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Journal of Animal Science - Animal Nutrition

Digestibility and metabolizable energy of raw soybeans manufactured with different processing treatments and fed to adult dogs and puppies


This article in JAS

  1. Vol. 91 No. 6, p. 2794-2801
    Received: Sept 2, 2011
    Accepted: Mar 12, 2013
    Published: November 25, 2014

    1 Corresponding author(s):

  1. A. P. Félix,
  2. C. P. Zanatta,
  3. C. B. M. Brito,
  4. C. M. L. Sá Fortes,
  5. S. G. Oliveira 1 and
  6. A. Maiorka
  1. Universidade Federal do Paraná, Rua dos Funcionários, 1540 Curitiba, Paraná, Brazil 80035-050


The objective of this study was to evaluate the apparent total tract digestibility (ATTD), ME, and fecal characteristics of adult dogs and puppies fed raw soybeans (RSB) and their by-products. Six treatments were evaluated: 1 reference diet (REF), based on a maize-poultry by-product meal, and 5 extruded diets containing 70% of the ingredients of the REF diet and 30% of a soybean processed product [defatted soybean meal (DSM), micronized soybeans (MSB), soybean meal (SBM), RSB, or toasted soybeans (TSB)]. Six adult dogs (5.8 yr old) and 6 puppies (5.1 mo old) were used in a study with a double Latin square design (6 × 6). Urease was reduced in all diets after extrusion, but trypsin inhibitor was reduced only in the diets containing SBM, DSM, and RSB. The ATTD of CP in DSM, SBM, MSB, TSB, and RSB were 85.1%, 85.2%, 88.4%, 84.7%, and 78.9%, respectively, for adult dogs. Soybean meal and DSM had the lowest ATTD of acid-hydrolyzed fat (AHF; 84.3% for both ingredients in adult dogs). The ATTD of DM and AHF in DSM and AHF in all soybean products were greater in puppies than adult dogs (P < 0.05). The ME content was greatest in MSB (21.39 MJ/kg) and least in DSM (15.23 MJ/kg). The feces of dogs fed soybean products were softer and had a lower pH (average of 5.91 vs. 6.05 for adult dogs fed soybean products and REF diets, respectively) and ammonia content (average of 3.82 vs. 4.32 g/kg for adult dogs fed soybean products and REF diets, respectively), except those fed RSB, which had similar fecal pH and ammonia values, compared with those fed the REF diet. Soybean products are good protein sources for both adult and growing dogs, provided they are heat treated before diet extrusion.


Most metabolism trials with dogs evaluate the effects of the inclusion of a specific ingredient on diet digestibility and not on the digestibility of the ingredient itself. Consequently, specific information on the nutritional quality of raw materials used in dog foods, particularly in puppy foods, is lacking. Among the protein feedstuffs used in commercial foods, for both adult dogs and puppies, soybean products, such as soybean meal (SBM), defatted soybean meal (DSM), toasted soybeans (TSB), and micronized soybeans (MSB), are a good supply of highly digestible protein (Clapper et al., 2001; Cavalari et al., 2006; Carciofi et al., 2009) and allow reduction in macromineral levels in complete commercial foods. The SBM is the most frequently used soybean product in dry extruded dog foods and included at rates ranging between 5.0% and 30.0% of the diet. However, because of its high demand for use in poultry and swine feed, this feedstuff is becoming increasingly expensive and difficult to obtain. One of the alternatives to SBM is full-fat soybeans, which have high protein (33.0 to 38.0%) and fat (18.0 to 22.0%) content (Mendes et al., 2004).

However, to be used in nonruminant diets, soybeans must be heat processed to deactivate the antinutritional factors present in raw soybeans (RSB). The trypsin inhibitor is the most important of these factors, as it impairs protein digestion (Rackis, 1972). The extrusion of commercial dog foods effectively inactivates protease inhibitors present in RSB because of the high temperatures used during this process (110 to 180°C; Purushotham et al., 2007). However, the extrusion of foods that already contain heat-treated soybean products, which are commonly used in commercial settings, may result in over heating these products, thus compromising the availability of their AA to animals. The present study was designed to test the hypothesis that the under heating or over heating of soybean products may reduce the use of their nutrients and energy by dogs. We also hypothesize that puppies have different food nutrient absorption capacity and fecal characteristics than adult dogs.


The experimental procedures were approved by the Animal Ethics Committee of the Sector of Agricultural Sciences of the Federal University of Paraná, Brazil.

Animals and Facilities

Twelve healthy Beagles, 6 adults (5.8 ± 0.1 yr old, 13.4 ± 1.7 kg BW, and 3 females and 3 males) and 6 puppies (5.1 ± 0.2 mo old, 5.6 ± 0.4 kg BW, and 3 females and 3 males) were used for the experiment. The dogs had previously been dewormed and vaccinated. During the digestibility trial, dogs were individually housed in 0.7-m long × 0-6 m high × 0.5-m wide stainless steel metabolic cages.


A reference diet (Table 1) was formulated with a nutritional profile for growing dogs in excess of Association of American Feed Control Officials (AAFCO, 2003) recommendations (Table 2). The other 5 diets (Table 2) included 70% of the ingredients of the reference diet and 30% of SBM, RSB, TSB, MSB, or DSM. The chemical composition and qualitative variables of soybean protein products included in the diets are shown in Table 3.

View Full Table | Close Full ViewTable 1.

Ingredients of the reference diet (as-fed basis)

Ingredients Content, %
Maize (yellow dent) 59.08
Poultry by-product meal 26.14
Corn gluten meal, 60% CP 5.71
Bovine fat 5.00
Dried hydrolyzed poultry liver 1.86
Calcium propionate 0.17
Potassium sorbate 0.03
Mineral and vitamin supplement1 1.30
Sodium chloride 0.71
1Provided per kilogram of feed: vitamin A (retinol) = 20,000 IU; vitamin D3 = 2,000 IU; vitamin E (α-tocopherol) = 48 mg; vitamin K3 = 48 mg; vitamin B1 = 4 mg; vitamin B2 = 32 mg; pantothenic acid = 16 mg; niacin = 56 mg; choline = 800 mg; Zn as zinc oxide = 150 mg; Fe as ferrous sulfate = 100 mg; Cu as copper sulfate = 15 mg; I as potassium iodide = 1.5 mg; Mn as manganous oxide = 30 mg; Se as sodium selenite = 0.2 mg; antioxidant = 240 mg.

View Full Table | Close Full ViewTable 2.

Analyzed DM content and chemical composition of experimental diets1

DM, % 90.97 91.78 91.85 92.94 92.08 92.44
Chemical composition (DM basis)
    CP, % 24.62 31.16 30.98 29.57 28.76 28.58
    Acid hydrolyzed fat (AHF), % 11.01 8.72 10.26 13.44 14.42 14.50
    Crude fiber, % 1.35 1.10 1.74 1.06 1.65 1.63
    NDF, % 3.78 3.42 4.53 3.13 4.38 4.35
    Ash, % 7.00 6.87 6.86 6.65 6.74 6.76
    Ca, % 1.43 1.51 1.23 1.24 1.43 1.39
    Total P, % 1.03 1.05 0.91 1.00 0.94 0.92
    N-free extract (NFE),3 % 56.02 52.15 50.16 49.28 48.43 48.53
    ME,4 MJ/kg 15.7 15.3 15.5 16.3 16.4 16.5
1REF = reference diet; DSM = defatted soybean meal; SBM = soybean meal; MSB = micronized soybeans; TSB = toasted soybeans; RSB = raw soybeans.
2The experimental diets consisted of 70% of the REF diet and 30% of the ingredient tested per kilogram (as-fed basis).
3Estimated as NFE = 100 – (ash + CP + AHF + crude fiber).
4ME = 0.01465 × CP + 0.03558 × AHF + 0.01465 × NFE.

View Full Table | Close Full ViewTable 3.

Analyzed DM content, chemical composition, and qualitative variables of soybean protein products1

Soybean products
DM, % 93.91 89.22 95.73 89.71 89.50
Chemical composition (DM basis)
    CP, % 52.41 46.72 40.80 37.62 37.64
    Acid-hydrolyzed fat (AHF), % 2.62 4.11 21.54 23.40 23.12
    Ash, % 6.14 6.10 4.70 4.98 4.90
    Crude fiber, % 1.60 5.34 1.53 4.13 4.05
    NDF, % 8.91 14.72 13.90 10.60 10.42
    N-free extract (NFE),2 % 37.23 37.73 31.43 29.87 30.29
    Ca, % 0.40 0.40 0.20 0.31 0.31
    Total P, % 0.90 0.63 0.60 0.61 0.62
    GE, MJ/kg 19.5 20.2 24.2 23.8 23.7
Qualitative variables (DM basis)
    Urease, Δ pH 0.22 0.05 0.04 0.07 1.74
    Trypsin inhibitor, mg/g 9.0 6.6 6.6 3.1 45.1
    Protein dispersibility index, % 8.56 10.74 13.03 10.31 54.24
1DSM = defatted soybean meal; SBM = soybean meal; MSB = micronized soybeans; TSB = toasted soybeans; RSB = raw soybeans.
2Estimated as NFE = 100 – (ash + CP + AHF + crude fiber).

The soybean by-products used in this study were derived from the same genetically modified soybean variety, which presented a semiearly maturing cycle (130 d, group VI), undetermined habitat, and a height of 60 to 80 cm. The SBM and DSM were produced by grinding RSB in a roller mill, dehulling, and grinding and toasting the hulls. The dehulled soybeans were treated with conditioning, flaking, oil extraction, desolventization (hexane), and toasting. Parts of the hulls were incorporated into the toasted soybeans to produce SBM with 42% CP. The toasted soybean flakes (with no hull inclusion) were ground in a roller mill to produce DSM. The TSB were obtained by heating whole soybeans with dry air at 120 to 125°C in a tunnel roaster. The MSB were produced by exposing soybeans to infrared radiation and indirect steam at 165°C for 2 to 3 min, followed by dehulling, flaking, and grinding in a roller mill. The TSB and RSB were previously ground in roller mill to a 2.5-mm particle size.

Diet ingredients were mixed in a vertical mixer, ground to 1-mm particle size in a hammer mill, and extruded in a single screw extruder (E-130; Ferraz, Ribeirão Preto, Brazil), at a rate of 1,890 to 2,000 kg/h and average water addition and preconditioner temperature occurred between 325 and 396 L/h and 83.5 and 96.0°C, respectively. Extrusion conditions were controlled every 10 min by adjusting the density of each food being processed. After extrusion, the prepared diets were dried in a triple-deck horizontal drier for 20 min at 100 to 110°C and sprayed with bovine fat. All flavoring was added after cooling.


The digestibility trial used the method of total feces collection conducted, according to recommendations of AAFCO (2003). The experimental diets were offered for a 5-d adaptation period, followed by 5 d of fecal collection, and fecal samples were pooled per individual animal. Feces were collected and weighed twice daily and individually stored in the freezer (–15°C) for subsequent analyses.

The diets were fed to dogs twice daily (at 0730 and 1630 h). The feed amount was calculated to supply ME requirements, according to the equations proposed by the NRC (2006) for adult dogs: MJ/day = 0.54 × BW0.75 and growing dogs: MJ/day = 0.54 × BW0.75 × 3.2 × (e–0.87w – 0.1), where w = current BW/mature BW. Water was offered ad libitum.

Fecal Characteristics

These fecal characteristics were evaluated in adult dogs and puppies: DM content, dry and wet fecal output/kg DM intake, fecal score, fecal pH [measured using a digital pH meter (331; Politeste Instrumentos de Teste LTDA, São Paulo, Brazil)], and ammonia. The fecal score was always determined by the same researcher, according to a scale from 1 to 5 as follows: 1 = watery feces; 2 = soft, unshaped stool; 3 = soft, shaped, and moist stool, leaving spots on the floor; 4 = firm, shaped, dry stool; and 5 = hard, dry pellets (small, hard masses).

Fecal pH and ammonia were measured in feces collected up to 15 min after excretion. The ammonia content was determined in 5 g of fresh feces, which were incubated in a 500-mL lidded glass balloon, containing 250 mL distilled water, for 1 h. Then, 3 drops of octyl alcohol (1-octanol) and 2 g of magnesium oxide were added to the solution, which was subsequently distilled in a macro-Kjeldahl apparatus and recovered in a beaker containing 50 mL of boric acid. Finally, the ammonia was titrated, using standardized sulfuric acid at 0.1N. The fecal ammonia concentration was calculated as: ammonia-N (g/kg) = N × correction factor × 17 × (volume of acid – blank)/sample weight (g). The fecal ammonia concentration was corrected to fecal DM.

Laboratory Analyses

After the collection period, feces from each replicate were thawed to room temperature and individually homogenized. Feces were then dried in a forced-ventilation oven at 55°C (320-SE; Fanem, São Paulo, Brazil) until a constant weight was achieved. The experimental diets, tested feedstuffs, and feces were ground (Arthur H. Thomas Co., Philadelphia, PA) to a 1-mm particle size and submitted for analysis to determine DM content at 105°C, CP (Method 954.01), crude fiber (CF; Method 962.10), acid-hydrolyzed fat (AHF; Method 954.02), and ash (Method 942.05), according to AOAC (1995). The GE was determined in a calorimetric bomb (Model 1261; Parr Instrument Co., Moline, IL). Nitrogen-free extract (NFE, g/kg) was estimated as: 1,000 – (moisture + CP + AHF + CF + ash). Urease activity and protein dispersibility index were analyzed according to The American Oil Chemists’ Society (AOCS, 1980a, b); trypsin inhibitor activity was analyzed according to Genovese and Lajolo (1998), and NDF was analyzed according to Silva and Queiroz (2002) in soybean by-products and experimental diets.

Calculations and Statistical Analysis

Based on the obtained analytical results, apparent total tract digestibility (ATTD) of DM, CP, AHF, OM, and NFE of the reference diet (ATTDrd), test diets (ATTDtd), and tested ingredients (ATTDing) were calculated, according to the substitution method proposed by Matterson et al. (1965), using these equations: ATTDrd/ATTDtd = [(g nutrient intake – g nutrient excretion)/g nutrient intake]; ATTDing = ATTDrd + [(ATTDtd – ATTDrd)/(g/kg ingredient substitution/1,000)]. Metabolizable energy was estimated according to AAFCO (2003): ME (kJ/g) = (kJ/g GE intake – kJ/g fecal GE – [(g CP intake – g fecal CP) × 5.23 kJ/g]/g DM intake.

The results of the digestibility trial with adult and growing dogs were analyzed, according to a double 6 × 6 (treatments × periods) Latin square experimental design in a 2 × 5 (ages × soybean by-products) factorial arrangement to determine digestibility and a 2 × 6 (ages × diets) factorial arrangement to analyze fecal characteristics. Data were analyzed for normality (Shapiro-Wilk test) first and, when this assumption was assumed, data were examined with an ANOVA, using GLM procedures (SAS Inst. Inc., Cary, NC). The experimental unit was individual dog. Sums of squares of ANOVA of the model were separated into animal, period, and treatment effects. Differences were considered significant when the F-test revealed differences at a 5% probability level. Means were compared by the Tukey test at a 5% probability level. The fecal score was analyzed by the Kruskal–Wallis test at a 5% probability level.


Table 4 shows urease and trypsin inhibitor activity values of the diets before and after extrusion. Urease activity was reduced in all diets after extrusion (P < 0.05), but trypsin inhibitor activity was reduced only in the diets containing DSM, SBM, and RSB (P < 0.05).

View Full Table | Close Full ViewTable 4.

Urease activity (Δ pH) and trypsin inhibitor activity (mg/g) of mash (MAS) and extruded (EXT) diets containing 30% soybean protein products (n = 3)1

Urease MAS 0.00 0.10a 0.05a 0.03a 0.06a 1.16a
EXT 0.00 0.04b 0.03b 0.00b 0.01b 0.03b
Trypsin inhibitor MAS 0.0 3.2a 3.5a 2.2 1.4 15.8a
EXT 0.0 1.7b 2.0b 2.2 1.5 4.1b
a,bMeans in the same column followed by different letters are different (P < 0.05).
1REF = reference; DSM = defatted soybean meal; SBM = soybean meal; MSB = micronized soybeans; TSB = toasted soybeans; RSB = raw soybeans.

No episodes of food rejection, emesis, or diarrhea were observed. Food intake was not different among treatments and between ages (Table 5). The interaction between dog age and soybean products affected ATTD of DM and AHF (P < 0.05). The MSB had the greatest ATTD of DM in both adult and growing dogs. The ATTD of DM among other soybean products was not different in adult dogs, but in puppies, RSB had the lowest DM digestibility. Soybean meal and DSM had the lowest ATTD of AHF ATTD in dogs of both ages, but the reduction was greater in adult dogs. Dry matter and AHF ATTD of DSM and AHF of all soybean products were greater in puppies compared with adult dogs.

View Full Table | Close Full ViewTable 5.

Dry matter intake (g/d), apparent total tract digestibility (ATTD) of nutrients and energy, digestible nutrients and energy, and ME of soybean protein products (SP) in dogs of different ages

SP1 P-value
DMI Adults 246 246 241 242 242 2 0.288 0.113 0.855
Puppies 260 260 253 254 254 2
    DM Adults 75.6Bb 75.8b 85.1a 76.7b 75.9b 0.7 <0.001 <0.001 0.048
Puppies 78.3Ab 77.3b 85.0a 78.4b 75.6c 0.6
    CP Adults 85.1b 85.2b 88.4a 84.7b 78.9c 0.5 <0.001 0.193 0.185
Puppies 84.8b 85.2b 87.4a 84.5b 76.4c 0.6
    AHF1 Adults 84.3Bb 84.3Bb 96.8Ba 96.6Ba 96.4Ba 1.0 <0.001 <0.001 <0.001
Puppies 93.9Ac 95.8Ab 98.2Aa 98.5Aa 99.0Aa 0.5
    NFE1 Adults 92.7 92.7 94.8 93.8 93.0 0.4 <0.001 0.607 0.200
Puppies 93.9 92.4 94.1 92.2 92.4 0.6
    GE Adults 79.8b 79.7b 88.8a 81.7b 79.6b 0.7 <0.001 0.095 0.121
Puppies 81.3b 80.1b 87.8a 82.4b 78.6b 0.8
Digestible nutrient, % DM, and energy, MJ/kg DM
    Protein Adults 44.60a 39.81b 36.07c 31.86d 29.70e 11.28 <0.001 0.897 0.923
Puppies 44.44a 39.81b 35.66c 31.79d 28.76e 12.14
    Fat Adults 2.21d 3.46c 20.85b 22.60a 22.29a 9.87 <0.001 0.356 0.423
Puppies 2.46d 3.94c 21.15b 23.05a 22.89a 8.56
    Energy Adults 15.59e 16.07d 21.46a 19.43b 18.89c 0.40 <0.001 0.178 0.231
Puppies 15.88e 16.15d 21.22a 19.60b 18.65c 0.38
ME, MJ/kg DM
    ME Adults 15.23e 17.04d 21.39a 19.19b 18.64c 0.25 <0.001 0.071 0.162
Puppies 15.55e 17.13d 21.20a 19.50b 18.44c 0.29
A,BMeans in the same column followed by different capital letters are different (P < 0.05).
a,bMeans in the same row followed by different lowercase letters are different (P < 0.05).
1DSM = defatted soybean meal; SBM = soybean meal; MSB = micronized soybeans; TSB = toasted soybeans; RSB = raw soybeans; AHF = acid-hydrolyzed fat; NFE = N-free extract.

There was no influence of dog age on the digestibility of CP, NFE, GE, or ME. The MSB had the greatest ATTD of CP and GE (P < 0.05), whereas CP in RSB was the least digestible (P < 0.05). The other soybean products were not different in CP or GE digestibility. The ATTD of NFE was similar among treatments. In both adult and growing dogs, ME of the soybean products decreased in the following order: MSB > TSB > RSB > SBM > DSM (P < 0.05). The DSM had the greatest digestible protein content and MSB the greatest DE level (P < 0.05). Micronized soybeans, TSB, and RSB had much greater digestible fat content in dogs of both ages than the DSM or SBM (P < 0.05).

Fecal characteristics of the included dogs are presented in Table 6. Dogs fed diets containing soybean products had greater fecal output and less fecal DM content, score, and pH, compared with those fed the REF diet (P < 0.05). Fecal ammonia content was less in dogs fed soybean products than those fed the REF diet (P < 0.05), except in dogs fed the diet containing RSB, which had similar fecal ammonia content compared with those fed the REF diet. Growing dogs produced feces with less DM content, lower fecal score and pH, and greater ammonia than adult dogs (P < 0.05).

View Full Table | Close Full ViewTable 6.

Fecal characteristics of dogs of different ages fed a reference (REF) diet and diets containing 30% soybean protein products

Item Age1 REF DSM SBM MSB TSB RSB SEM Diet Age Diet × Age
DM, % Adults 40.9Aa 31.5Ab 31.1Ab 31.5Ab 31.5Ab 31.9Ab 0.6 <0.001 <0.001 0.889
Puppies 36.6Ba 28.2Bb 28.7Bb 29.4Bb 28.3Bb 28.7Bb 0.6
Score Adults 4.0Aa 3.3Ab 3.1b 3.3b 3.4Ab 3.4Ab 0.1 <0.001 <0.001 0.978
Puppies 3.4Ba 2.8Bb 2.9b 3.0b 2.9Bb 2.9Bb 0.1
pH Adults 6.05Aa 5.89Ab 5.83Ab 5.86Ab 6.86Ab 6.87Ab 0.05 <0.001 <0.001 0.712
Puppies 5.86Ba 5.59Bb 5.58Bb 5.61Bb 6.60Bb 6.60Ba 0.05
NH32 Adults 4.32Ba 3.92Bb 3.75Bb 3.74Bb 3.88Bb 4.41Ba 0.06 <0.001 <0.001 0.354
Puppies 5.49Aa 5.23Ab 5.10Ab 5.16Ab 5.05Ab 5.62Aa 0.07
Wfo1 Adults 0.43b 0.62a 0.62a 0.59a 0.60a 0.59a 0.02 <0.001 0.858 0.503
Puppies 0.44b 0.59a 0.62a 0.59a 0.60a 0.64a 0.02
Dfo1 Adults 0.17 0.19 0.18 0.17 0.19 0.19 0.01 0.068 0.892 0.621
Puppies 0.16 0.18 0.18 0.17 0.18 0.19 0.02
A,BMeans in the same column followed by different capital letters are different (P < 0.05).
a,bMeans in the same row followed by different small letters are different (P < 0.05).
1DSM = defatted soybean meal; SBM = soybean meal; MSB = micronized soybeans; TSB = toasted soybeans; RSB = raw soybeans; Wfo = wet fecal output (g)/DMI (g); Dfo = dry fecal output (g)/DMI (g).
2g/kg DM.


The results of this study indicated that the extrusion process of the diet containing 30% RSB did not completely inactivate protease inhibitors, as shown by its trypsin inhibitor activity value and decreased ATTD of CP in both adult dogs and puppies. However, the urease activity value of the RSB diet was within the range considered optimal for heat-treated soybeans (<0.20 Δ pH; Butolo, 2002). Though more convenient and less expensive than trypsin inhibitor analysis, urease activity determination is less accurate because urease is more sensitive to temperature than the trypsin inhibitor (Purushotham et al., 2007).

The incomplete inactivation of the trypsin inhibitor may be due to the short residence time of the diet in the extruder (∼30 to 60 s) and high fat content of mash diets (MSB, 9.9%; TSB, 10.8%; and RSB, 10.9%). The trypsin inhibitor activity in these soybean protein products was greater compared with the diets containing defatted soybean protein products. The detrimental effects of dietary fat content on extrusion were reported by Lin et al. (1997), who observed a 100 to 55% reduction in the degree of starch gelatinization in extruded pet foods containing 0 to 7.5% AHF, respectively. The authors determined that the addition of fat during extrusion may affect starch gelatinization because of the lubricating effect of fat, which reduces the shear strength and temperature of the extruders, thereby making the absorption of water by starch granules more difficult.

In contrast to the results of the present experiment, Purushotham et al. (2007) reported efficient inactivation of the trypsin inhibitor (<2.0 mg/g) when extruding a complete dog food containing 15% RSB at 125 to 140°C. Although Purushotham et al. (2007) did not present the AHF content of the extruded food, it is possible that the decreased RSB level included in the evaluated food did not provide fat levels high enough to compromise extruder pressure and temperature, as opposed to the present study.

Studies with other species report that animal performance is not compromised when fed soybeans containing maximum trypsin inhibitor levels of 3.4 mg/g in poultry (Batal et al., 2000) and 2.0 mg/g in pigs (Webster et al., 2003). These values are less than those obtained in the present study with the extruded diet containing 30% RSB (4.1 mg/g). Therefore, considering ATTD of CP in the processed soybeans evaluated in the present study, it may be inferred that maximum trypsin inhibitor values of 2.2 mg/g do not compromise dietary protein digestibility in dogs. However, 4.1 mg of trypsin inhibitor/g of food reduced ATTD of CP in RSB.

Our results are consistent with other studies in adult dogs, which reported ATTD of CP to be between 80.3% and 87.3%, when SBM or DSM were used as the main protein source (Zuo et al., 1996; Clapper et al., 2001; Yamka et al., 2006; Carciofi et al., 2009), and ATTD of CP in TSB to be 83.7% (Cavalari et al., 2006). Among the soybean protein products evaluated in the present study, MSB had the greatest ATTD of CP, which was similar to the results reported by Carciofi et al. (2009) in dogs fed diets with MSB as the main protein source (87%). The greater digestibility of MSB is explained mainly by its decreased CF content (1.53%) and treatment. To produce MSB, soybeans are dehulled and processed with indirect steam and infrared radiation at 150 to 180°C for 2 to 3 min, followed by flaking. In addition to the inactivation of heat-sensitive, antinutritional factors, this process breaks down the cell wall, allowing greater exposure of soybean nutrients to digestive enzymes (Mendes et al., 2004).

Evaluating the digestibility of a diet containing SBM as the main protein source fed to growing dogs (∼5 mo old), Swanson et al. (2004) reported 76.7% dietary CP ATTD, which is less than that determined in the present study for SBM (85.2%). It is possible that the inclusion of 10% meat and bone meal in the diet of the aforementioned study, in addition to other protein ingredients, may have resulted in the difference when compared with our study. However, Swanson et al. (2004) did not find any differences in ATTD of CP between growing and adult dogs (11 yr old), either, but did report lower ATTD of AHF in growing dogs, which was not found in the present study.

Conflicting results on fat digestibility in dogs of different ages have been reported. For instance, Taylor et al. (1995) did not observe any influence of age on fat digestibility; however, older cats had less efficient lipid use. Peachey et al. (1999) also found reduced ATTD of AHF in older cats (11.6 yr old) relative to younger cats (3.0 yr old). Although the adult dogs (Beagles) used in the present study cannot be considered old (<7 yr), Burkholder (1999) asserts that the production of pancreatic enzymes and bile salts are reduced as dogs age. Handler et al. (1994) observed a 50% reduction in the secretion of bile salts of older rats compared with younger rats.

No studies on the evaluation of ME of soybean protein products for dogs were found in the literature. However, in pigs, the greatest ME was found in MSB (18.20 MJ/kg; Santos et al., 2005), followed by TSB (15.44 MJ/kg; NRC, 1998) and SBM (14.06 MJ/kg; Santos et al., 2005). The greater ME values obtained in dogs compared with pigs are most likely because dog foods were processed with extrusion, which increases nutrient availability (Kohlmeier, 1998). According to NRC (1998), extruded soybean ME for swine is 18.47 MJ/kg, which is ∼20% greater than that of TSB and similar to that found for dogs in the present study (average of 19.34 MJ/kg for both ages).

Despite the high nutritional value of soybeans, the negative effects of high soybean inclusion levels on dog fecal texture are well documented in the literature (Zuo et al., 1996; Clapper et al., 2001; Yamka et al., 2003; Swanson et al., 2004; Carciofi et al., 2009). Although no episodes of diarrhea were observed in adult or growing dogs, the dogs fed diets containing 30% soybean products had greater fecal output and looser stools, when compared with those fed the REF diet. Oligosaccharides and nonstarch polysaccharides (NSP) present in soybeans are highly fermentable in the large intestine, producing lactate and short-chain fatty acids. If excessive, these end-products of fiber fermentation increase osmotic pressure in the intestinal lumen and, together with the high water retention capacity of NSP, increase fecal volume and moisture content (Roberfroid, 1993; Silvio et al., 2000). The greater levels of fermentation of soybean carbohydrates by large intestine microbiota are confirmed by the lower fecal pH of dogs fed diets with soybean products relative to those fed the REF diet.

Intestinal pH, resulting from the fermentation of soybean carbohydrates, may have contributed to the decreased ammonia content of the dogs fed diets with soybean inclusion. Reduced intestinal ammonia production was also verified by Swanson et al. (2002) in dogs supplemented with fructooligosaccharides (FOS). Therefore, soybean oligosaccharides (particularly stachyose and raffinose), similar to FOS, may act as prebiotics by providing substrates to lactic acid-producing microorganisms and inhibiting the development of proteolytic microbes, including Clostridium spp., in the large intestine of dogs. The exceptions to this theory were the dogs fed the RSB diet, which presented greater fecal ammonia concentrations, most likely due to the greater availability of nondigested protein in the intestinal lumen caused by the incomplete inactivation of protease inhibitors.

Although NRC (2006) mentions the possible effect of the age of a dog on intestinal ammonia production, we did not find any studies on fecal ammonia levels in dogs of different ages in the literature that elucidated why growing dogs produced more fecal ammonia than adults. However, the greater concentration of Clostridium spp. in the colon of growing dogs than in adults (Buddington, 2003) may explain this fact.

In addition to the greater fecal ammonia content, growing dogs had lower fecal pH and looser stools, when compared with adults. Decreased fecal DM content in puppies, compared with adults, was also observed by Swanson et al. (2004). These fecal characteristics of growing dogs may be explained by their greater food intake/kg BW0.75 and greater digestion passage rate, compared with adults (Weber et al., 2003), as well as changes in intestinal microbiota during growth (Buddington et al., 2003).

The greater fermentation activity in the intestine of growing dogs, characterized by lower fecal pH and greater fecal ammonia content, may indicate the presence of a greater quantity of nondigested food in the intestines of these dogs. Therefore, this greater intestinal fermentation in puppies may have contributed to the similar apparent digestibility values in the total tract obtained in dogs of both ages. Therefore, these issues should be further investigated to elucidate the morphophysiological development of the digestive system of growing dogs and contribution of each intestinal segment in the digestion of different foodstuffs for dogs of different ages.

In conclusion, the high digestible protein content of heat-treated soybean products, together with the high digestible fat and energy contents of RSB, TSB, and MSB, indicate that these ingredients are well used by both growing and adult dogs. Therefore, soybean protein products may enrich commercial dog foods, especially when used with animal protein sources to obtain a better indispensable AA balance. However, these products must be processed with heat treatment before extrusion to completely inactivate trypsin inhibitors. Moreover, the inclusion of greater amounts of soybean products in diets of dogs may result in the output of looser stools, despite reducing fecal ammonia content. Dogs >5 yr have poorer dietary fat digestibility than 5- to 7-mo-old dogs.




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