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Studies carried out by Martínez-Pérez et al. (2021) demonstrated that palm kernel meal, fruit of royal palm (Roystonea regia H. B. K. Cook), is a good source of fiber and fat (NDF 72.55 %, ADF 55.84 % and EE 16.06 %). Due to these characteristics, up to 15 % can be used in the feeding of broilers, without affecting animal health and with economic improvements (Rodríguez et al. 2020).
Vives et al. (2021) observed an increase of relative weight of the cecum broilers that consumed palm kernel meal. According to these authors, this could indicate an increase of microbial activity in order to increase the fermentative capacity. In general, it is considered that the caecum of the digestive tract of broilers constitute the main site where fiber digestion occurs, due to their great fermentative activity because of the presence of microorganisms (Adebowale et al. 2019). According to Jiménez-Moreno et al. (2019), when animals consume bulky foods, tissue growth and distension of the large intestine of animals are stimulated. Therefore, the objective of this study was to determine the effect of palm kernel meal on the population of total and cellulolytic bacteria and cecal fermentative indicators of broilers.
The experimental study was carried out in the poultry unit of the Institute of Animal Science (ICA) of the Republic of Cuba. Twelve 21-day-old male broilers (HE21) were used, with a mean initial live weight of 750 ± 1.5 g. They were randomly housed individually in galvanized wire metabolic cages (40 x 40 x 80 cm). Each one had a feeder and two nipple-type drinkers. Water intake was ad libitum.
A completely randomized design was used, with four treatments and three repetitions. Each animal constituted an experimental unit. The control consisted of a conventional corn-soy bean diet. In the rest of the treatments, 5, 10 and 15 % of palm kernel meal was included, so that they remained isoproteic and isoenergetic throughout the experimental period. Diets and meal were prepared according to the procedure described by Rodríguez et al. (2020).
At 42 d of age, the animals were weighed and sacrificed according to the jugular vein exsanguination method, according to traditional procedures (stunning, exsanguination and evisceration), exactly two hours and thirty minutes after food ingestion. After sacrifice, cecum was separated and its contents were collected under a CO2 atmosphere.
The determination of microbial concentration was carried out by the method of serial dilutions of cecal samples, cultivated in tubes rolled with selective synthetic media, in anaerobiosis. They were incubated at 37 ºC and the visual colony count was expressed in colony forming units per gram (cfu.g-1). Total bacteria were cultivated in medium 10 (Elías 1971) from 10-10 to 10-12 dilutions. Cellulolytic bacteria, in a similar medium, in which the carbohydrate sources were replaced by 1 % powdered cellulose and 0.03 % cellobiose. It was cultivated in dilutions 10-4 to 10-6. Additionally, it was determined in the cecal content and dry matter (DM) according to AOAC (2019). Regarding the fermentative indicators, the digital pH meter (WPA brand, CD-70 series of English manufacture) was used to determine the pH and for ammonia (NH3), the procedure was followed according to Chaney and Marbach (1962).
For the analysis of the results corresponding to the concentrations of total bacteria and cellulolytic bacteria, the methodology proposed by Herrera et al. (2015) was used. Counting data were transformed by Ln and did not improve assumptions, so a completely randomized nonparametric Kruskall-Wallis ANOVA was used. For the comparison of mean ranges, Conover (1999) test was applied. In the case of fermentative indicators, simple classification analysis of variance was carried out. For the comparison between means, Duncan test was used (P<0.05). The Infostat (Di Rienzo et al. 2012) statistical package was used.
Table 1 shows the concentration of total and cellulolytic bacteria present in the cecum of animals that were fed different proportions of palm kernel meal. Total bacteria did not differ from the control with the inclusion of this source in the described quantities. However, from 10 % higher values were presented than those achieved with 5 % inclusion.
Table 1 Effect of the inclusion of palm kernel meal on microbial growth in the cecum of broilers.
| Bacteria | Control | Inclusion of palm kernel meal, % | p-value | ||
|---|---|---|---|---|---|
| 5 | 10 | 15 | |||
| Total (1011 cfu.g-1) | 18.67ab (45.00) SD = 27.72 | 7.06b (21.00) SD =11.40 | 26.22a (72.00) SD = 46.51 | 22.06a (52.00) SD = 47.80 | 0.0009 |
| Cellulolytic (106 cfu.g-1) | 17.78b (14.00) SD = 2.40 | 27.44a (16.00) SD = 2.32 | 19.67ab (14.00) SD = 2.65 | 9.11c (8.00) SD = 4.77 | 0.0030 |
a,b,c Mean ranges with different letters differ significantly according to Conover (1999)
() Original means between parenthesis, SD: standard deviation
According to Adebowale et al. (2019), with the increase of microbial concentration, the production of short-chain fatty acids also increases, especially butyric acid, which makes the organ mucosa grow. The increase in the relative weight of caeca (1.81 percentage units when including 15 % of palm kernel meal in the ration compared to control) obtained by Vives et al. (2021) suggests the increase of microbial activity. However, this is not demonstrated in the present study, which could be related to the ability of living organisms to maintain balance in the intestinal biota, even when the relative concentrations of some species are modified. It would be interesting for future studies to delve into this aspect with the application of molecular techniques that are more precise.
The adaptive response of cellulolytic microorganisms to the fiber values of the diet of broiler s is known (Rodríguez et al. 1996). However, cellulolytic bacteria showed the best values with 5 % of inclusion and the lowest with 15 %. Meanwhile, they were intermediate with 10 % and did not differ from those obtained with the control treatment and the lowest level of palm kernel meal. These results could be related, among other factors, to the physicochemical properties of the fibrous fraction of the source under study, the presence of secondary metabolites and the retention time.
The low solubility (12.77 %) of the studied source increases the transit speed and thus decreases the retention time of particles (Martínez-Pérez et al. 2021). Likewise, the high presence of soluble non-starch polysaccharides increases digesta viscosity, which also forms a barrier between enzymes and substrate, with an anti-nutritional effect (Jiménez-Moreno et al. 2019). All this could reduce, with the highest percentage of inclusion, food entry into the organ and with it, the accessibility of microorganisms to nutrients. The above can be related to the effect observed in the DM of cecal content, which was lower with respect to the rest of the treatments (table 2).
Table 2 shows fermentative indicators of cecum of broilers that consumed palm kernel meal. The pH was low, 5 and 15 % with respect to the 10 % of palm kernel meal inclusion and the control treatment did not differ from the rest. These variations are within normal values for the organ. In studies carried out by Martínez-Pérez et al. (2021), palm kernel meal showed greater buffering capacity against acids compared to bases. This property of the ingredient shows that it can contribute to maintaining pH conditions in the gastrointestinal tract of monogastric species. This is really important, since during the digestion process a pH change occurs, from very acid (1) to close to neutrality (6.8-7.2).
Table 2 Effect of the inclusion of palm kernel meal on dry matter and fermentative indicators of the caecum of broilers
| Indicators | Control | Inclusion of palm kernel meal, % | SE± | p-value | ||
|---|---|---|---|---|---|---|
| 5 | 10 | 15 | ||||
| DM, % | 19.72a | 23.10a | 24.71a | 8.28b | 1.97 | 0.0015 |
| pH | 7.13ab | 6.89b | 7.44a | 6.88b | 0.09 | 0.0110 |
| NH3, mg /L | 12.21a | 7.11c | 9.87b | 3.86d | 0.51 | <0.0001 |
a,b,c,d Means with different letters in the same row differ significantly according to Duncan at p<0.05
Regarding ammonia, all treatments differed from each other and were lower as the inclusion level of palm kernel meal increased. This decrease is perhaps the result of increases in absorption, the highest synthesis of microbial protein. Even the values reached with 15 % of inclusion could be the result of limitations in nutrient availability, considering the decrease of DM in the organ. This hypothesis needs to be confirmed.
In general, a reduction in the studied indicators was observed with the highest level of inclusion of palm kernel meal, except in total bacteria. This result could be explained because, unlike the first two, the concentration of insoluble fibrous material in the ration increases, which could cause an increase in the speed of intestinal transit, and thus reduces food stay time for the development of fermentative processes and changes in the concentrations of total and cellulolytic bacteria in the cecum.
It is concluded that the inclusion of 15 % of palm kernel meal maintains the concentration of total bacteria and decreases cellulolytic bacteria, as well as DM and NH3 in the cecum of broilers. As these are the first microbiological studies carried out with palm kernel meal, it would be of interest that future research, through the application of molecular techniques, delve deeper into the variations that occur in other species.
