INTRODUCTION
Soybean meal (SBM) is one of the most used protein sources in poultry and swine nutrition around the globe. Relatively high levels of crude protein (CP), an excellent profile of amino acids (AA), and high AA digestibility are some reasons for the universal acceptability of SBM [1]. Similar to most plant-derived feedstuff, SBM has some anti-nutritional factors such as trypsin inhibitor (TI) and non-starch polysaccharides (NSP) that can limit the bioavailability of SBM protein. Heat processing is one effective approach to eliminate or reduce the level of TI [1]. It has been reported that higher content of fiber in processed SBM leads to lower growth performance of broiler chickens [2]. Researchers stated that there is a relationship between water-soluble xylose content of SBMs and improvement in weight gain when sunflower meal is used to replace some SBM [3]. It has been reported that SBM contains 1.3% β-mannans [4]. β-Mannan, classified as a soluble NSP, is composed of sequential units of mannose complexed with galactose or glucose linked to β-mannan backbone [4–7]. Due to the lack of endogenous NSP degrading enzymes in poultry and swine, NSP cannot be hydrolyzed and digested in their gut [8]. The only known way to digest them is through bacterial fermentation. It has been well documented that the addition of exogenous enzymes such as β-mannanase, xylanase, α-amylase, and β-glucanase can alleviate the anti-nutritional effects of NSP [9,10]. Previously, other researchers reported the anti-nutritional effect of soybean oligosaccharides and soluble NSP in weaning piglets [11,12]. Reducing the fiber content of feedstuff and consequently reducing the concentration of NSP can also be beneficial in improving the performance of poultry and swine. It has been suggested that removing soluble NSP and oligosaccharide by ethanol/water extraction improved intestinal health and growth performance in weaning piglets [13]. Dehulling is another possible approach to reduce the NSP content of SBM. Researches have shown that NSP can increase the viscosity of digesta, modify the physiology of gastrointestinal tract, and change the ecosystem in the gut [14]. It has been stated that a commonly found NSP in feedstuff is β-mannan that can negatively affect the performance of animals [6]. However, there is some evidence showing that β-mannan can stimulate the innate immune system and is potentially capable of stimulating nonproductive energy draining innate immune responses [15–17]. Some researchers have suggested that dehulled SBM contains a higher concentration of protein and a lower level of fiber compared to hulled SBM [18]. Therefore, the objective of this study was to investigate the effect of using dehulled SBM and supplementing the diets with β-mannanase on growth performance, carcass characteristics, excreta microflora, blood constituents, and nutrient ileal digestibility in broiler chickens.
MATERIALS AND METHODS
Animal care
The experimental procedures used in this trial were approved by the Animal Care and Use Committee of Dankook University (approval number DK-1-1516).
Animals, diets, and management
In this experiment, 680 one-d-old Ross 308 (as hatched) broiler chicks with an average initial body weight (BW) of 43±0.54 g were used in a 35-d experimental period. The chicks were sorted into pens (17 birds per pen and 10 pens per treatment) and housed in battery cages (1.55×0.75×0.55 m/cage), in an environmentally controlled room (32°C to 24°C and 65% relative humidity). They were allowed free access to feed and water during the experiment. Each cage was equipped with two feeders (one feeder in each side) and two nipple drinkers.
The treatment diets were: i) T1, 44% CP SBM; ii) T2, T1 +0.05% β-mannanase; iii) T3, 48% CP SBM, and iv) T4, T3+0.05% β-mannanase. Four isocaloric diets were formulated to meet Ross 308 nutrient recommendations (Table 1) [19]. Enzyme preparation used in this study was provided by a local manufacturer (CTC Bio Inc., Seoul, Korea). It was produced by using Bacillus subtilis (WL-1) grown on Luria broth at guaranteed activity of 800 international unit/g (reducing sugar assay). One IU is defined as the quantity of enzyme required to generate 1 μmol of reducing sugar/min, at pH 6.0 and 50°C [19]. According to the manufacturer’s information, β-mannanase was purified from crude solution produced by Bacilli optimized to produce only β-mannanase.
The BW and remained feed were measured on d 0, 10, and 35 to calculate body weight gain (BWG), feed intake (FI), and feed conversion ratio (FCR). Mortality was recorded daily to allow the further correction of FCR. The finisher diet contained 0.2% of chromium oxide (Cr2O3) as marker to allow determination of apparent retention (AR) and digestibility of components.
Sampling
Excreta samples for AR assay; 10 samples per treatment) were collected in 3 consecutive days from d 32 to 35 and stored at −20°C until further analysis. On d 35, 160 birds (40 birds per treatment) were randomly selected (10 birds per treatment were bleed before euthanasia, samples were collected from a wing vein into vacuum tubes) and euthanized by cervical dislocation. Breast muscle, liver, abdominal fat, and empty gizzard were weighed. Ileal digesta (portion of the small intestine from Meckel’s diverticulum to approximately 1cm proximal to the ileo-cecal junction) were collected into sample bags and placed on ice and stored at −20°C.
Sample processing and analyses
Hulled and dehulled SBM were analyzed for CP (method 990.03) [21], ether extract (method 2003.06) [21], ash (method 942.05) [21], acid detergent fiber (ADF; method 973.18) [21], and neutral detergent fiber (NDF) [21]. To determine mannan content, test ingredients were hydrolyzed with 72% (w/w) H2SO4 for 1 h. Samples were then diluted with distilled water to final concentration of H2SO4 at 1N and incubated at 121°C for 45 min. Mannan contents in the hydrolysates were determined using an evaporative light scattering detector and a Shodex sugar column SP0810 (8.0 mm×300 mm; Showa Denko K.K., Tokyo, Japan; Table 2) [23].
Ileal digesta samples were weighed and then freeze-dried at −53°C for 72 h by freeze-dryer (FD5510, Freeze Dryer, Ilshin Lab, Dongducheon, Korea) after which they were finely ground to the size that could pass through a 1 mm screen. Chromium was analyzed by UV absorption spectrophotometry (Shimadzu, UV-1201, Shimadzu, Kyoto, Japan) [24]. The excreta samples were pooled and homogenized, and the moisture content determined by placing in an oven at 80°C for 48 h. Diet samples and air-dried excreta samples were finely ground. All the samples were analyzed for dry matter (DM, method 930.15) [21], N (Leco N analyzer; FP-528; Leco, Saint Joseph, MI, USA), gross energy (GE; IKA bomb calorimeter, C5000; IKA Works, Wilmington, NC, USA), and Cr2O3. The AR of components was calculated [25] as followed:
Where (NT/Cr) diet is ratio of component and chromium oxide in the diet, and (NT/Cr) excreta is ratio of component and chromium oxide in excreta. Component can be DM, N, or GE. The relative weights of breast meat, abdominal fat, and organs were expressed as percentage of live BW.
Serum samples were obtained by spinning blood samples at 3,000×g for 15 min at 4°C. Blood urea nitrogen (BUN), creatinine, and total protein concentrations were measured using an automatic biochemistry blood analyzer (Hitachi 747, Hitachi, Tokyo, Japan).
Statistical analysis
All data were subjected to statistical analysis in a completely randomized design, with a 2×2 factorial arrangement using general linear model procedures of SAS [26]. Each pen was used as an experimental unit. The main effects included different source of SBM and enzyme inclusion. The mean values and standard errors were reported. Probability values of less than 0.05 were considered as statistically significant.
RESULTS
Growth performance
Growth performance results are summarized in Table 3. Using different SBM containing 44% CP or 48% CP in the diets did not significantly affect BWG from d 1 to 10. But supplementing the diets with β-mannanase improved (p< 0.05) BWG during the starter phase. The results showed that feeding chickens with the diet containing SBM 48% CP significantly improved (p<0.05) FCR. It was also observed that the addition of β-mannanase to the diets improved (p = 0.05) FCR from d 1 to 10. The data showed that feeding the chickens with the diets containing SBM with 44% CP resulted in lower (p = 0.02) BWG but addition of β-mannanase to the diets improved (p = 0.02) BWG during the overall experimental period. Both FI and FCR were not significantly affected by treatment diets.
Relative organ weights
Results of the relative organ weights and breast meat yield are summarized in Table 4. The results showed that using different SBM containing 44% or 48% CP or adding β-mannanase to diets did not affect the relative weights of abdominal fat, liver, gizzard, and breast meat yield.
Nutrient digestibility
Results of AR of DM, N, and gross energy are shown in Table 5. The AR of DM, N, and GE were not significantly affected by treatment diets. Apparent ileal digestibility analysis also showed that DM, N, GE, and total essential amino acids (EAA) were not significantly influenced by dietary treatments. Among EAA valine and arginine showed a tendency (p<0.10) to be affected by treatments. Feeding the chickens with the diets composed of dehulled SBM and adding β-mannanase to the diets improved (p<0.05) the apparent ileal digestibility of valine and arginine (Table 5).
Excreta microbiota and blood constituents
The results of excreta microbiota assay presented in Table 4 showed that regardless of the CP level of SBM in the diet, addition of β-mannanase significantly increased (p = 0.002) the count of excreta Lactobacillus; these results also indicated that there was a trend (p = 0.06) in reducing the count of excreta Escherichia coli (E. coli).
Blood sample analysis showed that using different sources of SBM did not significantly affect the concentration of blood urea nitrogen, creatinine, and total protein. Supplementing diets with β-mannanase tended to increase (p = 0.06) the level of serum total protein. The levels of BUN and creatinine were not significantly affected by the addition of β-mannanase to the diets.
DISCUSSION
In general enzymes are added to feeds in order to augment low levels of natural endogenous enzymes or to add novel enzymatic systems not naturally produced by the bird [27]. Several researchers have reported that supplementing enzymes in the diets of broiler chickens improved overall performance [28–30]. It was also reported that replacing hulled SBM (44% CP) with de-hulled SBM (48% CP) led to an improvement in the performance of pigs [18]. Our findings in the current study showed that feeding broiler chickens with diets containing hulled or dehulled SBM supplemented with exogenous enzyme improved BWG at the starter phase. The results of the current study agreed with the results of previous studies showing beneficial effects of adding NSP degrading enzymes to the diet of broiler chickens [31]. Previous researchers reported that broiler chickens fed the diet supplemented with β-mannanase showed 3.5% higher BW compared to the birds fed diets without β-mannanase [32]. It has been reported that the content of β-mannan in broiler chickens starter diet containing 35% dehulled SBM is approximately 0.44% [6]. The results of previous studies suggested that simple sugars released from this amount (0.44% β-mannan) would not be enough to improve the performance of chickens [32]. They concluded that β-mannanase supplementation improved the apparent metabolizable energy (AME) of diets. The results of the current study were consistent with the findings of previous researchers showing that adding β-mannanase to the diet of broiler chickens improved BWG during the starter phase [4,6,33].
It has been reported that highly viscous nature of β-mannans is the main reason for their adverse effect on the function of digestive system. The high viscous nature of β-mannans results in lowering gastric emptying rate, restricting the contact of nutrients with absorptive epithelium [34].
Our findings showed that the addition of β-mannanase to the diets composed of different sources of SBM did not affect the retention of DM, N, and GE. The content of β-mannans in hulled and dehulled SBM was 1.7% and 1.6%, respectively, indicating that there was not a considerable difference between the content of substrate for β-mannanase. The low amount of β-mannans as the substrate of β-mannanase might be the reason why the retention of nutrients was not significantly improved. Previous researchers reported that mannans content of guar gum in broiler feed significantly increased intestinal supernatant viscosity [6]. It has been suggested that the effect of β-mannanase in reducing intestinal viscosity was the solution to overcome the anti-nutritional effect of mannans [35]. Supplementing the diets with β-mannanase resulted in preparing a suitable environment for the growth of Lactobacillus. It has been observed that supplementing broiler chicken diets with β-mannanase improved AR and apparent ileal digestibility of DM during the starter phase [32]. It can be speculated that the addition of β-mannanase to the diet of broiler chickens led to the generation of mannan-oligosaccharides, mannotriose, and mannobiose, as well as a small amount of mannose [32] which might have supported the improvement in BWG during starter phase.
Other researchers supplemented the diet of broiler chickens with β-mannanase and reported that addition of β-mannanase did not influence the blood proteins (albumin α1, albumin α2, albumin β, and γ-globulins) [36]. The results of current study agreed with previously published data. We did not observe any significant effect on BUN showing that source of SBM and/or addition of β-mannanase to the diets did not influence the efficiency of nitrogen which is consistent with the results of previous studies [31,37]. In a trial with growing pigs fed with diets containing hulled or dehulled SBM with or without β-mannanase the results showed that the count of excreta microbiota was not influenced by the source of SBM. They reported that addition of β-mannanase reduced the count of excreta E. coli [38].
CONCLUSION
In conclusion, the results of the current study showed that supplementing the diet of broiler chickens with β-mannanase improved their growth performance. Our findings indicated that dietary β-mannanase could improve the gut environment in favor of beneficial bacteria such as Lactobacillus while reducing the activity of E. coli which ultimately resulted in improving feed efficiency and BWG especially during the starter phase which is the critical stage of commercial broiler chickens.
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