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Relationships of Forage Compositions With Rates of Cell Wall Digestion and Indigestibility of Cell Walls
Relationships of Forage Compositions With Rates of Cell Wall Digestion and Indigestibility of Cell Walls L. W. SMITH, H. K. GOERING, and C. H. GORDON, Animal Science Research Division, USDA, Beltsville, Maryland 20705 Abstract
well established (2, 5, 7, and 9). Furthermore, Gill, Conrad, and Hibbs (3) showed that digestible dry matter intake was related to the rate of digestible cellulose disappearance in vitro. Smith et al. (8) reported that soluble forage dry matter did not contribute directly to faster rates of cell wall digestion although soluble dry matter content was highly correlated with cell wall digestion rates in a small group of hays. The objectives of these experiments were to determine over a great diversity of species and maturities in forage plants a) the validity of expressing cell wall digestion as a first-order reaction rate, b) the correlations of cell wall digestion rates with selected chemical entities, and c) the role of lignin in limiting the extent of cell wall digestion in a wide spectrum of forages.
Forage fiber digestion followed firstorder reaction kinetics even though individual forages differed widely in maturity, composition, and rate of fiber digestion. Linearity of individual semilog plots of remaining digestible cell walls on time and their individually high r z (mean ,978) for each of 112 different samples representing 15 species support this general model. Cell wall digestion rates were more highly correlated with soluble dry matter percentage (y = --.0299 -b .00261X, r =.72, P < .001) than with ]ignin percentage (r = --.47, P < .001), lignin-to-cellulose (r = --.18), log lignin-to-cellulose ratio (r = --.26, P < .01), or 72 hr in vitro cell wall indigestibility (r = --.38, P < .001). Similarity of mean lignin-to-cellulose ratios in theoretically indigestible residues from legumes (1.09) and grasses (.94) suggests a similar role of lignin in limiting extent of digestion. Lignin in dry matter, lignin:cellulose, or log lignin:cellulose seem to be equally suitable predictors of cell wall indigestibility in vitro. Legumes were higher in percentage soluble dry matter and lignin and lower in percentage hemicellulose than grasses. Legume cell walls were also more lignified and less digestible, but digested faster than grass cell walls.
Experimental Procedure
Introduction
The best measure of nutritive value of feed is animal productivity. Productivity is the result of intake, digestion, and efficiency of utilization of absorbed nutrients. Total nutrient intake and digestibility are highly related for ruminants consuming all forage diets, but intake appears more important in limiting productivity than digestibility (1). The relationship between soluble or insoluble forage dry matter and voluntary intake is Received for publication January 7, 1972.
Six legume and 9 grass species were harvested weekly to provide 112 samples. Approximately i kg samples were hande/ipped, leaving 4 cm of stubble, and dried in cloth bags. Drying was at 65 C in a circulating hot air drier. The total sample was ground through a 1 mm screen, allowed to equilibrate to air dry, and stored at room temperature. Compositional analyses were as outlined by Goering and Van Soest (4). Hemicellulose was the difference between neutral-detergent fiber and acid-detergent fiber, lignin was the weight loss due to permanganate treatment of aciddetergent fiber residue, and cellulose was the weight loss upon ashing the permanganate treated acid-detergent fiber residue. Forage samples of total dry matter, approximating .5 g were incubated (8) with ruminal fluid for 8, 6, 12, 18, 24, 30, 36, 48, and 72 hr. Digestion of cell walls approached an asymptote in 72 hr or less and was, thus, considered complete at 72 hr as was reported earlier (8). Rates of cell wall digestion were regressions of the natural logarithmic (In) transformation of percentages potentially digestible remaining on hours of fermentation. The digestion rate constant (lw -1) is the change in In concentration of cell walls (Y)/hr.
1140
1141
FORAGE COMPOSITION AND FIBER D I G E S T I O N
Results and Discussion
The range in harvest dates, growth stage descriptions, chemical compositions, 48 hr and 72 hr cell wall indigestibilities in vitro and rates of cell wall digestion for the 112 forages are in Table 1. Soluble d r y matter r a n g e d from 82 to 29; cellulose, 35 to 1I; hemicellulose, 28 to 1; and lignin, 16 to 1~ for the forages on a dry matter basis. Silica as well as lignin is
recognized for its role in depressing extent of digestion (11), b u t acid-insoluble ash content of all these forages was less than 1% of the dry matter. Therefore, neither these values nor this topic will b e discussed further. Forty-eight hour cell wall indigestibilities ranged from 73 to 9%; 72 h r cell wall indigestibflities, 73 to 8~; and cell wall digestion rate constants, .3090 to .0400 hr -1. The 72 hr indigestibflities
TABLE 1. Range in initial compositions, cell wall indigestibility, and rates constants of cell wall digestion. Forage and variety
Harvest date
Average height Descxiption
Alfalfa Saranae
5/5 6/16
(cm) 50 115
Birdsfoot trefoil Viking
5/5
6/16
30 80
Ladino clover Common
4/21 6/16
6 30
Red clover Kenland
4/21 6/16
18 110
Crown vetch Chemung
5/5 6/16
50 140
Vetch Common
5/26 6/16
170 190
Bluegrass 4/15 Common Kentucky 6/16
15 80
Prebloom Early pod, leaf loss, Prebloom Ear/y pod, leaf loss, Prebloom Dry heads
Soluble HemiIVICW ~ CW digesdry Cellu- cellu- Ligt_ionrate matter lose lose nin 48 hr 72 hr constant
--(% 66 some 47 lodge
66
21 30
of DM) 6 7 8 15
--(%)-46 45 60 58
(hr-l) b .191 _ .005 .073-+.009
56
19 25
9 7
6 12
51 63
51 61
.174-+.007 .060-+.005
82 62
11 21
4 4
3 13
19 44
19 40
.309_+.044 .063-+.014
Prebloom Dry heads, 90g~
53 54
23 25
21 8
2 13
22 57
22 54
.091-+ .008 .063+-.007
Budding Podded, some leaf loss, lodge Early pod Podded, some leaf loss, lodge Vegetative Mature
72 54
15 28
8 7
4 14
53 73
50 73
.103+-.017 .097+-.010
60 55
23 23
1 9
16 11
51 61
54 59
.118+-.004 .057+-.008
60 34
18 30
20 26
2 8
12 45
12 38
.153_.006 .048--_.001
some lodge
Brome grass Blair smooth
5/5 6/16
40 140
Vegetative Mature
50 32
24 33
23 24
3 10
19 50
15 50
.183-+.013 .073-+.003
Tall rescue Ky-31
5/5 6/16
25 120
Vegetative Mature
50 39
24 29
23 23
3 9
16 51
16 48
.131 +-.003 .058+- .004
Orchardgrass Potomac
4/15 6/16
15 120
Vegetative Mature, lodge
64 35
16 31
17 23
2 10
15 64
12 61
.128+- .014 .050+-.001
Orchardgrass Pennlate
5/19 6/16
70 140
Boot Mature, lodge
38 29
31 35
24 25
5 11
29 54
26 46
.077-+ .004 .046+-.004
Reed Canarygrass Common
4/28 6/16
25 140
Vegetative Mature
62 33
18 31
18 26
i 9
13 50
10 45
.183+-.006 .053_+.001
Barley Wong
4/15 7/8
8 75
Vegetative Mature
66 31
16 30
16 28
1 10
12 45
11 38
.169+ .006 .048- .003
Oats Gary
5/5 7/8
25 120
Vegetative Mature
67 41
16 26
14 25
1 8
9 58
8 51
.131 +-.008 .042 +-.003
Rye Balboa
4/7 7/8
30 180
Vegetative Mature
66 36
16 29
16 25
2 10
13 59
10 55
.160+- .002 .040+- .002
Wheat Common
5/19 6/16
68 110
Heading Mature
39 46
30 23
21 19
9 9
33 48
31 44
.078+-.001 .075+- .006
a In vitro cell wall indigestibilities. Rate constants for in vitro disappearances of digestible cell walls and their standard errors. JOURNAL OF DAIRY SCIENCE VOL. 55, NO. 8
1142
SMITH ET AL
generally decreased more relative to 48 hr indigestibilities for samples harvested at the later date. Immature oats had the lowest cell wall indigestibility (8%) and mature crown vetch had the highest cell wall indigestibility (73%). The April 21 sample of Ladino had the fastest rate of cell wail digestion (.3090 hr -1) which was comparable to .2705 hr "x previously reported for a fall sampled rye (8). Mature grains had the slowest rates of cell wall digestion (mean of .051 hr-X), The range in compositions, cell wall indigestibilities in vitro, and cell wall digestion rates were wider than for forages normally fed. Acquisition of a wide range was intended. Even though composition and observed rates of cell wall digestion varied widely, all of the 112 forages showed a high degree of linearity of the semilog plots of remaining digestible cell wall on time (r 2 -~ .978). This confirmed first-
order digestion kinetics for each of the forages. The mean r 2 was .991 for grasses and .965 for legumes. Correlations of cell wall digestion rates with chemical measurements are in Table 2 for individual forage species, all legumes, all grasses, and for all forages combined. The rates of cell wall digestion were highly correlated with soluble dry matter within species except for crown vetch, red clover, and wheat. The low correlations for crown vetch and wheat may have resulted from fewer samples which were confined to the latter part of the growth period. During this time seed formation contributes to the soluble dry matter content of the total plant, but seed sohibles are not associated with structural plant parts, thus might not be related to cell wall digestion rates in the same manner as soluble dry matter in the structural parts. In red clover the lower correlation of
TABLE 2. Correlations of the rates of cell wall digestion with chemieal measurements and in vitro cell wall indigestibility (72 hr). Hemicellulose
Forages
n
Soluble dry matter
Lignin
Lignin: cellulose
Acidlignin detergent Log fiber cellulose
Cell wall indigestibihty 72 hr
(% of D M ) - Legumes
Alfalfa Birdsfoot trefoil Ladino dover Red dover Crown vetch Vetch
6 6 8 8 6 3
.80 .98** .94*** ,55 .44 .93
--.74 --,90"* --.91"* --.25 -.48 .59
--.66 --.69 --.88** --.12 -.46 .72
9 6 6
.96*** .85* .90*
--.96*** --.83* --.81"
--.95*** --.81" --.83"
9
.82**
--.96***
4 7 10 9 II 4
.93 .94** .80*** .84** .77"* .04
37
.75***
.50 .42 .30 --.22 .20 -.54
--.69 --.71 --.88** .04 -.43 --.70
--.86 --.77* --.76* --.11 -.24 --.82
.86** .95** .83*
--.97*** --.84* --.88"
--.92*** --.69 --.80
".95***
.92***
--.95***
--.93***
--.99* --.91"* --.83"** --.90*** --.92"** .11
--.97" --.91"* --.83*** --.91"** --.93"** .24
.95 .94"* ,10 .29 .80"* --.17
--.96" --.93*** --.88"** --.93*** --,89"** --.25
--.96" --.89** --.92"** --.87** --.91"** --.25
--.61"**
--.44**
--.05
--.35*
--.58***
--.87***
--.79***
--,26**
--.38***
.27
.01
Grasses Bluegrass Brome Fescue Orchardgrass Potomac Orchardgrass Pennlate Reed canary Barley Oats Rye Wheat All legumes All grasses Legumes and grasses Hays
75
.76***
--.82***
--.83***
112
.72***
--.47***
--.18
42
.73***
--.16
.29
.64*** --.12 --.56***
* Indicates significance of correlations: * = P<.05; ** ~ P<.01; and *** ---- P<.001. JOURNAL OF DAIRY SCIENCE VOL. 55, NO.
1143
FOIRAGE C O M P O S I T I O N A N D FIBEIA D I G E S T I O N
soluble dry matter as a general predictor of cell wall digestion rates than other chemical measurements.
rate with soluble dry matter was the result of little change in the rate, even though soluble dry matter decreased with maturity. Rates of cell wall digestion were more highly correlated with soluble dry matter than with measurements of lignin in the individual legumes and the legume group. However, in the grasses, rates of cell wall digestion were more highly correlated with log lignin:cellulose or other lignin measurements than with soluble dry matter. Rates of cell wall digestion were also highly correlated with hemieellulose:acid-detergent fiber ratio in some of the grass species but not in any of the legumes. This correlation remained high and significant when analysis included all grasses. These positive correlations suggest that cell walls digested faster in some grasses containing proportionately greater amounts of hemicellulose. Relatively high negative correlations were revealed between rate of cell wall digestion and cell wall indigestibility for most of the individual species. This correlation remained high for the grass group (r = --.79) but was lower for the legume group (r = .58). Correlations in the next to the bottom line of Table 2 are the result of all samples. The higher coefficient for soluble dry matter over the others shows the greater applicability of
Three consecutive weekly samples were selected from each species to test whether solu-
ble dry matter would be as useful to predict digestion rates under more practical circumstances. Selections were based upon anatomical maturity criteria approximating hay harvesting maturities. Correlations using only these selected forages also substantiate the more general applicability of soluble dry matter for predicting forage cell wall digestion rates than other chemical measurements and are shown in the bottom line of Table 2. However, no direct causative relationship should be implied between solubles and rates of cell wall digestion in vitro (8). Relationships of cell wall digestion rates in vitro with soluble dry matter for the legume group, the grass group, and legumes and grasses together are in Figure 1 with respective regression equations. The slope of the regression line is steeper for legumes than for grasses or combined legumes and grasses. Therefore, separate prediction equations should estimate cell wall digestion rates of legumes and grasses with more precision. Cell wall digestion rates relative to log lignin:celhilose ratios are in Figure 2. This figure
O. 3 5
O. 5 0
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[]Y=-0.2(27 X T = -0. 032 y T : -0. 029
!
O. 25 []m
w 0.20
x x
x x
0.15 7~
x
[] []x[] [Ox [B
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- 0 . 0 0
-
20
30
,
liO 50 SOLUBLE
,
60
7(2 OIIY MflTTEH (Z]
~0
9(2
FIG. 1. Relationship of cell wall digestion rates and soluble dry matter contents. JOURNAL OF DAIRY SCIENCE VOL. 55. No.
R
1144
S M I T H E T AL
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13.t~0
Fr]T= D . 6 1 7 X T = 13. 313i yT=U.177
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LOG t L I G N I N / C E L L U L O S E
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FIc. 2. Relationship o£ cell wall digestion rates and log lignin: cellulose ratios. displays the possible usefulness of log lignin: cellulose for predicting cell wall digestion rates in grasses. The overall regression line is entirely unsuitable for predicting cell wall digestion rates o£ combined legumes or grasses. Correlations between 72 hr cell wall indigestibilities and chemical measurements are in Table 3 for individual forages, legumes and grasses, all forages, and selected hay maturities of all forages. In vitro cell wall indigestibilities were negatively correlated with soluble dry matter, r = --.44 to --.97 for the 15 individual species. Significance is indicated. Except for vetch and wheat, cell wall indigestibilities were positively correlated with lignin in dry matter (r = .32 to .96), lignin:eellulose (r = .56 to .97), and log lignin:cellulose (r = .62 to .98). Negative correlations for vetch and wheat may have been the result of samples representing only the latter growth stages. In the legume group, both soluble dry matter (r = --.74) and lignin in dry matter (r .78) were more highly correlated with cell wall JOURNAL OF DAIRY SCIENCE VOL. ~5, N o .
8
indigestibility than lignin:eellulose (r = .57) or log Iignin:cellulose (r = .59). While in the grasses all measures involving lignin were similarly correlated (r = .88) and more highly correlated with cell wall indigestibility than soluble dry matter (r = --.75). Lignification measures remained highly correlated for all forages in a single analysis, but the correlation of soluble dry matter versus cell wall indigestibility decreased (r = --.11).. Thus, soluble dry matter is o£ little general value in predicting cell wall indigestibilities of forage since differences between legumes and grasses are not resolved. Correlations where all forages were included in Table 3 show that cell wall indigestibility was equally and highly correlated with lignin in dry matter, lignin:cellulose, or log lignin: cellulose. Van Soest and Moore (10) reported that differences between legumes and grasses were resolved by relating cell wall digestibility to lignin concentrations in lignocellulose. They indicated that correlations were improved when lignin concentrations were converted to
F O R A G E COMPOSITION" A N D F I B E R D I G E S T I O N
1145
T~mi~ 3. Correlations of 72-hr cell wall indigestibilities with chemical measurements. Forages
n
Soluble dry matter ~(%
Lignin
Liguin: cellulose
lignin Log - cellulose
of D M ) - -
Legumes Alfalfa Birdsfoot trefoil Ladino clover Red dover Crown vetch Vetch
6 6 8 8 6 3
--.71 --.80* --.80* --.44 -.90 -- .97
.71 .84" .90** .95*** .85* - .95
.72 .56 .92** .90** .71 -- .99
.74 .62 .92** .83** .70 --.98
9 6 6
--.84"* --.84* --.70
.96"** .91"* .78
.96"** .90** .83*
.96"** .85* .82
9
-- .83"*
.32
.80"*
.94"**
4 7 10 9 11 4
-.95 --.94** --.90"** --.82** --.74** --.73
.92 .96*** .94"** .93*** .96*** -- ,12
.87 .97*** .88"** .96*** .96*** .74
.90 .94*** .91"** .84** .98*** .70
37
--.74***
.78***
.57***
.59***
--.75***
.89***
.88***
.87***
--.11
.88***
.82"**
.85***
.84***
.73***
.75***
Grasses Bluegrass Brome Fescue Orchardgrass -Potomac Orchardgrass Pennlate Reed canary Barley Oats Rye Wheat All legumes All grasses Legumes and grasses Hays
75 112 42
.33*
* Indicates significance of correlations: * = P<.05; ** ~ P<.01; and *** = P<.001.
logarithms. A similar conclusion cannot be drawn from our data even though ]ignification of structural carbohydrates can be more rationally associated with cell wall digestibility than can lignin content of total dry matter. The resuits of Mowat et al. (6) are in even greater disagreement with the concept that differences in cell wall digestibility of legumes and grasses are resolved by expressing lignin concentration in cell walls or ]ignocellulose rather than in dry matter. They reported a much higher correlation for lignin in dry matter (r = --.73) as opposed to lignin in cell walls (r = --.37) with cell wall digestibility in vitro. The apparent discrepancy in our work and that of Mowat et al. (6) may be the result 6f in vitro versus in vivo data. Also in this study samples were intentionally drawn from a wider range of maturities in order to establish extremes in particular variables. When correlations between cell wall indigestibility and measures of lignin were performed with selected forage samples of hay
maturity, the correlation remained high for lignin in dry matter (r = .84) while those for lignin:cellulose (r -~ .73) and log lignin: cellulose (r -- .75) decreased. The relationships of 72 hr cell wall digestibility with lignin in dry matter and log lignin: cellulose are in Figures 3 and 4. The relatively less scatter around the slopes in Figure 4 may lead one to elect log ]ignin:eellulose as the more appropriate predictor of cell wall digestibility. However, the relatively lower standard deviation for the legume and grass slope in Figure 3 as compared to that in Figure 4 suggests that lignin in dry matter is a better predictor of cell wall indigestibility in vitro than log lignin:cellulose. The smaller error of the estimate (8.1 versus 8.9) also supports this conclusion. Cellulose and lignin contents of residual cell wails after 72 hr of incubation were determined for hay maturity selections. The ratios of initial lignin:initial cellulose and initial lignin:72 hr cellulose were compared (TaJOURNAL OF DAIRY SCIENCE VOL. 55, No.
8
1146
SMITH ET AL B5
IT]T = 16. 392
*3.293
XT=3.137
-ttt. 1B2
y T = u,.B71
ai. 177
X : 5~ = O. tlt~tL82 X : 51 = 0 . 2 4 6 @ 5 X : 51 = O . 2 1 B I 6
75 []
65
[]
~
[] X
55 QEI
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S
. .95 (..)
X
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o
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LEGUME5 X GflR55E5 Y OVEflRLL REGRE55[ON
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1 2'
1 4'
15'
{Z]
Fie. 3. Relationship cell wall indigestibilities and lignin in dry ma~er. ble 4). The mean ratios for grasses and legumes were more similar in the residues than they were initially. The similarity of the lignin:cellulose ratios in the residues indicate that the extent of digestion is limited by liguin and that the limitation operates similarly in both groups. When these ratios and relationships were considered on the basis of observed liguin recoveries, legumes and grasses appeared less similar. Mean values of recoveries of lignin in the 72 hr incubation residues are in Table 4. t~ecovery was more complete for legume samples. This may be a reflection of the greater amount of lignin generally present in the legumes. Up to 75% of the variation of cell wall indigestibility in vitro can be attributed to lignification and about 50% of the variation in cell wall rates of digestion can be attributed to soluble dry matter content in this study. Differences in fiber that are not reflected in these chemical methods appear to be partially involved with the rate that cell walls digest. The high correlations of soluble forage dry matter (2, 5) or insoluble forage dry matter (7, 9) with voluntary intake appear to be an JOURNAL OF DAIRY SCIENCE VOL.
55, INTO. 8
additive result of less ruminal indigestible fiber from forages lower in cell wall content, and that these cell wails also digest faster. TABLE 4. Initial versus residual ]ignin:cellulose (L:C) in hays.
Hay
na
Initial L
Initial L
Initial C
72 hour C
Mean
Mean CV Mean CV
(~) (~) (z)
(g) Legumes 5 .48 Grasses 10 .24 Legumes and grasses 15 .32
nign~ recovery
20 42
1.09 .94
18 21
82 68
14 9
29
.99
20
72
11
Number of species represented, each with 3 stages of maturity. b Coefficient of variation. Acknowledgments
The authors acknowledge Dr. G. E. Carlson, Forage and Range Research Branch, for making plots available for this study, Dr. B. T. Weinland, BiometricaI Service Staff, for conducting the statistical analyses, J. O'Connor for the laboratory evaluations, and Elaine Bierman for tabulating this data.
1147
FORAGE COMPOSITION AND FIBER DIGESTION
9S
85 []T=-7tt.021 +7t~.737 LOGX: 5, =10.1~,g37 XY=-I~g.6S3 ~60.866 LOGX ; 5e =0,.01625
7S
YT=-48.
170
5e = 3 . 5 6 2 0 5
" 5 9 . 3 6 0 LOB X :
B
6S
X
010~] Cg
_~ss xX o ] X x
SX d 35
El m
> f.Sx Xx- oo
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x~ X
ls ×
0.6
x
0.90
x
~xx~
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--
1.00
×
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X
X
x X
x
~0:]
xxx
~m m
x
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-
x
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L~G[LIGNIN/CELLULOSE
1.60
~
_
1.70
1.BO
13VERFL i LPiEGflE55ION 1.go
X I00]
FIG. 4. Relationship of cell wall indigestibilities and log lignin:cellulose ratios.
References
( I ) Crampton, E. W., E. Donefer, and L. E. Lloyd. 1960. A nutritive value index for forages. J. Animal Sci., 21:628. (2) Donefer, E., E. W. Crampton, and L. E. Lloyd. 1966. The prediction of digestible energy intake potential (NVI) of forages using a simple in vitro technique. Proc. X Int. Grassland Congr., P. 442. (3) Gill, S. S., H. R. Conrad, and J. W. Hibbs. 1969. Relative rate of in vitro cellulose disappearance as a possible estimator of digestible dry matter intake. J. Dairy Sei., 52:1687. (4) Goering, H. K., and P. J. Van Soest. 1970. Forage fiber analyses (apparatus, reagents, procedures and some application). Agr. Handbook 379, USDA. (5) Johnson, R. R., B. A. Dehority, and J. L. Parsons. 1965. Relationships between in Vitro measurement on forages and their nutritive value. Proc. IX Int. Grassland Congr. Vol. 1:773. (6) Mowat, D. N., M. L. Kovain, and J. E. Winch. 1969. Lignification and in vitro
(7)
(8)
(9)
(10)
(11)
cell wall digestibility of plant parts. Canadian J. Plant Sei., 49:499. Osbourn, D. F., S. B. Cammell, R. A. Terry, and G. E. Outen. 1970. The effect of chemical composition and physical characteristics of forages on their voluntary intake by sheep. Annu. Rep., The Grassland Res. Institute. Hurley, England. Smith, L. W., H. K. Goering, D. R. Waldo, and C. H. Gordon. 1971. In vitro digestion rate of forage cell wall components. J. Dairy Sci., 54:71. Van Soest, P. J. 1965. Symposium on factors influencing the voluntary intake of herbage by ruminants: Voluntary intake in relation to chemical composition and digestibility. J. Animal Sci., 24:834. Van Soest, P. J., and L. A. Moore. 1965. New chemical methods for analysis of forages for the purpose of predicting nutritive value. Proe. Ninth Int. Grassl. Congr., 1:783. Van Soest, P. J., and L. H. P. Jones. 1968. Effect of silica in forages upon digestibility. J. Dairy Sei., 51:1644. JOURNAL OF DAIRY SCIENCE VOL. 55, NO.
well established (2, 5, 7, and 9). Furthermore, Gill, Conrad, and Hibbs (3) showed that digestible dry matter intake was related to the rate of digestible cellulose disappearance in vitro. Smith et al. (8) reported that soluble forage dry matter did not contribute directly to faster rates of cell wall digestion although soluble dry matter content was highly correlated with cell wall digestion rates in a small group of hays. The objectives of these experiments were to determine over a great diversity of species and maturities in forage plants a) the validity of expressing cell wall digestion as a first-order reaction rate, b) the correlations of cell wall digestion rates with selected chemical entities, and c) the role of lignin in limiting the extent of cell wall digestion in a wide spectrum of forages.
Forage fiber digestion followed firstorder reaction kinetics even though individual forages differed widely in maturity, composition, and rate of fiber digestion. Linearity of individual semilog plots of remaining digestible cell walls on time and their individually high r z (mean ,978) for each of 112 different samples representing 15 species support this general model. Cell wall digestion rates were more highly correlated with soluble dry matter percentage (y = --.0299 -b .00261X, r =.72, P < .001) than with ]ignin percentage (r = --.47, P < .001), lignin-to-cellulose (r = --.18), log lignin-to-cellulose ratio (r = --.26, P < .01), or 72 hr in vitro cell wall indigestibility (r = --.38, P < .001). Similarity of mean lignin-to-cellulose ratios in theoretically indigestible residues from legumes (1.09) and grasses (.94) suggests a similar role of lignin in limiting extent of digestion. Lignin in dry matter, lignin:cellulose, or log lignin:cellulose seem to be equally suitable predictors of cell wall indigestibility in vitro. Legumes were higher in percentage soluble dry matter and lignin and lower in percentage hemicellulose than grasses. Legume cell walls were also more lignified and less digestible, but digested faster than grass cell walls.
Experimental Procedure
Introduction
The best measure of nutritive value of feed is animal productivity. Productivity is the result of intake, digestion, and efficiency of utilization of absorbed nutrients. Total nutrient intake and digestibility are highly related for ruminants consuming all forage diets, but intake appears more important in limiting productivity than digestibility (1). The relationship between soluble or insoluble forage dry matter and voluntary intake is Received for publication January 7, 1972.
Six legume and 9 grass species were harvested weekly to provide 112 samples. Approximately i kg samples were hande/ipped, leaving 4 cm of stubble, and dried in cloth bags. Drying was at 65 C in a circulating hot air drier. The total sample was ground through a 1 mm screen, allowed to equilibrate to air dry, and stored at room temperature. Compositional analyses were as outlined by Goering and Van Soest (4). Hemicellulose was the difference between neutral-detergent fiber and acid-detergent fiber, lignin was the weight loss due to permanganate treatment of aciddetergent fiber residue, and cellulose was the weight loss upon ashing the permanganate treated acid-detergent fiber residue. Forage samples of total dry matter, approximating .5 g were incubated (8) with ruminal fluid for 8, 6, 12, 18, 24, 30, 36, 48, and 72 hr. Digestion of cell walls approached an asymptote in 72 hr or less and was, thus, considered complete at 72 hr as was reported earlier (8). Rates of cell wall digestion were regressions of the natural logarithmic (In) transformation of percentages potentially digestible remaining on hours of fermentation. The digestion rate constant (lw -1) is the change in In concentration of cell walls (Y)/hr.
1140
1141
FORAGE COMPOSITION AND FIBER D I G E S T I O N
Results and Discussion
The range in harvest dates, growth stage descriptions, chemical compositions, 48 hr and 72 hr cell wall indigestibilities in vitro and rates of cell wall digestion for the 112 forages are in Table 1. Soluble d r y matter r a n g e d from 82 to 29; cellulose, 35 to 1I; hemicellulose, 28 to 1; and lignin, 16 to 1~ for the forages on a dry matter basis. Silica as well as lignin is
recognized for its role in depressing extent of digestion (11), b u t acid-insoluble ash content of all these forages was less than 1% of the dry matter. Therefore, neither these values nor this topic will b e discussed further. Forty-eight hour cell wall indigestibilities ranged from 73 to 9%; 72 h r cell wall indigestibflities, 73 to 8~; and cell wall digestion rate constants, .3090 to .0400 hr -1. The 72 hr indigestibflities
TABLE 1. Range in initial compositions, cell wall indigestibility, and rates constants of cell wall digestion. Forage and variety
Harvest date
Average height Descxiption
Alfalfa Saranae
5/5 6/16
(cm) 50 115
Birdsfoot trefoil Viking
5/5
6/16
30 80
Ladino clover Common
4/21 6/16
6 30
Red clover Kenland
4/21 6/16
18 110
Crown vetch Chemung
5/5 6/16
50 140
Vetch Common
5/26 6/16
170 190
Bluegrass 4/15 Common Kentucky 6/16
15 80
Prebloom Early pod, leaf loss, Prebloom Ear/y pod, leaf loss, Prebloom Dry heads
Soluble HemiIVICW ~ CW digesdry Cellu- cellu- Ligt_ionrate matter lose lose nin 48 hr 72 hr constant
--(% 66 some 47 lodge
66
21 30
of DM) 6 7 8 15
--(%)-46 45 60 58
(hr-l) b .191 _ .005 .073-+.009
56
19 25
9 7
6 12
51 63
51 61
.174-+.007 .060-+.005
82 62
11 21
4 4
3 13
19 44
19 40
.309_+.044 .063-+.014
Prebloom Dry heads, 90g~
53 54
23 25
21 8
2 13
22 57
22 54
.091-+ .008 .063+-.007
Budding Podded, some leaf loss, lodge Early pod Podded, some leaf loss, lodge Vegetative Mature
72 54
15 28
8 7
4 14
53 73
50 73
.103+-.017 .097+-.010
60 55
23 23
1 9
16 11
51 61
54 59
.118+-.004 .057+-.008
60 34
18 30
20 26
2 8
12 45
12 38
.153_.006 .048--_.001
some lodge
Brome grass Blair smooth
5/5 6/16
40 140
Vegetative Mature
50 32
24 33
23 24
3 10
19 50
15 50
.183-+.013 .073-+.003
Tall rescue Ky-31
5/5 6/16
25 120
Vegetative Mature
50 39
24 29
23 23
3 9
16 51
16 48
.131 +-.003 .058+- .004
Orchardgrass Potomac
4/15 6/16
15 120
Vegetative Mature, lodge
64 35
16 31
17 23
2 10
15 64
12 61
.128+- .014 .050+-.001
Orchardgrass Pennlate
5/19 6/16
70 140
Boot Mature, lodge
38 29
31 35
24 25
5 11
29 54
26 46
.077-+ .004 .046+-.004
Reed Canarygrass Common
4/28 6/16
25 140
Vegetative Mature
62 33
18 31
18 26
i 9
13 50
10 45
.183+-.006 .053_+.001
Barley Wong
4/15 7/8
8 75
Vegetative Mature
66 31
16 30
16 28
1 10
12 45
11 38
.169+ .006 .048- .003
Oats Gary
5/5 7/8
25 120
Vegetative Mature
67 41
16 26
14 25
1 8
9 58
8 51
.131 +-.008 .042 +-.003
Rye Balboa
4/7 7/8
30 180
Vegetative Mature
66 36
16 29
16 25
2 10
13 59
10 55
.160+- .002 .040+- .002
Wheat Common
5/19 6/16
68 110
Heading Mature
39 46
30 23
21 19
9 9
33 48
31 44
.078+-.001 .075+- .006
a In vitro cell wall indigestibilities. Rate constants for in vitro disappearances of digestible cell walls and their standard errors. JOURNAL OF DAIRY SCIENCE VOL. 55, NO. 8
1142
SMITH ET AL
generally decreased more relative to 48 hr indigestibilities for samples harvested at the later date. Immature oats had the lowest cell wall indigestibility (8%) and mature crown vetch had the highest cell wall indigestibility (73%). The April 21 sample of Ladino had the fastest rate of cell wail digestion (.3090 hr -1) which was comparable to .2705 hr "x previously reported for a fall sampled rye (8). Mature grains had the slowest rates of cell wall digestion (mean of .051 hr-X), The range in compositions, cell wall indigestibilities in vitro, and cell wall digestion rates were wider than for forages normally fed. Acquisition of a wide range was intended. Even though composition and observed rates of cell wall digestion varied widely, all of the 112 forages showed a high degree of linearity of the semilog plots of remaining digestible cell wall on time (r 2 -~ .978). This confirmed first-
order digestion kinetics for each of the forages. The mean r 2 was .991 for grasses and .965 for legumes. Correlations of cell wall digestion rates with chemical measurements are in Table 2 for individual forage species, all legumes, all grasses, and for all forages combined. The rates of cell wall digestion were highly correlated with soluble dry matter within species except for crown vetch, red clover, and wheat. The low correlations for crown vetch and wheat may have resulted from fewer samples which were confined to the latter part of the growth period. During this time seed formation contributes to the soluble dry matter content of the total plant, but seed sohibles are not associated with structural plant parts, thus might not be related to cell wall digestion rates in the same manner as soluble dry matter in the structural parts. In red clover the lower correlation of
TABLE 2. Correlations of the rates of cell wall digestion with chemieal measurements and in vitro cell wall indigestibility (72 hr). Hemicellulose
Forages
n
Soluble dry matter
Lignin
Lignin: cellulose
Acidlignin detergent Log fiber cellulose
Cell wall indigestibihty 72 hr
(% of D M ) - Legumes
Alfalfa Birdsfoot trefoil Ladino dover Red dover Crown vetch Vetch
6 6 8 8 6 3
.80 .98** .94*** ,55 .44 .93
--.74 --,90"* --.91"* --.25 -.48 .59
--.66 --.69 --.88** --.12 -.46 .72
9 6 6
.96*** .85* .90*
--.96*** --.83* --.81"
--.95*** --.81" --.83"
9
.82**
--.96***
4 7 10 9 II 4
.93 .94** .80*** .84** .77"* .04
37
.75***
.50 .42 .30 --.22 .20 -.54
--.69 --.71 --.88** .04 -.43 --.70
--.86 --.77* --.76* --.11 -.24 --.82
.86** .95** .83*
--.97*** --.84* --.88"
--.92*** --.69 --.80
".95***
.92***
--.95***
--.93***
--.99* --.91"* --.83"** --.90*** --.92"** .11
--.97" --.91"* --.83*** --.91"** --.93"** .24
.95 .94"* ,10 .29 .80"* --.17
--.96" --.93*** --.88"** --.93*** --,89"** --.25
--.96" --.89** --.92"** --.87** --.91"** --.25
--.61"**
--.44**
--.05
--.35*
--.58***
--.87***
--.79***
--,26**
--.38***
.27
.01
Grasses Bluegrass Brome Fescue Orchardgrass Potomac Orchardgrass Pennlate Reed canary Barley Oats Rye Wheat All legumes All grasses Legumes and grasses Hays
75
.76***
--.82***
--.83***
112
.72***
--.47***
--.18
42
.73***
--.16
.29
.64*** --.12 --.56***
* Indicates significance of correlations: * = P<.05; ** ~ P<.01; and *** ---- P<.001. JOURNAL OF DAIRY SCIENCE VOL. 55, NO.
1143
FOIRAGE C O M P O S I T I O N A N D FIBEIA D I G E S T I O N
soluble dry matter as a general predictor of cell wall digestion rates than other chemical measurements.
rate with soluble dry matter was the result of little change in the rate, even though soluble dry matter decreased with maturity. Rates of cell wall digestion were more highly correlated with soluble dry matter than with measurements of lignin in the individual legumes and the legume group. However, in the grasses, rates of cell wall digestion were more highly correlated with log lignin:cellulose or other lignin measurements than with soluble dry matter. Rates of cell wall digestion were also highly correlated with hemieellulose:acid-detergent fiber ratio in some of the grass species but not in any of the legumes. This correlation remained high and significant when analysis included all grasses. These positive correlations suggest that cell walls digested faster in some grasses containing proportionately greater amounts of hemicellulose. Relatively high negative correlations were revealed between rate of cell wall digestion and cell wall indigestibility for most of the individual species. This correlation remained high for the grass group (r = --.79) but was lower for the legume group (r = .58). Correlations in the next to the bottom line of Table 2 are the result of all samples. The higher coefficient for soluble dry matter over the others shows the greater applicability of
Three consecutive weekly samples were selected from each species to test whether solu-
ble dry matter would be as useful to predict digestion rates under more practical circumstances. Selections were based upon anatomical maturity criteria approximating hay harvesting maturities. Correlations using only these selected forages also substantiate the more general applicability of soluble dry matter for predicting forage cell wall digestion rates than other chemical measurements and are shown in the bottom line of Table 2. However, no direct causative relationship should be implied between solubles and rates of cell wall digestion in vitro (8). Relationships of cell wall digestion rates in vitro with soluble dry matter for the legume group, the grass group, and legumes and grasses together are in Figure 1 with respective regression equations. The slope of the regression line is steeper for legumes than for grasses or combined legumes and grasses. Therefore, separate prediction equations should estimate cell wall digestion rates of legumes and grasses with more precision. Cell wall digestion rates relative to log lignin:celhilose ratios are in Figure 2. This figure
O. 3 5
O. 5 0
+O.UOS2gX: 5, = 0 . 0 0 0 7 6 +0.00279X: 5, = 0 . 0 0 0 2 8 +0.00261X: 5, =0.0002u~
[]Y=-0.2(27 X T = -0. 032 y T : -0. 029
!
O. 25 []m
w 0.20
x x
x x
0.15 7~
x
[] []x[] [Ox [B
x×
4 d
o.o
°
•
[] LEGUME5 X GRASSES Y OVEMFILL BEGBE55ION
- 0 . 0 0
-
20
30
,
liO 50 SOLUBLE
,
60
7(2 OIIY MflTTEH (Z]
~0
9(2
FIG. 1. Relationship of cell wall digestion rates and soluble dry matter contents. JOURNAL OF DAIRY SCIENCE VOL. 55. No.
R
1144
S M I T H E T AL
O.tLS
13.t~0
Fr]T= D . 6 1 7 X T = 13. 313i yT=U.177
O. 35
- - D . 3 0 3 S i L { ~ G X - 5~ = 1 3 . 1 3 7 5 1 3 - O . 1 6 S L I O L O G K : 5,~ = 0 . 1 3 1 0 7 3
-O-oSLi61L{~GX, 5~ = 0 . 0 1 8 ~ 7
I
O
0.30 bJ
e: 0 . 2 5
o.2o
4
d 13.is -
[]
[]
13.113
13.05 El LEGUME5 X Gflf155E5 Y OVEBRLL flEGflE55[aN
-13. 130 13.00
X
i
13.so
i.1313
Xx
i
i.113
i.~13
i.313
1.413
LOG t L I G N I N / C E L L U L O S E
X
i
i.so
l
i.oo
i
i . To
t.so
i.~13
X 11313)
FIc. 2. Relationship o£ cell wall digestion rates and log lignin: cellulose ratios. displays the possible usefulness of log lignin: cellulose for predicting cell wall digestion rates in grasses. The overall regression line is entirely unsuitable for predicting cell wall digestion rates o£ combined legumes or grasses. Correlations between 72 hr cell wall indigestibilities and chemical measurements are in Table 3 for individual forages, legumes and grasses, all forages, and selected hay maturities of all forages. In vitro cell wall indigestibilities were negatively correlated with soluble dry matter, r = --.44 to --.97 for the 15 individual species. Significance is indicated. Except for vetch and wheat, cell wall indigestibilities were positively correlated with lignin in dry matter (r = .32 to .96), lignin:eellulose (r = .56 to .97), and log lignin:cellulose (r = .62 to .98). Negative correlations for vetch and wheat may have been the result of samples representing only the latter growth stages. In the legume group, both soluble dry matter (r = --.74) and lignin in dry matter (r .78) were more highly correlated with cell wall JOURNAL OF DAIRY SCIENCE VOL. ~5, N o .
8
indigestibility than lignin:eellulose (r = .57) or log Iignin:cellulose (r = .59). While in the grasses all measures involving lignin were similarly correlated (r = .88) and more highly correlated with cell wall indigestibility than soluble dry matter (r = --.75). Lignification measures remained highly correlated for all forages in a single analysis, but the correlation of soluble dry matter versus cell wall indigestibility decreased (r = --.11).. Thus, soluble dry matter is o£ little general value in predicting cell wall indigestibilities of forage since differences between legumes and grasses are not resolved. Correlations where all forages were included in Table 3 show that cell wall indigestibility was equally and highly correlated with lignin in dry matter, lignin:cellulose, or log lignin: cellulose. Van Soest and Moore (10) reported that differences between legumes and grasses were resolved by relating cell wall digestibility to lignin concentrations in lignocellulose. They indicated that correlations were improved when lignin concentrations were converted to
F O R A G E COMPOSITION" A N D F I B E R D I G E S T I O N
1145
T~mi~ 3. Correlations of 72-hr cell wall indigestibilities with chemical measurements. Forages
n
Soluble dry matter ~(%
Lignin
Liguin: cellulose
lignin Log - cellulose
of D M ) - -
Legumes Alfalfa Birdsfoot trefoil Ladino clover Red dover Crown vetch Vetch
6 6 8 8 6 3
--.71 --.80* --.80* --.44 -.90 -- .97
.71 .84" .90** .95*** .85* - .95
.72 .56 .92** .90** .71 -- .99
.74 .62 .92** .83** .70 --.98
9 6 6
--.84"* --.84* --.70
.96"** .91"* .78
.96"** .90** .83*
.96"** .85* .82
9
-- .83"*
.32
.80"*
.94"**
4 7 10 9 11 4
-.95 --.94** --.90"** --.82** --.74** --.73
.92 .96*** .94"** .93*** .96*** -- ,12
.87 .97*** .88"** .96*** .96*** .74
.90 .94*** .91"** .84** .98*** .70
37
--.74***
.78***
.57***
.59***
--.75***
.89***
.88***
.87***
--.11
.88***
.82"**
.85***
.84***
.73***
.75***
Grasses Bluegrass Brome Fescue Orchardgrass -Potomac Orchardgrass Pennlate Reed canary Barley Oats Rye Wheat All legumes All grasses Legumes and grasses Hays
75 112 42
.33*
* Indicates significance of correlations: * = P<.05; ** ~ P<.01; and *** = P<.001.
logarithms. A similar conclusion cannot be drawn from our data even though ]ignification of structural carbohydrates can be more rationally associated with cell wall digestibility than can lignin content of total dry matter. The resuits of Mowat et al. (6) are in even greater disagreement with the concept that differences in cell wall digestibility of legumes and grasses are resolved by expressing lignin concentration in cell walls or ]ignocellulose rather than in dry matter. They reported a much higher correlation for lignin in dry matter (r = --.73) as opposed to lignin in cell walls (r = --.37) with cell wall digestibility in vitro. The apparent discrepancy in our work and that of Mowat et al. (6) may be the result 6f in vitro versus in vivo data. Also in this study samples were intentionally drawn from a wider range of maturities in order to establish extremes in particular variables. When correlations between cell wall indigestibility and measures of lignin were performed with selected forage samples of hay
maturity, the correlation remained high for lignin in dry matter (r = .84) while those for lignin:cellulose (r -~ .73) and log lignin: cellulose (r -- .75) decreased. The relationships of 72 hr cell wall digestibility with lignin in dry matter and log lignin: cellulose are in Figures 3 and 4. The relatively less scatter around the slopes in Figure 4 may lead one to elect log ]ignin:eellulose as the more appropriate predictor of cell wall digestibility. However, the relatively lower standard deviation for the legume and grass slope in Figure 3 as compared to that in Figure 4 suggests that lignin in dry matter is a better predictor of cell wall indigestibility in vitro than log lignin:cellulose. The smaller error of the estimate (8.1 versus 8.9) also supports this conclusion. Cellulose and lignin contents of residual cell wails after 72 hr of incubation were determined for hay maturity selections. The ratios of initial lignin:initial cellulose and initial lignin:72 hr cellulose were compared (TaJOURNAL OF DAIRY SCIENCE VOL. 55, No.
8
1146
SMITH ET AL B5
IT]T = 16. 392
*3.293
XT=3.137
-ttt. 1B2
y T = u,.B71
ai. 177
X : 5~ = O. tlt~tL82 X : 51 = 0 . 2 4 6 @ 5 X : 51 = O . 2 1 B I 6
75 []
65
[]
~
[] X
55 QEI
[]
[]rn
X
S
. .95 (..)
X
[]
[]
[] []
X
~ []
{ X
in
[] [ ] ~
X
xx []
:z
[] El
X
25 El
X
x
)¢
[].
XA
5
o
;•X
LEGUME5 X GflR55E5 Y OVEflRLL REGRE55[ON
X
~
d
B'
1
LIGNIN IN O.M.
1 2'
1 4'
15'
{Z]
Fie. 3. Relationship cell wall indigestibilities and lignin in dry ma~er. ble 4). The mean ratios for grasses and legumes were more similar in the residues than they were initially. The similarity of the lignin:cellulose ratios in the residues indicate that the extent of digestion is limited by liguin and that the limitation operates similarly in both groups. When these ratios and relationships were considered on the basis of observed liguin recoveries, legumes and grasses appeared less similar. Mean values of recoveries of lignin in the 72 hr incubation residues are in Table 4. t~ecovery was more complete for legume samples. This may be a reflection of the greater amount of lignin generally present in the legumes. Up to 75% of the variation of cell wall indigestibility in vitro can be attributed to lignification and about 50% of the variation in cell wall rates of digestion can be attributed to soluble dry matter content in this study. Differences in fiber that are not reflected in these chemical methods appear to be partially involved with the rate that cell walls digest. The high correlations of soluble forage dry matter (2, 5) or insoluble forage dry matter (7, 9) with voluntary intake appear to be an JOURNAL OF DAIRY SCIENCE VOL.
55, INTO. 8
additive result of less ruminal indigestible fiber from forages lower in cell wall content, and that these cell wails also digest faster. TABLE 4. Initial versus residual ]ignin:cellulose (L:C) in hays.
Hay
na
Initial L
Initial L
Initial C
72 hour C
Mean
Mean CV Mean CV
(~) (~) (z)
(g) Legumes 5 .48 Grasses 10 .24 Legumes and grasses 15 .32
nign~ recovery
20 42
1.09 .94
18 21
82 68
14 9
29
.99
20
72
11
Number of species represented, each with 3 stages of maturity. b Coefficient of variation. Acknowledgments
The authors acknowledge Dr. G. E. Carlson, Forage and Range Research Branch, for making plots available for this study, Dr. B. T. Weinland, BiometricaI Service Staff, for conducting the statistical analyses, J. O'Connor for the laboratory evaluations, and Elaine Bierman for tabulating this data.
1147
FORAGE COMPOSITION AND FIBER DIGESTION
9S
85 []T=-7tt.021 +7t~.737 LOGX: 5, =10.1~,g37 XY=-I~g.6S3 ~60.866 LOGX ; 5e =0,.01625
7S
YT=-48.
170
5e = 3 . 5 6 2 0 5
" 5 9 . 3 6 0 LOB X :
B
6S
X
010~] Cg
_~ss xX o ] X x
SX d 35
El m
> f.Sx Xx- oo
25
x~ X
ls ×
0.6
x
0.90
x
~xx~
×~-
--
1.00
×
1.10
X
X
x X
x
~0:]
xxx
~m m
x
i.20
-
x
1.30
1.U,O 1.50
L~G[LIGNIN/CELLULOSE
1.60
~
_
1.70
1.BO
13VERFL i LPiEGflE55ION 1.go
X I00]
FIG. 4. Relationship of cell wall indigestibilities and log lignin:cellulose ratios.
References
( I ) Crampton, E. W., E. Donefer, and L. E. Lloyd. 1960. A nutritive value index for forages. J. Animal Sci., 21:628. (2) Donefer, E., E. W. Crampton, and L. E. Lloyd. 1966. The prediction of digestible energy intake potential (NVI) of forages using a simple in vitro technique. Proc. X Int. Grassland Congr., P. 442. (3) Gill, S. S., H. R. Conrad, and J. W. Hibbs. 1969. Relative rate of in vitro cellulose disappearance as a possible estimator of digestible dry matter intake. J. Dairy Sei., 52:1687. (4) Goering, H. K., and P. J. Van Soest. 1970. Forage fiber analyses (apparatus, reagents, procedures and some application). Agr. Handbook 379, USDA. (5) Johnson, R. R., B. A. Dehority, and J. L. Parsons. 1965. Relationships between in Vitro measurement on forages and their nutritive value. Proc. IX Int. Grassland Congr. Vol. 1:773. (6) Mowat, D. N., M. L. Kovain, and J. E. Winch. 1969. Lignification and in vitro
(7)
(8)
(9)
(10)
(11)
cell wall digestibility of plant parts. Canadian J. Plant Sei., 49:499. Osbourn, D. F., S. B. Cammell, R. A. Terry, and G. E. Outen. 1970. The effect of chemical composition and physical characteristics of forages on their voluntary intake by sheep. Annu. Rep., The Grassland Res. Institute. Hurley, England. Smith, L. W., H. K. Goering, D. R. Waldo, and C. H. Gordon. 1971. In vitro digestion rate of forage cell wall components. J. Dairy Sci., 54:71. Van Soest, P. J. 1965. Symposium on factors influencing the voluntary intake of herbage by ruminants: Voluntary intake in relation to chemical composition and digestibility. J. Animal Sci., 24:834. Van Soest, P. J., and L. A. Moore. 1965. New chemical methods for analysis of forages for the purpose of predicting nutritive value. Proe. Ninth Int. Grassl. Congr., 1:783. Van Soest, P. J., and L. H. P. Jones. 1968. Effect of silica in forages upon digestibility. J. Dairy Sei., 51:1644. JOURNAL OF DAIRY SCIENCE VOL. 55, NO.