Directory UMM :Data Elmu:jurnal:A:Animal Feed Science and Technology:Vol81.Issue1-2.Sep1999:

Animal Feed Science and Technology
81 (1999) 1±16

Morphological fractions of maize stover harvested
at different stages of grain maturity and nutritive
value of different fractions of the stover
Adugna Toleraa,b,*, Frik Sundstùlc
a

Department of Animal Production and Rangeland Management, Awassa College of Agriculture,
P.O. Box 5, Awassa, Ethiopia
b
Department of Animal Science, Agricultural University of Norway, P.O. Box 5025, N-1432 AÊs, Norway
c
Agricultural University of Norway, Noragric, P.O. Box 5001, N-1432 AÊs, Norway
Received 21 December 1998; received in revised form 16 April 1999; accepted 3 June 1999

Abstract
The proportion of different morphological fractions of maize stover was assessed at three stages
of grain maturity and nutritive value of the morphological fractions was evaluated based on
chemical composition, in sacco dry matter (DM) degradability and in vitro gas production. Stem

proportion of the stover increased by 20%, whereas the proportions of tassel and leaf blades
decreased by 41.5 and 44%, respectively, as grain moisture content dropped from about 30±10%.
The crude protein (CP) content was highest in leaf blade and tassel. Leaf blade had the lowest
neutral detergent fibre (NDF), acid detergent fibre (ADF) and cellulose contents and the highest
ash, ADF-ash and total proanthocyanidins (TPA) contents. On the other hand, CP, ash, ADF-ash and
TPA contents were lowest and the NDF content was highest in husk. Stem had the highest ADF,
lignin and cellulose contents.
The overall in sacco DM degradability tended to be higher in leaf blades and lower in leaf sheaths
than in the other morphological fractions. The washing loss was highest (p < 0.05) in stem and
lowest in leaf blade and husk. On the other hand, leaf blade had significantly higher (p < 0.05)
degradability of the water insoluble fraction than leaf sheath, stem and whole stover. The lag time
was highest (p < 0.05) in stem and lowest in leaf blades. The morphological fractions differed in the
volume of gas produced in the following order: husk > whole stover > stem > leaf sheath > leaf
blade > tassel. Stem and whole stover showed rapid gas production in the early stage of
fermentation, although the gas production rate of stem started to decline earlier than that of the
other morphological fractions. Gas production due to fermentation of insoluble feed components,
which mostly occurred between 6 and 24 h of incubation, was highest in husk. In vitro gas
*

Corresponding author. Tel.: +251-6-200-221; fax: +251-6-200-072

E-mail address: aca@telecom.net.et (A. Tolera)
0377-8401/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 7 - 8 4 0 1 ( 9 9 ) 0 0 0 7 2 - 3

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A. Tolera, F. Sundstùl / Animal Feed Science and Technology 81 (1999) 1±16

production and in sacco DM degradability could not rank the morphological fractions in a similar
order which could be due to the effect of protein fermentation on gas production. Gas production
showed an inverse relationship with CP, ash, ADF-ash and TPA contents of the morphological
fractions. # 1999 Elsevier Science B.V. All rights reserved.
Keywords: Maize stover; Morphological fractions; Chemical composition; DM degradability; Gas production

1. Introduction
Variation in nutritional quality of crop residues could be due to differences in the
proportion and quality of the botanical fractions. Schulthess et al. (1995) observed big
differences in nutritive value of wheat straw fractions. Leaf blades have the lowest fibre
content, followed by leaf sheaths and stems (Schulthess et al., 1995; Tan et al., 1995).
Leaf blades also have the highest organic matter digestibility because of their high fibre

digestibility. Higher degradability of leaves and chaffs compared with stem were reported
for most cereals (Kernan et al., 1984; Ramanzin et al., 1986; Shand et al., 1988). Capper
et al. (1986) found higher in vitro cellulase digestibility in the leaves than in the stems
indicating a higher potential feeding value of leaves than stems. The crude protein (CP)
content of the leaf blades is almost twice as high as that of leaf sheaths and stems. The
leaf blades also have a much higher content of calcium, iron and manganese than leaf
sheaths and stems (Schulthess et al., 1995). Harika et al. (1995) also asserted that the
quality of maize stover depends on the proportions of leaf and stem fractions of the
stover. They indicated that the leaf fraction has a higher palatability and digestibility than
the stem fraction, as well as a higher protein and mineral content.
However, previous studies showed that the stem fraction comprised over 50% of the
whole plant of wheat (Pearce et al., 1988; Ohlde et al., 1992; Tan et al., 1995) oat, rye and
triticale (Ohlde et al., 1992) straws. Osafo (1993) also reported that the stem fraction
comprised about 50±70% of sorghum stover with the contribution of leaf blades and leaf
sheaths to the whole stover being only 14.8±29.3% and 14.1±24.8%, respectively. This
makes the nutritive value of cereal straws dependent on the proportion of stem in the
whole plant. The proportion of stem in wheat straw is negatively correlated with 48 h
nylon bag dry matter (DM) degradability values while the leaf-to-stem ratio and the
proportions of leaf blade or leaf sheath in the whole plant are positively correlated with
the DM degradability values (Tan et al., 1995). Stage of maturity at the time of harvest is

one management factor that influences the proportion of morphological fractions and
nutritive value of crop residues. Although the nutritive value of different morphological
fractions of most fine stemmed cereal straws has been studied and documented (Thiago
Ê man and Nordkvist, 1983; Kernan et al., 1984; Ramanzin et al.,
and Kellaway, 1982; A
1986; Bhargava et al., 1988; Ohlde et al., 1992; Schulthess et al., 1995; Tan et al., 1995),
relatively little has been done in this respect on coarse stemmed straws such as maize
stover. Therefore, this study was designed to assess the proportion of different
morphological fractions of maize stover harvested at three different stages of grain
maturity and to evaluate the comparative nutritive value of the different morphological
fractions.

A. Tolera, F. Sundstùl / Animal Feed Science and Technology 81 (1999) 1±16

3

2. Materials and methods
2.1. Planting, harvesting and sample preparation
Maize (Zea mays L.) was grown on the Research and Farm Centre of the
Awassa College of Agriculture in southern Ethiopia (78040 N and 388310 E; altitude

1650 m) during 1995 and 1996 cropping seasons. A maize variety known as
Beletech, released by Bako Agricultural Research Centre, Ethiopia, was used in the
experiment. Fertiliser was applied at sowing time [100 kg/ha of Diammonium phosphate
(DAP)] and when the plants reached knee height (50 kg urea/ha). Initial weed
clearing was accomplished with a cultivator followed by hand weeding. The maize
field was divided into three plots of 2 ha each which were harvested at different grain
maturity stages depending upon grain moisture content. Accordingly the first, second and
third stages were harvested at 30.2, 22.5 and 12.3% moisture content in the grain,
respectively, for the 1995 harvest. The 1996 plantings were harvested at 28, 20.2 and
9.8% moisture content in the grain which were designated as Stages I, II and III,
respectively.
For each maturity stage, 10 and 8 representative quadrats of 2  2 m2 were selected for
the 1995 and 1996 cropping seasons, respectively, and the total above ground biomass
was harvested. At harvest, the plants were separated into grain and crop residues. The
grain harvested during the first and second stages was air dried in the sun until the
moisture content dropped below 13% for safe storage. The stovers harvested during the
first and second stages were sun dried for about 3±4 days to avoid moulding. The effect of
stage of maturity, at the time of harvest, on yield and quality of maize grain and stover
was reported in another study (Tolera et al., 1998). The crop residues were further
separated into stem, leaf blade, leaf sheath, tassel and husk. Samples of the different

morphological fractions (10 samples in Year 1 and 8 samples in Year 2 for each fraction
at each stage) were oven dried at 608C for 72 h immediately after harvest to determine the
relative DM yield of the different fractions. After drying, the different morphological
fractions were bulked by stage of maturity and sub-samples were taken. The sub-samples
were divided into two portions and ground through 1 mm sieve (for chemical analyses
and in vitro gas production) and 2 mm sieve (for in sacco DM degradability
determination).

2.2. Chemical analyses
Dry matter was determined by oven drying the samples at 1058C overnight and ash was
determined by igniting the samples in a muffle furnace at 6008C for 6 h. Nitrogen (N)
content was determined by the micro-Kjeldahl method and CP was calculated as
N  6.25. Neutral detergent fibre (NDF), acid detergent fibre (ADF) and acid detergent
lignin (ADL) were determined according to Van Soest and Robertson (1985).
Hemicelluloses and cellulose were calculated as NDF ÿ ADF and ADFÿ(ADL + ADFash), respectively. The total proanthocyanidins (TPA) contents were determined using the
method of Porter et al. (1986).

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A. Tolera, F. Sundstùl / Animal Feed Science and Technology 81 (1999) 1±16

2.3. In sacco DM degradation
Dry matter degradation was determined by incubating about of 2.5 g of the dry samples
in nylon bags in three rumen fistulated sheep. The sheep were feeding on 500 g of
Desmodium intortum hay, 400 g maize stover, 400 g green alfalfa forage and 100 g of
concentrate mix (50% wheat bran and 50% linseed cake) which were offered in equal
proportions twice a day. The bags were incubated 1 h after the sheep were offered feed
and were withdrawn after 4, 8, 16, 24, 48, 72 and 96 h of incubation, washed for 20 min
in a washing machine and dried for 48 h at 608C. Washing losses were determined by
soaking two bags per sample in warm tap water (398C) for 1 h followed by washing and
drying as before. The DM degradation data was fitted to the exponential equation
p ˆ a ‡ b …1ÿeet †(érskov and McDonald, 1979) where p is DM degradation (%) at time
t. Since washing losses (A) were higher than the estimated rapidly soluble fraction (a), the
lag time was estimated according to McDonald (1981) by fitting the model
p ˆ A for tt0 ; p ˆ a ‡ b …1ÿeet † for t > t0 and the degradation characteristics of the
crop residues were defined as A is equal to washing loss (readily soluble fraction);
B = (a + b)ÿA, representing the insoluble but fermentable material; c = the rate of
degradation of B and the lag phase …L† ˆ …1=c† loge ‰b=…a ‡ bÿA†Š. The effective DM
degradability (ED) was calculated according to Dhanoa (1988) using the formula
ED ˆ A ‡ ‰Bc=…c ˆ k†Š where A, B and c are as described above and k is rumen outflow

rate assumed to be 0.03/h (érskov et al., 1988).
2.4. In vitro gas production
About 200  5 mg dry weight of the samples were weighed in duplicates into
calibrated glass syringes of 100 ml and incubated with rumen fluid following the
procedures of Menke and Steingass (1988) as described by Khazaal and érskov (1994).
Gas volume readings were recorded before incubation (0 h) and 3, 6, 12, 24, 48, 72 and
96 h after incubation. The results (means of three runs) were fitted to the exponential
equation of the form p ˆ a ‡ b …1ÿeet † (érskov and McDonald, 1979; BluÈmmel and
érskov, 1993) where p represents gas production at time t, (a+b) the potential gas
production, c the rate of gas production and a, b and c are constants in the exponential
equation. The a value is the intercept of the gas production curve whereas the b value
represents the fermentation of the insoluble but potentially fermentable fraction of the
feed. The short chain fatty acids (SCFA) content and pH of the supernatant were
determined after 96 h of incubation. The SCFA content was determined by gas liquid
chromatography (GLC) at the Nutrition Lab of the International Livestock Research
Institute, Addis Ababa.
2.5. Statistical analysis
Analysis of variance was carried out using the General Linear Models procedure of the
statistical analysis system (SAS). The model for analysis of the proportion of
morphological fractions of the stover included the effects of year and stage of maturity.

The chemical composition, in sacco DM degradability and in vitro gas production data

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A. Tolera, F. Sundstùl / Animal Feed Science and Technology 81 (1999) 1±16

Table 1
Proportion (%) of morphological fractions (on DM basis) of maize stover harvested at different stages of grain
maturity (average of 2 years, n = 18)a
Morphological fraction

Leaf blade
Leaf sheath
Stem
Husk
Tassel
a

Grain maturity stages

SE

Stage I

Stage II

Stage III

16.6 a
15.4
44.5 b
21.8
1.76 a

14.8 a
14.8
45.4 b
23.3
1.66 a

9.3 b
14.7
53.3 a
21.7
1.03 b

0.8
0.6
1.5
1.4
0.2

Means with the same or no alphabets within a row are not significantly different (p > 0.05).

were analysed in a factorial analysis of variance and the model included the effects of
year, stage of maturity, morphological fractions and their interactions. The statistical
significance of the differences between means were tested using the Student±Newman±
Keuls (SNK) test (SAS, 1985).

3. Results and discussion
3.1. Proportion of morphological fractions
Table 1 shows the proportion of morphological fractions of maize stover harvested at
three stages of grain maturity. The leaf blade and tassel fractions showed a decreasing
trend, whereas the stem fraction showed an increasing trend with increasing stage of
maturity. However, there was no significant difference in the proportion of morphological
fractions of maize stover harvested at Stages I and II. Maize stover harvested at Stage III
(at grain moisture content of about 10±12%) had significantly lower proportion of leaf
blade and tassel and higher proportion of stem (p < 0.05). In general, the proportion of
stem increased by 20%, whereas, the proportions of leaf sheaths, tassel and leaf blades
decreased by 4.5, 41.5 and 44%, respectively, as the grain moisture content at harvesting
decreased from about 30±10%. Russell (1986) reported an increase in stem proportion
and a linear decrease in the proportion of leaves with increased stage of maturity of maize
from 3 weeks pre- to 5 weeks post-physiological maturity. Shattering due to over drying
and brittleness could explain the decline in the proportion of leaf blade and tassel as the
stage of maturity increased. Similarly, Harika and Sharma (1994) reported that the
number of leaves per plant and the leaf: stem ratio decreased with the delay in harvesting
from physiological maturity to the dead ripe stage which could be attributed to the loss of
leaves due to drying and detachment from the stem. Moreover, more leaf material than
stem is lost in the process of harvesting and handling the stover. Therefore, the stover
offered to animals may contain a larger proportion of stem than indicated here by
Ê man (1984) indicated that botanical composition of cereal
fractionation. Theander and A
straws depends on several factors, such as length of stubble, stage of ripeness and the way
the cereal is grown.

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A. Tolera, F. Sundstùl / Animal Feed Science and Technology 81 (1999) 1±16

The stem fraction comprised the highest proportion (about 45±53% depending upon
stage of maturity) of the total stover followed by husk (22±23%). Thus, stem is the most
important fraction influencing the nutritive value of the stover. The proportion of leaf
blade and leaf sheath was similar at early stage of maturity (Stages I and II). However, the
proportion of leaf blade significantly decreased in Stage III. Tassel made the lowest
contribution (1±1.8%) to the total stover. Previous studies (Thiago and Kellaway, 1982;
Pearce et al., 1988; Tan et al., 1995) on wheat straw also showed that the stem comprises
a much higher proportion of the straw than the leaves. Tan et al. (1995) reported that stem
constituted over 50% of the total wheat straw, whereas leaf blades and leaf sheaths
constituted about 25% each. According to Pearce et al. (1988) stem, leaf sheath and leaf
blade comprised 58, 24 and 18% of the wheat straw, respectively. Bhargava et al. (1988)
also reported that leaf blade, leaf sheath, stem and chaff of barley straw comprised 12.8,
31.4, 50.0 and 5.8%, respectively. Ohlde et al. (1992), studying eight different cereal
straw species, found the following ranges in proportion of morphological fractions: stem
(nodes + internodes) 53.5% (spring barley) ± 72.7% (rye), leaf sheaths 16.5% (rye) ±
33.1% (spring barley) and leaf blades 10.8% (rye) ± 18.1% (oat).

3.2. Chemical composition
Chemical composition of the different morphological fractions of maize stover is
shown in Table 2. The DM content at harvesting time was significantly lower (p < 0.05)
in stem than in the other morphological fractions. However, there was no difference in the
DM content of the morphological fractions after sun drying. The ash content was highest
in leaf blade and lowest in husk (p < 0.05). Leaf sheath, whole stover, tassel and stem did
not significantly differ (p > 0.05) from one another in their ash contents.
In general, the different morphological fractions of maize stover differed in their
chemical composition. The highest CP and lowest fibre (NDF, ADF and cellulose)
content in leaf blades compared to other morphological fractions is consistent with
previous studies on other cereal straws (Bhargava et al., 1988; Goto et al., 1992;
Schulthess et al., 1995). Tan et al. (1995) also reported higher CP and total ash and lower
NDF and ADF contents in leaf blades than in leaf sheaths and stems of wheat straw. The
results of our study revealed that the CP, ash, ADF-ash and TPA contents were lowest and
the NDF content was highest in husk. The stem fraction had the highest ADF, lignin and
cellulose contents. Verbic et al., (1995) also found a higher NDF content in husk than in
leaves and stems and a higher ADF and lignin contents in stems than in leaves and husk
of maize stover. Similarly, Goto et al. (1992) reported that the stem of barley straw
contained more lignin and cellulose than the other morphological fractions. The higher
contents of ash and ADF-ash in leaf blades than in the other fractions could be an
Ê man and Nordkvist, 1983; Kernan et al., 1984;
indicative of higher silica content (A
Ê
Capper et al., 1986). Aman and Nordkvist (1983) reported higher CP, ash and silica
contents and a lower cellulose content in the leaves than in the stems of wheat and barley
straws. Capper et al. (1986) also reported significantly lower NDF, ADF and lignin
contents and significantly higher CP and silica contents in the leaves than in the stems of
three varieties of barley straws from Syria.

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A. Tolera, F. Sundstùl / Animal Feed Science and Technology 81 (1999) 1±16
Table 2
Chemical composition of the morphological fractions at three maturity stagesa
Chemical
components

Maturity
stage

Leaf
blade

Leaf
sheath

Stem

Husk

Tassel

Whole
stover

DM at harvest (g/kg)

I
II
III
Mean
SE

804
895
935
878 a
25

718
882
928
842 a
48

436
506
760
567 b
91

761
816
926
834 a
35

801
890
923
871 a
23

629
717
918
754 a
78

Ash (g/kg DM)

I
II
III
Mean
SE

205
209
168
194 a
9

97
104
100
100 b
4

69
96
74
80 b
10

41
43
46
43 c
3

76
87
112
92 b
13

105
93
85
94 b
6

CP (g/kg DM)

I
II
III
Mean
SE

63
58
48
56 a
5

42
39
38
39 b
3

38
35
31
34 bc
2

33
30
28
30 c
2

62
60
52
58 a
3

48
44
37
43 b
4

NDF (g/kg DM)

I
II
III
Mean
SE

639
656
694
663 d
11

805
794
800
800 b
7

744
777
800
774 bc
17

872
882
887
880 a
6

755
774
796
775 bc
10

746
757
769
757 c
7

ADF (g/kg DM)

I
II
III
Mean
SE

319
322
362
334 c
9

372
382
387
380 b
5

414
465
475
451 a
19

379
370
383
377 b
3

372
380
396
383 b
5

371
393
408
390 b
7

Lignin (g/kg DM)

I
II
III
Mean
SE

41
43
44
42 d
2

40
41
48
43 d
3

51
65
64
60 a
3

29
30
29
29 e
1

51
54
59
55 b
3

46
49
52
49 c
2

ADF-ash (g/kg DM)

I
II
III
Mean
SE

114
135
103
118 a
8

44
53
54
50 b
4

11
15
14
13 c
2

17
17
20
18 c
2

44
50
47
47 b
3

50
38
36
41 b
5

Cellulose (g/kg DM)

I
II
III
Mean
SE

164
144
215
174 d
15

288
288
284
287 c
4

352
385
397
378 a
15

332
323
335
330 b
4

277
276
290
281 c
6

275
307
320
301 c
11

Hemicellulose (g/kg DM)

I
II
III

319
336
332

433
412
413

330
312
325

493
512
503

383
394
400

375
364
362

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A. Tolera, F. Sundstùl / Animal Feed Science and Technology 81 (1999) 1±16

Table 2 (Continued )
Chemical
components

TPAb (A550

a
b

nm/g

DM)

Maturity
stage

Leaf
blade

Leaf
sheath

Stem

Husk

Tassel

Whole
stover

Mean
SE

329 e
4

419 b
9

322 e
4

503 a
8

392 c
7

367 d
7

I
II
III
Mean
SE

16
14
14
15 a
1

11
11
12
12 b
1

10
9
8
9c
1

5
7
6
6d
0.6

8
8
9
8c
0.6

12
12
11
12 b
1

Means with the same alphabets within a row are not significantly different (p > 0.05).
TPA: total proanthocyanidins.

The DM content at the time of harvest showed an increase with increasing stage of
maturity. Overall, there was a decrease in CP content and an increasing trend in NDF,
ADF, lignin and cellulose contents with increasing stage of maturity, but morphological
fractions did not show a clear pattern in ash, ADF-ash, hemicellulose and TPA contents
with changes in stage of maturity. However, there was a decrease in ash and ADF-ash
content of whole stover with increasing stage of maturity which could be attributed to
decreased proportion of leaf blades in the whole stover with advanced stage of maturity
(Table 1). The decrease in CP and increase in DM, NDF, ADF, lignin and cellulose
contents with increasing stage of maturity are consistent with the results of previous
studies (Russell, 1986; Harika and Sharma, 1994; Tolera et al., 1998).
3.3. In sacco DM degradability
The in sacco DM disappearance (%) of the different morphological fractions of maize
stover is shown in Fig. 1. The DM disappearance was higher in leaf blade and relatively
lower in leaf sheath than the other morphological fractions after 24, 48, 72 and 96 h of
incubation. In general, leaf blade had higher DM disappearance values than leaf sheath,
husk and stem at all rumen incubation times (4±96 h). The higher DM disappearance of
leaf blades is consistent with previous findings (e.g., Bhargava et al., 1988), although the
ranking order of the morphological fractions was different from our results. According to
Bhargava et al. (1988) DM degradability of barley straws decreased in the order leaf
blades > leaf sheath > whole plant > chaff > stems. Tan et al. (1995) reported that the
48 h DM disappearance of wheat straw was highest in leaf blades and lowest in stems
with intermediate values in leaf sheaths and whole straw. But in the present study leaf
sheath had the lowest degradability, which indicates that the coarse and fine stemmed
straws could be different in the nature of their cell wall structure. According to Doyle and
Oosting (1994) the differences in digestibility between leaves and stems in stovers are not
as great as in straws of wheat and barley crops.
The washing loss (A) was highest (p < 0.05) in stem followed by whole stover and leaf
sheath in a decreasing order and the lowest values were in leaf blade and husk (Table 3).
This is consistent with the findings of Verbic et al. (1995). The 0 h washing loss in stem
was higher than its DM disappearance after 4 h of incubation (Fig. 1) which could be due

A. Tolera, F. Sundstùl / Animal Feed Science and Technology 81 (1999) 1±16

9

Fig. 1. In sacco DM disappearance and in vitro gas production from different morphological fractions of maize
stover.

to microbial contamination. The higher lag time in this fraction also explains the same.
The lag time was highest (p < 0.05) in stem, followed by leaf sheath whereas no lag phase
was observed in the leaf blade. The lag time in the degradation of fibrous feeds is caused
by the time taken for adherence of cellulolytic organisms to the substrate (érskov, 1991)
and long lag time is one of the factors limiting intake and utilization of straws and stovers

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A. Tolera, F. Sundstùl / Animal Feed Science and Technology 81 (1999) 1±16

Table 3
In sacco DM degradability characteristics of the morphological fractions of maize stover at three maturity stages
(average of 2 years)a
Degradability
Characteristics

Stage of
maturity

Leaf
blade

Leaf
sheath

Stem

Husk

Whole
stover

A (washing loss), %

I
II
III
Mean
SE

10.3
6.8
6.7
7.9 d
0.6

16.3
11.9
9.5
12.6 c
1.0

17.5
15.3
15.8
16.2 a
0.3

11.2
5.9
8.2
8.4 d
0.8

16.2
15.7
13.6
15.2 b
0.3

B (insoluble
but slowly degradable), %

I
II
III
Mean
SE

72.1
69.6
66.9
69.5 a
1.8

58.1
53.0
55.3
55.5 b
2.9

61.0
59.6
54.3
58.3 b
3.3

69.6
61.7
62.0
64.4 ab
2.8

61.0
58.2
56.5
58.5 b
2.0

A + B (potential degradability), %

I
II
III
Mean
SE

82.4
76.3
73.7
77.5 a
2.1

74.4
64.9
64.8
68.0 b
3.2

78.5
74.9
70.0
74.5 ab
3.4

80.8
67.6
70.2
72.8 ab
3.2

77.2
73.9
70.1
73.7 ab
2.0

c (degradation rate), /h

I
II
III
Mean
SE

0.031
0.029
0.027
0.029 a
0.001

0.030
0.028
0.027
0.028 a
0.001

0.029
0.026
0.025
0.027 a
0.002

0.031
0.028
0.026
0.028 a
0.002

0.030
0.026
0.025
0.027 a
0.002

L (lag time), h

I
II
III
Mean
SE

0.0
0.0
0.0
0.0 d
0.0

4.1
6.0
3.2
4.5 b
0.9

5.0
5.7
7.3
6.0 a
0.7

3.0
2.2
3.1
2.8 c
0.4

1.3
1.7
1.7
1.6 c
0.3

ED (effective degradability), %

I
II
III
Mean
SE

47.0
41.1
38.6
42.2 a
2.0

45.7
37.2
35.9
39.6 a
2.2

47.0
41.2
40.2
42.8 a
1.8

46.3
35.5
36.7
39.5 a
2.2

46.0
42.3
38.7
42.3 a
1.1

a

Means with the same alphabets within a row are not significantly different (p > 0.05).

(Van Soest, 1988). The longer lag time in stem could be a reflection of its higher
lignocellulose content than the other morphological fractions. The washing loss of the
morphological fractions, with the exception of whole stover, was significantly higher
(p < 0.05) in maize stover harvested at Stage I than at Stages II and III with a general
decreasing trend with increasing stage of maturity whereas the lag time showed an
increasing trend with increasing stage of maturity. However, the washing loss in husk was
relatively higher at Stage III than at Stage II and the lag time in leaf sheath was relatively
shorter at Stage III than at Stages I and II.
Leaf blade had significantly higher (p < 0.05) degradability of the water insoluble but
potentially degradable fraction (B) than leaf sheath, stem and whole stover. The extent of

A. Tolera, F. Sundstùl / Animal Feed Science and Technology 81 (1999) 1±16

11

digestion of the insoluble components depends on its lignin content and on the nature of
the lignin (érskov, 1991). The potential degradability (A + B) was significantly higher
(p < 0.05) in leaf blade than in leaf sheath with intermediate values in the other
morphological fractions. Osafo (1993) reported a similar pattern of higher potential
degradability in leaf blades than in other morphological fractions of sorghum stover.
Bhargava et al. (1988) also showed that the leaf blades of barley straw had the
highest potential degradability and rate of degradation compared to the other
morphological fractions. The lowest potential degradability was observed in leaf sheath
followed by husk (p > 0.05). The rate of degradation (c) and the effective degradability
(ED) of the morphological components did not differ significantly (p > 0.05). Similarly,
Verbic et al. (1995) found no significant differences in ED among morphological
fractions of maize stover, although they found a higher rate of degradation in leaves
than in stems and husks. The slowly degradable fraction, potential degradability,
degradation rate and effective degradability values showed a decreasing trend with
increasing stage of maturity. However, the potential and effective degradability in husk
and the slowly degradable fraction in leaf sheath were relatively higher at Stage III than at
Stage II.
The lower DM degradability of leaf sheaths than leaf blades could reflect the higher
NDF, cellulose and hemicellulose contents of leaf sheaths than leaf blades. However, the
lower DM degradability of leaf sheaths compared to other morphological fractions could
not be explained by chemical composition. Other factors like variations in physical
structure such as distribution of lignified cells within the tissues of the different fractions
(Ramanzin et al., 1986) and the manner in which cell wall constituents are chemically
and physically linked (Doyle, 1994) might be responsible for the lowered degradability in
leaf sheaths. Visual assessment indicates that stem internodes in maize stover are filled
with soft pith cells and this condition might have contributed to the relatively higher
degradability of stem than leaf sheath. In general, stovers of maize, sorghum and millet
differ from other cereal straws in having a filled pithy stem that serves as a reserve for
soluble carbohydrates (Van Soest, 1994). The digestibility of thin-walled cell types
(mesophyll, phloem, inner cell walls of the epidermis, stem pith parenchyma) is high
compared to that of the thick-walled cells of the parenchyma bundle sheath,
sclerenchyma, the outer cuticular part of the epidermis and lignified vascular tissue
(Wilson, 1991). Thus, determination of the physical structure of the cell-walls and the
proportion of different tissues in cross-sectional area of the different morphological
fractions might be necessary to explain the comparative degradability of these
morphological fractions.
In describing the nutritive value of different morphological fractions of cereal straws,
the leaf blades and leaf sheaths are sometimes lumped together (Harika and Sharma,
1994; Verbic et al., 1995) as leaves. This may lead to a generalization that they have
similar nutritive values. However, the leaf sheaths in this study had consistently lower
degradability than the leaf blades. The chemical analyses of the two fractions (Table 2)
also showed higher CP, ash, ADF-ash, and TPA and lower NDF, hemicellulose and
cellulose contents in leaf blades than in leaf sheaths. This indicates the necessity of
considering the two fractions separately in assessing the nutritive value of morphological
fractions of cereal straws and stovers.

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A. Tolera, F. Sundstùl / Animal Feed Science and Technology 81 (1999) 1±16

Table 4
In vitro gas production characteristics of the morphological fractions of maize stover at three maturity stages
(average of 2 years)a
Gas production
characteristics

Stage

Leaf
blade

Leaf
sheath

Stem

Husk

Tassel

Whole
stover

a

I
II
III
Mean
SE

ÿ3.7
ÿ4.5
ÿ6.1
ÿ4.8 c
0.4

ÿ4.5
ÿ5.0
ÿ6.1
ÿ5.2 c
0.5

1.6
ÿ3.9
ÿ2.2
ÿ1.5 a
1.2

ÿ9.8
ÿ7.0
ÿ9.1
ÿ8.6 d
0.5

ÿ2.5
ÿ2.1
ÿ2.6
ÿ2.4 b
0.4

ÿ2.1
ÿ2.8
ÿ4.3
ÿ3.0 b
0.8

b

I
II
III
Mean
SE

50.7
48.4
46.4
48.5 c
2.4

58.3
51.1
53.0
54.1 bc
2.1

54.1
53.5
47.0
51.5 bc
2.4

79.2
66.7
72.4
72.8 a
3.4

52.2
47.5
46.8
48.8 c
2.2

61.7
58.8
54.9
58.5 b
1.7

a+b

I
II
III
Mean
SE

47.0
43.9
40.3
43.7 c
2.2

53.8
46.1
46.9
48.9 bc
2.1

55.7
49.5
44.9
50.0 bc
2.3

69.4
59.7
63.3
64.1 a
3.2

49.7
45.3
44.2
46.4 c
2.3

59.7
56.0
50.6
55.4 b
1.4

c

I
II
III
Mean
SE

a

0.041
0.037
0.037
0.038 b
0.001

0.040
0.036
0.038
0.038 b
0.002

0.043
0.040
0.040
0.041 a
b0.001

0.046
0.042
0.046
0.045 a
0.001

0.029
0.025
0.026
0.027 c
0.002

0.041
0.041
0.040
0.040 a b
0.001

Means with the same alphabets within a row are not significantly different (p > 0.05).

3.4. In vitro gas production
The morphological fractions differed in the volume of gas produced in the following
decreasing order: husk > whole stover > stem > leaf sheath > leaf blade > tassel (Fig. 1).
Stem and whole stover showed rapid gas production in the early stage of fermentation
which indicates a higher content of rapidly fermentable soluble components in these
fractions. However, the gas production rate of stem started to decline earlier than that of
the other morphological fractions. Gas production due to fermentation of insoluble but
fermentable feed components was highest in husk and was more rapid between 6 and 24 h
of incubation. The insoluble feed components need to be hydrated and colonized by
rumen micro-organisms before they can be fermented (Van Milgen et al., 1993; Groot et
al., 1996). Thus, before digestion of the insoluble feed components proceeds, the
microbial population has to multiply and colonize the substrate which results in increased
rate of gas production in the early stages of incubation. The a value (the intercept of the
gas production curve) was lowest in husk and highest in stem (Table 4). The negative
values of all the intercepts could be due to a lag phase in the fermentation of the insoluble
feed components and may indicate a deviation from exponential course of fermentation
(BluÈmmel and Becker, 1997). On the other hand, the b value (the fermentation of the
insoluble but potentially fermentable fraction) and the potential gas production (a + b)

13

A. Tolera, F. Sundstùl / Animal Feed Science and Technology 81 (1999) 1±16

were higher (p < 0.05) in husk than in the other morphological fractions. The rate of gas
production was highest in husk and lowest in tassel (p < 0.05). The intercept and slope of
the gas production curve, the potential gas production and the gas production rate showed
a decrease with increasing stage of maturity of the stover. However, the gas production
profile of husk showed higher b, a + b and c values at Stage III than at Stage II.
The in vitro gas production could not rank the morphological fractions in the same
order as the in sacco DM degradability. Cone and van Gelder (1999) indicated that the in
vitro gas production may not necessarily reflect the degradability measured with the
nylon bag technique. In the present study, a negative relationship was observed between
gas production and CP content of the morphological fractions. Gas production was lowest
in tassel and leaf blades, morphological fractions with the highest CP content, and it was
highest in husk, a fraction with the lowest CP content. Abreu and Bruno-Soares (1998)
reported similar negative correlation between gas production and CP content of nine
legume grains. Cone and van Gelder (1999) also showed that the fermentation of protein
causes less gas production than carbohydrate fermentation. The ash, ADF-ash and TPA
contents of the morphological fractions, in the present study, showed a general trend of
inverse relationship with gas production. BluÈmmel and Becker (1997) reported that more
gas was produced from 200 mg NDF than from 200 mg whole roughage and this was
attributed to proportionally less microbial biomass yield and higher SCFA production
from NDF. BluÈmmel et al. (1997) showed that substrates with proportionally high gas
volumes (i.e., higher SCFA production) had comparatively low microbial biomass yields.
They also indicated that in vitro gas tests need to be complemented by a quantification of
substrate concomitantly truly degraded to avoid selection of a substrate with
proportionally higher SCFA production and lower microbial biomass yield. They
proposed an in vitro method which combines gas volume and substrate degradability
measurements to estimate microbial yield.
Table 5 shows the pH and SCFA content of the supernatant after 96 h of incubation.
The pH varied within a narrow range of 6.79±6.96. The highest pH was in the supernatant
from leaf blade and tassel, whereas the lowest was in husk. The supernatant from stem
had lower pH than leaf blade and tassel and higher than that of husk. The concentration of
total SCFAs was higher (p < 0.05) in the supernatant from husk than all the other
morphological fractions which corresponds with higher gas production from husk than
Table 5
Short chain fatty acids content and pH of the supernatant after incubation of 30 ml rumen liquor/buffer and
200 mg DM of different morphological fractions of maize stover for 96 h for in vitro gas productiona
Parameter

Leaf
blade

Leaf
sheath

Stem

Husk

Tassel

Whole
stover

pH
Total SCFAs (mmol/l)
Molar proportion ( %)
Acetic acid
Propionic acid
Butyric acid

6.96 a
40.8 b

6.93 ab
43.2 b

6.91 b
42.7 b

6.79 c
55.8 a

6.96 a
33.4 b

6.93 ab
37.4 b

0.01
3.13

62.7
19.4 c
11.0

63.1
20.8 bc
10.4

59.4
24.3 a
10.8

61.3
22.3 abc
10.6

61.5
19.9 c
11.8

60.7
23.3 ab
10.5

1.46
0.85
0.87

a

Means with similar or no alphabets within a row are not significantly different (p > 0.05).

SE

14

A. Tolera, F. Sundstùl / Animal Feed Science and Technology 81 (1999) 1±16

the other fractions. On the other hand, the molar proportion of propionic acid was lowest
for leaf blade and tassel and it was highest for stem. However, the supernatants from the
different morphological fractions did not differ in the molar proportions of acetic and
butyric acids. The relatively higher proportion of propionic acid in stem could be due to a
higher content of rapidly fermentable soluble substrates in this fraction. BluÈmmel and
érskov (1993) indicated that incubation of a rapidly fermentable substrate would
probably yield a higher proportion of propionate and could lead to a lower gas volume per
unit of SCFA generated.

4. Conclusions
The proportion of the morphological fractions was influenced by the stage of maturity
at the time of harvest. Delayed harvesting at Stage III resulted in reduced proportion of
leaf blades and tassels with concomitant increase in stem proportion. The morphological
fractions differed in chemical composition and in vitro gas production and to some extent
in DM degradability, although the difference in the latter was relatively low. In sacco DM
degradability and in vitro gas production could not rank the morphological fractions in a
similar order which could be due to the difference in CP content of the morphological
fractions and the effect of protein fermentation on gas production. Gas production showed
an inverse relationship with CP, ash, ADF-ash and TPA contents of the morphological
fractions.

Acknowledgements
Technical assistance of Alemayehu Kidane, Tadesse Bokore and Fiseha Gebre is highly
appreciated. We would like to thank the Norwegian Universities Committee for
Development Research and Education (NUFU) and Centre for International Environment
and Development Studies (Noragric) for financial support. We thank the International
Livestock Research Institute (ILRI) Nutrition Lab, Addis Ababa, for ADF and SCFA
determination.
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