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Effect of soybean meal particle size on amino acid andenergy digestibility in grower-finisher swine^1,2,3,4

Department of Animal Sciences, The Ohio State University and Ohio Agricultural Research and Development Center, Columbus 43210

⁵ Correspondence:
205 Plumb Hall, 2027 Coffey Road (phone: 614-292-6987; fax: 614-292-7116; E-mail:
mahan.3{at}osu.edu).

	Abstract

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A study was conducted using the ileal digestibility technique with grower-finisher pigs to evaluate the effects of particle size reduction of soybean meal (SBM) on amino acid and energy digestibility. Soybean meal was processed through a hammer mill to achieve average particle sizes of 900, 600, 300, and 150 µm. The treatments included the use of two soybean meal sources and soy protein concentrate. One source of SBM was ground to four different particle sizes (i.e., 949, 600, 389, 185); a second source was a common SBM source used in other trials (i.e., 800 µm). The soy protein concentrate had an average particle size of 385 µm. A low-protein (5% casein) diet was fed to determine endogenous amino acid losses. This experiment was conducted in a 7 x 7 Latin Square design in two replicates using 14 crossbred barrows ([Landrace x Yorkshire] x Duroc) that averaged 28 kg BW and 60 d of age. Animals were surgically fitted with a T-cannula at the distal ileum. Treatment diets were fed in meal form, initially at 0.09 kg BW^0.75 and at graded increases at each subsequent period. Pigs within replicate were fed a constant quantity of their treatment diet for a 5-d adjustment period followed by a 2-d collection of ileal digesta samples. Apparent and true digestibility of amino acids was calculated by use of chromic oxide (0.5%) as an indigestible marker. Apparent digestibility of isoleucine, methionine, phenylalanine, and valine increased linearly (P < 0.05) as particle size decreased. True digestibility of isoleucine, methionine, phenylalanine, and valine increased linearly (P < 0.05) as particle size decreased. When the essential amino acids were averaged, apparent digestibility increased (P < 0.10) from 83.5% to 84.9% as particle size decreased, whereas, nonessential amino acid digestibility increased only slightly (P > 0.15). Essential amino acid true digestibility increased numerically from 91.0% to 92.4% as particle size decreased. Energy digestibility was not affected by particle size (P > 0.15). These results suggest that a reduction in particle size of soybean meal resulted in a small increase in the digestibility of its amino acids with the essential amino acids being affected more than the nonessential amino acids. The largest improvement in digestibility, however, was obtained when the particle size was reduced to 600 µm.

Key Words: Amino Acids • Digestibility • Ileum • Particle Size • Pigs • Soybeans

	Introduction

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Dietary particle size affects the digestibility of feed nutrients (Wondra et al., 1995a,b,c,d). Smaller particles have an increased surface area thus allowing the digestive enzymes to more effectively release the nutrients. This generally results in higher digestibility coefficients and improved feed efficiency responses (Owsley et al., 1981). Average particle sizes between 600 to 700 µm are generally recommended for most swine diets (Wondra et al., 1995a). A preliminary survey of several soybean meal (SBM) samples demonstrated a particle size between 800 to 900 µm, suggesting that the digestibility of commercial SBM might be improved with a smaller particle size.

Although N digestibility measurements have previously used total fecal and urinary collections, colonic and large intestine microbial populations metabolize the N fractions, resulting in an amino acid profile that differs from that which exits the ileum. This indicates that terminal tract amino acid compositions and the resulting digestibility values do not accurately reflect amino acid digestibilities in the pig. The ileal digestibility technique has subsequently been used to reduce the confounding effects of the microbial populations (Low and Zebrowska, 1989).

Amino acids present in the ileal digesta are, however, not completely of dietary origin. The endogenous fraction of the upper digestive tract also contains microbial protein, sloughed cells, muco-proteins, and digestive enzymes (Moughan and Schuttert, 1991). The quantity and composition of this endogenous fraction is influenced by dietary protein, fiber, and dry matter intake. Adjusting for this fraction in the digesta gives a more accurate assessment of the true dietary amino acid digestibility.

Because the effect of differing particle sizes of SBM on amino acid digestibility is currently unknown, we therefore examined the amino acid and energy digestibilities using the ileal digestibility technique in grower-finisher pigs.

	Materials and Methods

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The experiment evaluated the ileal digestibility coefficients of commercial SBM processed to four different particle sizes and to compare the results to an unground SBM from another processor and to a soy protein concentrate source.

One source of SBM (48% CP) had an average particle size 949 µm and served as one treatment group. This meal was then processed through a 60 horsepower hammer mill (model 1040; Schutte, Buffalo, NY) to attain three other mean particle sizes of approximately 600, 300, and 150 µm; the final four products having average sizes of 949, 600, 329, and 185 µm, respectively. Another 48% CP SBM (i.e., 800 µm) and a soy protein concentrate (i.e., 383 µm) source both from a different processor than the initial SBM source was also evaluated. The soy protein concentrate was evaluated to determine the effects of its lower antinutritional factor content on amino acid digestibility. A low-protein (5% casein) diet served as a seventh treatment group to determine the endogenous amino acid secretions within each study period. We assumed that the casein was completely digestible thus reducing the animals’ need to catabolize body protein reserves during the period of inadequate protein intake (Low, 1980). The amino acids in the digesta from this low-protein casein diet were considered to be of endogenous origin.

Fourteen crossbred barrows ([Yorkshire x Landrace] x Duroc) averaging 28 kg BW and 60 d of age were surgically fitted with a simple T-cannula at the distal ileum after a 24-h fast. Four additional animals were cannulated to replace animals if complications required their removal from the experiment. These pigs were fed the same quantity of a control corn SBM diet and were similarly managed in the digestion crates as experimental animals. During an approximate 10-d recovery period, all pigs were fed a corn-SBM grower diet formulated to 1.00% lysine (total).

The experiment was conducted in a 7 x 7 factorial arrangement of treatments in a Latin-square design. Pigs within each of two replicates were allotted to treatment diet sequences before the experiment began, such that each animal received the seven treatment diets in a different order and at different times. Each study period consisted of a 5-d adjustment to the treatment diet followed by a 2-d collection. The quantity of feed provided daily during the initial collection period was calculated to the average animals’ metabolic body weight (0.09 BW^0.75) within each replicate. The amount of treatment diet fed during each subsequent period was increased by approximately 175 g/d or to a level where all pigs consumed the same quantity of diet during that collection period. Diets were provided in meal form twice daily in equal amounts at 12-h intervals (i.e., 0700 and 1900) with an approximate water to diet ratio of 1.5:1. Fresh water was provided to appetite during the 24-h period. During wk 5 of the second replicate, the animals experienced a slight diarrhea, whereupon they were removed from their treatment diets, fed a 1.00% lysine corn-SBM diet, and injected with an antibiotic. They were returned to the experiment within a 7-d period and resumed their diet sequence.

With the exception of the low-protein casein diet, all diets were formulated to 17% crude protein (Table 1). Chromium oxide was added to the diets at 0.50% as an indigestible marker. All diets met or exceeded vitamin and mineral nutrient requirements for growing pigs (NRC, 1998).

Digesta was collected for two 12-h (0700 to 1900) periods on d 6 and 7 after the 5-d adjustment period. Plastic collapsible tubes (50-mm inside diameter) were attached to the cannula with digesta collected at 20- to 30-min intervals for the 12-h period. Care was taken to ensure that digesta flow into the tube was unobstructed during collection. Samples collected were placed in a freezer (-20°C), pooled by pig for the 2-d period, mixed, freeze-dried, and analyzed for their amino acid and Cr contents.

Analyses.

Particle size distributions of the SBM samples were determined by the method outlined by the American Society of Agricultural Engineers (ASAE, 1995) using a Ro-tap style shaker (W. S. Tyler, Mentor, OH). Nine sieves used for particle size determination ranged from 2,000 to 105 µm. The geometric mean diameter of the samples was calculated according to ASAE (1995).

Soybean meal samples of each treatment group and the soy protein concentrate were analyzed for their CP content using a Perkin-Elmer 2410 Series II N analyzer (Perkin-Elmer, Norwalk, CT). Digesta samples were freeze-dried and analyzed for their N, amino acid, and Cr contents. Amino acids of the digesta and the soybean protein sources were determined using a Beckman 6300 (Beckman Coulter, Inc., Fullerton, CA) amino acid analyzer by the method outlined by AOAC (1995). The analytical amino acid values for the soybean protein sources were used to calculate the dietary contribution for digestibility determination. Chromium was determined by atomic absorption after wet ashing in HCl acid. Gross energy was analyzed on the freeze-dried treatment diets and digesta using a Parr (Moline, IL) model 1241 adiabatic oxygen bomb calorimeter.

Apparent digestibility (AD) values were calculated using the Cr concentration in the feed and digesta by the equation: AD = 100 - [(N_D/N_F) x (Cr_F/Cr_D) x 100]. In this equation, N_D is the nutrient concentration present in the ileal digesta, N_F is the nutrient concentration in the feed, Cr_F is the Cr concentration in the feed, and Cr_D is the Cr concentration in the ileal digesta. Endogenous amino acid losses (EAL) were calculated according to the equation reported by Moughan et al. (1992): EAL = [N_D x (Cr_F/Cr_D)]. True digestibility (TD) values were calculated using the equation: TD = AD + (EAL/N_F) x 100.

The data were statistically analyzed using the General Linear Model procedure of SAS (SAS Inst. Inc., Cary, NC). A Latin Square design was followed according to the method of Steel and Torrie (1980) using the individual pig as the experimental unit. The low-protein treatment diet was not used in statistical evaluation because its value would skew the variation and was used only for adjusting for TD within that study period. The statistical model included replicate, period, treatment, and pig nested within replicate. Linear regression analysis evaluated the effect of particle size of the four ground soybean meals. The common source soybean meal and the soy protein concentrate treatment groups were each contrasted to the 900-µm soybean meal treatment group.

	Results

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The analyzed lysine contents of the various soy protein sources and the subsequent calculated dietary lysine levels are presented in Table 1. Soy protein concentrate had a higher lysine content than the two soybean meal samples, consistent with NRC (1998) published values.

Individual and cumulative percentages of particle size weights in each sieve of the various soy protein source samples are presented in Table 2. Figure 1 presents the relative distribution of particles when the unground SBM sample (949 µm) was compared with the SBM processed to an average 600 µm. The greatest change in particle size seemed to be due to a reduction in the larger sized particles.

View larger version (12K): [in this window] [in a new window]   Figure 1. Particle size distribution in soybean meal treatment sizes.

Soy protein concentrate had a higher (P < 0.05) AD coefficient of several essential amino acids (i.e., isoleucine, leucine, lysine, phenylalanine, threonine, and tyrosine) and the entire group of essential amino acids (P < 0.10) than did either SBM source. Soy protein concentrate would be expected to have a higher amino acid digestibility due to its lower antinutritional content and lower oligosaccharide fraction, but it also had a particle size smaller than commercial SBM. Whether the improved digestibility coefficient response was due to the smaller particle size or the reduction in the level of antinutritional factors cannot be determined from our results.

The digestible energy content of SBM decreased numerically by 1.0% as particle size decreased, but the difference was not statistically significant (P > 0.15; Table 3). Both the unground SBM source used for the particle size study and soy protein concentrate, however, had higher digestible energy contents than the common source SBM, implying that differences may exist in the carbohydrate contents of the two SBM sources. Carbohydrates, such as stachyose and raffinose, present in the soluble fraction of SBM, have been shown to resist digestion in the small intestine but seem to be better utilized by microorganisms in the large intestine (Clarke and Wiseman, 1998).

Endogenous amino acid losses (milligrams per kilogram feed) determined from the low-protein 5% casein diet fed during each collection period are presented in Table 4. There was no significant difference in any of the specific amino acids, nor was there a difference in the total essential or nonessential amino acid content of the digesta between each period or between replicates (P > 0.15).

The common source SBM had higher (P < 0.05) TD coefficients of methionine, valine, and alanine than the unground soybean meal used for the particle size study. This was largely attributed to its smaller particle size (800 µm) compared with the larger (949 µm) particle size of the unground treatment SBM.

	Discussion

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The reduction in particle size resulted in somewhat different responses on specific amino acids, but there was a general improvement in ileal digestibility coefficients of many essential amino acids, whether expressed either on an AD or TD basis. The largest increase in amino acid digestibility seemed to occur, however, when the particle size was reduced from 949 to 600 µm. Figure 1 and the distribution of the particle sizes on different sieves in Table 2 largely indicate that the reduction in the larger particles in SBM may explain the increase in digestibility coefficients. The difference between essential or nonessential amino acid digestibilities may be due to a greater number of amino acid transporters specific for the absorption of the essential amino acids in the small intestine (Mailliard et al., 1995).

Wondra et al., (1995a,b,c,d) demonstrated that reducing the particle size of corn from 1200 to 400 µm in finishing and lactation diets also improved the digestibility of dry matter, N, and energy. Owsley et al. (1981) reported that reducing the particle size of sorghum in grower-finisher diets resulted in a 7.6% increase in apparent amino acid digestibility. Although commercial SBM is not normally processed further once received from the processor, our study suggests that an improvement in amino acid digestibility may occur if the particle size of SBM component of a diet is less than 600 µm. An increased digestibility could result in a decrease in amino acids excreted in the feces.

The apparent and true amino acid digestibility values achieved by reducing particle size of the SBM used in this study are similar to those reported by the NRC (1998) and Grala et al. (1998), although the particle size in these reports were not reported. Our values are, however, somewhat higher than those reported by Sohn et al. (1994) and Caine et al. (1997) possibly due to the difference in SBM level in their diets, differences in production phase evaluated, and the possibility that the particle size of their SBM source was higher. Both Sohn et al. (1994) and Caine et al. (1997) evaluated dietary levels >40%, whereas our study incorporated a level of 35%. They also conducted their studies with weanling pigs and provided higher dietary amino acid levels.

The common source SBM generally had higher numerical apparent and TD coefficients for most of the essential amino acids compared to the other source of SBM that we used, but the differences were not significant (P > 0.15). The slightly smaller average particle size (i.e., 800 µm) of the common source SBM than the unground (949 µm) SBM used in our particle size study suggests that the larger particle sizes in both samples had the same digestibility.

The soy protein concentrate that we evaluated, however, had a lower digestibility of amino acids than reported by NRC (1998), but it generally had similar true amino acid digestibility coefficients compared to the two soybean protein sources evaluated, except for glycine, which had a higher digestibility (P < 0.05). The NRC (1998) reports that soy protein concentrate digestibility coefficients of most essential amino acids are approximately 5.6% higher than SBM.

Endogenous amino acid excretion was not affected by replicate or period. Our values, expressed as milligrams per kilogram dry feed intake, are similar to those reported by Traylor et al. (2001) and Hodgkinson et al. (2000) but higher than Hess and Seve (1999). The latter workers used a protein-free diet that resulted in lower EAL. The low-protein casein diet that we used would, however, be expected to have higher EAL than if a protein-free diet had been fed. Hodgkinson et al. (2000) demonstrated that proteins and peptides present in low-protein diets stimulates the secretion of endogenous amino acids in a dose dependent manner that would increase the amount of endogenous secretions. This would seem to better simulate the endogenous secretion when a diet containing protein is fed.

	Implications

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Particle size reduction increased the digestibility of some amino acids in the grower pig, particularly the essential amino acids isoleucine, leucine, methionine, phenylalanine, and valine. Overall, average essential amino acid digestibility increased by 1.4% for apparent and increased by 1.3% for true digestibility as particle size decreased to 185 µm. The largest increase in digestibility was evident when the particle size was reduced to 600 µm.

	Footnotes

 
¹ Salaries and research support were provided by state and federal funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State University.

² Funded in part by the National Soybean Research Laboratory and the Illinois Soybean Operating Board, Urbana, IL.

³ Animal care and procedures followed were approved by the Ohio State University Animal Care and Use Committee.

⁴ Appreciation is expressed to J. Hanson for the surgical preparation of animals, M. Watts for animal care and sample collections, B. Bishop for statistical analysis, J. Bardall for feed manufacture and C. Gains, and H. Keener for the use of particle size analysis equipment.

Received for publication October 11, 2001. Accepted for publication October 4, 2002.

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