J Sci Food Agric 1998, 76, 357È363

Response of Nutrient Digestibilities to FeedingDiets with Low and High Levels of SoybeanTrypsin Inhibitors in Growing PigsShaoyan Li, Willem C Sauer* and William R Caine

Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta,Canada T6G 2P5

(Received 29 July 1996 ; revised version received 15 May 1997 ; accepted 30 June 1997)

Abstract : Studies were carried out to determine the e†ect of dietary soybeantrypsin inhibitor (SBTI) content on nutrient and energy digestibilities in growingpigs. Six barrows, average initial body weight (BW) 47É8 ^ 4É0 kg, were Ðttedwith a simple T-cannula at the distal ileum and fed two diets according to acrossover design. Two maize starch-based diets were formulated to contain 200 gcrude protein (CP) kg~1 from either Nutrisoy (a food-grade defatted soy Ñour) orautoclaved Nutrisoy. The contents of SBTI in the Nutrisoy and autoclavedNutrisoy diets were 13É4 and 3É0 g kg~1, respectively. The experiment consistedof two periods of 12 days each. The average BW at the start of the Ðrst andsecond experimental periods were 53É3 ^ 3É7 and 61É0 ^ 5É1 kg, respectively. Theaverage BW at the conclusion of the experiment was 71É8 ^ 7É6 kg. The ilealdigestibilities of dry matter (DM), organic (OM), energy, CP and all amino acidsmeasured were higher (P\ 0É01) in pigs fed the autoclaved Nutrisoy diet. Theenergy digestibility increased from 66É0 to 77É9%, and the CP digestibilityincreased from 37É4 to 77É1%. The increases in ileal digestibilities of the indis-pensable amino acids ranged from 27É0 (methionine) to 49É2 (leucine) percentageunits. The increases in ileal digestibilities of the dispensable amino acids rangedfrom 30É2 (aspartic acid) to 50É8 (tyrosine) percentage units. The faecal digest-ibilities of all criteria measured were also higher (P\ 0É01) in pigs fed the auto-claved Nutrisoy diet. Furthermore, there was a greater (P\ 0É01) netdisappearance (g kg~1 DM intake) of DM, OM, CP, energy and all amino acidsin the large intestine of pigs fed the Nutrisoy diet. In conclusion, feeding dietscontaining high levels of SBTI decreased both ileal and faecal digestibilities of allcriteria measured. The formation of SBTI-enzyme complexes is likely to beresponsible for the reduction in protein digestion and amino acid absorption.( 1998 SCI.

J Sci Food Agric 76, 357È363 (1998)

Key words : pigs ; soybean trypsin inhibitors ; amino acids ; digestibility

INTRODUCTION

Feeding raw soybean or soybean trypsin inhibitors(SBTI) to chicks (Chernick et al 1948 ; Yen et al 1973 ;Herkelman et al 1993) and rats (Rackis 1972 ; Rackis et

* To whom correspondence should be addressed.Contract/grant sponsor : Natural Sciences and EngineeringResearch Council of Canada.

al 1979 ; Liener and Kakade 1980) causes hypertrophyof the pancreas, hypersecretion of pancreatic enzymesand a reduction in nutrient and energy digestibilities.Studies with pigs fed raw soybean did not show pancre-atic hypertrophy (Yen et al 1977) and hypersecretion ofpancreatic enzymes (Li et al 1997). Studies with growingpigs (Li et al 1997) showed that hypersecretion of pan-creatic juice occurred only in terms of volume, but nottotal enzyme activities. A decrease in performance (eg

3571998 SCI. J Sci Food Agric 0022È5142/98/$17.50. Printed in Great Britain(

358 S L i, W C Sauer, W R Caine

Yen et al 1974 ; Cook et al 1988) and in faecal crudeprotein (CP) and energy digestibilities (P\ 0É05) wasalso observed in pigs fed raw soybean (Combs et al1967).

The objectives of this study (a follow up to that of Liet al 1997) were to determine the e†ects of feeding dietscontaining Nutrisoy and autoclaved Nutrisoy, whichcontain relatively high and low levels of SBTI, respec-tively, on ileal as well as faecal digestibilities of energyand nutrients in growing pigs.

EXPERIMENTAL PROCEDURES

Animals and diets

Six PIC (Pig Improvement Canada) barrows(Camborough ] Canabrid), average initial body weight(BW) 47É8 ^ 4É0 kg, were obtained from the Universityof Alberta Swine Research Unit. The barrows werehoused individually in stainless-steel metabolism crates(length 140 cm; height 85 cm; width 80 cm) in atemperature-controlled barn (20 ^ 1¡C). The pigs weregiven ad libitum access to a grower diet containing160 g CP kg~1 (Sauer et al 1983). Water was availablefrom a low-pressure drinking nipple.

Four days later, the barrows were Ðtted with a simpleT-cannula at the distal ileum according to proceduresadapted from Sauer (1976). The cannulas were modiÐedaccording to De Lange et al (1989). The barrows werereturned to the metabolism crates after surgery andfasted that same day. The next day they were givenapproximately 200 g of a starter diet containing 180 gCP kg~1 (Sauer et al 1983). The feed allowance wasgradually increased until the pigs consumed the diet ata daily rate of 4% of their individual BW. The barrowswere allowed a 9-day recuperation period. A detaileddescription of pre- and post-operative care was pre-viously presented by Li et al (1994).

Two maize starch-based diets were formulated tocontain 200 g CP kg~1 (dry matter (DM) basis) fromeither Nutrisoy or autoclaved Nutrisoy. The prep-aration of autoclaved Nutrisoy, formulation of theexperimental diets and chemical composition of auto-claved Nutrisoy and Nutrisoy were presented pre-viously (Li et al 1997).

The experiment was carried out according to a two-period crossover design (Petersen 1985). Each experi-mental period lasted 12 days. The pigs were fed twicedaily, equal amounts, at 08 :00 and 20 :00 h, at a dailyrate of 4% of BW which was determined at the begin-ning of each experimental period. The average BW ofthe pigs at the start of the Ðrst and second experimentalperiods was 53É3 ^ 3É7 and 61É0 ^ 5É1 kg, respectively.The average BW at the conclusion of the experimentwas 71É8 ^ 7É6 kg. The average daily gain (ADG) and

feed/gain ratio (F/G) were determined for each pigduring each experimental period. The collection offaeces was initiated at 08 :00 h on day 8 of each experi-mental period and continued for 48 h. Ileal digesta werecollected for a total of 24 h from 08 :00 to 20 :00 h onday 10 and from 20 :00 h on day 11 to 08 :00 h on day12.

The procedure for the collection of digesta and faeceswas described previously by Li et al (1994). Faeces anddigesta were frozen at [20¡C immediately after collec-tion. The samples were pooled, leaving one sample offaeces and digesta for each pig for each experimentalperiod. The experimental proposal and surgical pro-cedures were reviewed and approved by the AnimalCare Committee of the Faculty of Agriculture, Forestryand Home Economics at the University of Alberta. Theanimals were cared for in accordance with the guide-lines established by CCAC (1980).

Chemical analyses

Samples of digesta and faeces were freeze-dried andground in a Wiley mill through a 0É8-mm mesh screenbefore analysis. Analyses for DM, organic matter (OM),energy and CP were conducted according to AOAC(1990) procedures. Analysis of chromic oxide wascarried out using procedures described by Fenton andFenton (1979). Amino acids in digesta and faeces wereanalysed according to principles outlined by Jones andGilligan (1983) using a Varian 5000 high performanceliquid chromatography system (Varian Canada, Mis-sissauga, ON, Canada). This procedure was described inmore detail by Sedgwick et al (1991) and Li and Sauer(1994).

Statistical analysis

Data were subjected to analysis of variance using theGeneral Linear Model procedure of SAS (1990) accord-ing to a two-period crossover design (Petersen 1985).The statistical model included dietary treatments(df\ 1), experimental periods (df\ 1) and the inter-actions of treatment and period (df \ 1) as sources ofvariation. For parameters where the interaction was notsigniÐcant, the variation was combined into the errorterm. Least square means of treatments and periodswere compared using the probability of di†erence(PDIFF) procedure (SAS 1990).

RESULTS AND DISCUSSION

All pigs consumed their meal allowances within 30 minafter feeding throughout the experiment. Post-mortem

Soybean trypsin inhibitors and nutrient digestibilities 359

examinations conducted at the conclusion of the experi-ment showed no intestinal adhesions or other abnor-malities.

The incorporation of autoclaved Nutrisoy comparedwith Nutrisoy into the maize starch-based dietincreased (P\ 0É01) the apparent ileal digestibilities ofDM, OM, energy, CP and all amino acids (Table 1).The digestibility of methionine in the diets was cor-rected for methionine that was supplemented (0É15%)which is completely absorbed in the small intestine(Huisman J unpublished results, 1996). The energydigestibility increased from 66É0 to 77É9% and the CPdigestibility increased from 37É4 to 77É1%. The increasein digestibility ranged from 27É0 (methionine) to 49É2(leucine) percentage units for the indispensable aminoacids. The increase in digestibility ranged from 30É2(aspartic acid) to 50É8 (tyrosine) percentage units for thedispensable amino acids.

With the exception of methionine (58É9%), the appar-ent ileal digestibilities of the indispensable amino acidsin the Nutrisoy diet were uniformly low, ranging from37É1 (leucine) to 45É4% (arginine ; Table 1). These resultsare di†erent from studies with many other protein-

containing ingredients including soybean meal andpeas. For soybean meal-based diets, the apparent ilealdigestibilities of arginine and lysine are relatively high,whereas that of threonine is relatively low (eg Li andSauer 1994). Similar Ðndings were also reported instudies with peas (eg Fan et al 1994). Low (1980) report-ed that arginine and lysine would be expected to appearÐrst after enzymatic hydrolysis and threonine last, basedon the known speciÐcity of the proteases and peptidasesinvolved in protein digestion. With the exception oflysine, this pattern was also observed for the autoclavedNutrisoy diet. The relatively lower lysine digestibilitymay have resulted from the process of autoclavingwhich may reduce its digestibility as a result of theMaillard reaction (Hurrell and Carpenter 1977). Fromthe aforementioned results, it can be speculated thatSBTI and trypsin (also chymotrypsin) combined to formcomplexes which resulted in an overall decrease in theefficiency of protein digestion in the Nutrisoy diet. TheenzymeÈinhibitor complex is very strong due to theclose complementary Ðt of these two interacting mol-ecules. The binding force is further reinforced by a largenumber of non-covalent hydrophobic and hydrogen

TABLE 1E†ect of experimental diet on ileal and faecal digestibilities (%) of nutrients and energy in growing pigs

Items Ileal digestibilities Faecal digestibilities

Nutrisoy Autoclaved Nutrisoy SEa Nutrisoy Autoclaved Nutrisoy SEa

Dry matter 64É3c 73É9b 1É11 87É2c 89É7b 0É47Organic matter 67É0c 76É2b 0É95 89É2c 91É3b 0É43Crude protein 37É4c 77É1b 2É69 77É3c 90É2b 1É38Energy 66É0c 77É9b 1É12 87É6c 90É9b 0É60

Amino acidsIndispensable

Arginine 45É4c 90É0b 2É34 84É8c 95É6b 1É02Histidine 43É9c 82É5b 2É88 85É1c 94É7b 1É19Isoleucine 40É4c 86É3b 2É81 74É2c 91É3b 1É70Leucine 37É1c 86É3b 2É43 75É2c 91É8b 1É67Lysine 40É8c 79É6b 3É09 79É6c 90É2b 1É58Methionined 58É9c 85É9b 3É87 72É2c 89É4b 2É57Phenylalanine 39É1c 87É8b 1É98 77É0c 92É8b 1É55Threonine 36É5c 73É3b 3É96 72É4c 88É9b 2É01Valine 38É2c 83É6b 2É46 73É6c 90É9b 1É84

DispensableAlanine 43É4c 81É1b 2É78 71É9c 89É3b 2É29Aspartic acid 42É4c 72É6b 1É92 81É1c 92É5b 1É48Cysteine 35É5c 67É7b 4É43 76É8c 85É6b 1É23Glutamic acid 48É6c 83É7b 1É60 86É0c 95É4b 1É03Glycine 29É4c 70É2b 4É18 75É4c 89É7b 1É97Serine 36É8c 80É6b 2É16 80É2c 92É9b 1É36Tyrosine 34É1c 84É9b 3É06 72É9c 91É4b 2É27

a Standard error of the mean (n \ 6).b,c Means in the same row, within ileal or faecal digestibilities, with di†erent superscripts di†er (P\ 0É01).d Digestibility after correction for dietary supplementation of methionine.

360 S L i, W C Sauer, W R Caine

TABLE 2E†ect of experimental diet and experimental period on the net disappearance (g kg~1 DM intake) of

nutrients and energy in the large intestine of growing pigs

Items Dietary treatment Experimental period

Nutrisoy Autoclaved Nutrisoy SEa Period 1 Period 2 SEa

Dry matter 229É2b 157É4c 9É21 179É1 7É5 9É21Organic matter 209É9b 143É3c 7É25 163É5 89É7 7É25Crude protein 81É0b 27É1c 3É19 46É5c 61É7b 3É19Energy (MJ kg~1) 4É1c 2É5c 0É17 2É9 3É3 0É17

Amino acidsIndispensable

Arginine 5É3b 0É7c 0É21 2É4c 3É6b 0É21Histidine 1É9b 0É6c 0É09 1É2 1É3 0É10Isoleucine 3É2b 0É5c 0É19 1É3c 2É4b 0É19Leucine 5É7b 0É8c 0É20 2É4c 4É0b 0É20Lysine 4É7b 1É2c 0É22 2É6 3É3 0É22Methionined 2É8b 0É3c 0É09 1É2c 2É0b 0É09Phenylalanine 3É8b 0É5c 0É12 1É6c 2É6b 0É11Threonine 2É6b 1É1c 0É21 1É4c 2É2b 0É21Valine 3É3b 0É7c 0É12 1É4c 2É6b 0É12

DispensableAlanine 2É4b 0É7c 0É13 1É1c 2É0b 0É13Aspartic acid 7É6b 4É0c 0É26 4É8c 6É9b 0É26Cysteine 3É6b 0É6c 0É13 1É5c 2É7b 0É13Glutamic acid 13É1b 4É1c 0É33 6É9c 10É3b 0É33Glycine 3É9b 1É6c 0É28 2É3 3É2 0É28Serine 4É0b 1É1c 0É13 2É1c 3É0b 0É13Tyrosine 2É1b 0É3c 0É10 0É9c 1É5b 0É09

a Standard error of the mean (n \ 6).b,c Means in the same row, within treatment or periods, with di†erent superscript letters di†er(P\ 0É01).d Net disappearance after correction for dietary supplementation of methionine.

bonds (Blow et al 1974 ; Sweet et al 1974). Based on thevery low ileal amino acid digestibilities in the Nutrisoydiet, only small amounts of free trypsin and chymotryp-sin may have been available for digestion.

The incorporation of autoclaved Nutrisoy comparedwith Nutrisoy also increased (P\ 0É01) the apparentfaecal digestibilities of all parameters measured (Table1). The energy digestibility increased from 87É6 to90É9% and the CP digestibility increased from 77É3 to90É2%. The increase in digestibility ranged from 9É6(histidine) to 17É3 (valine) percentage units for the indis-pensable amino acids. The increase in digestibilityranged from 8É8 (cysteine) to 18É5 (tyrosine) percentageunits for the dispensable amino acids.

As pointed out previously, the ileal rather than faecalanalysis method should be used to determine aminoacid digestibilities in feedstu†s for pigs because of themodifying action of the microÑora in the large intestine(Sauer and Ozimek 1986). The di†erences in amino aciddigestibilities between dietary treatments measured withthe ileal analysis method were considerably larger than

those measured with the faecal analysis method (Table1), which shows that the ileal analysis method is moresensitive than the faecal analysis method for detectingdi†erences in amino acid digestibilities. The improvedsensitivity of the ileal analysis method was alsoobserved by Sauer et al (1977) and Vandergrift et al(1983) in studies with feedstu†s that were processed dif-ferently. In contrast, the digestibility of energy shouldbe determined with the faecal analysis method, becausethere can be considerable disappearance of energy inthe large intestine in the form of volatile fatty acids. Theextent of the disappearance is dependent on diet com-position (eg Sauer et al 1980 ; Fan et al 1994).

Compared with pigs fed the autoclaved Nutrisoy diet,pigs fed the Nutrisoy diet had a lower ADG and higherF/G ratio. The ADG for pigs fed the Nutrisoy andautoclaved Nutrisoy diets were 523 and 780 g, respec-tively. In the same order of the dietary treatments, theF/G ratio was 5É0 and 2É8, respectively. Some cautionshould be exercised in the interpretation of the per-formance results, as these are based only on a 14-day

Soybean trypsin inhibitors and nutrient digestibilities 361

TABLE 3E†ects of experimental period on ileal and faecal digestibilities (%) of nutrients and energy

in growing pigs

Items Ileal digestibilities Faecal digestibilities

Period 1 Period 2 SEa Period 1 Period 2 SEa

Dry matter 69É6 68É6 1É11 87É6c 89É3b 0É47Organic matter 72É1 71É1 0É95 89É4c 91É1b 0É43Crude protein 59É7 54É8 2É69 82É5 85É1 1É38Energy 72É6 71É3 1É13 88É5 90É0 0É60

Amino acidsIndispensable

Arginine 70É6 64É8 2É34 88É4 92É0 1É02Histidine 62É5 64É0 2É88 88É1 91É7 1É19Isoleucine 66É8 59É8 2É81 80É2 85É3 1É69Leucine 64É7 59É0 2É43 80É9 86É0 1É67Lysine 61É3 59É1 3É09 82É8 87É0 1É57Methionined 74É8 70É0 3É88 77É1b 84É5a 2É57Phenylalanine 66É1 60É8 1É98 82É5 87É3 1É55Threonine 58É3 51É5 3É97 78É5 82É8 2É01Valine 64É9 56É9 2É46 79É9 84É6 1É84

DispensableAlanine 65É3 59É2 2É78 78É1 83É1 2É29Aspartic acid 61É6b 53É3c 1É92 85É6 88É0 1É48Cysteine 57É8 45É3 4É75 76É9c 85É5b 1É23Glutamic acid 69É7b 62É5c 1É60 89É4 91É9 1É03Glycine 52É9 46É7 4É17 80É7 84É4 1É97Serine 62É0 55É4 2É16 84É8 88É3 1É36Tyrosine 62É5 56É5 3É06 79É9 84É4 2É27

a Standard error of the mean (n \ 6).b,c Means in the same row, within ileal or faecal digestibilities, with di†erent superscriptsdi†er (P\ 0É05).d Digestibility after correction for dietary supplementation of methionine.

measurement. However, the di†erences in performancereÑect di†erences in energy and amino acid digest-ibilities between dietary treatments. The results are inagreement with those reported in the literature in whichdiets containing raw soybean were compared withsoybean meal. A decrease in performance, nitrogenretention and nutrient digestibilities were reported instudies with young (Yen et al 1974 ; Cook et al 1988),growing (Combs et al 1967) and growing-Ðnishing pigs(Jimenez et al 1963 ; Young 1967) when diets containingraw soybean were fed.

There was a higher (P\ 0É01) net disappearance(g kg~1 DM intake) of DM, OM, CP, energy and allamino acids in the large intestine of pigs fed the Nutri-soy than with the autoclaved Nutrisoy diet (Table 2).For CP, the net disappearance was 53É9 g greater ; forenergy, the net disappearance was 1É6 MJ kg~1 greater.The disappearance of energy in the large intestine corre-sponded to 21É8% of the energy intake in pigs fed theNutrisoy and only 13É3% in pigs fed the autoclavedNutrisoy diet, respectively. For the indispensable aminoacids, the di†erences in net disappearance between

dietary treatments ranged from 1É3 (histidine) to 4É9 g(leucine) and for the dispensable amino acids from 1É7(alanine) to 9É0 g (glutamic acid). The larger disap-pearance of amino acids in the large intestine of pigs fedthe Nutrisoy as opposed to the autoclaved Nutrisoydiet will have no e†ect on the protein utilisation by thepig. Zebrowska et al (1973) showed that microbial fer-mentation of undigested protein (and unabsorbedamino acids and peptides) in the large intestine yieldednitrogen-containing compounds, mainly in the form ofammonia. Once absorbed, ammonia will be convertedto urea and excreted in urine. However, the higher dis-appearance of energy in the large intestine in pigs fedthe Nutrisoy diet will be of partial beneÐt, becausemicrobial fermentation of undigested energy sources inthe large intestine will yield volatile fatty acids whichwill be used by the pig as supplementary energy source.In terms of energy utilisation, these results indicate acompensatory e†ect by the action of microÑora. Thiscompensatory e†ect will likely be important in pigs feddiets high in non-starch polysaccharides, in particularwhen dietary energy supply is limiting.

362 S L i, W C Sauer, W R Caine

There was a greater (P\ 0É05) net disappearance ofCP and most of the amino acids during period 2 (Table2). With the exception of aspartic and glutamic acid,there were no di†erences (P[ 0É05) in the apparent ilealdigestibilities of all amino acids between experimentalperiods. With the exception of cysteine and methionine,there were no di†erences between periods in the faecaldigestibilities of all amino acids (Table 3). These resultsare in agreement with previous studies using pigs of thesame BW range that showed no period e†ects on appar-ent ileal (eg Fan et al 1994) or faecal digestibilities (egImbeah and Sauer 1991).

As shown in a previous study (Li et al 1997), thefeeding of Nutrisoy compared with autoclaved Nutrisoydid not a†ect (P[ 0É05) the total activities of the exo-crine pancreatic enzymes in growing pigs, indicating theabsence of hypersecretion of pancreatic enzymes, whichwas demonstrated in studies with chicks (eg Chernick etal 1948) and rats (eg Rackis et al 1979). The largeincrease in the recovery of amino acids in ileal digestawhen the Nutrisoy diet was fed is likely caused by theformation of complexes between SBTI and trypsin andchymotrypsin. The formation of these complexes willincrease both endogenous (of animal origin) and exoge-nous (of dietary origin) recoveries of amino acids. Theincreased endogenous recovery of amino acids likelyresults from a decrease in autodigestion of theseenzymes in the small intestine. The increased exogenousrecovery of amino acids results from a decrease indigestion of exogenous protein due to a decrease in theavailable (not complexed) supply of trypsin and chy-motrypsin.

In conclusion, the ileal as well as faecal digestibilitiesof energy and amino acids were considerably lower inpigs fed the Nutrisoy compared with the autoclavedNutrisoy diet. There was also a greater disappearance ofenergy and amino acids in the large intestine of pigs fedthe Nutrisoy diet. The lower ileal digestibilities in pigsfed the Nutrisoy diet are likely due to the formation ofcomplexes between SBTI and trypsin and chymotryp-sin, which results in a decrease in the available supply ofthese enzymes and a decrease in the efficiency of proteindigestion. Further studies may be warranted to parti-tion the e†ect of SBTI on exogenous and endogenousamino acid losses.

ACKNOWLEDGEMENTS

Financial support provided by the Natural Sciences andEngineering Research Council of Canada is gratefullyacknowledged. The authors also thank Brenda Tchir,Charlane Gorsack, Suxi Huang and Rick Allan forassistance in animal surgery and Gary Sedgwich andMargaret Micko for assistance in laboratory analyses.

REFERENCES

AOAC 1990 Official Methods of Analysis (15th edn). Associ-ation of Official Analytical Chemists, Arlington, VA, USA.

Blow D M, Janin J, Sweet R M 1974 Mode of action ofsoybean trypsin inhibitor (Kunitz) as a model for speciÐcproteinÈprotein interactions. Nature (London) 249 54È57.

CCAC 1980 Guide to the Care and Use of ExperimentalAnimals (Vol 1) (with addendum). Canadian Council onAnimal Care, Ottawa, ON, Canada.

Chernick S S, Lepkovsky S, Chaiko† I L 1948 A dietaryfactor regulating the enzyme content of the pancreas :changes induced in size and proteolytic activity of the chickpancreas by the ingestion of raw soybean meal. Am JPhysiol 155 34È41.

Combs G E, Conness R G, Berry T H, Wallace H D 1967E†ect of raw and heated soybeans on gain, nutrient digest-ibility, plasma amino acids and other blood constituents ofgrowing swine. J Anim Sci 26 1067È1071.

Cook D A, Jensen A H, Fraley J R, Hymowitz T 1988 Uti-lization by growing and Ðnishing pigs of raw soybeans oflow Kunitz trypsin inhibitor content. J Anim Sci 66 1686È1691.

De Lange C F M, Sauer W C, Mosenthin R, Sou†rant W B1989 The e†ect of feeding di†erent protein-free diets on therecovery and amino acid composition of endogenousprotein collected from the distal ileum and feces in pigs. JAnim Sci 67 746È754.

Fan M Z, Sauer W C, Jaikaran S 1994 Amino acid and energydigestibility in peas (Pisum sativum) from white-Ñoweredspring cultivars for growing pigs. J Sci Food Agric 64 249È256.

Fenton T W, Fenton M 1979 An improved procedure for thedetermination of chromic oxide in feed and faeces. Can JAnim Sci 59 631È634.

Herkelman K L, Cromwell G L, Cantor A H, Stahly T S,Pfei†er T W 1993 E†ects of heat treatment on the nutri-tional value of conventional and low trypsin inhibitor soy-beans for chicks. Poultry Sci 72 1359È1369.

Hurrell R F, Carpenter K J 1977 Mechanisms of heat damagein proteins. 8. The role of sucrose in the susceptibility ofprotein foods to heat damage. Br J Nutr 38 285È297.

Imbeah M, Sauer W C 1991 The e†ect of dietary level of faton amino acid digestibilities in soybean meal and canolameal and on rate of passage in growing pigs. L ivestock ProdSci 29 227È239.

Jimenez A A, Perry T W, Pickett R A, Beeson W M 1963 Rawand heat-treated soybeans for growing-Ðnishing swine andtheir e†ect on fat Ðrmness. J Anim Sci 22 471È475.

Jones B N, Gilligan J P 1983 o-Phthaldialdehyde precolumnderivatization and reversed-phase high-performance liquidchromatography of polypeptide hydrolysates and physio-logical Ñuids. J Chromatogr 266 471È482.

Li S, Sauer W C 1994 The e†ect of dietary fat content onamino acid digestibility in young pigs. J Anim Sci 72 1737È1743.

Li S, Sauer W C, Hardin R T 1994 E†ect of dietary Ðbre levelon amino acid digestibility in young pigs. Can J Anim Sci74 327È333.

Li S, Sauer W C, Huang S X, Hardin R T 1997 Response ofpancreatic secretions to feeding diets with low and highlevels of soybean trypsin inhibitors in growing pigs. J SciFood Agric 76 347È356.

Liener I E, Kakade M L 1980 Protease Inhibitors. In T oxicConstituents of Plant Foodstu†s, ed Liener I E. AcademicPress, New York, USA, pp 7È71.

Low A G 1980 Nutrient absorption in pigs. J Sci Food Agric31 1087È1130.

Soybean trypsin inhibitors and nutrient digestibilities 363

Petersen R G 1985 Design and Analysis of Experiments.Marcel Dekker, New York, USA, pp 305È306.

Rackis J J 1972 Biologically active components. In Soybeans :Chemistry and T echnology (Vol 1, Proteins), eds Smith A K& Circle S J. AVI Publishing Co, Westport, CT, USA, pp158È202.

Rackis J J, Mcgee J E, Gumbmann M R, Booth A N 1979E†ects of soy proteins containing trypsin inhibitors in longterm feeding studies in rats. J Am Oil Chem Soc 56 162È168.

SAS 1990 SAS/STAT UserÏs Guide (Release 6.08). SAS Insti-tute, Cary, NC, USA.

Sauer W C 1976 Factors a†ecting amino acid availabilities forcereal grains and their components for growing mono-gastric animals. PhD dissertation, University of Manitoba,Winnipeg, Manitoba, Canada.

Sauer W C, Ozimek L 1986 Digestibility of amino acids inswine. Results and their applications. A review. L ivestockProd Sci 15 367È388.

Sauer W C, Stothers S C, Parker R J 1977 Apparent and trueavailabilities of amino acids in wheat and milling by-products for growing pigs. Can J Anim Sci 57 775È784.

Sauer W C, Just A, Jorgensen H H, Fekadu M, Eggum B O1980 The inÑuence of diet composition on the apparentdigestibility of crude protein and amino acids at the distalileum and overall in pigs. Acta Agric Scand 30 449È459.

Sauer W C, Jorgensen H, Berzins R 1983 A modiÐed nylonbag technique for determining apparent digestibilities ofprotein in feedstu†s for pigs. Can J Anim Sci 63 233È237.

Sedgwick G, Fenton T W, Thompson J R 1991 E†ect ofprotein precipitating agents on the recovery of plasma freeamino acids. Can J Anim Sci 71 953È957.

Sweet R M, Wright H T, Janin J, Chochia C H, Blow D M1974 Crystal structure of the complex of porcine trypsinwith soybean trypsin inhibitor (Kunitz) at 2.6-C resolution.Biochemistry 13 4212È4228.

Vandergrift W L, Knabe D A, Tanksley Jr T D, Anderson S A1983. Digestibility of nutrients in raw and heated soyÑakesfor pigs. J Anim Sci 57 1215È1224.

Yen J T, Jensen A H, Hymowitz T, Baker D H 1973 Uti-lization of di†erent varieties of raw soybeans by male andfemale chicks. Poultry Sci 52 1875È1882.

Yen J T, Hymowitz T, Jensen A H 1974 E†ect of soybeans ofdi†erent trypsin-inhibitor activities on performance ofgrowing swine. J Anim Sci 38 304È309.

Yen J T, Jensen A H, Simon J 1977 E†ect of dietary rawsoybean trypsin inhibitor on trypsin and chymotrypsinactivities in the pancreas and in the small intestinal juice ofgrowing swine. J Nutr 107 156È165.

Young L G 1967 Raw soybeans in swine growing-Ðnishingrations. Can J Anim Sci 47 227È234.

Zebrowska T 1973 Digestion and absorption of nitrogenouscompounds in the large intestine of pigs. Rocz Nauk Roln,Ser B, Zootech 95 85È90.