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J. Anim. Sci. 2002. 80:2493-2501
© 2002 American Society of Animal Science

The effects of dry extrusion temperature of whole soybeans on digestion of protein and amino acids by steers1

F. Orias2,3, C. G. Aldrich4, J. C. Elizalde5, L. L Bauer and N. R. Merchen6

Department of Animal Sciences, University of Illinois, Urbana 61801

6 Correspondence:
162 Animal Sciences Laboratory, 1207 W. Gregory Drive (phone: (217) 333-4189; fax (217) 244-3169; E-mail:
nmerchen{at}uiuc.edu).


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 
Five Holstein steers (450 kg) with cannulas in the rumen, proximal duodenum, and terminal ileum were used in a 5 x 5 Latin square design to study the effects of extrusion temperature on site of digestion of nitrogenous compounds in whole soybeans. The basal diet contained 50% corn silage, 24% alfalfa hay, 16.6% corn starch, 4.05% ground corn, 1% urea, and 3.4% soybean oil. Raw soybeans or soybeans extruded at 116, 138, or 160°C (diets 116, 138, and 160, respectively) replaced the soybean oil and most of the corn starch in the test diets. Total N (g/d) reaching the duodenum was 232, 293, 285, 308, and 299 for the basal, raw, 116, 138, and 160 diets, respectively. No differences were observed between the raw and extruded soybeans (P = 0.81), or for the linear or quadratic effects of extrusion temperature (P = 0.56 and P = 0.45, respectively). Nonbacterial N (g/d) reaching the duodenum was 63.1, 104.6, 106.7, 101.9, and 113.9 for the same diets, respectively, and was not influenced by extrusion or extrusion temperature. Nitrogen disappearance from the small intestine (g/d) was 150 for the basal diet, 194 for the raw soybean diet, and 187, 221, and 213 for the 116, 138, and 160°C extruded diets, respectively; no differences were observed between the raw and the extruded soybeans, or for diets containing soybeans extruded at different temperatures. Nitrogen disappearance (% of N entering) from the small intestine was lower (P < 0.05) for steers fed the basal diet than for steers fed the soybean-supplemented diets (64.1 vs 68.5%). No differences (P > 0.10) due to extrusion temperature were detected for flows of individual, essential AA, nonessential AA, and total AA at the duodenum. As extrusion temperatures increased, there were linear increases (P < 0.10) in disappearance (g/d) of all individual AA from the small intestine except for methionine and glycine. Essential, nonessential, and total AA disappearance from the small intestine were increased linearly (P < 0.10) with increasing extrusion temperature. Extrusion of soybeans can protect soy protein against extensive ruminal degradation without compromising intestinal digestibility.

Key Words: Amino Acids • Digestion • Extrusion • Soybeans • Steers


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 
The advantages of soybeans in providing highly digestible AA in ruminants are offset due to the extensive degradation of the soy protein by the ruminal microbes. Using values reported by Sniffen et al. (1992), 100 g of CP from soy protein would provide approximately 10 g of CP as nonprotein nitrogen (NPN) with a degradation rate of 400%/h, and 35 g as true soluble protein (Fraction B1) with a degradation rate of 100 to 200%/h. Thus, almost half of the CP in raw soybeans will be converted to ammonia (NH3) almost instantaneously by the ruminal microbes. More than half of the insoluble protein (51.4 g) corresponds to Fraction B2, which has a degradation rate of 8 to 10%/h. Thus, almost 86% of the true protein in soybeans (Fractions B1 and B2) might be manipulated to increase resistance to ruminal digestion, and increase the post-ruminal supply of UIP. Heat processing of soybeans is widely used to decrease ruminal degradation of CP in soybeans (Oldham and Tamminga, 1980). However, excessive heat application can damage dietary proteins and form compounds that cannot be digested (Van Soest, 1982). Roasting has been demonstrated to have limitations as a method for optimal heat processing (Merchen et al., 1997). The synergistic effect of heating and high pressure imposed during extrusion might offer an alternative to fully enhance the digestion characteristics of whole soybeans by ruminants. However, in vivo data are largely unavailable regarding the optimal temperature and duration of heat application that maximize ruminal escape protein and AA availability in the small intestine (Aldrich et al., 1995).

The objective of this experiment was to determine the effects of different temperatures of dry extrusion of whole soybeans on protein and AA digestion in steers. We hypothesized that there is an optimal level of heat exposure during dry extrusion of whole soybeans at which ruminal escape and intestinal digestibility of soy protein are maximized.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results and Discussion
 Implications
 Literature Cited
 
Animals.
Five Holstein steers, averaging 450 kg BW and surgically fitted with open-flange T-type cannulas in the rumen, proximal duodenum, and terminal ileum were used. Animals were tethered in individual stalls with rubber comfort mats during the experiment. Surgical procedures were performed at the Large Animal Clinic, Department of Veterinary Clinical Medicine, University of Illinois. Surgical and experimental procedures were approved by the University of Illinois Laboratory Animal Care Advisory Committee before any procedures were performed.

Diets and Feeding Regimen.
Dietary treatments were arranged in a 5 x 5 Latin square design with treatment periods of 14 d. The first 10 d of each period were for adaptation to the diets; samples were collected during the last 4 d of each period. Ingredient composition and chemical composition of the experimental diets are presented in Table 1Go. During the experimental periods, DMI was restricted to 2.2% of BW for all steers to minimize feed refusals and to avoid day-to-day fluctuations in intake. Steers were fed twice daily (0900 and 2100) a total mixed ration containing corn silage (50% of diet DM) and alfalfa hay (24% of diet DM) as the forage sources. A concentrate mix providing 4.05% ground corn, 1% urea, and a vitamin and mineral premix was also included in the diet. The basal diet contained no soybeans, but it contained soybean oil in an amount calculated to be equivalent to the quantity of oil provided in the soybean-supplemented diets and also corn starch grits to minimize differences in energy intake between the basal and soybean-supplemented diets. Soybean-containing diets included 16% of the diet DM as raw soybeans or soybeans extruded at temperatures (final chamber of extruder) of 116, 138, or 160°C, and steeped for 30 min (Triple "F" Inc., Des Moines, IA). These temperatures were selected because they represent the low and high end of temperatures commonly used for extruding soybeans. Raw soybeans were ground through a hammermill for use in the raw soybean diet or prior to extrusion treatment. Chromic oxide (15 g/hd/d) was used as an external marker to measure digesta flow and fecal output. Marker was dosed twice daily (7.5 g in gel caps) at 0900 and 2100 via the ruminal cannula.


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  		Table 1. Ingredient (% of DM) and chemical composition of the experimental diets
 
Sample Collection, Processing, and Laboratory Analyses.
Samples of individual feed ingredients and feed refusals were collected once daily at 0900 on d 11 to 14 of each period and composited by periods prior to laboratory analyses. Samples were analyzed for DM, OM, ash, and total N (AOAC, 1990).

Ruminal samples were collected four times on the last day of collection at 4-h intervals. A 50-mL aliquot of ruminal fluid was collected at each time and immediately analyzed for pH on a pHITM 31 pH meter (Beckman Instruments, Inc., Fullerton, CA), acidified with 2 mL of 6 N HCl and stored frozen. Samples were thawed, centrifuged at 25,000 x g for 20 min, and analyzed for NH3N according to Chaney and Marbach (1962) and for VFA (Merchen et al., 1986). One liter of whole rumen contents (approximately 4 L total) also was collected at each time, placed in a Waring blender (Waring Products Division, New Hartford, CT), and homogenized for 2 min to dislodge adherent bacteria, filtered through eight layers of cheesecloth, composited, and frozen as described by Firkins et al. (1986). At the end of each period, the frozen bacteria-rich sample was thawed, centrifuged at 500 x g for 20 min to remove coarse particles, and then recentrifuged at 25,000 x g for 20 min. The bacteria-rich pellet was washed with 0.9% saline, freeze-dried, and broken down in a mortar and pestle. Freeze-dried bacteria were analyzed for DM, OM, total N (AOAC, 1990), and purine concentrations (Zinn and Owens, 1986).

Twenty-four samples of duodenal and ileal digesta (approximately 300 mL each) were taken during d 11 through 14 (six times daily). Within each period, samples were taken at each 30-min interval between the 0900 and 2100 feedings. Samples were stored frozen until the end of the period and then thawed and composited for each steer within each period. A homogenous subsample was obtained, freeze-dried, and ground (Wiley mill, 1-mm screen). Duodenal and ileal samples were analyzed for DM, OM, and total N (AOAC, 1990). Duodenal samples also were analyzed for purine concentrations (Zinn and Owens, 1986). Because the purine:N ratio of isolated bacteria was not affected by treatment (P = 0.45), animal (P = 0.27), or period (P = 0.66), the overall bacterial purine:N ratio (0.77) was used to estimate the quantity of bacterial N reaching the small intestine. Duodenal and ileal samples were prepared (Williams et al., 1962) for Cr determination on an atomic absorption spectrophotometer (air acetylene flame, Model 2380; Perkin-Elmer Corp., Norwalk, CT). For AA analysis, duodenal and ileal samples were prepared according to Spitz (1973). Briefly, samples (150 mg) were hydrolyzed in screw-cap culture tubes (20 x 150 mm) in 15 mL of 6 N HCl for 22 h at 105°C. Cooled hydrolyzates were filtered through Whatman 541 filter paper and centrifuged at 29,000 x g for 20 min. Clarified hydrolyzates were neutralized with NaOH and sodium citrate buffer before analysis. Concentrations of AA were measured using a Beckman 126 AA analyzer (Beckman Instruments, Inc., Fullerton, CA), using ninhydrin detection with Na citrate buffers in a 12-cm ion-exchange column. Feces were collected by rectal grab samples at every other duodenal/ileal collection (three times daily) during d 11 through 14. Fecal samples were composited for each steer within each period and stored frozen. A homogenous subsample was obtained, oven-dried at 55°C, and ground (Wiley mill, 1-mm screen). Fecal samples were analyzed for DM, OM, and total N (AOAC, 1990). Samples also were prepared (Williams et al., 1962) for Cr determination as described previously. Total fecal output was calculated by reference to the concentration of Cr in feces using the formula: fecal output = Cr administered (g/d) || Cr in feces (g/g).

Calculations and Statistical Analyses.
Nutrient flows at the duodenum and ileum and fecal output were calculated by reference to Cr. The proportion of bacterial N reaching the small intestine was calculated by dividing the bacterial purine:N ratio by the purine:N ratio in duodenal digesta samples. Statistical analyses were performed using Proc GLM and Proc MIXED (SAS Inst. Inc., Cary, NC). Data were analyzed by analysis of variance for a 5 x 5 Latin square design. Model sums of squares were separated into animal, period, and treatment effects. Treatment comparisons were conducted by preplanned orthogonal contrasts: 1) basal diet vs soybean-containing diets, 2) raw vs extruded (116, 138, 160°C) soybeans, 3) the linear effect of extrusion temperature, and 4) the quadratic effect of extrusion temperature. Ruminal pH, NH3N, and VFA data were analyzed as a repeated measures analysis with time, and the interaction of treatment x time tested. No treatment x time interactions were detected (P > 0.05) for the variables measured; therefore, treatment effects were compared across sampling times using the described contrasts.


   Results and Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
Implications
 Literature Cited
 
Results for ruminal pH, NH3N, and VFA concentrations are presented in Table 2Go. Ruminal pH averaged 6.3 across treatments and was not affected by treatment (P > 0.05). Ammonia concentrations were greater for the soybean-supplemented diets than for the basal diet (P < 0.01), which is due to the higher N concentrations of the diets and higher N intakes when the steers were fed the soybean-supplemented diets. There was no difference in ruminal NH3N concentration between the steers fed raw vs extruded soybeans (19.8 vs average 18.2 mg/100 mL; P = 0.57). Although not significant (P = 0.18), there was a slight decrease in ruminal NH3N concentration as the temperature of extrusion increased (20.7, 17.8, and 16.1 mg/100 mL for 116, 138, and 160°C, respectively). Other researchers have shown decreases in ruminal NH3N concentrations when feeding extruded whole soybeans (Block et al., 1981) or roasted SBM (Plegge et al., 1985). Because our soybeans were ground before extrusion, some of the expected reduction in ruminal protein degradability conferred by heating might have been offset by the reduction in particle size. In agreement with our results, Stern et al. (1985), Tice et al. (1993), and Aldrich et al. (1995) found no decline in ruminal NH3N concentrations when steers were fed roasted or extruded vs raw soybeans. These discrepancies among research reports might be due to differences in the type (roasted vs extruded) or level of heat application (temperature and time of exposure).


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  		Table 2. Ruminal pH, ammonia (NH3), and VFA concentrations in steers fed diets supplemented with soybeans that were raw or extruded at different temperatures
 
Total VFA concentration tended (P = 0.07) to be greater when steers were fed the soybean-supplemented diets compared with the basal diet (average 71.8 vs 67.1 mM). No differences were detected in VFA concentrations for the contrasts between raw and extruded soybeans (P = 0.32) or for the linear (P = 0.82) or quadratic (P = 0.43) effects of extrusion temperature. Molar proportions of acetate and propionate in ruminal fluid were not affected by feeding raw or extruded soybeans, although the proportion of acetate tended to be greater (P = 0.14) and proportion of propionate tended to be lower (P = 0.11) when steers consumed the soybean-supplemented diets. This is probably a reflection of the higher percentage of corn starch grits present in the basal diet. Molar proportion of butyrate was greater (P = 0.04) when steers were fed the basal diet than when fed diets containing soybeans. Molar proportions of isobutyrate, valerate, and isovalerate in ruminal fluid were increased with the addition of soybeans to the diet. This increase in the concentration and molar proportions of the branched-chain VFA reflects increased concentration of branched-chain AA in the soybean-supplemented diets.

Because DMI was restricted at 2.2% BW among treatments, small numerical differences in DM and OM intakes are due to nonsignificant differences in their dietary concentrations and to small amounts of feed refusals during the experimental periods (Table 3Go). No differences due to treatment were detected for OM flow at the duodenum or for OM apparently digested in the stomach (g/d or as a percentage of OM intake). No differences due to treatment were detected for OM truly digested in the stomach (g/d or as a percentage of OM intake). There was an increase in total tract OM digestibility for steers fed the soybean-supplemented diets vs steers fed the basal diet (P = 0.03).


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  		Table 3. Organic matter intake and digestion by steers fed diets supplemented with soybeans that were raw or extruded at different temperatures
 
Nitrogen intake was a function of dietary N concentration and was greater for the soybean-supplemented diets than for the basal diet (P = 0.01; Table 4Go). Nitrogen intake averaged 175, 276, and 285 g/d for steers fed the basal, raw, and extruded soybean diets, respectively. No differences were observed between steers fed raw or extruded soybeans (P = 0.44). There was a linear decrease in N intake as the temperature of extrusion increased (P = 0.10). Because soybean-containing diets were formulated to be isonitrogenous, these differences in N intake are the result of sorting and feed refusals.


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  		Table 4. Nitrogen intake and digestion by steers fed diets supplemented with soybeans that were raw or extruded at different temperatures
 
Total N reaching the duodenum was greater (P < 0.01) for steers fed the soybean-supplemented diets than for those fed the basal diet. No differences (P = 0.81) in duodenal N flow were observed when steers were fed raw vs extruded soybeans, nor were linear or quadratic effects of extrusion temperature (P = 0.56 and P = 0.45, respectively) detected for duodenal N flows. Even though we observed a difference in ruminal NH3 N concentrations between steers fed the basal (6.2 mg/100 mL) vs steers fed the soybean-supplemented diets (18.6 mg/100 mL; P < 0.01), N of bacterial origin reaching the duodenum was not affected by diet (P = 0.25). Moreover, no differences were observed between feeding raw or extruded soybeans (P = 0.91) or for the linear or quadratic effects of temperature of extrusion (P = 0.75 and P = 0.21, respectively) in the amount of bacterial N reaching the duodenum. This agrees with the findings of Satter and Slyter (1974), who suggested that NH3N concentrations above 5 mg/100 mL of ruminal fluid do not limit microbial protein production. Bacterial N represented approximately 73% of the N entering the small intestine for the basal diet, 64% for the raw soybean-supplemented diet, and 62, 66, and 61% for diets containing soybeans extruded at 116, 138, and 160°C, respectively. Flow of nonbacterial N at the duodenum was greater (P < 0.01) for steers fed the soybean-supplemented diets than for those fed the basal diet. No differences were observed in duodenal nonbacterial N flow between steers fed raw vs extruded soybeans (P = 0.80) or for the linear (P = 0.62) or quadratic (P = 0.51) effects of extrusion temperature. The hypothesis of this experiment was that there is an optimal temperature of extrusion at which the postruminal availability and digestibility of protein are maximized. We expected that, at the higher temperatures of extrusion, the ruminal degradability of the soybean protein would be decreased, and more dietary N would reach the small intestine. We expected an increase in the amount of nonbacterial N reaching the small intestine as the temperature of extrusion increased; however, this did not occur. It appears that extruding soybeans at 116 or 138°C may not be sufficient to protect soy proteins from ruminal degradation and thus did not increase the amount of nonbacterial N reaching the small intestine.

Small-intestinal N disappearance was greater (P < 0.01) for steers fed the soybean-supplemented diets than for those fed the basal diet. Nitrogen disappearance from the small intestine was 150 g/d for the basal diet, 194 g/d for the raw soybean diet, and 187, 221, and 213 g/d for the 116, 138, and 160°C extruded diets, respectively. Even though N disappearance was greatest for the 138 and 160°C diets, no differences (P >= 0.20) were observed between diets containing raw vs extruded soybeans or for the linear or quadratic effects of extrusion temperature. Intestinal disappearance, expressed as a percentage of N intake, was higher for the basal diet than for the soybean supplemented diets (P = 0.03). Nitrogen disappearing in the small intestine was equivalent to almost 90% of N intake for steers fed the basal diet, 70% for those fed raw soybeans, and averaged 73% for the steers fed extruded soybeans. Nitrogen disappearing in the small intestine (% of N intake) tended (P = 0.11) to increase linearly as the temperature of extrusion increased. In contrast, steers fed the basal diet had a lower disappearance of N in the small intestine than the soybean-supplemented diets (64.1 vs 68.5% of N entering; P = 0.01). Extruding soybeans increased (P = 0.04) the disappearance of N in the small intestine as a percentage of N reaching the duodenum compared with raw soybeans. Small intestinal N disappearance averaged 69.4% of the N entering the small intestine for the extruded soybean diets. The quadratic effect of extrusion temperature was significant (P = 0.08), so increasing the temperature of extrusion above 160°C may not have any beneficial effects on small intestine N disappearance. This is in agreement with findings by Stern et al. (1985) and Aldrich et al. (1995), who also reported an increase in small intestine N disappearance when feeding extruded or roasted soybeans compared with raw soybeans. Lower N disappearance could be due to higher trypsin inhibitor (TI) activity in the raw soybean diet. Mielke and Schingoethe (1981) reported that extruding soybeans at 160°C reduced TI activity compared with raw soybeans (2.7 vs 24.0 TIU/mg, respectively). The TI complex has received little attention in ruminant nutrition because it was assumed to be largely degraded by ruminal microbes. However, Aldrich et al. (1994) measured TI activities before and after in vitro incubations and found that in vitro incubations in ruminal fluid did not decrease TI activity in raw soybeans, nor did it increase AA digestibilities in cecectomized roosters.

Bacterial N synthesis (grams entering the small intestine per kilogram of OM apparently or truly digested in the rumen) was not affected by feeding raw or extruded soybeans (P = 0.46 and P = 0.73, respectively; data not shown) or by increasing the temperature of extrusion from 116 to 160°C (P = 0.85 and P = 0.94, respectively). Extruding soybeans at these temperatures did not affect bacterial N synthesis, because it did not negatively affect OM digestion or N availability for the ruminal microbes.

Flows of essential (EAA) and nonessential (NEAA) amino acids to the small intestine of steers fed diets supplemented with raw or extruded soybeans are presented in Table 5Go. Quantities of all individual EAA except for methionine (P = 0.14) reaching the duodenum were increased with the addition of raw or extruded soybeans to the diets. Likewise, flows of all individual NEAA were increased except for glycine (P = 0.32). No differences (P > 0.10) were observed between raw and extruded soybeans in duodenal flows of any of the individual EAA or NEAA. As temperature of extrusion increased, there was a linear increase in the amounts of arginine and glutamate (P = 0.07 and P = 0.09, respectively) flowing to the small intestine. The amount of arginine reaching the small intestine increased 22%, and glutamate increased 23%, when temperature of extrusion was raised from 116 to 160°C. There also were important numerical increases for the EAA isoleucine, leucine, phenylalanine, histidine, lysine, and for the NEAA aspartate, serine, and proline as extrusion temperature increased. However, these numerical differences were not statistically significant (P values between 0.10 and 0.15).


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  		Table 5. Flow (g/d) of essential (EAA) and nonessential (NEAA) amino acids to the small intestine of steers fed diets supplemented with soybeans that were raw or extruded at different temperatures
 
Total EAA and NEAA reaching the duodenum tended to be increased with the addition of soybeans to the diets (P = 0.07 and P = 0.08, respectively). Total EAA and NEAA averaged 684 and 791 g/d for the soybean-supplemented diets vs 561 and 653 g/d, respectively, for the basal diet. No differences were observed between diets containing raw vs extruded soybeans for total EAA or NEAA (P = 0.65 and P = 0.68, respectively), but there were increases in EAA and NEAA flows as the temperature of extrusion increased. Total EAA averaged 645, 664, and 766 g/d for the 116, 138, and 160°C extruded soybean diets. Corresponding NEAA flows were 749, 771, and 878 g/d, respectively. Even though there were no statistical differences (P = 0.15 and P = 0.19), these values represent an increase of about 18% in the flow of total EAA and NEAA to the small intestine when the temperatures of extrusion were increased from 116 to 160°C, and an increase in 15% if compared with the raw soybean treatment. In contrast with our observations, Aldrich et al. (1995) reported an increase in total EAA and NEAA reaching the small intestine when they fed diets containing roasted soybeans compared with a diet containing raw soybeans. It is noteworthy that their diets contained soybeans roasted at 141, 149, and 157°C. It seems that extruding soybeans at 116 and 138°C was insufficient to cause significant increases in the amounts of total EAA and NEAA reaching the duodenum.

Small-intestinal disappearance (g/d) of EAA and NEAA in steers fed diets supplemented with raw or extruded soybeans are presented in Table 6Go. There were increases in the quantities of individual and total EAA and NEAA disappearing in the small intestine between steers fed the basal diet and those fed the soybean-supplemented diets. The only AA that did not demonstrate an increased small intestine disappearance was glycine (P = 0.18). No differences (P > 0.10) were detected for individual AA, total EAA, total NEAA, or total AA disappearance between diets containing raw vs extruded soybeans. As temperature of extrusion increased from 116 to 160°C, there was a linear increase (P < 0.10) in the small intestine disappearance of all individual AA except methionine and glycine (P = 0.38 and P = 0.73, respectively). Total EAA, NEAA, and total AA disappearance in the small intestine also increased linearly as the temperature of extrusion increased (P < 0.10). Total EAA disappearance averaged 450, 485, and 582 g/d for the 116, 138, and 160°C extruded soybean diets. Total NEAA disappearance was 511, 556, and 659 g/d for the same diets. Total AA disappearance averaged 962, 1,042, and 1,242 g/d for the 116, 138, and 160°C extruded soybean diets. This represents an increase of 30% in the total AA disappearance in the small intestine between the 116 and the 160°C extruded diets.


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  		Table 6. Small intestinal disappearance (g/d) of essential (EAA) and nonessential (NEAA) amino acids in steers fed diets supplemented with soybeans that were raw or extruded at different temperatures
 
When evaluating small intestine flows and availability of AA provided by protein supplements for ruminants, special attention must be given to methionine and lysine because they are often implicated as limiting production responses in ruminants (Schwab et al., 1992; Rulquin et al., 1993). In our study, methionine was the only EAA that did not demonstrate an increased duodenal flow when diets were supplemented with raw or extruded soybeans. Similar results were reported by Aldrich et al. (1995). Methionine represents approximately 2.2% of the EAA in raw and extruded soybeans. Regardless of the method of processing, soybeans are a relatively poor source of methionine for ruminants. On the other hand, lysine represented about 15% of the EAA in the soybeans fed to the steers, but it is also the AA that is most susceptible to heat damage. Even though we did not detect a statistical difference between steers fed raw soybeans vs those fed extruded soybeans, the supply of lysine to the small intestine was increased by increasing the temperature of extrusion. Lysine flowing to the duodenum was 15% greater for the diet that included soybeans extruded at 160°C than for the diet that included raw soybeans (124.9 vs 108.2 g/d). This result is consistent with the findings of Stern et al. (1985) and Aldrich et al. (1995), who reported increases of 10% in the flow of lysine reaching the duodenum when soybeans were extruded at 149°C or roasted at 157°C when compared with raw soybeans. Lysine loss due to heat damage and Maillard product formation does not seem to be a problem at these temperatures of extrusion or roasting.

Small-intestinal disappearance of lysine also was positively affected by the temperature of extrusion. Lysine disappearance increased 23% (98.7 vs 80.3 g/d) when the soybeans were extruded at 160°C compared with the raw soybeans. Aldrich et al. (1995) reported an increase in lysine disappearance of 25% when they fed roasted soybeans compared with raw soybeans, and Stern at al. (1985) reported an increase of 15% in lysine disappearing in the small intestine when extruded vs raw soybeans were fed.

Heating of soybeans at these temperatures did not negatively affect AA digestion in the small intestine; on the contrary, it actually increased it. The increased disappearance from the small intestine of all EAA and NEAA, except methionine and glycine, suggests that perhaps more extensive denaturation of the heat-labile TI complex may be necessary in order to maximize AA absorption when feeding soybeans.


   Implications
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
 Implications
Literature Cited
 
When steers were fed soybeans that were extruded to a final chamber temperature of 160°C, followed by steeping for 30 min, total amino acids disappearing from the small intestine were increased compared with raw soybeans or soybeans extruded at 116°C. Extruding soybeans at 160°C did not decrease the availability of any individual amino acid, nor was there a reduction in amino acid disappearance from the small intestine as a result of damage to dietary protein from excess heat application. Extruding soybeans to final chamber temperatures of 116 and 138°C was not sufficient to protect soy proteins from ruminal degradation and to maximize postruminal utilization of amino acids. Heat treatment of soybeans before feeding to ruminants may be essential due to the persistence of anti-nutritional factors present in soybeans that can limit amino acid utilization.


   Footnotes
 
1 Research reported herein was supported in part by a grant from the Illinois Soybean Program Operating Board, Bloomington, IL, and by a gift from Triple "F", Inc., Des Moines, IA. Back

2 The senior author acknowledges support during his graduate program from the H.H. Mitchell Fellowship in Animal Nutrition, Department of Animal Sciences, University of Illinois. Back

3 Current address: Universidad Nacional de Rio Cuarto, Rio Cuarto, Argentina. Back

4 Current address: Menu Foods, P.O. Box 1046, 1400 E. Logan Ave., Emporia, KS 66801. Back

5 Current address: National University of Mar del Plata, C.C. 276 (7620) Balcarce, Argentina. Back

Received for publication June 22, 2001. Accepted for publication May 7, 2002.


   Literature Cited
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 


Aldrich, C. G., N. R. Merchen, H. S. Hussein, and S. Ingram. 1994. Assessment of postruminal amino acid digestibility of roasted whole soybeans by the precision-fed rooster assay. J. Anim. Sci. 72(Suppl. 2):87 (Abstr.).[Abstract]

Aldrich, C. G., N. R. Merchen, D. R. Nelson, and J. A. Barmore. 1995. The effect of roasting temperature applied to whole soybeans on site of digestion by steers: II. Protein and amino acid digestion. J. Anim. Sci. 73:2131–2140.[Abstract]

AOAC. 1990. Official Methods of Analysis. 15th ed. Association of Official Analytical Chemists, Washington, DC.

Block, E., L. D. Muller, L. C. Griel, Jr., and D. L. Garwood. 1981. Brown midrib-3 corn silage and heat extruded soybeans for early lactating dairy cows. J. Dairy Sci. 64:1813–1825.[Abstract/Free Full Text]

Chaney, A. L., and E. P. Marbach. 1962. Modified reagents for determination of urea and ammonia. Clin. Chem. 8:130–132.[Abstract]

Firkins, J. L., L. L. Berger, N. R. Merchen, and G. C. Fahey, Jr. 1986. Effects of forage particle size, level of feed intake and supplemental protein degradability on microbial protein synthesis and site of nutrient digestion in steers. J. Anim. Sci. 62:1081–1094.[Abstract/Free Full Text]

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