D. Hong*, D. Ragland, and O. Adeola* 2. Departments of *Animal Sciences and Veterinary Clinical Sciences, Purdue University, West Lafayette, IN 47907

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1 Additivity and associative effects of metabolizable energy and amino acid digestibility of corn, soybean meal, and wheat red dog for White Pekin ducks 1 D. Hong*, D. Ragland, and O. Adeola* 2 Departments of *Animal Sciences and Veterinary Clinical Sciences, Purdue University, West Lafayette, IN ABSTRACT: The additivity of true amino acid digestibility (TAAD) and true metabolizable energy (TME) values in corn, soybean meal, and wheat red dog for White Pekin ducks was investigated. Differences between observed values for the complete diets and values predicted from measurements of individual ingredients were used to test additivity. Eight ducks were each assigned to the following dietary treatments: corn, soybean meal (44% CP), wheat red dog (wheat by-product with less than 4% fiber), complete Diet 1 (cornsoybean meal), complete Diet 2 (corn-red dog-soybean meal), and dextrose. Dextrose-fed ducks were used to estimate endogenous losses. The nitrogen-corrected TME (TMEn) in corn, soybean meal, wheat red dog, and two complete diets were 3.411, 2.919, 2.502, 3.148, and kcal/g, respectively. In general, the TME and TMEn values observed in the two complete diets were not different (P > 0.05) from predicted values and indicated that the TME and TMEn in corn, soybean meal, and wheat red dog were all additive. The mean TAAD of corn, soybean meal, wheat red dog, and the two complete diets were not different, and were 87.03, 88.15, 90.58, 85.83, and 87.02%, respectively. The differences in TAAD between observed and predicted values were significant (P < 0.05) only for arginine, lysine, and aspartate in complete Diet 1, and for arginine, histidine, lysine, and aspartate in complete Diet 2. These results indicated that TME and TMEn values for corn, soybean meal, and wheat red dog were all additive in the two complete diets, but digestibilities of some amino acids were not additive and demonstrated some associative effects. Key Words: Amino Acids, Metabolizable Energy, Soyabean Oilmeal, Wheat Milling Residues 2002 American Society of Animal Science. All rights reserved. J. Anim. Sci : Introduction Additivity of nutrients is a crucial consideration in the formulation of diets for poultry. It is assumed that the supply of nutrients in a complete diet is equal to the sum of the supply based on the digestibility values determined from the single ingredients. However, few studies have confirmed this assumption. There are some reports of associative (nonadditivity) effects in ruminant animals when they are fed a diet composed of forage or low quality roughage and a readily available energy source because of changes in ph value, VFA concentration, and microbial activity in the rumen 1 The financial support Maple Leaf Farms, Inc., Syracuse, Indiana, is thankfully acknowledged. The authors wish to thank Pat Jaynes and Jason Sands for their assistance. Journal paper No of the Purdue University Agricultural Research Programs. 2 Correspondence: Phone: ; Fax: ; E- mail: ladeola@purdue.edu. Received October 31, Accepted July 11, (Thompson et al., 1991; Kienzle, 1994). Information regarding additivity from pig (Imbeah et al., 1988; Furuya and Kaji, 1991) and broiler (Angkanaporn et al., 1996) experiments showed that nutrients may not always be additive under certain circumstances. In our previous study (Hong et al., 2001), we investigated the possible nutrient additivity and associative effects for barley and canola meal with White Pekin ducks and observed that the ME values of barley and canola meal were additive in a complete diet but digestibilities of some amino acids were not additive. Therefore, in order to further improve the practical formulation of diets for ducks, studies with a wider variety of feedstuffs are warranted. Corn and soybean meal are two of the most common feed ingredients used to formulate diets for poultry in the United States. Wheat red dog, a low fiber by-product composed primarily of wheat offal is often used in duck diets in a variety of situations. The objective of the present study was to determine the additivity and associative effects of the metabolizable energy and amino acid digestibility of corn, soybean meal, and wheat red dog for ducks. 3222

2 Additivity of feedstuffs for ducks 3223 Table 1. The composition and chemical analysis of diets (as-fed) basis Complete Complete Item Corn Soybean meal Wheat red dog Diet 1 Diet 2 Ingredients (g/kg) Corn 1, Wheat red dog , Soybean meal (44% CP) 0.0 1, Dicalcium phosphate a Limestone Salt Vitamin mineral premix b Total 1, , , , ,000.0 Dry matter, g/kg Gross energy, kcal/kg 4, , , , ,283.9 Crude protein, g/kg a Contained 20% calcium and 18% phosphorus. b Supplied the following per kilogram of diet: vitamin. A, 5,484 IU; vitamin. D 3, 2,643 ICU; vitamin. E, 11 IU; menadione sodium bisulfite, 4.38 mg (2.19 mg of vitamin K 3 ); riboflavin, 5.49 mg; D-pantothenic acid, 11 mg; niacin, 44.1 mg; choline chloride, 771 mg (670 mg of choline); vitamin. B 12, 13.2 g; biotin, 55.2 g; thiamine mononitrate, 2.2 mg (2.1 mg of thiamine); folic acid, 990 g; pyridoxine hydrochloride, 3.3 mg (3.2 mg of pyridoxine); I (as calcium iodate), 1.11 mg; Mn (as manganous oxide), mg; Cu (as copper sulfate), 4.44 mg; Fe (as ferrous sulfate), 44.1 mg; Zn (as zinc oxide), 44.1 mg; Se (as sodium selenite), 300 g. Materials and Methods Management of Ducks and Diets Forty-eight 11-wk-old White Pekin male ducks with an average weight of 3.8 kg were sorted according to initial weights and assigned to five dietary treatment groups and one feed-deprived group such that average weights across the six groups were similar (eight ducks per group). The ducks were randomly assigned to individual cages ( m) and housed in an environmentally controlled room maintained at 25 C. Fluorescent bulbs provided 24-h lighting. The eight ducks assigned to the feed-deprived group were fed dextrose for estimation of endogenous losses of amino acids, energy, and nitrogen (Sibbald, 1979). The experimental diets consisted of corn, soybean meal, and wheat red dog and two complete diets composed of corn and soybean meal, and corn, soybean meal, and wheat red dog (Table 1). Excreta Collection Equipment and Methodology Traditional excreta collection methods, where trays are placed under cages, are not appropriate for ducks because they consume much greater quantities of water than chickens (Siregar and Farrell, 1980). As such, ducks often produce highly liquid excreta, which results in dry matter losses due to splatter from the contact of forcefully ejected excreta with collection trays. Therefore, the attachment of a collection apparatus to ducks was devised for collection of contaminant-free excreta to accurately estimate nutrient and energy output. The surgical collection method was described previously by Adeola et al. (1997). In summary, approximately 3 d before the start of the experiment, each duck was surgically fitted with modified plastic retainer lids from a Playtex nurser bottle set (Playtex Products, Dover, DE). The ducks were placed in a restraint box, and feathers around the vent were removed. Four milliliters of 2% lidocaine hydrochloride (Phoenix Pharmaceutical, Inc., St. Joseph, MO) were injected around the vent to desensitize the area (1 ml in each of four regions). A continuous suture was applied to secure the retainer ring to the skin of the ducks. The plastic bottle of the nurser set was measured and cut to a length of 3 cm below the threads on the bottle and Whirl-Pak bags (NASCO, Ft. Atkinson, WI) inserted into the bore of the bottle, so that the edges of the bags hung over the threads of the bottle. The bottle and Whirl-Pak bag were then screwed onto the modified retainer ring attached to the duck and the collection apparatus was completed. Feeding Techniques and Experimental Procedures The ME assay used in the present experiment followed the standard techniques devised by Sibbald (1976) and the modifications suggested by McNab and Blair (1988). The modified true metabolizable energy (TME) bioassay improved the precision of Sibbald s method by extending the collection time from 24 to 48 h and decreased the stress on the birds used for the determination of endogenous losses by administering dextrose instead of no feed at all. In using the assay for ducks, it became necessary to modify the techniques again, because many ducks were observed to regurgitate a generous portion of the test ingredient when 50 g was tube-fed at one time. We found that it was necessary to feed the test ingredients in two equal portions, 6 h apart, and to extend excreta collection time from a total of 48 to 54 h. The experimental protocol is summarized in Table 2. Each experiment lasted 102 h with an initial 48-h feeddeprived period and a 54-h excreta collection period.

3 3224 Hong et al. Time (h) and operation Table 2. The experimental protocol 0 Feed withdrawn 24 Each bird fed dextrose solution (25 g/100 ml water) 30 Each bird fed dextrose solution (25 g/100 ml water) 48 Birds fed appropriate feedstuff (25 g/100 ml water). Birds used for fasting energy loss, fasting nitrogen, and amino acid loss calculation were fed dextrose solution (25 g/100 ml water). Collection bags screwed into affixed lids. 54 Birds fed appropriate feedstuff (25 g/100 ml water). Birds used for fasting energy loss, fasting nitrogen, and amino acid loss calculation were fed dextrose solution (25 g/100 ml water). Collection bags screwed into affixed lids. 60 Excreta collected and frozen, new collection bags screwed into lids. 72 Excreta collected and frozen, new collection bags screwed into lids. 84 Excreta collected and frozen, new collection bags screwed into lids. 96 Excreta collected and frozen, new collection bags screwed into lids. 102 Excreta collected and frozen, lids removed from the ducks. All ducks were fitted with their respective collection apparatus at the time of the first feeding of experimental diets. The Whirl-Pak bags containing excreta were changed within the first 6 h after placement and every 12 h thereafter during the 54-h collection period. All of the feeding and surgical collection protocols in the present study were approved by the Purdue University Animal Care and Use Committee. Chemical Analysis All excreta samples were frozen immediately after collection. At the completion of the experiment, all samples were transferred to aluminum pans and placed in a forced-air oven for 96 h. A temperature of 55 C was chosen to minimize the volatilization of fatty acids. After drying, excreta samples were ground through a 0.5-mm screen prior to analysis. Dry matter was determined by drying the samples at 110 C for 24 h. Nitrogen determination was by the combustion method with the LECO model FP-2000 Nitrogen Analyzer (LECO, St. Joseph, MI). The energy contents of the feedstuffs and excreta samples were determined by bomb calorimetry with benzoic acid as a standard (Parr, Moline, IL). Amino acid compositions were determined at the University of Missouri Chemistry Laboratory (AOAC, 1995). Calculations and Statistical Analysis The TME contents of the feedstuffs were calculated using methods described by Sibbald (1976). The TME and nitrogen-corrected TME (TMEn) in kcal/g were calculated as follows: TME = [(EI EO)/FI] + (FEL/FI) TMEn = TME (8.22 ANR/FI) (8.22 FNL/FI) where EI is gross energy intake (kcal); EO is gross energy output (kcal); FI is the intake of the feedstuffs (50 g); ANR is apparent nitrogen retention (g); FEL is fasting energy loss (kcal) from the feed-deprived ducks, and FNL is fasting nitrogen loss (g). Nitrogen retained in tissues can be catabolized to yield energy-containing excretory compounds that contribute to fasting energy loss. Therefore, the gross energy excreted was corrected to zero-nitrogen balance using a factor of 8.22 kcal/g (Hill and Anderson, 1958). The true amino acid digestibility (TAAD) was calculated as [(AA intake - AA output)/aa intake] + (endogenous AA output/aa intake). Additivity was tested by comparing the differences between observed digestibility coefficients of the complete diet and predicted values from measurements determined with individual ingredients (corn, wheat red dog, and soybean meal). Analysis of variance using the General Linear Model (GLM) procedure of SAS (SAS Inst., Inc., Cary, NC) was performed as a randomized complete block design. Treatment mean differences were tested with the least significant difference procedure (Steel and Torrie, 1980). Results and Discussion The crude protein, nitrogen, and amino acid contents of the ingredients and the complete diets used in the study are shown in Table 3. The crude protein and amino acid composition of corn, soybean meal, and wheat red dog were similar to those listed by NRC (1994). The two complete diets were formulated to contain 16% crude protein. The rapid assay was devised for the determination of TME (Sibbald, 1976) and apparent metabolizable energy (AME) (Farrell, 1978), which are mathematically related to each other. However, because the former is corrected for metabolic fecal and endogenous urinary losses and are not affected by the level of feed intake (Sibbald, 1976), the TME values should be an improvement over AME. The endogenous output of nitrogen, energy, and amino acids are presented in Table 4. The fasting energy loss (37.01 kcal) during the 54-h collection period was in agreement with that of previous studies (Adeola et al., 1997), but the endogenous nitrogen loss was higher than previously reported. The endogenous amino acid outputs in the present study were in agreement with those reported by Ragland et al. (1999). Studies by Siriwan et al. (1994) demonstrated a relatively high content of threonine and serine in the endogenous secretion of chickens and pigs fed a protein-free diet because the muco-protein and pancreatic secretion of the small intestine are rich in threonine and serine in addition to proline. However, the observation was not evident in the present study with feed-deprived ducks. The true metabolizable energy and amino acid digestibilities are shown in Table 5. The TME values were highest for corn, intermediate for the two complete diets and wheat red dog, and lowest for soybean meal (P < 0.05). The values of the two complete diets were not

4 Additivity of feedstuffs for ducks 3225 Table 3. Nitrogen and amino acid composition (%) of ingredients and diets (as-fed basis) Complete Complete Diet Corn Soybean meal Wheat red dog Diet 1 Diet 2 Crude protein Nitrogen Arginine Histidine Isoleucine Leucine Lysine Methionine Phenylalanine Threonine Tryptophan Valine Alanine Aspartate Cysteine Glutamate Proline Serine Tyrosine different. Correction of TME to zero-nitrogen balance for the five diets resulted in 7 to 10% reduction, which were the same in our previous study with ducks (Hong et al., 2001), and was higher than the 2 to 4% reduction in cockerels (McNab and Blair, 1988) and the 2 to 5% reduction in other studies with ducks (Adeola et al., 1997). Nitrogen-corrected TME is not affected by the physiological state of the bird; therefore, it is the most useful estimation of bioavailable energy under low feed intake conditions (Wolynetz and Sibbald, 1984). The TME and TMEn values of corn were in agreement with previous determinations in ducks (Adeola et al., 1997; King et al., 1997; Ragland et al., 1997). However, the TME value of corn was lower than observations by Sibbald (1976) for roosters. After nitrogen correction, the TMEn value of corn paralleled those listed by NRC (1994). For example, the TMEn value listed in the NRC (1994) in cockerel assays is kcal/g, while our results for ducks was kcal/g. When compared with that listed in the NRC (1994) for 48% CP soybean meal, Table 4. Endogenous output of nitrogen, energy, and amino acids for feed-deprived ducks a Item Mean SD Range Nitrogen output, g , Energy output, kcal Indispensable AA output, g Arginine Histidine Isoleucine Leucine Lysine Methionine Phenylalanine Threonine Tryptophan Valine Dispensable AA output, g Alanine Aspartate Cysteine Glutamate Proline Serine Tyrosine a All data based on a 54-h collection period from seven or eight ducks.

5 3226 Hong et al. Table 5. True metabolizable energy (TME) (kcal/g) and amino acid digestibilities (%) of diets Complete Complete Item Corn Soybean meal Wheat red dog Diet 1 Diet 2 SD TME, kcal/g w z y x x TMEn, kcal/g w z y x x Indispensable amino acids, % Arginine z y yz z z 3.95 Histidine yz z yz yz y 3.57 Isoleucine z y yz yz yz 6.39 Leucine Lysine z w yz xy wx 4.10 Methionine Phenylalanine Threonine yz y yz z yz 5.65 Tryptophan y z z z yz 2.91 Valine Dispensable amino acids, % Alanine y yz z yz y 5.29 Aspartate z y z yz yz 5.22 Cysteine Glutamate yz y y z yz 3.10 Proline Serine xyz x xy z yz 9.35 Tyrosine yz y yz z z 6.01 Average of all amino acids N w,x,y,z Means in the same row with no common superscript differ significantly (P < 0.05). the TMEn value of soybean meal was not different (2.502 vs kcal/g). These suggest that the energy in corn and soybean meal were metabolized with similar efficiency in ducks and chickens. There is limited information about the metabolizable energy of wheat red dog in ducks. The TME and TMEn values observed in the current study are and kcal/g, respectively. The basic method of the TME assay can be applied successfully to the measurement of amino acid digestibility because the energy input level does not influence amino acid excretion (Sibbald, 1979). The ducks used in the current study were not cecectomized because our previous study (Ragland et al., 1999) showed that there is no difference between cecectomized and noncecectomized ducks in TAAD for these ingredients. The mean TAAD value of soybean meal (90.58%) was highest among the five dietary treatments. The complete diet that consisted of corn and soybean meal had the lowest mean TAAD value (85.83%). The true digestibility values of many amino acids were similar for corn and wheat red dog (P > 0.05), except that digestibility of tryptophan and alanine in corn were higher. Poor apparent digestibilities of lysine and threonine in corn were also reported by Green et al. (1987) in cockerels and by Imbeah et al. (1988) in pigs because of their slow rate of absorption and high concentration in endogenous secretion. Lysine in cereals is mainly deposited in the poorly digested aleurone layer and little is found in the digestible prolamine fraction. The TAAD values for soybean meal in the present study were close to the results of Angkaporn et al. (1996) in broilers, but lower than those of Likuski and Dorrell (1978) and Cave (1988) in roosters. Such species differences could be accounted for by the different collection periods and statistical methods. For example, the collection period after force-feeding was 24 h in the experiment of Likuski and Dorrell (1978), but 54 h in the present study for ducks. Cave (1988) used regression methods to determine endogenous amino acid losses, which generally were higher than those obtained from feed-deprived birds. When further compared with the NRC (1994) for roosters, the true digestibilities of amino acids for soybean meal were in good agreement. However, the results for corn were inconsistent. Most of the TAAD for corn were about 3 to 8 percentage units lower than the coefficients listed by NRC (1994), except methionine and threonine were similar and tryptophan and histidine were higher than those listed in the NRC (1994). Early studies adopting the 24-h collection for determination of the TAAD values in corn with roosters resulted in higher TAAD values than those listed by NRC (1994). The TAAD values of corn in the present study were also lower than those observed by Parsons (1992) with roosters. The number of studies conducted to determine the TAAD of wheat red dog in poultry are sparse. Mean TAAD value for wheat red dog (88.15%) was in agreement with a previous study by Ragland et al. (1999) in ducks (87.52%). The differences between observed ME values and predicted values from measurement determined with individual ingredients (corn, soybean meal, and wheat red dog) are shown in Table 6. Predicted values were calculated from the digestibility values determined with the

6 Additivity of feedstuffs for ducks 3227 Table 6. Differences between observed and predicted true metabolizable energy (TME) (kcal/g) and amino acid digestibility values (%) for complete diets a Diet Complete Diet 1 Complete Diet 2 Difference SD Difference SD TME TMEn Indispensable amino acids (%) Arginine 3.55* * 2.38 Histidine * 2.08 Isoleucine Leucine Lysine 6.99* * 3.41 Methionine Phenylalanine Threonine Tryptophan Valine Dispensable amino acids (%) Alanine Aspartate 4.94* * 3.13 Cysteine Glutamate Proline Serine Tyrosine Average of all amino acids (%) N 8 8 a Differences obtained by subtraction of predicted values of corn, soybean meal, and wheat red dog from observed values of the complete diet. *Differences between observed and predicted values are significant (P < 0.05). individual ingredients and their relative proportions in the complete diets. For complete Diet 1, the respective TME and TMEn values observed were and kcal/g higher than predicted values. Corresponding values for complete Diet 2 were and kcal/g. These differences between observed and predicted values were not significant (P > 0.05), which indicated that the TME and TMEn of corn, soybean meal, and wheat red dog were all additive and did not have any significant associative effects in ducks. The nitrogen correction can result in less difference between observed and predicted metabolizable energy values. Recent studies indicated that the nonstarch polysaccharides of wheat have antinutritive properties, which may depress the ME value, starch digestibility, and nitrogen retention in chickens. Mollah and Annison (1981) reported that there was a synergistic effect in the dietary ME when wheat cultivars having an abnormally low AME value were combined with either corn or oat hulls for broilers. Because the hemicellulose from corn and oat hulls is more digestible than that from wheat, their presence in the intestinal tract may favor an optimal microflora for the digestion of starch, thereby improving the ME. However, this was not observed in the present study with wheat red dog (wheat by-product with less than 4% fiber) for ducks. The differences between the observed and predicted TAAD values for the two complete diets are also presented in Table 6. The observed and predicted true digestibilities of most dispensable amino acids were similar (P > 0.05) in the two complete diets with the exception of aspartate. For indispensable amino acids, only digestibilities of arginine and lysine in complete Diet 1 and arginine, histidine, and lysine in complete Diet 2 were significantly different (P < 0.05). The differences between observed and predicted TAAD ranged from 5.87 (serine) to 6.99 (lysine) in complete Diet 1 and 8.89 (cysteine) to 9.82 (lysine) in complete Diet 2. The results of the present study indicated that there were some associative effects on TAAD for ducks fed a corn-soybean meal-wheat red dog-based diet. Several factors in the complete diets that not only affect the amino acid digestibility, but also influence the endogenous amino acid losses may account for the observation. Corn and soybean meal usually contain 6 to 7% crude fiber, and stachyose and raffinose, none of which are well digested by birds. These compounds can significantly increase the excretion of some endogenous amino acids because they can adsorb peptides, amino acids, and digestive enzymes and increase pancreatic juices and mucin production in the lumen (Coon et al., 1990). The study of de Lange et al. (1989) demonstrated that purified pectin affect endogenous losses much larger than that of purified cellulose in pigs fed protein-free diets and indicated that different kinds of fibers have different effects on endogenous nitrogen losses. Wheat red dog often contains high non-starch polysaccharides (pentosans), which consist of a (1-4)-β-xy-

7 3228 Hong et al. lan chain with arabinose units substituted at the 2 and 3 positions of the xylose and arabinoxylan. Nonstarch polysaccharides can decrease the apparent amino acid digestibility through two ways. First, it can directly complex with digestive enzymes and reduce their activity or indirectly increase the bulk and viscosity of the intestinal contents and inhibit the rate of diffusion of substrates and enzymes (Angkanaporn et al., 1994). Second, the presence of pentosans accelerates the proliferation of microflora and the secretion of peptide hormone in the digestive tract and also increases endogenous amino acid output (Low, 1989). Although heating soybean meal can denature the hemagglutinin and trypsin inhibitor to improve their nutritive values, overprocessing by autoclaving can result in nonenzymatic Maillard reactions between lysine and oligosaccharides and reduce the availability of lysine (Parsons, 1992). It should be noted that dietary crude protein level might affect the amino acid digestibility values of ingredients. The study conducted by Fan et al. (1994) demonstrated that apparent ileal digestibility values of amino acids in feedstuffs are quadratically affected by their respective dietary levels. Thus, the different dietary protein levels of the five diets in the present experiment appeared to have an effect on the additivity of amino acids for ducks. Although many studies indicate that arginine can antagonize the utilization of lysine via a selective system, this may have exerted very little effect on the amino acid addititity in the present study. In balance studies, there are three major methods most often used to measure digestibility the direct method, the difference method, and the regression method. Fan and Sauer (1995) indicated that the direct method might not be suitable for the determination of digestibility values of feedstuffs with low-protein and poor palatability, such as barley and so on. However, the direct method was appropriate for the determination of energy and nutrient digestibilities in corn and soybean meal. Implications The study provided important information on the energy and amino acid utilization of corn, soybean meal, and wheat red dog by White Pekin ducks and indicated that the true metabolizable energy and nitrogen-corrected true metabolizable energy values of the three ingredients were all additive. However, the digestibilities of some amino acids were not additive and demonstrated some associative effects. Literature Cited Adeola, O., D. Ragland, and D. King Feeding and excreta collection techniques in metabolizable energy assays for ducks. Poult. Sci. 76: Angkanaporn, K., M. Choct, W. L. Bryden, and E. F. Annison Effects of wheat pentosans on endogenous amino acid losses in chickens. J. Sci. Food. Agric. 66: Angkanaporn, K., V. Ravindran, and W. L. Bryden Additivity of apparent and true ileal amino acid digestibilities in soybean meal, sunflower meal, and meat and bone meal for broilers. Poult. Sci. 75: AOAC Official Methods of Analysis. 16th ed. Assoc. Offic. Anal. Chem., Arlington, VA. Cave, N. A Bioavailability of amino acids in plant feedstuffs determined by In Vitro digestion, chick growth assay, and true amino acid availability methods. Poult. Sci. 67: Coon, C., K. L. Leske, O. Akavanichan, and T. K. Cheng Effect of oligosaccharide free soya bean meal on true metabolisable energy and fibre digestion in adult roosters. Poult. Sci. 69: de Lange, C. F. M., W. C. Sauer, R. Mosenthin, and W. B. Souffrant The effect of feeding different protein-free diets on the recovery and amino acid composition of endogenous protein collected from the distal ileum and feces in pigs. J. Anim. Sci. 67: Fan, M. Z., and W. C. Sauer Determination of apparent ileal amino acid digestibility in barley and canola meal for pigs with the direct, difference, and regression methods. J. Anim. Sci. 73: Fan, M. Z., W. C. Sauer, R. T. Hardin, and K. A. Lien Determination of apparent ileal amino acid digestibility in pigs: effect of dietary amino acid level. J. Anim. Sci. 72: Farrell, D. J Rapid determination of metabolisable energy of foods using cockerels. Br. Poult. Sci. 19: Furuya, S., and Y. Kaji Additivity of the apparent and true ileal digestible amino acid supply in barley, maize, wheat or soya-bean meal based diets for growing pigs. Anim. Feed Sci. Technol. 32: Green, S., S. L. Bertrand, M. J. C. Duron, and R. Maillard Digestibilities of amino acids in maize, wheat and barley meals, determined with intact and cecectomised cockerels. Br. Poult. Sci. 28: Hill, F. W., and D. L. Anderson Comparison of metabolizable energy and productive determinations with growing chicks. J. Nutr. 64: Hong, D., D. Ragland, and O. Adeola Additivity and associative effects of metabolizable energy and amino acid digestibility in barley and canola meal for White Pekin ducks. Poult. Sci. 80: Imbeah, M., W. C. Sauer, and R. Mosenthin The prediction of the digestible amino acid supply in barley-soybean meal or canola meal diets and pancreatic enzyme secretion in pigs. J. Anim. Sci. 66: Kienzle, E Small intestinal digestion of starch in the horse. Rev. Méd. Vét. 14: King, D., D. Ragland, and O. Adeola Apparent and true metabolizable energy values of feedstuffs for ducks. Poult. Sci. 75: Likuski, H. J. A., and H. G. Dorrell A bioassay for rapid determinations of amino acid available values. Poult. Sci. 57: Low, A. G Secretory response of the pig gut to non-starch polysaccharides. Anim. Feed. Sci. Technol. 23: McNab, J. M., and J. C. Blair Modified assay for true and apparent metabolizable energy based on the tube feeding. Br. Poult. Sci. 26: Mollah, Y., and E. F. Annison Wheat: Maize interactions in the bioassay of metabolisable energy in poultry diets in relation to the digestibility of starch. Proc. Nutr. Soc. Aust., Brisbane, OLD, Australia. 6:138. NRC Nutrient Requirements of Poultry. 9th rev. ed. Natl. Acad. Press, Washington, DC. Parsons, C. M Application of the concept of amino acid availability in practical feed formulation. In: International Technical Symposia proceedings, NOVUS International, St. Louis, MO. Ragland, D., D. King, and O. Adeola Determination of metabolizable energy contents of feed ingredients for ducks. Poult. Sci. 76:

8 Additivity of feedstuffs for ducks 3229 Ragland, D., C. R. Thomas, R. G. Elkin, D. J. Shafer, and O. Adeola The influence of cecectomy on metabolizable energy and amino acid digestibility of select feedstuffs for White Pekin ducks. Poult. Sci. 78: Sibbald, I. R A bioassay for true metabolizable energy in feedstuffs. Poult. Sci. 55: Sibbald, I. R A bioassay for available amino acids and true metabolizable energy in feedingstuffs. Poult. Sci. 58: Siregar, A. P., and D. J. Farrell A comparison of the energy and nitrogen metabolism of fed duckings and chickens. Br. Poult. Sci. 21: Siriwan, P., W. L. Bryden, and E. F. Annison Use of guanidinated dietary protein to measure losses of endogenous amino acids in poultry. Br. J. Nutr. 71: Steel, R. G. D., and J. H. Torrie Principles and Procedure of Statistics: A Biometrical Approach. 2nd ed. McGraw-Hill Book Co., New York. Thompson, K. N., S. G. Jackson, and J. P. Baker Apparent digestion coefficients and associative effects of varying hay: grain ratios fed to horses. Nutr. Rep. Int. 30: Wolynetz, M. S., and I. R. Sibbald Relationship between apparent and true metabolizable energy and the effects of a nitrogen correction. Poult. Sci. 63: