiperistaltic reflux in the gastrointestinal tract, improves enzymatic activity because it increases retention time in the digestive tract and, consequently, nutrient digestibility.
The greater mechanical function of the gizzard is physically reflected in its increased weight. Jiménez-Moreno et al. (2019) reported that fibrous sources promote morphological modifications of the gastrointestinal tract. In addition, insoluble fibers of palm kernel are more resistant to grinding in this organ, so they remain in it for longer than the rest of the food particles. As this organ is the mechanical stomach of birds, it acts as a filter for the passage of particles to the duodenum depending on their characteristics, so they remain retained for a longer time until reaching a size determined by the diameter of pylorus, a reason that provokes a greater distention (Mtei et al. 2019).
Nevertheless, the reduction of particle size of fiber favors depolymerization and partial solubilization of the polysaccharides of the cell wall, which could increase viscosity and, therefore, remaining time of food in this organ. Gastric emptying slows down and, as a consequence, its distension increases (Jiménez-Moreno et al. 2019). The size of the particles will always depend on the source of origin and will influence, for this type of insoluble polysaccharides, water adsorption capacity (WAC) of fiber, which is moderate in palm kernel (Caro et al. (2015).
Insoluble structural polysaccharides behave like sponges, so that their WAC depends more on intermolecular spaces than on the surface of contact with water. In this way, they can include water into their matrix and swell, produce a more voluminous digest and, with it, distension of the organ. Scholey et al. (2020) observed a similar performance with the inclusion of oats (Avena sativa) in different percentages of shell in the ration intended for broilers. These authors pointed out that fiber swells during its transit through the GIT in a variable degree, increases bulkiness and chyme weight, and its effect varies depending on its characteristics.
Dietary fiber reaches large intestine of birds and it is digested in the caeca, to a greater or lesser extent, depending on its structure. Therefore, relative weight increase of caeca of animals (table 1) is related to the increase of microbial activity, in function of increasing fermentative capacity before the bulky feed (Jiménez-Moreno et al. 2019), which stimulates tissue growth and distension.
Fermentation produces short chain fatty acids (SCFA), which are absorbed in the caeca and metabolized in the liver to obtain energy (Jiménez-Moreno et al. 2019). Although the pattern of individual SCFAs was not studied in this experiment, weight increase of this organ could indicate an increase of its production. This is the case of butyric acid, which constitutes the main substrate for growth and metabolism of cells of the colonic mucosa.
Regarding accessory organs, no differences were found among treatments for their relative weight. Vives et al. (2020) reported an increase in the activity of pancreatic lipase enzyme with the inclusion of palm kernel meal in the ration of broilers, and, although macroscopically they did not report changes in morphometry, they did indicate an increase of specific functions, which favors digestion process.
It is concluded that the inclusion of palm kernel meal in diets for broilers increased the weight of gizzard and empty caeca, which is related to mechanical and fermentative digestion, for the best use of the fibrous fraction by the animal.
