Interactions between dietary fat type and enzyme
supplementation in broiler diets with high pentosan
contents: effects on precaecal and total tract
digestibility of fatty acids, metabolizability
of gross energy, digesta viscosity
and weights of small intestine
S. DaÈnicke
a,*, H. Jeroch
a, W. BoÈttcher
a, O. Simon
b aInstitut fuÈr TierernaÈhrung und Vorratshaltung, Landwirtschaftliche FakultaÈt, Martin-Luther-UniversitaÈtHalle-Wittenberg, Emil-Abderhalden-Straûe 26, 06108 Halle (Saale) Wittenberg, Germany
bInstitut fuÈr TierernaÈhrung, Freie UniversitaÈt Berlin, BruÈmmerstraûe 34, 14195 Berlin, Germany
Received 15 June 1999; received in revised form 19 October 1999; accepted 10 January 2000
Abstract
One hundred grams fat, based on blends of beef tallow and soya oil (0:100, 20:80, 40:60, 60:40, 80:20 and 100:0) which corresponded to dietary ratios of unsaturated to saturated fatty acids (U:S ratio) of 5.47, 3.23, 2.11, 1.45, 1.00 and 0.69, respectively, was incorporated into a rye-based broiler diet (610 g kgÿ1). All diets were tested without or with supplementation of a xylanase-containing
enzyme preparation in a combined growth test and balance study. Increasing the proportion of dietary tallow resulted in depression in live-weight gain and increased feed conversion ratio, exponential increase in jejunal supernatant viscosity, increased empty weight of small intestine and reduced digestibility of fat and fatty acids and metabolizability of gross energy. The bene®cial effect of addition of xylanase was greatest at higher tallow concentrations.
In diets which were not supplemented with enzyme, it was shown that small incremental increases in U:S ratio up to1.5 resulted in distinct incremental increases in digestibility, whereas ratios exceeding this value had virtually no further bene®t. Such effects were less pronounced in enzyme supplemented groups.
Animal Feed Science and Technology 84 (2000) 279±294
*Corresponding author. Present address: Institute of Animal Nutrition, Federal Agricultural Research Centre,
Braunschweig (FAL), Bundesallee 50, D-38 116 Braunschweig, Germany; Tel.:49-531-596-436; fax: 49-531-596-376.
E-mail address: [email protected] (S. DaÈnicke)
In conclusion, addition of an exogenous xylanase to a rye-based broiler diet containing 100 g added fat/kg gives results which very much depend on the U:S ratio. As the ratio increases up to maximum of 1.5 U:S, which corresponds to 40 g soya oil and 60 g tallow/kg, the enzyme effects are greatly pronounced in terms of growth performance, fat and fatty acid digestibility and metabolizability of energy. Beyond this U:S ratio effects of xylanase are more limited.# 2000 Elsevier Science B.V. All rights reserved.
Keywords:Broiler; Tallow; Soya oil; Xylanase; Fat digestibility
1. Introduction
Several investigators have shown that the effect of exogenously supplied xylanases is dependent on the dietary fat type when diets with high concentrations of soluble pentosans are fed to broilers. It was demonstrated that enzyme effects were much more pronounced in diets containing animal fat compared with plant oil (Smulikowska and Mieczkowska, 1996; DaÈnicke et al., 1997a; Langhout et al., 1997). Intestinal viscosity was found to interfere with digestibility of saturated fatty acids more dramatically than with digestibility of unsaturated fatty acids in rye-based diets (DaÈnicke et al., 1997b) which partially explained the greater effectiveness of xylanase addition in such diets. Furthermore, fatty acid utilization from saturated fat sources, such as beef tallow, can be improved by the addition of small amounts of an unsaturated fat, such as plant-derived oils (Garrett and Young, 1975; Leeson and Summers, 1976; Sibbald, 1978; Sibbald and Kramer, 1978; Muztar et al., 1981; Wiseman and Lessire, 1987). Moreover, the ratio of unsaturated-to-saturated fatty acids (U:S ratio) in the diet in¯uences digestibility of fat and metabolizable energy (ME)-contents of fats or fat blends (Ketels and De Groote, 1989; Wiseman et al., 1991). This relationship was described by non-linear regression models characterized by saturation kinetics. Such models considered that, at low ratios (more saturated fats), fat digestibility and ME responded more dramatically to addition of unsaturated fats than at higher ratios (mainly unsaturated fats). The physiological basis of this synergistic effect between unsaturated and saturated fatty acids is most likely due to the greater emulsifying capacities of the former and the dependence of long chain saturated fatty acids on such emulsifying agents for effective digestion and absorption. However, it is not known how dietary fat blends, which differ markedly in their U:S ratios, interact with intestinal viscosity and effectiveness of xylanase addition. This is of particular importance from a practical point of view, since fat blends are commonly used in broiler diets rather than pure plant oils or fats of animal origin.
The aim of the current study was to investigate the effects of increasing U:S ratios by blending soya oil with beef tallow under conditions of high intestinal viscosity, as mediated by dietary rye inclusion, on the performance of male broilers, precaecal and total tract digestibility of fat and fatty acids and ME-values of experimental diets. A further aim was to study the ef®cacy of xylanase addition under the various conditions on the same parameters.
2. Material and methods
2.1. Experimental design
A rye-based diet (610 g kgÿ1) varying in dietary proportions of beef tallow and soya oil
(beef tallow:soya oil, g kgÿ10:100, 20:80, 40:60, 60:40, 80:20 and 100:0, respectively)
was tested in an intestinal milieu differing largely in viscosity which, in turn, was markedly altered through absence or presence of a xylanase-containing enzyme preparation (Avizyme
1300, Finnfeeds, Marlborough, UK). Hence, each diet was tested without (ÿ) or with ()
xylanase supplementation. The xylanase preparation was obtained from Trichoderma
longibrachiatumand contained 3000 IU/g, measured at pH 6.0 on the basis of formation of reducing substances. û-Glucanase and cellulase activities measured under the same conditions were very low, 40 and 14 IU/g, respectively. The inclusion rate of the enzyme
preparation was 1 g kgÿ1of the diet. The composition of the experimental diets is given in
Table 1 and fatty acid pro®le of beef tallow and soya oil is shown in Table 2.
2.2. Combined growth and balance study
Birds used in the experiment were male broilers of the Lohmann strain. They were obtained from a local hatchery. A total of 288 day-old broilers was divided into 12 groups
of 24 birds, having an initial mean body weight of 420.4 g. The birds were placed in
metabolism cages (4 birds per cage6 replicates per group) and the experimental diets
and water were provided for ad libitum consumption. Feed intake and live weight were
recorded at 14 and 28 days of age. TiO2was added as an indigestible marker to all diets at
a rate of 5 g kgÿ1from Day 24 of age until the end of the experiment. Excreta were
collected from each cage from 26 to 28 days of age. Digesta from segments of the small intestine were collected at 29 days of age. Broilers were killed by decapitation after cervical dislocation, the intestines were quickly excised and subsequent segments (duodenum, jejunum, ileum) were ligated. The duodenum was de®ned from the pylorus to the entrance of the main pancreatic and biliary ducts, the jejunum from the end of the
duodenum to Meckel's diverticulum and the ileum from the end of the jejunum to1 cm
from the ileo-caecal-junction. Jejunal and ileal digesta of birds from each cage were pooled and thoroughly mixed. Pooled jejunal samples were divided in two sub-samples, one was kept on ice before being centrifuged for viscosity measurements. The remaining jejunal digesta sample and the whole ileal digesta sample were frozen immediately in liquid nitrogen and kept frozen until used for determination of digestibility of fat and fatty acids. The empty weights of the duodenum, jejunum and ileum were also recorded.
2.3. Sample preparation
The pooled and homogenized digesta samples for viscosity measurements were
centrifuged at 15 000gfor 15 min.
Excreta and digesta samples for lipid analysis were freeze-dried. The samples were ground in a freezer mill (Model 6700-230, SPEX CertiPrep, NJ) under liquid nitrogen in order to produce a ®ne powder and to minimize nutrient losses during preparation.
Table 1
Composition of the experimental diets (g kgÿ1)
Group T0ÿ/T0a T20ÿ/T20a T40ÿ/T40a T60ÿ/T60a T80ÿ/T80a T100ÿ/T100a
aAvizyme 1300, 1 g/kg, at the expense of starch or cellulose.
bVariety `Marder,' insoluble pentosans, 66.2 g/kg DM, soluble pentosans, 35.5 g/kg DM.
cVitamin±mineral premix provided per kg of diet: Fe, 60 mg; Cu, 5 mg; Zn, 51.4 mg; Mn, 60.8 mg; Se, 0.2 mg; I, 0.6 mg; Vitamin A, 12 000 IU; Vitamin D 3,
3000 IU; Vitamin E, 42 mg; Vitamin B1, 2.1 mg; Vitamin B2, 6.6 mg; Vitamin B6, 4.1 mg; Vitamin B12, 20.7mg; pantothenic acid, 15 mg; nicotinic acid, 36 mg; folic
acid, 1 mg; biotin, 102mg; choline chloride, 700 mg; ethoxyquin, 120 mg; and Zn-bacitracin, 50 mg.
2.4. Analysis
Viscosity of jejunal and ileal digesta supernatant was measured using a Brook®eld
viscometer (model DV-IILV) at 408C.
Crude fat content of diets, digesta and excreta samples were analyzed according to the methods of the Verband Deutscher Landwirtschaftlicher Untersuchungs-und Forschungs-anstalten (Naumann and Bassler, 1993) by extraction with chloroform±methanol, following acidi®cation with 6 N HCl. Fatty acids from the fat extracts were methylated with trimethylsulphoniumhydroxide and the resulting methyl esters were identi®ed from their retention time using a gas chromatography system which consisted of a HP 5890 gas-chromatograph, HP 7673 autosampler and HP 3365 data-station. The FFAP-fused silica column used for separation had a length of 30 m and an inner diameter of 0.53 mm. He was used as a carrier gas with a ¯ow of 9 ml/min. A ¯ame ionization detector was
used for detecting the fatty acid derivatives. TiO2 in feed, digesta and excreta was
analyzed according to Brandt and Allam (1987). Gross energy of diets and excreta were measured using an adiabatic bomb calorimeter (model C 4000, Heitersheim, Germany). Total nitrogen in diets and excreta was analyzed as Kjeldahl-N and free fatty acids of soya oil and beef tallow as described by Naumann and Bassler (1993).
2.5. Calculations and statistics
Apparent digestibility coef®cient of fat and fatty acids and metabolizability of gross energy were calculated as follows:
Digestibility 1ÿ TiO2 in diet
TiO2 in digesta or excreta
Nutrient or energy in digesta or excreta
Nutrient or energy in diet
The values calculated for fat digestibility at the faecal level also include fat excretions and should, strictly speaking, be considered as utilization values. However, in view of the fact that urinary fat excretion in broilers is negligible, the digestibility term will be used. Digestibility coef®cients are apparent and not true as no allowance was made for endogenous fat. In addition, no allowance for possible fatty acid transformations was made.
Table 2
Pro®le of main fatty acids of rye oil, soya oil and tallow (g kgÿ1)
Rye oil Soya oil Beef tallow
C 16:0 188.2 94.9 253.8
C 18:0 15.4 31.6 204.5
C 18:1 n-9 150.7 245.7 272.6
C 18:2 n-6 434 439.8 18.2
C 18:3 n-3 49.3 65.1 2
Free fatty acids N.A.a 21.3 34.7
aN.A., not analyzed.
Metabolizable energy (ME) was corrected for zero-nitrogen retention using a factor of
36.5 kJ gÿ1N retained (Titus et al., 1959) to yield N-corrected apparent metabolizable
energy (AMEN).
A complete six-by-two two-factorial arrangement of analysis of variance (ANOVA) was applied:
yijkmaibj axbijeijk
whereyijkis thekth observation subjected to fat proportioniand enzyme supplementation
j, ai the tallow proportion (g kgÿ1; 0, 20, 40, 60, 80 and 100), bj the enzyme
supplementation (without or with supplementation), (axb)ij the interactions between ai
andbj, andeijkthe error term.
Non-linear regressions were estimated in order to describe the relationships between the dietary ratio of unsaturated to saturated fatty acids and several parameters (viscosity,
AMENand fatty acid digestibility coef®cients). All statistics were carried out using the
Statistica for the Windows operating system (StatSoft Inc., 1994).
3. Results
3.1. Performance
Live-weight gain and feed-to-gain ratio for the experimental period are shown in Table 3. Signi®cant tallow proportion and enzyme effects were detected, both for live weight gain and for feed-to-gain ratio. Both parameters were improved as the dietary soya oil proportion increased (and tallow proportion decreased) and enzyme supplementation generally exerted a positive effect on growth performance. The bene®t of enzyme addition tended to be greater as the tallow proportion increased (interactions between tallow proportion and enzyme addition, signi®cant for live-weight gain).
3.1.1. Intestinal viscosity and weight of segments of small intestine and AMENcontent of diets
Jejunal viscosity and empty weights of segments of the small intestine are shown in Table 3. Viscosity was found to be highly variable even within dietary treatments which resulted in non-signi®cant differences for the effect of tallow proportion despite the fact that an exponential increase was observed as dietary tallow proportion was increased. However, enzyme supplementation signi®cantly reduced viscosity in jejunal supernatant. Enzyme supplementation signi®cantly reduced the empty weights of small intestine. Increasing the proportion of tallow in the diet resulted in increased weight of small
intestine. Metabolizability of gross energy and AMEN of experimental diets decreased
with increasing tallow inclusion rate, an effect which was especially pronounced in the non-supplemented groups (Table 3). In groups receiving xylanase, this trend became
obvious only when >60 g kgÿ1tallow was included.
3.1.2. Fat and fatty acid digestibility coef®cients at different sites of the intestinal tract Apparent fat digestibility at the jejunal, ileal and total tract level was found to signi®cantly decrease as tallow proportion increased (Table 4). This trend was also found
Table 3
Effects of different tallow proportions in fat blends on viscosity in jejunal supernatant, performance, AMENcontents of the diets and on weights of small intestine of
male broilers (Day 28 of age)a
Tallow content of added dietary fat (g kgÿ1) Pooled
SEMb
209 59 272 67 241 111 551 106 626 115 1335 184 269.3 0.447 0.029 0.654 0.054 0.372
Small intestine(g kgÿ1live weight)
59.1 39.4 70.3 41.7 63.4 42.1 74.6 47 71.4 50.1 90.6 47.2 3.6 0.001 <0.001 0.111 <0.001 0.494
Live-weight gain(g per bird)
793 990 648 959 682 939 589 945 538 865 418 815 27 <0.001 <0.001 0.007 <0.001 0.123
Feed to gain(g gÿ1)
1.671 1.471 1.909 1.697 1.787 1.602 2.020 1.587 2.237 1.788 2.617 1.718 0.136 <0.001 <0.001 0.088 <0.001 0.317
AMEN(MJ/kg)
12.43 13.11 12.03 12.9 12.37 13.24 11.24 13.1 11.11 12.46 10.72 12.47 0.24 <0.001 <0.001 0.071 <0.001 0.137
aValues represent mean values of six replications per group. bPooled standard error of means.
cWithout enzyme supplementation.
dSupplemented with 1 g/kg diet of a xylanase containing enzyme preparation (Avizyme 1300, Finnfeeds, Marlborough, UK).
Table 4
Effects of different tallow proportions in fat blends on apparent digestibility coef®cients of crude fat and fatty acids as determined at different measurement sites of male broilers (Day 28 of age)a
Tallow content of added dietary fat (g kgÿ1) Pooled
SEMb
ANOVA (p-values) effects of: Orthogonal effects ofT(p-values)
Jejunum 0.688 0.841 0.616 0.746 0.549 0.732 0.361 0.644 0.326 0.503 0.260 0.420 0.025 <0.001 <0.001 0.063 <0.001 0.368 Ileum 0.745 0.861 0.649 0.787 0.599 0.764 0.500 0.645 0.411 0.625 0.309 0.512 0.026 <0.001 <0.001 0.376 <0.001 0.269 Total tract 0.751 0.858 0.674 0.778 0.646 0.799 0.566 0.708 0.490 0.680 0.365 0.613 0.025 <0.001 <0.001 0.047 <0.001 0.087
Total fatty acids
Ileum 0.752 0.869 0.609 0.775 0.542 0.733 0.415 0.582 0.298 0.537 0.188 0.435 0.030 <0.001 <0.001 0.277 <0.001 0.827 Total tract 0.821 0.873 0.683 0.814 0.634 0.762 0.542 0.682 0.432 0.631 0.342 0.572 0.028 <0.001 <0.001 0.048 <0.001 0.986
Palmitic acid
Ileum 0.432 0.733 0.426 0.592 0.410 0.537 0.308 0.426 0.195 0.465 0.126 0.374 0.035 <0.001 <0.001 0.039 <0.001 0.897 Total tract 0.587 0.733 0.507 0.648 0.463 0.695 0.317 0.545 0.288 0.527 0.195 0.524 0.042 <0.001 <0.001 0.244 <0.001 0.793
Stearic acid
Ileum 0.349 0.418 0.166 0.453 0.144 0.398 0.051 0.332 0.011 0.272 ÿ0.11 0.197 0.045 <0.001 <0.001 0.108 <0.001 0.572 Total tract 0.339 0.461 0.342 0.485 0.282 0.421 0.177 0.437 0.163 0.408 0.079 0.412 0.058 0.034 <0.001 0.387 <0.001 0.779
Oleic acid
Ileum 0.668 0.856 0.612 0.770 0.614 0.807 0.549 0.684 0.478 0.686 0.413 0.612 0.033 <0.001 <0.001 0.877 <0.001 0.331 Total tract 0.808 0.876 0.729 0.832 0.737 0.835 0.734 0.799 0.657 0.801 0.629 0.744 0.025 <0.001 <0.001 0.630 <0.001 0.660
Linoleic acid
Ileum 0.845 0.882 0.710 0.859 0.657 0.876 0.544 0.754 0.435 0.721 0.313 0.784 0.057 <0.001 <0.001 0.012 <0.001 0.922 Total tract 0.863 0.879 0.739 0.884 0.738 0.834 0.728 0.831 0.578 0.812 0.514 0.788 0.046 <0.001 <0.001 0.069 <0.001 0.601
Linolenic acid
Ileum 0.899 0.923 0.791 0.908 0.740 0.913 0.643 0.808 0.530 0.807 0.168 0.824 0.036 <0.001 <0.001 <0.001 <0.001 0.004 Total tract 0.879 0.899 0.798 0.909 0.805 0.882 0.769 0.866 0.580 0.849 0.512 0.850 0.057 0.002 <0.001 0.051 <0.001 0.331
aValues represent mean values of six replications per group. bPooled standard error of means.
cWithout enzyme supplementation.
dSupplemented with 1 g/kg diet of a xylanase-containing enzyme preparation (Avizyme 1300, Finnfeeds, Marlborough, UK).
for the enzyme supplemented treatments, but absolute digestibility values were all signi®cantly greater when xylanase supplementation was used. Enzyme effects increased with tallow proportion (interactions between fat and enzyme, signi®cant for total tract digestibility). Fat digestibility generally increased from the jejunal to the excreta level (total tract digestibility), the increment being more pronounced in high tallow-containing diets. The digestibility of individual fatty acids and of sum of individual fatty acids as measured at the ileal and total tract level followed generally a similar pattern as described for total fat (Table 4) with respect to main effects and interactions.
3.2. Regressions
The ratio between the sum of unsaturated fatty acids to the sum of dietary saturated fatty acids (U:S ratio) in the diet was used (variability as a result of feeding different proportions of tallow and soy oil) for regression analysis. The relationship between this ratio and viscosity of jejunal digesta can be described conveniently by a multiple non-linear regression model (Fig. 1). The ®rst term of regression describes the relationship at
low U:S ratios (up to 1.5), where low incremental changes in this ratio results in
geometrical incremental viscosity changes. The second term of the regression is characterized by saturation kinetics, in which larger incremental U:S ratio changes provoke only small incremental viscosity changes. This relationship is valid for both, xylanase supplemented or non-supplemented treatments.
The effect of U:S ratio on apparent digestibility of individual fatty acids at the total tract level is shown in Fig. 2. It can be seen that small incremental changes in U:S ratio up
to2.1 result in relatively large incremental increases in fatty acid digestibility, but in
non-supplemented diets only. The results of all regressions are summarized in Table 5.
Fig. 1. Effect of dietary ratio of unsaturated to saturated fatty acids on jejunal viscosity Without xylanase,
y349xÿ2.457214(1eÿ1.712*x),r20.9843, (±*±); with xylanase,y24xÿ3.93879(1eÿ40.178x),
r20.8842, (±&±).
The maximum enzyme response compared with the control for each parameter was calculated from the regression equations (using the most extreme U:S ratio of 5.47).
Furthermore, the ratio between y(5.47) of non-supplemented variant and the
corresponding xylanase supplemented variant was calculated (Table 5).
4. Discussion
Water-soluble and high-molecular weight fractions of non-starch polysaccharides, such as arabinoxylans, as present in rye and to a lesser extent in wheat, are known to exert adverse effects on the performance and nutrient digestibility in broilers. These effects are mediated by an increased intestinal viscosity (for recent reviews see Bedford and Schulze, 1998; Simon, 1998). The current results con®rm these general effects. Signi-®cant interactions between dietary fat type and carbohydrase addition were observed by DaÈnicke et al. (1999a) in that enzyme effects were more pronounced in tallow-fed birds. Interactions between tallow proportion and xylanase addition failed to reach signi®cance in the current study, despite the fact that differences in jejunal viscosity between
non-supplemented, and xylanase non-supplemented, diets containing pure soya oil (T0ÿ/T0) or
beef tallow (T100ÿ/T100) amounted 150 mPas and 1151 mPas, respectively. This might
be due to a high individual variation in intestinal viscosity even within the same experimental group when the diets contained high amounts of tallow, i.e. at low U:S ratios (Fig. 1). It can be seen from the shape of the curve of the non-supplemented groups (Fig. 1) that, at low U:S ratios, small incremental changes in this ratio caused large incremental changes in intestinal viscosity. It might be concluded that tallow itself, and other unabsorbed material, contributed to intestinal viscosity. The melting point of tallow
Fig. 2. Effect of dietary ratio of unsaturated-to-saturated fatty acids on apparent total tract digestibility of fatty acids. Stearic acid, without xylanase (±&±); stearic acid with xylanase (±&±); linolenic acid, without xylanase (±*±) ; linolenic acid, with xylanase (±*±).
Table 5
Regression of dietary ratio of unsaturated to saturated fatty acids (x) on apparent total tract digestibility coef®cient of fat and fatty acid, on metabolizability of energy or on performance (y):ya(1ÿeÿbx)
c
y Enzymea a b c y(5.47) Relative enzyme caused improvement aty(5.47)
r2
RSDb Total fat ÿ 0.696 0.705 0.051 0.747 0.154 0.992
SEc 0.018 0.046 0.023 0.019 0.555 0.733 0.307 0.862 0.967
SE 0.019 0.058 0.024 0.029
Palmitic acid (C16:0) ÿ 0.618 0.605 ÿ0.011 0.584 0.244 0.980
SE 0.015 0.048 0.021 0.028
0.341 0.494 0.409 0.727 0.828
SE 0.013 0.096 0.021 0.050
Stearic acid (C18:0) ÿ 0.489 0.837 ÿ0.133 0.351 0.342 0.964
SE 0.022 0.063 0.024 0.026
0.103 0.554 0.373 0.471 0.673
SE 0.014 0.192 0.015 0.022
Oleic acid (C18:1) ÿ 0.255 0.692 0.539 0.788 0.099 0.849
SE 0.022 0.174 0.032 0.032
0.180 0.680 0.691 0.867 0.881
SE 0.020 0.213 0.028 0.020
Linoleic acid (C18:2 n-6)
ÿ 0.582 0.920 0.247 0.825 0.071 0.895
SE 0.032 0.085 0.037 0.053
0.154 0.651 0.734 0.884 0.930
SE 0.016 0.167 0.020 0.013
Linolenic acid (C18:3 n-3)
ÿ 0.898 1.358 ÿ0.045 0.852 0.062 0.938
SE 0.030 0.053 0.032 0.046
0.103 0.681 0.805 0.906 0.909
SE 0.023 2.067 0.072 0.010
Metabolizability of energy
ÿ 0.177 0.913 0.494 0.671 0.055 0.873
SE 0.018 0.198 0.020 0.018
0.145 1.925 0.563 0.708 0.704
SE 0.041 0.423 0.041 0.013
Live weight gain ÿ 545 0.761 220 765 0.275 0.900
SE 2 0.080 3 53
ÿ, Without enzyme supplementation,, supplemented with 1 g/kg diet of a xylanase containing enzyme preparation (Avizyme 1300, Finnfeeds, Marlborough, UK).
bResidual standard deviation of regression. cStandard errors of parameter estimation.
is only slightly higher than the body temperature of the chicken (Hakansson, 1974) and might be, consequently, more viscous at such a temperature than soya oil. Viscosity of
pure soya oil and tallow measured at 408C was 26 and 42 mPas, respectively. Therefore,
the presence of high amounts of relatively solid saturated fatty acids in the gut might amplify the effects of soluble pentosans in increasing intestinal viscosity. Moreover, it might be suggested that the slower feed passage time observed in birds fed non-supplemented rye-based diets (DaÈnicke et al., 1997a) could have increased the degree of solubilization of pentosans, whereby intestinal viscosity would have further increased.
The empty weight of the small intestine was signi®cantly reduced in enzyme-treated birds (Table 3), whilst it increased with tallow inclusion. As dietary inclusion of soluble NSP increased, so did intestinal viscosity and, as a result, the length, absolute and relative weights of small intestine in rats (Johnson et al., 1984; Johnson and Gee, 1986) and broilers (Simon, 1998; DaÈnicke et al., 1999b) also increased. It was suggested by Johnson et al. (1984) that the presence of large amounts of unabsorbed material in the intestine exerts trophic effects on the intestinal mucosa. The hypothesis is supported by the work of DaÈnicke et al. (1999b) who reported an increased fractional rate of protein synthesis of small intestinal tissues of birds fed a rye-based, tallow-containing diet as compared to their xylanase supplemented counterparts. Soya oil was found to be unresponsive to enzyme addition under such conditions, which would partially explain the fat effects observed in the present experiment.
The digestibility of a more saturated fat was depressed to a greater extent in the presence of a highly viscous intestinal environment than digestibility of a more unsaturated fat (Smulikowska and Mieczkowska, 1996; DaÈnicke et al., 1997b). Consequently, xylanase effects were found to be greater under such conditions, resulting in signi®cant interactions between dietary fat type and enzyme supplementation. Similar relationships were found with many of the determined parameters of birds in the present study. For example, the relative improvement in total tract digestibility of fat due to xylanase addition was 14 and 68% in soya oil- and tallow-fed birds, respectively. Corresponding increases in live-weight gain (Day 1 to Day 28) were approximately 25 and 95%, respectively. In the present study, the fatty acid composition was modi®ed systematically by stepwise blending of soya oil and tallow, yielding U:S ratios between 0.69 and 5.47. Total tract digestibility of fat and feed to gain ratio in the presence or absence of enzyme were found to exhibit a non-linear relationship with the U:S ratio as shown in Fig. 3. The greatest improvement in these
parameters occurred between U:S ratios of1±1.45, which is equivalent to a proportional
0.02±0.04 replacement of tallow by soya oil. Incremental improvements in this range of U:S ratios produced a geometrical pattern, whereas improvements at higher ratios were characterized by an exponential saturation behaviour. The improvement in both the feed
conversion ratio (FCR) and total tract digestibility of fat was11±12% for a rye based diet
containing 100 g kgÿ1 soya oil (Fig. 3) as compared with the respective tallow-fed
counterparts (35 and 68% respectively). These observations are further supported by consideration of individual fatty acid digestibility (Table 5 and Fig. 2). Enzyme addition resulted in improvements in digestibility of approximately 24, 34 and 7±10% for palmitic acid, stearic acid and unsaturated fatty acids, respectively, in the case of the maximum U:S ratio of 5.47 (Table 5). The potential for improved digestibility of fatty acids upon enzyme addition relates very much to their digestibility in the ®rst place, which ranged from
(overall means for total tract level): palmitic acid, 0.502; stearic acid, 0.334; oleic acid, 0.765; linoleic acid, 0.766; and linolenic acid, 0.8. These ®ndings are in accordance with those of others (Renner and Hill, 1961a, b; Young, 1961; Garrett and Young, 1975; Ketels and De Groote, 1987; Blanch et al., 1995; Vila and Esteve-Garcia, 1996a, b; DaÈnicke et al., 1997b). These general relationships were maintained with each fat blend in spite of large differences in the absolute digestibility ®gures for any particular fatty acid between blends. It can be stated for fatty acid digestibility that enzyme effects increased with tallow inclusion and were more pronounced for saturated than for unsaturated fatty acids (Table 4). There was generally a decrease in digestibility of all fatty acids with increasing inclusion rates of tallow. However, this decrease was less obvious for unsaturated fatty acids than for saturated fatty acids in enzyme-supplemented groups. These trends underline the fact that absorption of saturated fatty acids is a physiologically more demanding process and, hence, less ef®cient than that of unsaturated fatty acids, especially under conditions of high digesta viscosity. Furthermore, differences in fatty acid digestibility between the ileal and total tract levels decreased with soya oil inclusion and after xylanase addition, which would indicate that hindgut fermentation will remove considerable amounts of fat if it is not absorbed from the small intestine. Moreover, it is well known from ruminant nutrition that higher concentrations of unsaturated fatty acids (mainly oleic, linoleic and linolenic acids) in the diet change the microbial composition and activity in the rumen markedly (for reviews, see Jilg et al., 1988; Demeyer and van Nevel, 1995). One effect is that unsaturated fatty acids become saturated. Thus, hindgut fermentation in broilers Ð albeit less important Ð might also contribute to changes in fatty acid pattern of excreta and, consequently, bias digestibility coef®cients of individual fatty acids. Therefore, digestibility of stearic acid might be systematically underestimated, whereas digestibility of unsaturated fatty acids of the C18-family will be overestimated.
Fig. 3. Improvement of total tract digestibility of fat and feed-to-gain ratio after xylanase addition (relative to the non-supplemented counterparts). Total tract digestibility of fat,y17.7xÿ2.52612.3(1eÿ0.355x),r20.9935, (±&±); feed conversion,y11.3xÿ1.91610.9(1eÿ70.01x),r20.9074, (±*±).
The physiological basis of fat and fatty acid absorption in broilers has been extensively discussed by Renner and Hill (1961a, b), Garrett and Young (1975), and Krogdahl (1985), and under the conditions of a highly viscous intestinal milieu by DaÈnicke et al. (1997a, b) and Smulikowska (1998).
The non-linear relationship between digestibility of added fat and added fat ME with U:S ratio, as found in the present study, has also been reported by other investigators (Ketels and De Groote, 1989; Wiseman et al., 1991). The greatest increase in digestibility of most of the measured fatty acids (with the exception of oleic acid) was observed as the U:S ratio approached 2.0 in birds fed the non-supplemented diets (Fig. 2), a fact which
can be deduced from their steeper regression slopes (Parameterbin Table 5). In contrast,
it would appear that when intestinal viscosity is much reduced, the U:S ratio is only of minor importance for digestibility of these fatty acids. Only small differences were detected between these slopes for birds fed the xylanase supplemented or non-supplemented for oleic acid and total fat which would indicate that the U:S ratio plays an important role independent of intestinal viscosity. The threshold of a U:S ratio of 2.0 corresponds to the range of the respective curves reported by Ketels and De Groote (1989) and Wiseman et al. (1991), a ratio beyond which fat-ME and fat digestibility responded less dramatically. Similarly shaped curves, but differing in absolute values, were described for young and old birds and for diets with low and high contents of free fatty acids (Wiseman et al., 1991). Free fatty acid concentrations of soya oil and beef
tallow used in the present study were 21 and 35 g kgÿ1 fat, respectively. It might be
deduced from the experiments of Wiseman et al. (1991) that these levels are suf®ciently low, so as to minimize the effects on AME.
A metabolizability of gross energy of0.7±0.72 was measured in diets T0, T2, T4
and T6 (enzyme-supplemented groups up to 60 g kgÿ
1
tallow) which corresponded
to AMEN values of 12.9 to 13.24 MJ/kg diet. Only in these groups was the calculated
target AMEN value of 13.3 MJ achieved. The shortfall from this target value increased
with increasing dietary tallow. This failure to achieve the target AMEN was the result
of a viscosity-mediated depression in nutrient digestibility, and clearly suggests that simple additivity in AME values of raw materials derived from tables is not valid under the conditions of the present study. Similar conclusions were drawn by Smulikowska and Mieczkowska (1996), who found the difference between calculated
and measured AMEN to increase with dietary rye proportion and, thus, intestinal
viscosity.
In conclusion, the effect of xylanase addition to a rye-based broiler diet containing
100 g kgÿ1of an added fat is mediated by dietary U:S ratio. Up to a ratio of1.5, which
corresponds to 40 g kgÿ1 soya oil and 60 g kgÿ1 tallow, enzyme effects are greatly
pronounced in terms of performance, fat and fatty acid digestibility and metabolizability of energy.
Acknowledgements
The authors are grateful to Finnfeeds International, Marlborough, UK, for ®nancial support of the experiment.
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