Morphological fractions of maize stover harvested
at different stages of grain maturity and nutritive
value of different fractions of the stover
Adugna Tolera
a,b,*, Frik Sundstùl
caDepartment of Animal Production and Rangeland Management, Awassa College of Agriculture,
P.O. Box 5, Awassa, Ethiopia
bDepartment of Animal Science, Agricultural University of Norway, P.O. Box 5025, N-1432 AÊ s, Norway cAgricultural University of Norway, Noragric, P.O. Box 5001, N-1432 AÊ s, 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
81 (1999) 1±16
*Corresponding author. Tel.: +251-6-200-221; fax: +251-6-200-072
E-mail address: aca@telecom.net.et (A. Tolera)
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 and Kellaway, 1982; AÊ man and Nordkvist, 1983; Kernan et al., 1984; Ramanzin et al., 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.
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 (78040N and 388310E; 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 22 m2were 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 N6.25. 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 NDFÿADF and ADFÿ(ADL +
ADF-ash), respectively. The total proanthocyanidins (TPA) contents were determined using the method of Porter et al. (1986).
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 intortumhay, 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
pab 1ÿeet(érskov and McDonald, 1979) wherepis 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
pAfortt0; pab 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 1=clogeb= abÿA. The effective DM degradability (ED) was calculated according to Dhanoa (1988) using the formula EDA Bc= ckwhereA,Bandcare as described above andkis rumen outflow rate assumed to be 0.03/h (érskov et al., 1988).
2.4. In vitro gas production
About 2005 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 pab 1ÿeet (érskov and McDonald, 1979; BluÈmmel and érskov, 1993) where p represents gas production at time t, (a+b) the potential gas production,cthe rate of gas production and a,b andcare constants in the exponential equation. The avalue is the intercept of the gas production curve whereas the bvalue 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
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 fractionation. Theander and AÊ man (1984) indicated that botanical composition of cereal straws depends on several factors, such as length of stubble, stage of ripeness and the way the cereal is grown.
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 Grain maturity stages SE Stage I Stage II Stage III
Leaf blade 16.6 a 14.8 a 9.3 b 0.8
Leaf sheath 15.4 14.8 14.7 0.6
Stem 44.5 b 45.4 b 53.3 a 1.5
Husk 21.8 23.3 21.7 1.4
Tassel 1.76 a 1.66 a 1.03 b 0.2
aMeans with the same or no alphabets within a row are not significantly different (p> 0.05).
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 indicative of higher silica content (AÊ man and Nordkvist, 1983; Kernan et al., 1984; Capper et al., 1986). AÊ man 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.
Table 2
Chemical composition of the morphological fractions at three maturity stagesa
Chemical DM at harvest (g/kg) I 804 718 436 761 801 629
II 895 882 506 816 890 717 III 935 928 760 926 923 918 Mean 878 a 842 a 567 b 834 a 871 a 754 a III 694 800 800 887 796 769 Mean 663 d 800 b 774 bc 880 a 775 bc 757 c
SE 11 7 17 6 10 7 ADF (g/kg DM) I 319 372 414 379 372 371
II 322 382 465 370 380 393 III 362 387 475 383 396 408 Mean 334 c 380 b 451 a 377 b 383 b 390 b Cellulose (g/kg DM) I 164 288 352 332 277 275
II 144 288 385 323 276 307 III 215 284 397 335 290 320 Mean 174 d 287 c 378 a 330 b 281 c 301 c
SE 15 4 15 4 6 11 Hemicellulose (g/kg DM) I 319 433 330 493 383 375
II 336 412 312 512 394 364 III 332 413 325 503 400 362
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
Table 2 (Continued)
aMeans with the same alphabets within a row are not significantly different (p> 0.05). bTPA: total proanthocyanidins.
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
Fig. 1. In sacco DM disappearance and in vitro gas production from different morphological fractions of maize stover.
(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
Table 3
In sacco DM degradability characteristics of the morphological fractions of maize stover at three maturity stages (average of 2 years)a
B(insoluble I 72.1 58.1 61.0 69.6 61.0 but slowly degradable), % II 69.6 53.0 59.6 61.7 58.2 III 66.9 55.3 54.3 62.0 56.5 Mean 69.5 a 55.5 b 58.3 b 64.4 ab 58.5 b
SE 1.8 2.9 3.3 2.8 2.0
A+B(potential degradability), % I 82.4 74.4 78.5 80.8 77.2 II 76.3 64.9 74.9 67.6 73.9 III 73.7 64.8 70.0 70.2 70.1 Mean 77.5 a 68.0 b 74.5 ab 72.8 ab 73.7 ab
SE 2.1 3.2 3.4 3.2 2.0
c(degradation rate), /h I 0.031 0.030 0.029 0.031 0.030 II 0.029 0.028 0.026 0.028 0.026 III 0.027 0.027 0.025 0.026 0.025 Mean 0.029 a 0.028 a 0.027 a 0.028 a 0.027 a ED (effective degradability), % I 47.0 45.7 47.0 46.3 46.0 II 41.1 37.2 41.2 35.5 42.3 III 38.6 35.9 40.2 36.7 38.7 Mean 42.2 a 39.6 a 42.8 a 39.5 a 42.3 a
SE 2.0 2.2 1.8 2.2 1.1
aMeans with the same alphabets within a row are not significantly different (p> 0.05).
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.
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. Theavalue (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)
Table 4
In vitro gas production characteristics of the morphological fractions of maize stover at three maturity stages (average of 2 years)a III 46.4 53.0 47.0 72.4 46.8 54.9 Mean 48.5 c 54.1 bc 51.5 bc 72.8 a 48.8 c 58.5 b
SE 2.4 2.1 2.4 3.4 2.2 1.7
a+b I 47.0 53.8 55.7 69.4 49.7 59.7
II 43.9 46.1 49.5 59.7 45.3 56.0 III 40.3 46.9 44.9 63.3 44.2 50.6 Mean 43.7 c 48.9 bc 50.0 bc 64.1 a 46.4 c 55.4 b
SE 2.2 2.1 2.3 3.2 2.3 1.4
c I 0.041 0.040 0.043 0.046 0.029 0.041 II 0.037 0.036 0.040 0.042 0.025 0.041 III 0.037 0.038 0.040 0.046 0.026 0.040 Mean 0.038 b 0.038 b 0.041 a 0.045 a 0.027 c 0.040 a b
SE 0.001 0.002 b0.001 0.001 0.002 0.001
aMeans with the same alphabets within a row are not significantly different (p> 0.05).
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 higherb,a+bandcvalues 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
Acetic acid 62.7 63.1 59.4 61.3 61.5 60.7 1.46 Propionic acid 19.4 c 20.8 bc 24.3 a 22.3 abc 19.9 c 23.3 ab 0.85 Butyric acid 11.0 10.4 10.8 10.6 11.8 10.5 0.87
aMeans with similar or no alphabets within a row are not significantly different (p> 0.05).
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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