Protein quality of linseed for growing broiler chicks
J. TrevinÄo
*, M.L. RodrõÂguez, L.T. Ortiz, A. ReboleÂ, C. Alzueta
Departamento de ProduccioÂn Animal, Facultad de Veterinaria, Universidad Complutense, Ciudad Universitaria, 28040 Madrid, Spain
Received 19 July 1999; received in revised form 19 October 1999; accepted 2 March 2000
Abstract
The protein quality of linseed was assessed and compared to that of soybean meal (SBM) in two experiments using growing broiler chicks. In the ®rst experiment, the protein ef®ciency ratio (PER) and net protein ratio (NPR) were calculated for diets (90 g crude protein kgÿ1) containing the following sources of protein: SBM and SBM diet modi®ed to contain 10, 30 and 100% of the total protein from linseed. There was a signi®cant (P<0.001) relationship between performance data or PER and NPR values and inclusion rate of linseed in the diet. When fed as the sole source of protein, linseed caused a drastic reduction in weight gain (97.6 versus 6.9 g) and PER (3.38 versus 0.40) and NPR (4.29 versus 1.96) values compared to those for SBM. In the second experiment, apparent and true total tract retentions (ATTR and TTTR) for nitrogen and amino acids were determined for diets (150 g crude protein kgÿ1) containing SBM as reference protein or SBM protein replaced 10 and 30% by linseed protein. The ATTR values were 15.3 and 24.8% lower for nitrogen and 4.6 and 15.5% lower for total amino acids for diets containing linseed to compared to SBM diet. The response of individual amino acids to inclusion level of linseed was linear (P<0.001). The TTTR values followed a similar trend than that found for apparent retention. In conclusion, utilization of linseed protein by broiler chicks was worse than that from SBM and it attributable in all probability to the presence in linseed of antinutritional factors, such as mucilage, which can depress the retention of protein in chicks.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Linseed; Broiler chicks; Protein quality; Amino acid retention
1. Introduction
The seed of the ¯ax plant (Linum usitatissimum L.) is an important oilseed crop in some countries as Canada, China, Argentina and India. Linseed contains approximately
84 (2000) 155±166
*Corresponding author. Tel.:34-913943849; fax:34-913943849.
E-mail address: arebole@eucmax.sim.ucm.es (J. TrevinÄo)
200±250 g kgÿ1crude protein and 400±430 g kgÿ1oil (Lee et al., 1991), constituting a potential source of protein and energy to be used in animal feeding. It is also an excellent source of omega-3 fatty acids, particularly a-linolenic acid, which are currently of
interest in both human and animal nutrition (Bhatty, 1995; Cunnane, 1995; Wood and Enser, 1997; Doreau and Chilliard, 1997).
Linseed and solvent extracted seed meal (linseed meal) have been used predominantly as a ruminant feed. Information on the nutritional value of linseed in broiler diets is limited, and most studies relate to the effects of linseed on animal performance. Lee et al. (1991) reported growth depression and reduced feed ef®ciency in chickens fed diets containing 100 and 200 g linseed kgÿ1. Ajuyah et al. (1991) observed that birds receiving a diet with 200 g linseed kgÿ1had signi®cantly lower live weight than birds fed other diets. Roth-Maier and Kirchgessner (1995) recommended a maximum of 50 g linseed kgÿ1to be used in broiler diets. Bond et al. (1997) observed that the inclusion of linseed in the diet resulted in a signi®cant reduction in growth rate as the level of linseed increased from 100 to 300 g kgÿ1, attributable to antinutritional factors including mucilage, vitamin B6 antagonist (linatine), cyanogenic glycosides, trypsin inhibitors,
phytic acid, allergens and goitrogens (Madhusudhan et al., 1986; Bhatty, 1995). The aim of the current program was to evaluate the protein quality of linseed with growing broiler chicks using soybean meal (SBM) as a reference protein. The response of chicks to dietary protein was determined in terms of protein ef®ciency ratio (PER), net protein ratio (NPR) and apparent and true total tract retentions (ATTR and TTTR) for nitrogen and amino acids.
2. Materials and methods
2.1. Materials
The linseed (golden variety, c. Royal) and SBM used in this study, were obtained as single batches from commercial suppliers in Spain. Linseed was ground through a ultracentrifuge mill with a 4 mm sieve before being incorporated into the experimental diets. Linseed (LS) and (SBM) samples were analyzed in duplicate by the procedures described below.
2.2. Experiment 1
In this experiment, the protein quality and SBM-replacement value of linseed were evaluated using the PER and NPR bioassay.
Day-old male broiler chicks (Cobb) were fed a commercial starter diet from 0 to 7 days post-hatching, after which they were assigned to the dietary treatments. A total of 60 chicks were distributed at random to ®ve treatments, in four groups of three birds each. The mean group initial weights were similar (111.0 g). All the chicks were housed in an environmentally controlled room in heated, thermostatically controlled starter batteries with raised wire ¯oors. Feed and water were offered ad libitum and light was provided continuously. The test diets were fed from 8 to 17 days post-hatching. The feed intake and weight for each replicate were recorded.
The ®ve dietary treatments consisted of a protein-free diet and four test diets containing the following protein sources: reference SBM diet, and SBM diet modi®ed to contain 10 (SBM-10LS diet), 30 (SBM-30LS diet) and 100% (LS diet) of the total crude protein from linseed. The test diets were formulated to contain 90 g crude protein kgÿ1, the concentration of ground linseed being increased by decreasing the amount of SBM, glucose and starch in the reference diet. The composition of experimental diets appears in Table 1.
2.3. Experiment 2
The objective of this experiment was to estimate the effect of replacement in part of SBM protein by linseed protein on the ATTR and TTTR for nitrogen and amino acids.
Day-old male broiler chicks (Cobb) were maintained on a commercial diet until the beginning of the trial. On the 28th day, 48 birds of similar weight were randomly divided Table 1
Composition (g kgÿ1) of diets used in Experiments 1 and 2
Experiment 1 Experiment 2
Protein-Soybean meal 204.9 183.6 142.8 ± 341.0 307.0 239.0 ±
Ground linseed ± 40.9 122.8 411.0 ± 68.5 205.0 ±
Starch 340.5 330.0 310.0 273.5 293.5 274.0 244.5 432.5
Glucose 340.0 330.0 318.9 260.0 280.0 270.0 256.0 442.0
Cellulose 20.0 20.0 20.0 10.0 10.0 10.0 ÿ 40.0
Sun¯ower oil 50.0 50.0 40.0 ± 30.0 25.0 10.0 40.0
Dicalcium phosphate 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0
Calcium carbonate 15.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0
Sodium chloride 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0
Antioxidant (BHT) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5
Premixa 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0
Composition
Crude proteinb 91.0 91.3 90.2 90.3 152.2 149.4 149.4 3.2
ME (MJ kgÿ1diet)c 14.2 14.2 14.3 14.4 13.1 13.2 13.3 14.8
aVitamin and mineral mixture provided (mg kgÿ1diet): calcium carbonate, 14 g; dicalcium phosphate, 20 g;
sodium chloride, 4.2 g; retinol, 3 mg; cholecalciferol, 55 mg; dl-tocopheryl acetate, 25 mg; menadione, 2.5 mg; thiamine, 3 mg; ribo¯avin, 6 mg; pyridoxine, 7.5 mg; folic acid, 0.2 mg; cyanocobalamin, 0.02 mg; biotin, 0.2 mg; calcium pantothenate, 25 mg; niacin, 50 mg; choline chloride, 1300 mg; CuSO45H2O, 29.5 mg;
FeSO47H2O, 375.0 mg; ZnSO4H2O, 138.0 mg; MnSO4H2O, 277.0 mg; Na2SeO3, 0.35 mg; KI, 0.20 mg;
CoCl26H2O, 2.1 mg; Na2MoO42H2O, 0.50 mg and BHT, 1.5 g. bDetermined values.
cCalculated values. ME attributed to linseed, 15.7 MJ kgÿ1(Lee et al., 1991). SBM: Soybean meal;
SBM-10LS: Linseed protein replacing SBM protein by 10%; SBM-30LS: Linseed protein replacing SBM protein by 30% and LS: Linseed.
into 16 groups of three chickens each. Four such groups were assigned to each of the following dietary treatments: protein-free diet and three test diets in which 0 (SBM diet), 10 (SBM-10LS diet) and 30% (SBM-30LS diet) of the SBM protein was replaced by linseed protein. A diet containing linseed as the sole source of protein was not tested due to the very poor performance of chicks fed on the LS diet in experiment 1. A protein level of 150 g kgÿ1 diet, which is approximately 25% lower than that recommended for broilers from 3 to 6 weeks of age (National Research Council, 1994), was chosen since it was thought that a suboptimal protein level would result in a greater effect of the quality of protein provided. The composition of the experimental diets is given in Table 1.
Diets were evaluated in a balance trial by the total collection procedure. After 3 days of adaptation, birds were starved for 24 h, then fed on experimental diets ad libitum for 3 days and starved again for the following 24 h. Feed intake and excreta output for each cage were measured quantitatively. Excreta samples were collected twice daily and frozen; at the end of the balance period, they were freeze-dried, weighed and ground to pass a 1 mm sieve.
2.4. Analyses
Dry matter, crude protein (KjeldahlN6.25), crude ®ber, ether extract and ash were analyzed using the standard methods of the Association of Of®cial Analytical Chemists (AOAC International, 1995). Neutral-detergent ®ber was determined as described by Van Soest et al. (1991), acid-detergent ®ber and acid-detergent lignin following the sequential procedure proposed by Robertson and Van Soest (1981). Mucilage content of the linseed was estimated by extracting the seeds with boiling water (1:20, w:v) for 4 h; the extract was precipitated by adding ethanol, separated by centrifugation and, ®nally, freeze-dried (Mazza and Biliaderis, 1989; Bhatty, 1993). Amino acid analysis was performed by HPLC after 22 h hydrolysis with boiling 6 N HCl under re¯ux conditions; nitrogen was bubbled through the mixture during the hydrolysis period. A large acid:sample ratio (400 ml:200 mg, v/w) was used to reduce amino acid losses in the presence of carbohydrates. Protein hydrolyzates and amino acid calibration mixture were derivatized by OPA-pthaldialdehyde and amino acids were measured in a HPLC system (Hewlett-Packard 1100, Waldbronn, Germany) following the procedure described by Jones et al. (1981). Cystine was determined as cysteic acid (Moore, 1963) and tryptophan after alkaline hydrolysis (AOAC International, 1995).
2.5. Calculations and statistics
PER and NPR were computed by the following formulas:
PERGain body weight
Protein consumed
NPRWeight gain of test groupÿWeight loss of protein-free group
Protein consumed
ATTR and TTTR (corrected for metabolic and endogenous losses) for nitrogen and amino
acids were calculated as follows:
ATTRTNCÿTNE
TNC
TTTRTNCÿ TNEÿTNEF
TNC
where TNC is the total nutrient consumed, TNE the total nutrient excreta and TNEF the total nutrient excreta of protein-free group.
Results obtained in the two experiments were subjected to regression analysis using the linear and non-linear models (Statistical Analysis Systems Institute, 1988).
3. Results
The chemical and amino acid composition of linseed and SBM used in this study are shown in Table 2. In general, the composition in essential amino acids of linseed compared very well to that of reference SBM except for its low content of lysine. Expressed as g amino acid 16 gÿ1N, the concentration of lysine in linseed was about
Table 2
Composition (g kgÿ1DM) of Soybean meal and linseed samples
Component Soybean meal Linseed
Crude protein (N6.25) 493.0 238.8
Ether extract 18.2 441.4
Crude ®ber 79.3 53.2
Neutral detergent ®ber 132.3 131.6
Acid detergent ®ber 69.0 66.2
Acid detergent lignin 2.1 18.7
Mucilage ± 75.0
Ash 68.7 42.6
Amino acids (g 16 gÿ1N)
Aspartic acid 11.7 10.6
Glutamic acid 21.0 19.2
Serine 5.7 5.6
Histidine 2.8 2.6
Glycine 4.3 5.5
Threonine 4.5 4.8
Alanine 4.8 4.6
Arginine 8.3 10.0
Tyrosine 3.9 3.3
Valine 5.2 5.2
Methionine 1.8 1.9
Phenylalanine 6.1 6.1
Isoleucine 5.0 5.2
Leucine 8.5 6.4
Lysine 6.2 3.8
Cystine 1.5 2.4
Tryptophan 1.5 1.7
Table 3
Evaluation of protein quality and SBM replacement value of linseed (LS)a
Diet Weight gain Feed intake Feed to gain ratio PERd NPRd
SBM 97.6 304.2 3.12 3.38 4.29
SBM-10LSb 75.3 286.7 3.81 3.00 4.05
SBM-30LSc 57.3 281.7 4.91 2.21 3.26
LS 6.9 192.5 27.81 0.40 1.96
Regression analysis
Model Square root-Y Reciprocal-Y Exponential Square root-Y Reciprocal-Y
R2 0.927 0.869 0.933 0.944 0.950
Intercept 9.62 3.2210ÿ3 1.08 1.84 2.2710ÿ1
S.E. 0.330 0.123103 0.102 0.049 0.108101
P-value *** *** *** *** ***
Slope ÿ0.172 0.04610ÿ3 0.056 ÿ0.030 0.06910ÿ1
S.E. 0.0153 0.005710ÿ3 0.0047 0.0023 0.005010ÿ1
P-value *** *** *** *** ***
aAll values are the mean of four replicates of three birds from 8 to 17 days of age. bLinseed protein replacing SBM protein by 10%.
cLinseed protein replacing SBM protein by 30%. dPER: protein ef®ciency ratio and NPR: net protein ratio. ***P<0.001.
Table 4
Metabolic plus endogenous losses of nitrogen and amino acids in excreta (mg per bird per day) for growing chicks fed a protein-free dieta
Mean S.E.
TotalN 184.1 20.26
Amino acids
Asp 59.3 6.53
Glu 48.5 5.36
Ser 30.7 2.74
His 6.1 0.32
Thr 24.2 1.11
Ala 28.2 0.53
Arg 34.6 1.30
Tyr 11.3 0.00
Val 23.3 1.31
Met 10.7 0.01
Phe 17.2 0.76
Ile 16.2 0.52
Leu 28.2 0.96
Lys 16.9 0.53
Cys 28.4 4.06
Total amino acids 424.7 17.81
aMean data from four pulled samples each from three animalsS.E.
39% lower than the corresponding value for SBM. In contrast, linseed contained a considerable higher amount of sulfur-containing amino acids (methionine plus cystine). Results of the protein quality bioassay are presented in Table 3. The inclusion of linseed in the reference diet depressed body weight gain and feed utilization (feed intake:weight gain ratio). Regression analysis revealed a statistically signi®cant (P<0.001) relationship between performance parameters and rate of linseed inclusion. Of the models ®tted, the square root-Yfor weight gain, reciprocal-Yfor feed intake and exponential for feed to gain ratio yielded the highestR2values. These values (Table 3) were 4.2, 3.7 and 14.0%, respectively, higher than the corresponding values obtained applying the linear model. The results of PER and NPR were in accordance with the performance data. Both conventional indexes ranked the protein sources in the same order to that shown by performance data. The PER and NPR values obtained in the bioassay also showed a statistically signi®cant (P<0.001) relationship with the inclusion level of linseed in the diet. Curvilinear models were better ®tted to data than linear model, and they explained 89.1 and 95.0% of the variability in PER and NPR, respectively.
Table 4 shows the metabolic plus endogenous losses of nitrogen and amino acids found in the excreta of birds given the protein-free diet. Losses for individual amino acids varied between 59.3 mg per bird per day for aspartic acid and 6.1 mg per bird per day for histidine. The data on the total tract retention for nitrogen and individual amino acids are
Table 5
ATTR and TTTR coef®cients of nitrogen and amino acids for chicks fed diets containing SBM or SBM partly replaced by linseed (SBM-10LSaand SBM-30LSb) as the sole source of proteinc
Diet Apparent retention True retention
SBM SBM-10LS SBM-30LS SBM SBM-10LS SBM-30LS
TotalN 0.549 0.465 0.413 0.657 0.577 0.538
Amino acids
Asp 0.851 0.820 0.755 0.897 0.860 0.808
Glu 0.920 0.856 0.780 0.929 0.900 0.801
Ser 0.852 0.817 0.733 0.894 0.857 0.776
His 0.893 0.870 0.790 0.915 0.887 0.811
Thr 0.838 0.792 0.687 0.876 0.830 0.729
Ala 0.797 0.765 0.667 0.840 0.794 0.711
Arg 0.883 0.855 0.740 0.919 0.884 0.771
Tyr 0.914 0.880 0.759 0.929 0.897 0.786
Val 0.877 0.822 0.695 0.909 0.852 0.732
Met 0.927 0.881 0.706 0.942 0.909 0.754
Phe 0.897 0.837 0.750 0.918 0.858 0.775
Ile 0.886 0.839 0.752 0.912 0.864 0.780
Leu 0.872 0.829 0.756 0.897 0.854 0.783
Lys 0.869 0.848 0.784 0.890 0.870 0.809
Cys 0.736 0.686 0.646 0.818 0.767 0.732
Total mean 0.867 0.827 0.733 0.903 0.859 0.767
aLinseed protein replacing SBM protein by 10%. bLinseed protein replacing SBM protein by 30%. cAll values are the mean of four replicates of three chicks.
summarized in Table 5. The ATTR for nitrogen and individual amino acids was impaired by the inclusion of linseed in the SBM reference diet. For each amino acid, the ATTR coef®cient decreased as the inclusion level of linseed in the SBM diet increased. Of the amino acids considered essential to poultry, methionine followed by valine were the most affected by the use of linseed. The TTTR coef®cients for nitrogen and single amino acids were higher than those of the, respective values, for ATTR, and they were also markedly lowered by the effect of linseed. In general, the decrease in the TTTR value for each
Table 6
Linear regression analysis relating ATTR and TTTR coef®cients of nitrogen and amino acids to rate of inclusion of linseed in the diet
Coeficient R2 Intercept S.E. P-value Slope S.E. P-value
TotalN ATTRa 0.957 0.550 0.0057 *** ÿ0.0364 0.00245 ***
TTTRa 0.924 0.649 0.0069 *** ÿ0.0326 0.00296 ***
Amino acids
Asp ATTT 0.716 0.854 0.0183 *** ÿ0.0079 0.00158 ***
TTTR 0.781 0.894 0.0089 *** ÿ0.0071 0.00119 ***
Glu ATTR 0.942 0.907 0.0062 *** ÿ0.0106 0.00083 ***
TTTR 0.866 0.935 0.0100 *** ÿ0.0107 0.00133 ***
Ser ATTR 0.886 0.854 0.0082 *** ÿ0.0097 0.00110 ***
TTTR 0.894 0.895 0.0079 *** ÿ0.0096 0.00105 ***
His ATTR 0.880 0.898 0.0075 *** ÿ0.0086 0.00100 ***
TTTR 0.895 0.918 0.0070 *** ÿ0.0087 0.00094 ***
Thr ATTR 0.901 0.840 0.0097 *** ÿ0.0124 0.00130 ***
TTTR 0.904 0.877 0.0093 *** ÿ0.0120 0.00124 ***
Ala ATTR 0.882 0.801 0.0093 *** ÿ0.0107 0.00124 ***
TTTR 0.815 0.851 0.0131 *** ÿ0.0116 0.00175 ***
Arg ATTR 0.931 0.894 0.0079 *** ÿ0.0122 0.00105 ***
TTTR 0.938 0.925 0.0075 *** ÿ0.0123 0.00100 ***
Tyr ATTR 0.903 0.921 0.0100 *** ÿ0.0129 0.00133 ***
TTTR 0.894 0.936 0.0098 *** ÿ0.0120 0.00130 ***
Val ATTR 0.952 0.880 0.0079 *** ÿ0.0149 0.00106 ***
TTTR 0.952 0.909 0.0076 *** ÿ0.0144 0.00102 ***
Met ATTR 0.896 0.939 0.0149 *** ÿ0.0185 0.00198 ***
TTTR 0.874 0.960 0.0147 *** ÿ0.0163 0.00196 ***
Phe ATTR 0.937 0.892 0.0072 *** ÿ0.0118 0.00096 ***
TTTR 0.936 0.913 0.0071 *** ÿ0.0115 0.00095 ***
Ile ATTR 0.942 0.885 0.0064 *** ÿ0.0109 0.00086 ***
TTTR 0.943 0.910 0.0062 *** ÿ0.0107 0.00083 ***
Leu ATTR 0.919 0.870 0.0066 *** ÿ0.0094 0.00088 ***
TTTR 0.917 0.895 0.0065 *** ÿ0.0092 0.00087 ***
Lys ATTR 0.836 0.873 0.0074 *** ÿ0.0071 0.00099 ***
TTTR 0.836 0.893 0.0071 *** ÿ0.0067 0.00094 ***
Cys ATTR 0.688 0.726 0.0092 *** ÿ0.0057 0.00122 ***
TTTR 0.754 0.813 0.0095 *** ÿ0.0070 0.00127 ***
Total mean ATTR 0.925 0.865 0.0072 *** ÿ0.0107 0.00096 ***
TTTR 0.938 0.899 0.0064 *** ÿ0.0105 0.00086 ***
***P<0.001.
aIn these cases square root-Xmodel ®tted better than linear model.
amino acid was similar to that observed for the mean value of total amino acids. Regression analysis was carried out with the ATTR and TTTR coef®cients for nitrogen and amino acids against the rate of linseed inclusion. The results (Table 6) indicated a signi®cant (P<0.001) effect of inclusion level. For nitrogen the response was curvilinear for both ATTR and TTTR, the selected model explaining 95.7 and 92.4%, respectively, of the variability found. For individual amino acids, however, the responses were linear, explaining 92.5% (ATTR) and 93.8% (TTTR) of the variability for the total mean of amino acids.
4. Discussion
The chemical and amino acid composition of linseed used in the current study is within the range of published values (Mazza and Biliaderis, 1989; Barbour and Sim, 1991; Bhatty, 1995; Lee et al., 1995). A comparison of the amino acid composition found for linseed with the amino acid requirements of broiler chicks during the three weeks of age (National Research Council, 1994), on the basis of g amino acid gÿ1 crude protein, indicated that the most limiting amino acid was lysine, which just satis®ed 79% of the nutritive requirements of chicks.
The biological data from Experiment 1 indicated that the protein quality of linseed was in¯uenced by the level at which it is found in the diet. When fed as the sole source of protein in a diet containing 90 g crude protein kgÿ1, linseed caused a drastic reduction in the growth rate of chicks and also in the PER and NPR values compared to those obtained for SBM. When linseed was used at a low level (replacing 10% of SBM protein in the reference diet), weight gain was depressed but PER and NPR were unaffected. The decrease observed in PER and NPR values with increasing level of linseed in the diet is attributable more to lower ef®ciency of feed utilization than to reduced feed (protein) intake. This is consistent with previous reports in which detrimental effects on growth and feed ef®ciency of chickens were observed when linseed was used at dietary levels in excess of 100 g kgÿ1(Ajuyah et al., 1991; Lee et al., 1991; Bond et al., 1997).
Data for PER and NPR were further supported by the retention data for nitrogen and amino acids determined with the fecal collection method in Experiment 2. The mean values for ATTR and TTTR of amino acids of SBM are close to those reported by Green and Kiener (1989) with adult male birds given a diet based on SBM and containing 144 g crude protein kgÿ1 (ATTR and TTTR coef®cients of 0.85 and 0.92, respectively). The replacement in part of the SBM protein by linseed protein resulted in a decrease of the ATTR values for nitrogen and amino acids, and it was more marked as the replacement level increased. As expected, the ATTR values for nitrogen were lower than those for single amino acids as nitrogen retention measures the difference between dietary nitrogen intake and nitrogen in excreta from both urinary and digestive tract origin. Urinary amino acid excretion does not signi®cantly affect amino acid retention values since the major nitrogen compounds in urine are uric acid or urates and ammonia, while amino acids are presented in very small amounts (O'Dell et al., 1960; McNabb and McNabb, 1975). It might also explain the current different response to inclusion level of linseed, which was curvilinear for nitrogen and linear for all the amino acids. Data for glycine are not
presented since the formation of this amino acid from urinary uric acid and from degradation of uric acid during acid hydrolysis of the excreta samples produces erratic balance values (Soares et al., 1971; Slump et al., 1977).
Adjustment of apparent to true total tract retention of nitrogen and amino acids was made by applying the values for metabolic plus endogenous losses determined using a group of birds fed a protein-free diet. In this work, values for these losses were 184 and 425 mg per bird per day for total nitrogen and amino acids, respectively. These ®gures agree with those reported by Salter et al. (1974) and Green and Kiener (1989). The TTTR values were higher than the corresponding ATTR values for nitrogen and all the amino acids in the test diets. However, correction of apparent to true total tract retention did not in¯uence the order in which the diets could be ranked. As concluded from regression analysis results (Table 6), the ATTR and TTTR data of each amino acid ®tted the same regression line (that is, a linear mode), giving in most cases a similar precise estimation of the effect produced by the rate of inclusion of linseed in the diet.
The results obtained in this work are evidence of poor utilization of linseed protein compared to that of SBM. This may be attributable to the presence of various antinutritional factors in linseed, the main ones being mucilage and linatine (Madhusudhan et al., 1986; Bhatty, 1995). The mucilage content of the linseed used in the current experiments was 75.0 mg kgÿ1DM, which was similar to that reported by Mazza and Biliaderis (1989) for another linseed cultivar. Linseed mucilage is a gumlike material associated with the hull of linseed (BeMiller, 1986), which can reduce nutrient availability in broiler chicks. It has been assumed that soluble ®bre constituents such as gum or pectins and other polysaccharide substances increase the viscosity of the digesta, and they reduce digestion and absorption of nutrients (Marquardt et al., 1979; Ward and Marquardt, 1983; Angkanaporn et al., 1994). It is also suggested that the increased ¯ow of undigested nutrients to the lower gut, as a result of the increased digesta viscosity, stimulates bacterial activity and it leads to the proliferation of micro¯ora and to a greater competition with the host animal for nutrients (Bedford, 1995; Choct et al., 1996). Linatine, a dipeptide of glutamic acid and proline, has been reported to depress growth of chickens as a vitamin B6 antagonist (Mandokhot and Singh, 1983; Bhatty, 1995). In the current study, however, test diets were supplemented with 75 mg kgÿ1pyridoxine, being more than twice the estimated requirements of broiler chicks (3.5 mg kgÿ1diet; National Research Council, 1994). Consequently, there was probably suf®cient pyridoxine present. In conclusion, the current study indicated that the utilization of protein from linseed by broiler chicks was considerably worse than that from SBM. Performance of chicks, PER and NPR and nitrogen and amino acid retention were impaired as the replacement level of SBM protein by linseed protein increased, due primarily to a reduction in the retention of nitrogen and amino acids caused probably by the presence of antinutritional factors, for example mucilage, in linseed.
Acknowledgements
The authors gratefully acknowledge the ®nancial support given by the ComisioÂn Interministerial de Ciencia y TecnologõÂa (Project AGF98-0688).
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