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Nutritive value of danish-grown barley varieties, II, effect of carbohydrate composition on digestibility of energy and protein
Journal of Cereal Science 6 (1987) 187-195
Nutritive Value of Danish-grown Barley Varieties, II, Effect of Carbohydrate Composition on Digestibility of Energy and Protein* K. E. BACH KNUDSENt, B. O. EGGUM and INGEBORG JACOBSEN National Institute of Animal Science, Department of Animal Physiology and Biochemistry, Rolighedsvej 25, DK-1958 Frederiksherg C, Denmark Received 9 December 1986 The nutritive value of nine barley varieties grown in three successive years on good quality clay soil and poor quality sandy soil were investigated. The barleys were subdivided into four spring feeding, three spring malting and two winter feeding varieties. Digestible energy (DE), true protein digestibility (TD), biological value (BV) and net protein utilisation (NPU) were determined in rats. DE was 1'3 % higher in spring malting compared to spring feeding varieties. The higher DE was accounted for by a higher starch and lower dietary fibre (DF) content. For all barleys DE varied [rom 73·1 to 83·1 %. This variation was attributed to the composition of the carbohydrate fraction. Dietary starch was positively correlated and total and insoluble DF (IOF) negatively correlated with DE content. The reason for the strong negative correlation with IDF to DE was that IDF accounts for most of the grain cellulose, hemicellulose and lignin. These DF constituents are highly resistant to microbial degradation. In contrast, neither ~-glucan nor SDF had any impact on DE. TD was affected by year and type of barley variety and BV by type of barley variety. However, the variation in TD was not correlated with any of the OF constituents. It is concluded that more detailed carbohydrate analyses would be informative in the estimation of metabolisable or net energy of feed and foodstuffs from chemical data.
Introduction It is generally accepted that starch, quantitatively the most important energy source for man and animals, and most sugars are broken down by a combination of pancreatic and mucosal enzymes in the small intestine. Studies with man and monogastric animals have shown that less than 5 % of dietary starch escapes digestion in the small intestine l-4. The endproducts of enzymic digestion of carbohydrates are monosaccharides (glucose, galactose and fructose), which are rapidly absorbed. The monosaccharides thus provide the most important fuel for the turnover of cellular nutrients. In contrast to starch and Abbreviations used: DE = digestible energy; DF = dietary fibre; TD = true protein digestibility; BV = biological value; NPU = net protein utilisation; ME = metabolisable energy; NE = net energy; TDF = total dietary fibre; IDF = insoluble dietary fibre; SDF = soluble dietary fibre; NSP = non-starch polysaccharides; NFE = nitrogen free extracts. SCFA = short chain fatty acids. >I< Part I: J. Cereal Sci. 6 (1987) 173-186. t To whom correspondence should be addressed. © 1987 Academic Press Limited 0733-5210/87/050187 + 10 $03.00/0
188
K. E. BACH KNUDSEN ET AL.
sugars, much less is known about the fate of dietary fibre (DF = non-starch polysaccharides (NSP) + lignin) in the gastro-intestinal tract. Recent studies with man and pigs, have shown that most cereal DF escapes digestion in the small intestine 2 • 4 • In the hindgut, however, a significant amount of NSP is broken down by microbial enzymes. The extent of microbial breakdown of NSP is influenced by the composition of the NSP-fraction and the degree oflignification 5 • The main endproducts of microbial fermentation are short chain fatty acids (SCFA) which provided the host with energy. However, the net energy (NE) of carbohydrates fermented and absorbed as SCFA from the hindgut is appreciably lower than that of carbohydrates hydrolysed and absorbed as monosaccharides from the small intestine 6 • Therefore, a higher DF at the expense of starch and sugars has two negative implications for the NE value. Firstly, digestible energy (DE) and metabolisable energy (ME) are reduced in proportion to the ratio of DF to starch and sugar, and secondly, the utilisation of ME is lower as more DE derives from SCFA. Since the carbohydrate fraction makes up approximately 80 % of barley dry matter, the composition of this fraction and particularly the ratio of starch plus sugar to DF will have a major influence on the nutritive value. Recent studies 7- 9 have shown considerable variation in carbohydrate composition of barley varieties grown in Sweden, the U.S.A. and Denmark. In the Danish work 9 , starch varied from 50·7 to 64·2% and DF from 18·6 to 27·0 %. This variation was attributed to growing conditions e.g. weather and type of barley variety. Moreover, in barley varieties suitable for malting, higher starch and soluble dietary fibre (SDF) and insoluble dietary fibre (IDF) contents were found. These factors might have nutritional implications. The aim of the present investigation was to study the nutritive value of barley in relation to type of barley variety, year and locality. The chemical composition of the barley samples are reported in the previous paper 9 • Experimental Material
The barley material comprised the nine barley varieties characterised with regard to chemical composition in the accompanying paper9 • The varieties were subdivided into three groups and referred to as spring feeding (SF), spring malting (8M) and winter feeding (WF) varieties. The barley samples were grown in three successive years; 1981, 1982 and 1983 at two localities; a good quality clay (C) soil in 8ealand and a poor quality sand (8) soil in Jutland. (See ref. 9 for further details.) Diets
The diets were composed of barley grains adjusted to a nitrogen content of 1'5 % by the addition of a N-free mixture. To all diets were added mineral and vitamin mixtures as described by EggumlO • The composition of the experimental diets is shown in Table 1.
DIGESTIBILITY OF BARLEY
189
T ABLE I. Composition of experimental diets (% of dry matter) Range
Barley grain" N-free mixture" Mineral mixture" Vitamin mixture'
Mean
Min
Max
78·3 16·1 4·0 1·6
63'5 0 4·0 1·6
94-4 30·9 4·0 Jo6
• Different barley varieties. t> Composition (% dry matter) in the N-free mixture: sucrose (9'0); cellulose powder (5,2); soya bean oil (5,2); potato starch (autoc1aved) (80'6). As described by Eggum lO • C
Nutritional experiments with rats The experimental procedures for digestibility trials and nitrogen balance have been described by Eggum 1o . Groups of five male Wistar rats, each weighing approximately 70 g, were used in the experiments with preliminary feeding periods of four days and balance periods of five days. Each animal received 150 mg ofN and 10 g dry matter daily throughout the preliminary and the balance periods. Digestible energy (DE), true protein digestibility (TD), biological value (BV), and net protein utilisation (NPU) were used as criteria in the biological study. DE was calculated by difference after correction of digestibility of the N-free mixture of 91'7 ± 1'2 %.
Chemical methods Nitrogen in feed and faeces was determined by the Kjeldahl method using a Kjel-Foss 16200 Autoanalyser (N. Foss Electric A/S, Denmark), while energy in feed and faeces was determined in a IKA calorimeter C 400 (Janke & Kuntkel KG IKA-werk, BRD).
Statistical analysis The data from the rat nutritional experiments were examined by a three-way analysis of variance model, as outlined by Snedecor and Cochran ll :
where XiJkl is the dependent variable (Le. DE, TD, BV, NPU); I! is the overall mean; at is the effect of locality; ~J is the effect of year; and 'Y k is the effect of the barley variety. E ijkl is a normal distributed random variable. The varieties within subclasses e.g. SF, SM and WF varieties were pooled and used to estimate error (B ljkZ)' However before doing so we estimated the homogeneity of the material within subclasses. The part of the total variation within subclasses which could be attributed to the effect of variety was estimated using the following model:
The estimated variance for Yk within subclass was tested by Barlett's test l l for homogeneity according to:
190
K. E. BACH KNUDSEN ET AL.
with m-l degrees of freedom, (e.g. three groups) where
and
It = n
j
-1 degree of freedom group no. i (e.g. SF
= 3; SM = 2; WF = 1).
However, since we did not find any three-way interaction in the complete model, this effect was omitted from the model and included in the estimate for error (B jlkl ). The relationship between IDF and DE was described by a linear regression model l l : Yj
Standard error of intercept
(s~o)
Variety differences
= ~O+~lXll+Bj
and standard error of slope
(S~l)
were calculated.
Results
Digestible energy of SM varieties was 1·3 % higher than in SF varieties (Table II, Fig. 1). The higher DE was accounted for by a higher starch content of 1'7 % and a lower SDF and IDF content of 0·9 and 1·3 % respectively. True protein digestibility was also higher in 8M than in SF feeding varieties, whereas the opposite was the case with biological value. Hence, net protein utilisation was virtually unchanged. The difference in DE between SF and WF barley varieties was less significant. In the WF varieties TD was higher and BV lower than in SF varieties. Ranking with regard to DE of the individual barley varieties within year and locality, shows that Gunhild was at the top of the higher range (ranking 1-3) and Lami at the bottom of the lower range (ranking 8-9) for each year and locality. On average, the difference in DE between the two varieties was 2·8 % (Table III). The reason being that DF was increased at the expense of starch. Protein, HCl-fat and ash, on the other hand, were affected less. Effect of year and locality DE was significantly lower for the barley varieties grown in 1983 compared to the previous two years. This effect was especially pronounced for the barley varieties grown on sandy soil in 1983. On this locality DE values were 80'9, 80·4 and 76·1 % in 1981,1982 and 1983 respectively (Table II). Comparative values for barleys grown on the clay soil were 80'0, 79'5 and 79·3 %. Moreover, the interaction between locality and year and locality and variety were significant. The reason for the latter interaction was that the starch content of the spring varieties (malting and feeding) was much more severely affected by drought on the sandy soil than the winter varieties. (Values for DE of individual barley varieties were as low as 73·1 %.) The variation in TD was attributed to the same factors, year and barley variety, as was DE. However, in contrast to DE, TD was lowest in 1982 and higher in 1981.
DIGESTIBILITY OF BARLEY
191
TABLE II. Effect of locality, year and barley variety on digestible energy, true protein digestibility, biological value and net protein utilisation of barley DE
Locality Clay soil Sandy soil Year 1981 1982 1983 Type of variety' SF SM WF Pooled s F value for Locality (L) Year (Y) Type of variety (1) L*Y L*T Y*T
(%)
(%)
TD
(%)
BV
NPU
79-6 79-0
85·9 86·7
77-6 75·7
66·7 65·6
80-4 79·9 77·7
86'8 85-6 86'7
77-2 77-0 75·8
67-0 65·9 65·7
78·9 80'2 78-6 2·13
85·8 86-9 86-8 2-08
77-6 75'9 75-6 3-06
66·0 65-6 3-14
1·92 7-33*** 9'48***
1·75 0·89 15,73***
0·15 0·52 3,28*
27,96*** 2·36 2,42*
4'80** 7'24*** 0·55
21'36*** 11'90*** 8-08***
8-04*** 3,51* 5,84***
p < 0-05; **p < 0·01; ***p < 0·001. • SF = spring feeding; SM '" spring malting; WF
DE
66-6
0'35 26,78*** 11,42***
*
2
(%)
Protein
= winter feeding.
Starch
SDF
IOF
~
~
LJ.. CIl
I
:::;; ~
0
-I
-2
FIGURE 1. Differences (i1(SM - SF), %) in DE, protein, starch, SDP and IDP dietary fibre of spring malting and spring feeding barley varieties.
192
K. E. BACH KNUDSEN ET AL.
TABLE III. Chemical composition, digestible energy, true protein digestibility, biological value and net protein utilisation of two varieties of barley, Gunhild and Lami
No. of samples Chemical composition (% dry matter) Low-molecularweight sugars a Starch NSP Cellulose ~-Glucan
Hemicellulose Total NSP Klason lignin
DF TDP
Protein (N x 6'25) HC1-fat Ash DE(%) TD (%) BV (%) NPU (%) N.S.
Gunhild
Lami
6
6
1·9 5%
2'0 56·9
4·3 4·3 7-7 16·3 2'7 19'0 20·8 12·0 3·5 2·2 80·8 86·5 76·9 66·5
5·0 4·6 8·7 18·3 2·9 21'2 23-6 11·9 3·7 2-4 78·0 85·7 75-6 64·8
6 (%)
0'1 2'7 -0,7 -0,3 -1,0 -2·0 -0'2 -2,2 -2,8 +0·1 -0'2 -0,2 2,8*** 0·8N.S. 1·3N.S. 1·7N.S.
= not significant
*** p < 0·001.
• Sum of glucose, fructose, sucrose and fructans. Dietary fibre determined gravimetrically.
b
TABLE IV. Correlation coefficients between starch, soluble dietary fibre, insoluble dietary fibre, total dietary fibre and total ~-glucan vs. digestible energy and true protein digestibility DE
Starch SDP lOF
TDF
~-Glucan N.S. =
*....
P
not significant.
< 0'001.
+0'770*** -0·160N.S. -0'786*** -0,761 *** -0·034N.S.
TD -O·257N.S. -O·157N.S. -0·013N.S. -0·040N.S. -0·218N.S.
DIGESTIBILITY OF BARLEY
193
Correlation between dietary carbohydrate constituents and DE and TD
The variation in DE was negatively correlated with IDF and TDF and positively correlated with dietary starch (Table IV). The relationship between DE and IDF, shown in Fig. 2 can be expressed as: DE = 97·6 -1,02 x IDF; R 2 ±2'09±0'12
= 0·58
In contrast, neither SDF nor p-glucan content affected the DE value.
o
~
",
~ c
OJ OJ
;e
78
en
OJ
Ci"" 74
...
o
•
•
~ 1---,'16':----'-----=2'=-0_ _.L.-_---:'':-..c..----J IDF(%)
FIGURE 2. Relationship between DE and IDF; e, spring feeding variety; 0, spring malting variety; 0, winter feeding variety.
TD varied from 81·6 to 89·9%. However, the variation was not correlated with any of the dietary carbohydrate constituents listed in Table IV. Discussion
The results in the present investigation show that DE is strongly correlated with the composition of the carbohydrate fraction, in particular the ratio of starch to DF. Hence the higher energy digestibility of 1·3 % in spring malting varieties compared to spring feeding varieties is accounted for by the higher starch and lower DF content 9 • The significantly lower energy digestibility of barley varieties grown on the poor quality sandy soil in 1983 compared with the previous two years, and to varieties grown on clay soil, is attributed to the same factors 9 • In both cases the variation in DE can be explained by the relatively more dry matter located in the cell wall compared to the cell contents, primarily endosperm. The variation in DE of individual varieties was from 73·1 to 83·1 %. This is within the
194
K. E. BACH KNUDSEN ET AL.
same range as the variation in starch and Dp·9 and demonstrates that the barley kernel is not as homogeneous as previously believed. The chemical composition, and thus the nutritive value, are affected by factors such as variety, cultivation conditions and year. For the individual varieties, the difference between the highest and lowest DE value was 2·8 %. Here, the ratio of starch to DF also explains the differences. It is not possible from the data in the present study to estimate the effect of carbohydrate composition on metabolisable or net energy. However, DE is the factor of most importance for the variation in ME and NE12. It therefore seems reasonable to assume that the variation in ME and NE can be attributed to the same factors as for DE. The high negative correlation between IDF and DE are in agreement with results from an earlier study13. In that study rats were fed different decortication fractions from barley. The slope and the intercept of the regression between DE and IDF in the earlier study were 1·00 ± 0·10 and 99·3 ± 2·81 respectively compared to 1·01 ± 0·12 and 97·6 ± 2·09 respectively in this study. The reason for the strong negative correlation between DE and IDF is that IDF primarily measures cellulose, hemicellulose and lignin 9 • These DF constituents are found in high levels in the lignified secondary cell walls of husk tissues 14 and are highly resistant to microbial degradation. Recently, we have estimated the digestibility of cellulose, hemicellulose and lignin from barley in rats to be 31 ± 3, 55 ± 2 and 5 ± 14 % respectivelys. In contrast to the high resistance to microbial degradation of cellulose, hemicellulose and lignin, the p-glucans found in the unlignified cell walls of endosperm and aJeurone 15 , are readily available substrates for the hindgut microflora 2 • 5 • Hence, neither p-glucan nor SDF are negatively correlated with DE. In this respect, rats and other monogastric animals with active microbial fermentation, are very different to broiler chickens. In broiler chickens, the p-glucans are found to be responsible for the low productive value of oats and barley, mainly because of their influence on starch and protein digestion 16. The variation in TD was not correlated with any of the dietary carbohydrate constituents. This is in contrast to results from several studies in which reasonably high correlations between fibre levels and protein digestibility were identified l 7-19. However, the correlation between DF and cell wall bound proteins is quite different in the two situations. In barley, the husk accounts for the major variation in DFD. The protein content of the husk is, however, very low ls and the weak correlation between TD and DF is therefore easily understood. In studies with different plant materials l 7-lD a higher fibre level was associated with relatively more ct!l1 wall bound protein. This protein is poorly digested by microbial flora in the gut 1S . Hence, the effect of fibre level on protein digestibility is quite different in some types of plant (e.g. barley) compared to plant materials in general. Although we could not explain the variation in TD by any of the DF constituents, the TD values were affected significantly by the year grown and by the barley variety. BV and NPU were higher in the spring feeding than in the winter feeding varieties. Using the conventional method of calculating ME or NE of feed and foodstuffs, the material is analysed for ash, fat, protein and crude fibre and nitrogen-free extract (NFE), calculated by difference 2D . In barley and other cereals the NFE fraction often comprises 75-80 % of the total dry matter. As shown in the accompanying paper D, Swedish
DIGESTIBILITY OF BARLEY
195
work? and work from Montana, U.S.A. B, the composition of this fraction varies significantly, in particular with regard to starch and DF. In agreement with the variation in carbohydrate composition, DE and, probably, ME and NE varied significantly. On the basis of the results in the present study, it can be concluded that more detailed carbohydrate analyses should be included in the analytical scheme when estimating ME and NE of feed and foodstuffs from chemical data. The project was supported by a grant to the senior research worker by the Royal Veterinary and Agricultural University, Copenhagen and technical assistance was supported by the Carlsberg Foundation, Copenhagen. Birgit S. Jensen is thanked for technical assistance.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. II. 12. 13. 14. 15. 16. 17. 18. 19. 20.
Keys, J. E. and De Barthe, J. V. J. Anim. Sci. 39 (1974) 53-56. Graham, H., Hesselman, K. and Aman, P. J. Nutr. 116 (1986) 242-251. Englyst, H. and Cummings, J. H. Am. J. Clin. NUlr. 42 (1986) 778-787. Sandberg, A.-S., Andersson, H., Kivesta, B. and Sandstmm, B. Br. J. Nutr. 55 (1986) 245-254. Bach Knudsen, K. E., Agergaard, N. and Olesen, H. P. Short communication No. 633, National Institute of Animal Science, Copenhagen (1986). (In Danish.). Just, A., Fernandez, J. A. and Jargensen, H. Livestock Prod. Sci. 10 (1983) 171-186. Aman, P., Hesselman, K. and Tilly, A.·C. J. Cereal Sci. 3 (1985) 73-77. Aman, P. and Newman, C. W. J. Cereal Sci. 4 (1986) 133-141. Bach Knudsen, K. E., Aman, P. and Eggum, B. O. J. Cereal Sci. 6 (1987) 173-186. Eggum, B. O. A Study of Certain Factors Influencing Protein Utilization in Rats and Pigs (TheBis) Report No. 406, National Institute of Animal Science, Copenhagen (1973). Snedecor, G. W. and Cochran, W. G. 'Statistical Methods', 6th ed. The Iowa State University Press, Ames, Iowa (1973). Just, A. Livestock Prod. Sci. 8 (1982) 541-555. Bach Knudsen, K. E. and Eggum, B. O. Z. Tierphysiol., Tierernahrg. Futterm-ittelkde. 51 (1984) 130148. Salomonsson, A.-C., Theander, O. and Arnan, P. Swedish J. Agric. Res. 10 (1980) 11-16. Bacic, A. and Stone, B. A. Aust. J. Plant Physiol. 8 (1981) 475-495. Hesselman, K. PhD Thesis, Swedish University of Agricultural Sciences, Department of Animal Husbandry, Report No. 112 (1983). Just, A. Livestock, Prod. Sci. 9 (1982) 569-580. Bach Knudsen, K. E. Z. Tierphysiol., Tierernahrg. u. Futtermittelkde. 48 (1982) 90-104. Taverner, M. R. and Farrell, D. J. Br. J. Nutr. 46 (1981) 173-192. Just, A. J0rgensen, H. and Fernandez, J. A. Livestock Prod. Sci. 11 (1984) 105-128.
Nutritive Value of Danish-grown Barley Varieties, II, Effect of Carbohydrate Composition on Digestibility of Energy and Protein* K. E. BACH KNUDSENt, B. O. EGGUM and INGEBORG JACOBSEN National Institute of Animal Science, Department of Animal Physiology and Biochemistry, Rolighedsvej 25, DK-1958 Frederiksherg C, Denmark Received 9 December 1986 The nutritive value of nine barley varieties grown in three successive years on good quality clay soil and poor quality sandy soil were investigated. The barleys were subdivided into four spring feeding, three spring malting and two winter feeding varieties. Digestible energy (DE), true protein digestibility (TD), biological value (BV) and net protein utilisation (NPU) were determined in rats. DE was 1'3 % higher in spring malting compared to spring feeding varieties. The higher DE was accounted for by a higher starch and lower dietary fibre (DF) content. For all barleys DE varied [rom 73·1 to 83·1 %. This variation was attributed to the composition of the carbohydrate fraction. Dietary starch was positively correlated and total and insoluble DF (IOF) negatively correlated with DE content. The reason for the strong negative correlation with IDF to DE was that IDF accounts for most of the grain cellulose, hemicellulose and lignin. These DF constituents are highly resistant to microbial degradation. In contrast, neither ~-glucan nor SDF had any impact on DE. TD was affected by year and type of barley variety and BV by type of barley variety. However, the variation in TD was not correlated with any of the OF constituents. It is concluded that more detailed carbohydrate analyses would be informative in the estimation of metabolisable or net energy of feed and foodstuffs from chemical data.
Introduction It is generally accepted that starch, quantitatively the most important energy source for man and animals, and most sugars are broken down by a combination of pancreatic and mucosal enzymes in the small intestine. Studies with man and monogastric animals have shown that less than 5 % of dietary starch escapes digestion in the small intestine l-4. The endproducts of enzymic digestion of carbohydrates are monosaccharides (glucose, galactose and fructose), which are rapidly absorbed. The monosaccharides thus provide the most important fuel for the turnover of cellular nutrients. In contrast to starch and Abbreviations used: DE = digestible energy; DF = dietary fibre; TD = true protein digestibility; BV = biological value; NPU = net protein utilisation; ME = metabolisable energy; NE = net energy; TDF = total dietary fibre; IDF = insoluble dietary fibre; SDF = soluble dietary fibre; NSP = non-starch polysaccharides; NFE = nitrogen free extracts. SCFA = short chain fatty acids. >I< Part I: J. Cereal Sci. 6 (1987) 173-186. t To whom correspondence should be addressed. © 1987 Academic Press Limited 0733-5210/87/050187 + 10 $03.00/0
188
K. E. BACH KNUDSEN ET AL.
sugars, much less is known about the fate of dietary fibre (DF = non-starch polysaccharides (NSP) + lignin) in the gastro-intestinal tract. Recent studies with man and pigs, have shown that most cereal DF escapes digestion in the small intestine 2 • 4 • In the hindgut, however, a significant amount of NSP is broken down by microbial enzymes. The extent of microbial breakdown of NSP is influenced by the composition of the NSP-fraction and the degree oflignification 5 • The main endproducts of microbial fermentation are short chain fatty acids (SCFA) which provided the host with energy. However, the net energy (NE) of carbohydrates fermented and absorbed as SCFA from the hindgut is appreciably lower than that of carbohydrates hydrolysed and absorbed as monosaccharides from the small intestine 6 • Therefore, a higher DF at the expense of starch and sugars has two negative implications for the NE value. Firstly, digestible energy (DE) and metabolisable energy (ME) are reduced in proportion to the ratio of DF to starch and sugar, and secondly, the utilisation of ME is lower as more DE derives from SCFA. Since the carbohydrate fraction makes up approximately 80 % of barley dry matter, the composition of this fraction and particularly the ratio of starch plus sugar to DF will have a major influence on the nutritive value. Recent studies 7- 9 have shown considerable variation in carbohydrate composition of barley varieties grown in Sweden, the U.S.A. and Denmark. In the Danish work 9 , starch varied from 50·7 to 64·2% and DF from 18·6 to 27·0 %. This variation was attributed to growing conditions e.g. weather and type of barley variety. Moreover, in barley varieties suitable for malting, higher starch and soluble dietary fibre (SDF) and insoluble dietary fibre (IDF) contents were found. These factors might have nutritional implications. The aim of the present investigation was to study the nutritive value of barley in relation to type of barley variety, year and locality. The chemical composition of the barley samples are reported in the previous paper 9 • Experimental Material
The barley material comprised the nine barley varieties characterised with regard to chemical composition in the accompanying paper9 • The varieties were subdivided into three groups and referred to as spring feeding (SF), spring malting (8M) and winter feeding (WF) varieties. The barley samples were grown in three successive years; 1981, 1982 and 1983 at two localities; a good quality clay (C) soil in 8ealand and a poor quality sand (8) soil in Jutland. (See ref. 9 for further details.) Diets
The diets were composed of barley grains adjusted to a nitrogen content of 1'5 % by the addition of a N-free mixture. To all diets were added mineral and vitamin mixtures as described by EggumlO • The composition of the experimental diets is shown in Table 1.
DIGESTIBILITY OF BARLEY
189
T ABLE I. Composition of experimental diets (% of dry matter) Range
Barley grain" N-free mixture" Mineral mixture" Vitamin mixture'
Mean
Min
Max
78·3 16·1 4·0 1·6
63'5 0 4·0 1·6
94-4 30·9 4·0 Jo6
• Different barley varieties. t> Composition (% dry matter) in the N-free mixture: sucrose (9'0); cellulose powder (5,2); soya bean oil (5,2); potato starch (autoc1aved) (80'6). As described by Eggum lO • C
Nutritional experiments with rats The experimental procedures for digestibility trials and nitrogen balance have been described by Eggum 1o . Groups of five male Wistar rats, each weighing approximately 70 g, were used in the experiments with preliminary feeding periods of four days and balance periods of five days. Each animal received 150 mg ofN and 10 g dry matter daily throughout the preliminary and the balance periods. Digestible energy (DE), true protein digestibility (TD), biological value (BV), and net protein utilisation (NPU) were used as criteria in the biological study. DE was calculated by difference after correction of digestibility of the N-free mixture of 91'7 ± 1'2 %.
Chemical methods Nitrogen in feed and faeces was determined by the Kjeldahl method using a Kjel-Foss 16200 Autoanalyser (N. Foss Electric A/S, Denmark), while energy in feed and faeces was determined in a IKA calorimeter C 400 (Janke & Kuntkel KG IKA-werk, BRD).
Statistical analysis The data from the rat nutritional experiments were examined by a three-way analysis of variance model, as outlined by Snedecor and Cochran ll :
where XiJkl is the dependent variable (Le. DE, TD, BV, NPU); I! is the overall mean; at is the effect of locality; ~J is the effect of year; and 'Y k is the effect of the barley variety. E ijkl is a normal distributed random variable. The varieties within subclasses e.g. SF, SM and WF varieties were pooled and used to estimate error (B ljkZ)' However before doing so we estimated the homogeneity of the material within subclasses. The part of the total variation within subclasses which could be attributed to the effect of variety was estimated using the following model:
The estimated variance for Yk within subclass was tested by Barlett's test l l for homogeneity according to:
190
K. E. BACH KNUDSEN ET AL.
with m-l degrees of freedom, (e.g. three groups) where
and
It = n
j
-1 degree of freedom group no. i (e.g. SF
= 3; SM = 2; WF = 1).
However, since we did not find any three-way interaction in the complete model, this effect was omitted from the model and included in the estimate for error (B jlkl ). The relationship between IDF and DE was described by a linear regression model l l : Yj
Standard error of intercept
(s~o)
Variety differences
= ~O+~lXll+Bj
and standard error of slope
(S~l)
were calculated.
Results
Digestible energy of SM varieties was 1·3 % higher than in SF varieties (Table II, Fig. 1). The higher DE was accounted for by a higher starch content of 1'7 % and a lower SDF and IDF content of 0·9 and 1·3 % respectively. True protein digestibility was also higher in 8M than in SF feeding varieties, whereas the opposite was the case with biological value. Hence, net protein utilisation was virtually unchanged. The difference in DE between SF and WF barley varieties was less significant. In the WF varieties TD was higher and BV lower than in SF varieties. Ranking with regard to DE of the individual barley varieties within year and locality, shows that Gunhild was at the top of the higher range (ranking 1-3) and Lami at the bottom of the lower range (ranking 8-9) for each year and locality. On average, the difference in DE between the two varieties was 2·8 % (Table III). The reason being that DF was increased at the expense of starch. Protein, HCl-fat and ash, on the other hand, were affected less. Effect of year and locality DE was significantly lower for the barley varieties grown in 1983 compared to the previous two years. This effect was especially pronounced for the barley varieties grown on sandy soil in 1983. On this locality DE values were 80'9, 80·4 and 76·1 % in 1981,1982 and 1983 respectively (Table II). Comparative values for barleys grown on the clay soil were 80'0, 79'5 and 79·3 %. Moreover, the interaction between locality and year and locality and variety were significant. The reason for the latter interaction was that the starch content of the spring varieties (malting and feeding) was much more severely affected by drought on the sandy soil than the winter varieties. (Values for DE of individual barley varieties were as low as 73·1 %.) The variation in TD was attributed to the same factors, year and barley variety, as was DE. However, in contrast to DE, TD was lowest in 1982 and higher in 1981.
DIGESTIBILITY OF BARLEY
191
TABLE II. Effect of locality, year and barley variety on digestible energy, true protein digestibility, biological value and net protein utilisation of barley DE
Locality Clay soil Sandy soil Year 1981 1982 1983 Type of variety' SF SM WF Pooled s F value for Locality (L) Year (Y) Type of variety (1) L*Y L*T Y*T
(%)
(%)
TD
(%)
BV
NPU
79-6 79-0
85·9 86·7
77-6 75·7
66·7 65·6
80-4 79·9 77·7
86'8 85-6 86'7
77-2 77-0 75·8
67-0 65·9 65·7
78·9 80'2 78-6 2·13
85·8 86-9 86-8 2-08
77-6 75'9 75-6 3-06
66·0 65-6 3-14
1·92 7-33*** 9'48***
1·75 0·89 15,73***
0·15 0·52 3,28*
27,96*** 2·36 2,42*
4'80** 7'24*** 0·55
21'36*** 11'90*** 8-08***
8-04*** 3,51* 5,84***
p < 0-05; **p < 0·01; ***p < 0·001. • SF = spring feeding; SM '" spring malting; WF
DE
66-6
0'35 26,78*** 11,42***
*
2
(%)
Protein
= winter feeding.
Starch
SDF
IOF
~
~
LJ.. CIl
I
:::;; ~
0
-I
-2
FIGURE 1. Differences (i1(SM - SF), %) in DE, protein, starch, SDP and IDP dietary fibre of spring malting and spring feeding barley varieties.
192
K. E. BACH KNUDSEN ET AL.
TABLE III. Chemical composition, digestible energy, true protein digestibility, biological value and net protein utilisation of two varieties of barley, Gunhild and Lami
No. of samples Chemical composition (% dry matter) Low-molecularweight sugars a Starch NSP Cellulose ~-Glucan
Hemicellulose Total NSP Klason lignin
DF TDP
Protein (N x 6'25) HC1-fat Ash DE(%) TD (%) BV (%) NPU (%) N.S.
Gunhild
Lami
6
6
1·9 5%
2'0 56·9
4·3 4·3 7-7 16·3 2'7 19'0 20·8 12·0 3·5 2·2 80·8 86·5 76·9 66·5
5·0 4·6 8·7 18·3 2·9 21'2 23-6 11·9 3·7 2-4 78·0 85·7 75-6 64·8
6 (%)
0'1 2'7 -0,7 -0,3 -1,0 -2·0 -0'2 -2,2 -2,8 +0·1 -0'2 -0,2 2,8*** 0·8N.S. 1·3N.S. 1·7N.S.
= not significant
*** p < 0·001.
• Sum of glucose, fructose, sucrose and fructans. Dietary fibre determined gravimetrically.
b
TABLE IV. Correlation coefficients between starch, soluble dietary fibre, insoluble dietary fibre, total dietary fibre and total ~-glucan vs. digestible energy and true protein digestibility DE
Starch SDP lOF
TDF
~-Glucan N.S. =
*....
P
not significant.
< 0'001.
+0'770*** -0·160N.S. -0'786*** -0,761 *** -0·034N.S.
TD -O·257N.S. -O·157N.S. -0·013N.S. -0·040N.S. -0·218N.S.
DIGESTIBILITY OF BARLEY
193
Correlation between dietary carbohydrate constituents and DE and TD
The variation in DE was negatively correlated with IDF and TDF and positively correlated with dietary starch (Table IV). The relationship between DE and IDF, shown in Fig. 2 can be expressed as: DE = 97·6 -1,02 x IDF; R 2 ±2'09±0'12
= 0·58
In contrast, neither SDF nor p-glucan content affected the DE value.
o
~
",
~ c
OJ OJ
;e
78
en
OJ
Ci"" 74
...
o
•
•
~ 1---,'16':----'-----=2'=-0_ _.L.-_---:'':-..c..----J IDF(%)
FIGURE 2. Relationship between DE and IDF; e, spring feeding variety; 0, spring malting variety; 0, winter feeding variety.
TD varied from 81·6 to 89·9%. However, the variation was not correlated with any of the dietary carbohydrate constituents listed in Table IV. Discussion
The results in the present investigation show that DE is strongly correlated with the composition of the carbohydrate fraction, in particular the ratio of starch to DF. Hence the higher energy digestibility of 1·3 % in spring malting varieties compared to spring feeding varieties is accounted for by the higher starch and lower DF content 9 • The significantly lower energy digestibility of barley varieties grown on the poor quality sandy soil in 1983 compared with the previous two years, and to varieties grown on clay soil, is attributed to the same factors 9 • In both cases the variation in DE can be explained by the relatively more dry matter located in the cell wall compared to the cell contents, primarily endosperm. The variation in DE of individual varieties was from 73·1 to 83·1 %. This is within the
194
K. E. BACH KNUDSEN ET AL.
same range as the variation in starch and Dp·9 and demonstrates that the barley kernel is not as homogeneous as previously believed. The chemical composition, and thus the nutritive value, are affected by factors such as variety, cultivation conditions and year. For the individual varieties, the difference between the highest and lowest DE value was 2·8 %. Here, the ratio of starch to DF also explains the differences. It is not possible from the data in the present study to estimate the effect of carbohydrate composition on metabolisable or net energy. However, DE is the factor of most importance for the variation in ME and NE12. It therefore seems reasonable to assume that the variation in ME and NE can be attributed to the same factors as for DE. The high negative correlation between IDF and DE are in agreement with results from an earlier study13. In that study rats were fed different decortication fractions from barley. The slope and the intercept of the regression between DE and IDF in the earlier study were 1·00 ± 0·10 and 99·3 ± 2·81 respectively compared to 1·01 ± 0·12 and 97·6 ± 2·09 respectively in this study. The reason for the strong negative correlation between DE and IDF is that IDF primarily measures cellulose, hemicellulose and lignin 9 • These DF constituents are found in high levels in the lignified secondary cell walls of husk tissues 14 and are highly resistant to microbial degradation. Recently, we have estimated the digestibility of cellulose, hemicellulose and lignin from barley in rats to be 31 ± 3, 55 ± 2 and 5 ± 14 % respectivelys. In contrast to the high resistance to microbial degradation of cellulose, hemicellulose and lignin, the p-glucans found in the unlignified cell walls of endosperm and aJeurone 15 , are readily available substrates for the hindgut microflora 2 • 5 • Hence, neither p-glucan nor SDF are negatively correlated with DE. In this respect, rats and other monogastric animals with active microbial fermentation, are very different to broiler chickens. In broiler chickens, the p-glucans are found to be responsible for the low productive value of oats and barley, mainly because of their influence on starch and protein digestion 16. The variation in TD was not correlated with any of the dietary carbohydrate constituents. This is in contrast to results from several studies in which reasonably high correlations between fibre levels and protein digestibility were identified l 7-19. However, the correlation between DF and cell wall bound proteins is quite different in the two situations. In barley, the husk accounts for the major variation in DFD. The protein content of the husk is, however, very low ls and the weak correlation between TD and DF is therefore easily understood. In studies with different plant materials l 7-lD a higher fibre level was associated with relatively more ct!l1 wall bound protein. This protein is poorly digested by microbial flora in the gut 1S . Hence, the effect of fibre level on protein digestibility is quite different in some types of plant (e.g. barley) compared to plant materials in general. Although we could not explain the variation in TD by any of the DF constituents, the TD values were affected significantly by the year grown and by the barley variety. BV and NPU were higher in the spring feeding than in the winter feeding varieties. Using the conventional method of calculating ME or NE of feed and foodstuffs, the material is analysed for ash, fat, protein and crude fibre and nitrogen-free extract (NFE), calculated by difference 2D . In barley and other cereals the NFE fraction often comprises 75-80 % of the total dry matter. As shown in the accompanying paper D, Swedish
DIGESTIBILITY OF BARLEY
195
work? and work from Montana, U.S.A. B, the composition of this fraction varies significantly, in particular with regard to starch and DF. In agreement with the variation in carbohydrate composition, DE and, probably, ME and NE varied significantly. On the basis of the results in the present study, it can be concluded that more detailed carbohydrate analyses should be included in the analytical scheme when estimating ME and NE of feed and foodstuffs from chemical data. The project was supported by a grant to the senior research worker by the Royal Veterinary and Agricultural University, Copenhagen and technical assistance was supported by the Carlsberg Foundation, Copenhagen. Birgit S. Jensen is thanked for technical assistance.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. II. 12. 13. 14. 15. 16. 17. 18. 19. 20.
Keys, J. E. and De Barthe, J. V. J. Anim. Sci. 39 (1974) 53-56. Graham, H., Hesselman, K. and Aman, P. J. Nutr. 116 (1986) 242-251. Englyst, H. and Cummings, J. H. Am. J. Clin. NUlr. 42 (1986) 778-787. Sandberg, A.-S., Andersson, H., Kivesta, B. and Sandstmm, B. Br. J. Nutr. 55 (1986) 245-254. Bach Knudsen, K. E., Agergaard, N. and Olesen, H. P. Short communication No. 633, National Institute of Animal Science, Copenhagen (1986). (In Danish.). Just, A., Fernandez, J. A. and Jargensen, H. Livestock Prod. Sci. 10 (1983) 171-186. Aman, P., Hesselman, K. and Tilly, A.·C. J. Cereal Sci. 3 (1985) 73-77. Aman, P. and Newman, C. W. J. Cereal Sci. 4 (1986) 133-141. Bach Knudsen, K. E., Aman, P. and Eggum, B. O. J. Cereal Sci. 6 (1987) 173-186. Eggum, B. O. A Study of Certain Factors Influencing Protein Utilization in Rats and Pigs (TheBis) Report No. 406, National Institute of Animal Science, Copenhagen (1973). Snedecor, G. W. and Cochran, W. G. 'Statistical Methods', 6th ed. The Iowa State University Press, Ames, Iowa (1973). Just, A. Livestock Prod. Sci. 8 (1982) 541-555. Bach Knudsen, K. E. and Eggum, B. O. Z. Tierphysiol., Tierernahrg. Futterm-ittelkde. 51 (1984) 130148. Salomonsson, A.-C., Theander, O. and Arnan, P. Swedish J. Agric. Res. 10 (1980) 11-16. Bacic, A. and Stone, B. A. Aust. J. Plant Physiol. 8 (1981) 475-495. Hesselman, K. PhD Thesis, Swedish University of Agricultural Sciences, Department of Animal Husbandry, Report No. 112 (1983). Just, A. Livestock, Prod. Sci. 9 (1982) 569-580. Bach Knudsen, K. E. Z. Tierphysiol., Tierernahrg. u. Futtermittelkde. 48 (1982) 90-104. Taverner, M. R. and Farrell, D. J. Br. J. Nutr. 46 (1981) 173-192. Just, A. J0rgensen, H. and Fernandez, J. A. Livestock Prod. Sci. 11 (1984) 105-128.