Forage nutritive values of vetch species and their accessions grown under nitosol and vertisol conditions in the central highlands of Ethiopia

Gezahagn Kebede, Getnet Assefa*, Alemayehu Mengistu** and Fekede Feyissa

Ethiopian Institute of Agricultural Research, Holetta Research Center, P.O.Box 2003, Addis Ababa, Ethiopia
gezk2007@yahoo.co.uk
* Ethiopian Institute of Agricultural Research, P.O. Box 2003 Addis Ababa, Ethiopia
** Urael Branch PO Box 62291, Addis Ababa, Ethiopia

Abstract

Twenty accessions of five different vetch species were evaluated for their nutritional differences at Holetta and Ginchi in the central highlands of Ethiopia during 2009 cropping season. The experiment was conducted in randomized complete block design with three replications. At sowing, DAP fertilizer at a rate of 100 kg ha^-1 was applied for all treatments. Data on nutritive values were collected and analyzed using the general linear model procedures of SAS and Duncan multiple range tests was used for mean comparison. Most measured nutritional parameters showed significant difference among the tested vetch species and their accessions at both locations. The two locations also displayed significant differences for most measured parameters for vetch species and their accessions.  

The nutritive values of vetch species and their accessions were comparatively better at Holetta than Ginchi for most analyzed parameters. Intermediate maturing and creeping growth habit vetch species had relatively better ash%, CP% and CP yield than early maturing and erect growth habit vetch species at both locations. The result also showed that higher NDF, ADF, ADL, cellulose and hemicellulose contents were recorded for early maturing and erect growing type of vetch species than intermediate maturing and creeping type of vetch species. Likewise, the species which had creeping growth habit had better IVDMD than vetch species which had erect growth habit. The result generally indicated that creeping vetch species (V. dasycarpa) had comparatively higher ash (10.4 and 9.5%), CP (25.8 and 26.0%), CP yield (1.4 and 1.8 t ha^-1) and IVDMD (73.4 and 73.2%) contents at Holetta and Ginchi respectively, but lower fiber and cell wall constituents than other vetch species. Correlation analysis was also done for major nutritional traits to select the accessions for desired traits. The result revealed that the IVDMD had a significant positive correlation with ash and CP contents but negatively correlated with NDF, cellulose and hemicelluloses contents. Generally, vetch species and their accessions had a great variation in most measured nutritional parameters. Variation in genetic (days to maturity, growth habit, morphological fractions), and environment (soil types and climatic conditions) are some of the important causes of nutritional difference among the tested vetch species and their accessions in the central highlands of Ethiopia.

Keywords: fiber chemical composition, in-vitro dry matter digestibility, non-fiber chemical composition, relationships between traits

Introduction

Poor nutrition is one of the major constraints to livestock productivity in sub-Saharan Africa (Osuji et al 1993) and it results in low rates of production, often defined by growth rates and reproduction (Getu et al 2012). This is because animals thrive predominantly on high fiber feeds which are deficient in essential nutrients for microbial fermentation. Higher concentration of cell wall constituents were related to reduced intake (Sarwar et al 2002) and low digestibility in ruminants. Concentration of cell wall is generally regarded as the most important factor affecting forage quality. The variation in morphological characteristics such as leaf, stem, pod and flower fractions of forage accounts for parts of the difference in feed quality. Riday et al (2002) argued that the genetic variation of fiber content is one of the main reasons of forage quality variation. This variation in morphological characteristic is important in the selection of forage crops, which are agronomically suitable and used for various purposes such as for hay, silage, and grazing (Getnet and Ledin 2001).  

Among many annual forage legumes, adaptation of vetch is better and promising than the others in the central highlands of Ethiopia. Vetch is an annual forage legume widely adapted to the highlands of Ethiopia. It grows well on the reddish brown clay soils and the black soils of the highland areas. It has been grown successfully in areas of acid soil with pH of 5.5-6. It is reported that vetches are rich in protein, minerals, and have lower fiber content. With the highest level of crude protein (CP), vetch could be used as supplement to roughages for dairy cows. Forages which are moderate to high in CP reduce the need for supplemental purchased protein. As forage plants mature, the CP level in the plant declines, however, total CP per unit area may increase as forage quantity increases with maturity (Osuji et al 1993). Once mature, declines in nutrient composition and leaching are especially serious in the case of herbaceous plants (Alemayehu 2006). Legumes in general and vetch in particular are excellent sources of N for livestock feed and the importance of forage legumes in livestock and crop production is well recognized.    

Herbage yield in combination with other characteristics like maturity, proportions of morphological fractions and nutritive value of the herbage yield are useful considerations in selecting the best variety for forage production (Arelovich et al 1995). Research results indicate that variation in lucerne forage digestibility and forage intake is correlated with the presence of significant genetic variation (Katic et al 2008). Species of vetch have different characteristics in terms of growth habit, days to maturity, morphological fractions, and climatic adaptation. In general, growth habit of vetch species can be broadly grouped as erect, creeping or climbing. For instance, Vicia dasycarpa, Vicia villosa and Vicia atropurpurea have creeping or climbing growth habit, whereas Vicia narbonensis and Vicia sativa have erect growth habit. These differences in genetic characteristics are the basis for variation in nutritive values and also determine the production, utilization and the various management practices. This shows that the different vetch species and their accessions need to be assessed for the nutritional quality differences under the different soil types and climatic conditions. Therefore, this experiment was designed to assess the nutritive values of different vetch species and their accessions grown under nitosol and vertisol conditions in the central highlands of Ethiopia.

Materials and methods

Description of the study sites 

The experiment was conducted at Holetta Agricultural Research Center (HARC) and Ginchi sub center in the central part of Ethiopia. HARC is located at 9°00'N latitude, 38°30'E longitude at an altitude of 2400 m above sea level. It is 34 km west of Addis Ababa on the road to Ambo and is characterized with the long term average annual rainfall of 1055.0 mm, average relative humidity of 60.6%, and average maximum and minimum air temperature of 22.2°C and 6.1°C respectively. The rainfall is bimodal and about 70% of the precipitation falls in the period from June to September, while the remaining 30% falls in the period from March to May. The soil type of the area is predominantly red nitosol, which is characterized by an average organic matter content of 1.8%, total nitrogen 0.17%, pH 5.24, and available phosphorus 4.55 ppm (Gemechu 2007). Ginchi sub center is located at 75 km west of Addis Ababa in the same road to Ambo. It is situated at 9°02'N latitude and 38°12'E longitude with an elevation of 2200 m above sea level, and characterized with the long term average annual rainfall of 1095.0 mm, average relative humidity of 58.2%, and average maximum and minimum air temperature of 24.6°C and 8.4°C respectively. The site has a bimodal rainfall pattern, with the main rain from June to September and short rain from March to May. The soil of the area is predominantly black clay vertisol with organic matter content of 1.3%, total nitrogen 0.13%, pH 6.5 and available phosphorus 16.5 ppm (Getachew et al 2007).   

Figure 1. Map of the experimental sites, at Holetta (Welmera) and Ginchi (Dendi) in the central highlands of Ethiopia.

Experimental treatments and design 

The study was executed using twenty accessions from five different vetch species (Table1). All accessions of Vicia narbonensis, Vicia villosa, and Vicia sativa were introduced from International Center for Agricultural Research in the Dry Areas (ICARDA); Vicia dasycarpa and Vicia atropurpurea accessions were initially introduced from Australia. The experiment was conducted on a Randomized Complete Block Design (RCBD) with three replications. Seeds were sown in rows with 30 cm spacing on a plot size of 2.4 m x 4 m = 9.6 m². The treatments were sown according to their recommended seeding rates: 25 kg ha^-1 for Vicia villosa, Vicia dasycarpa and Vicia atropurpurea; 30 kg ha^-1 for Vicia sativa and 75 kg ha^-1 for Vicia narbonensis. At sowing, 100 kg ha^-1 diammonium phosphate (DAP) fertilizer was uniformly applied for all treatments at both locations. At Ginchi site, sowing was done on camber-beds to improve drainage and reduce water-logging problems of vertisol.  

Chemical analysis and in-vitro dry matter digestibility 

Forage sample was taken from each plot and oven dried for 72 hours at a temperature of 65 ^oC. The oven dried samples were ground to pass through a 1 mm sieve size for laboratory analysis to determine their nutritive values. Before scanning, the samples were dried at 60 ^oC overnight in an oven to standardize the moisture and then 3 g of each sample was scanned by the Near Infra Red Spectroscopy (NIRS) with an 8 nm step. This is one of the recent techniques that uses a source of producing light of known wavelength pattern (usually 800- 2500 nm) and that enables to obtain a complete picture of the organic composition of the analyzed substances (Van Kempen 2001). The technique is noted to be one of the more multifaceted robust applications to estimate chemical entity and parameters like digestibility of organic matter in the dry matter which is usually estimated by bioassays (Fekadu et al 2010). It is now recognized as a valuable tool in the accurate determination of the chemical composition, digestibility parameters and gas production parameters of a wide range of forages (Givens et al 1997). The samples were analyzed in % DM basis for ash, crude protein (CP), neutral detergent fiber (NDF), acid detergent fiber (ADF), acid detergent lignin (ADL), and in-vitro dry matter digestibility (IVDMD) using a calibrated NIRS (Foss 5000 apparatus and Win ISI II software). Hemicellulose and cellulose contents were estimated from subtracting ADF from NDF and ADL from ADF respectively. The CP yield was calculated by multiplying CP with total dry biomass yield and then divided by 100.

Statistical analysis

Analysis of variance (ANOVA) procedures of SAS general linear model (GLM) was used to compare treatment means (SAS 2002). Bartlett’s test for homogeneity of variance was carried out to determine the validity of individual experiment. Square root transformation method was used for data which couldn’t exhibit homogeneity of variance for nutritional parameters according to Gomez and Gomez (1984). Accordingly, data based on CP, NDF, ADL and hemicellulose contents were square root transformed for statistical analysis and untransformed means were presented according to Gomez and Gomez (1984). Duncan Multiple Range Test (DMRT) at 5% significance was used for comparison of means. The Pearson correlation analysis procedure of the SAS statistical package was also applied to measure the strength of linear dependence between two measured variables. The data were analyzed using the following model: Y_ijk = µ + T_i + L_j + (TL)_ij + B_k(j) + e _ijk Where, Y_ijk = measured response of treatment i in block k of location j, µ = grand mean, T_i = effect of treatment i, L_j = effect of location j, TL= treatment and location interaction, B_{k (j)} = effect of block k in location j, and e _ijk = random error effect of treatment i in block k of location j.

Results and Discussion

Effect of location and interaction effects   

The two locations displayed differences for a number of nutritional characters for different vetch species and their accessions (Table 2). Generally, nutritive values of vetch species and their accessions were comparatively better at Holetta than Ginchi for most measured parameters. Species by location interaction and accessions by location interaction effects also revealed differences for most analyzed nutritional characters. When significant, the interaction effects were mostly a “cross-over” type; i.e. interactions were associated with rank order changes among the species and accessions of vetch. This indicated that the two locations were distinctly different for most of the nutritional parameters and that better vetch species and their accessions at one location may not also be better performing at another. According to Gemechu (2012), when genotypes perform consistently across locations, on the other hand, breeders are able to effectively evaluate germplasm with a minimum cost in a few locations for ultimate use of the resulting varieties across wider geographic areas. However, with high genotype by location interaction effects, genotypes selected for superior performance under one set of environmental conditions may perform poorly under different environmental conditions (Ceccarelli 1997). Therefore, it could be implicated that selection of better performing genotypes at one location may not enable the identification of genotypes that can repeat nearly the same performances at another location.

Photo 1. Accessions of different vetch species

Forage chemical compositions and in-vitro dry matter digestibility

Non-fiber chemical compositions  

The ash content of vetch species in this study showed difference at both locations, ranged from 7.7 to 10.4% with a mean of 8.8% and from 6.7 to 9.5% with a mean of 8.3% at Holetta and Ginchi respectively (Table 3). The average ash content was the highest in Vicia dasycarpa, and the lowest in Vicia sativa at both locations. The highest ash content in Vicia dasycarpa could be an indication of better mineral concentration. Intermediate to late maturing vetch species had relatively higher ash content than early maturing species, which could be due to differences in proportions and composition of morphological fractions. Fekede (2004) also reported that late maturing or low grain producing oats varieties had comparatively higher ash content in their whole forage DM than early maturing or high grain producing oats varieties. The ash content was also different among the accessions of different vetch species at both locations (Table 4). The result revealed that Lana (V. dasycarpa) had comparatively higher, whereas accession 61212 (V. sativa) lower in ash content at both locations. As the plant mature, mineral content declines due to a natural dilution process and the translocation of nutrients to the root system. In most circumstances P, K, Mg, Na, Cl, Cu, Co, Fe, Se, Zn and Mo declines as the plant matures (Ford et al 1979). According to Jennings (2004), herbaceous forage legumes have higher content of some minerals like calcium, sulfur and possibly phosphorus than grasses, and well nodulated legumes contain large amount of calcium, magnesium and other essential elements. Concentration of minerals in forage varies due to factors like plant developmental stage, morphological fractions, climatic conditions, soil characteristics and fertilization regime (Jukenvicius and Sabiene 2007). McDonald et al (2002) also reported that mineral concentration declines with age and is also influenced by soil type, soil nutrient levels and seasonal conditions. 

Protein is the limiting nutrient for grazing animal productivity, a deficiency being manifested in poor overall production by the animal, such as low live weight gain, poor reproduction rate and low forage hay intake owing to the inability to provide enough nitrogen for the microbes in the rumen to break down cellulose. The crude protein (CP) content also showed difference among vetch species at both locations (Table 3). The CP content of the species ranged from 18.9 to 25.8% with a mean of 22.4% and from 18.9 to 26.0% with a mean of 22.5% at Holetta and Ginchi respectively. Vicia dasycarpa had higher CP content followed by Vicia atropurpurea, Vicia narbonensis, Vicia villosa and Vicia sativa at both locations. The results in this study indicated that intermediate maturing species such as Vicia dasycarpa and Vicia atropurpurea had comparatively higher CP content whereas early and late maturing species such as Vicia sativa, Vicia narbonensis and Vicia villosa had comparatively lower CP content at both locations. It has also been observed that CP content was different among the accessions at both locations (Table 4). Lana and Namoi had higher CP content, whereas all Vicia sativa accessions had lower CP content at both locations. Getnet and Ledin (2001) reported that vetch has a higher CP content compared to many other tropical herbaceous legumes. They found that the CP content of vetch was 18.9%, which is similar to good alfalfa forage and with this level of CP vetch could be used as a supplement to roughages for dairy cows. The CP content of all forage legumes is highly varied with genetic factor, environmental factor and the interaction of both and the dilution of CP is increased with increasing plant age. Other study has shown that lucerne quality depends on phenological stage and decreases with plant age (Rotili et al 2001). Most of the herbaceous legumes have CP content of >15%, a level which is usually required to support lactation and growth, which suggests the adequacy of herbaceous legumes to supplement basal diets of predominately low quality pasture and crop residues (Norton 1982).  

In evaluating the nutritive values of forage legumes, CP content should not be used as the only parameter to be considered. It is the CP yield, which describes the overall and actual productivity of quality forage. The CP yield of vetch species differed at both locations and varied from 0.3 to 1.4 t ha^-1 with a mean of 1.0 t ha^-1 and from 0.5 to 1.8 t ha^-1 with a mean of 1.4 t ha^-1 at Holetta and Ginchi respectively (Table 3). Vicia dasycarpa had the highest CP yield at both testing sites, while the lowest was recorded from Vicia narbonensis. The result generally indicated that Vicia dasycarpa had comparatively higher in both CP content and CP yield at both locations. The CP yield had also different among the accessions, this generally related to the biomass yield at both locations (Table 4). Getnet and Ledin (2001) reported that vetch was the highest in nutritional parameters analyzed but, lower in dry matter (DM) forage yield per hectare. Generally, legumes have higher feeding values due to their higher protein content and this experiment also confirms that vetch has a higher CP content at forage harvesting stage. It confer several advantages in the context of animal nutrition; their higher protein content relative to that of grasses has long been recognized, and provide quality feed for livestock due to higher CP content (Getnet and Ledin 2001).

Fiber chemical compositions  

It is very important to note that total neutral detergent fiber (NDF) concentration of forage is a dominant factor in determining forage quality. The NDF content of vetch species differed at both locations, which varied from 36.5 to 55.2% with a mean of 48.5% and from 39.5 to 54.3% with a mean of 43.8% at Holetta and Ginchi respectively (Table 5). Vicia sativa had higher NDF content than Vicia dasycarpa and Vicia atropurpurea at Holetta, whereas Vicia narbonensis had the highest NDF content of all the other vetch species at Ginchi. There were variations in the NDF content among all the tested accessions at both locations (Table 6). In general, early maturing and erect growing type of vetch species had comparatively higher NDF content than intermediate to late maturing and creeping type of vetch species. The NDF contents above the critical value of 60% results in decreased voluntary feed intake, feed conversion efficiency and longer rumination time (Shirley 1986; Hoffman et al 2001). However, the NDF content of all the tested vetch species was found below this threshold level which indicates higher digestibility. As stems mature, protein content decreases and carbohydrate content increases and at maturity, stems make up as much as 80% of the total DM and NDF, which generally estimates the percentage of total fiber increase due to increase in xylem tissue (Jung and Engles 2002). However, high amount of protein is associated with NDF, increasing the ruminal and total tract digestibility (Mustafa et al 2000). As the plants mature, there is an increase in the proportion of fiber in the herbage, which has a strong influence on digestibility (McDonald et al 2002). The stems have higher NDF concentrations than leaves, which is due to higher concentration of fiber and lignin (Karachi 1997). Increasing dietary NDF concentration most often has a negative impact on the amount of DM consumed by lactating dairy cows, which generally translates into reduced milk production (Allen 2000). However, legume fibers ferment more rapidly in the rumen that is why ruminants can consume larger amounts of legumes than grasses (Hinders 1995).  

The fiber content of a feed is particularly important for determining quality within the parameter of digestibility. Acid detergent fiber (ADF) is the percentage of highly indigestible and slowly digestible material in a feed or forage. This fraction includes cellulose, lignin and pectin. Lower ADF indicates more digestible forage and is more desirable. The result indicated that the ADF contents of vetch species differed at both locations (Table 5). The ADF content ranged from 22.7 to 38.1% with a mean of 33.2% and from 24.7 to 32.6% with a mean of 28.5% at Holetta and Ginchi respectively. The result revealed that Vicia narbonensis and Vicia villosa had the highest ADF content at Holetta and Ginchi respectively. On the other extreme, Vicia atropurpurea and Vicia narbonensis had the lowest ADF content at Holetta and Ginchi respectively. The ADF contents also showed variations among the tested accessions at both locations (Table 6). The nutrient composition of forage crops is variable depending on many factors such as genotypic characteristics, environmental conditions and harvesting stages of the plants (Pascual et al 2000; Rotili et al 2001). There was a significant increase in NDF, ADF and ADL in plants with increased maturity (Kallenbach et al 2002). Generally, grasses contain higher concentrations of NDF and ADF than do legumes. The higher fiber concentration is found in both the leaf and stem fractions of grasses compared to legumes. 

Lignin is a component which attributes erectivety, strength and resistance to plant tissue thereby limiting the ability of rumen microorganisms to digest the cell wall polysaccharides, cellulose and hemicellulose (Reed et al 1988). The acid detergent lignin (ADL) contents of vetch species significantly differed at both locations (Table 5). The ADL content ranged from 8.5 to 13.7% with a mean of 11.0% and from 6.4 to 9.1% with a mean of 8.2% at Holetta and Ginchi respectively. The result indicated that Vicia narbonensis (13.7%) and Vicia dasycarpa (9.1%) gave the highest ADL content at Holetta and Ginchi respectively, whereas Vicia atropurpurea and Vicia sativa gave the lowest ADL content at Holetta and Ginchi respectively. The ADL contents also showed variations among the tested accessions at both locations (Table 6). The ADL contents were high in accessions 2384 (V. narbonensis) and 2565 (V. villosa) at Holetta and Ginchi respectively. Digestibility decreased with advancing age and this decline resulted from the interaction of factors such as increased fiber concentration in plant tissue, increased lignifications during plant development and decreased leaf to stem ratio (Wilson et al 1991). Generally, the presence of insoluble fiber, particularly lignin, lowers the overall digestibility of the feed by limiting nutrient availability (Mustafa et al 2000). 

The cellulose content of vetch species varied at both locations, which varied from 14.2 to 24.7% with a mean of 12.2% and from 17.4 to 23.5% with a mean of 20.3% at Holetta and Ginchi respectively (Table 7). Vicia narbonensis and Vicia villosa had comparatively higher cellulose content at Holetta and Ginchi respectively, whereas Vicia atropurpurea had lower cellulose content at both locations. When compared at accession level, the higher cellulose contents were recorded for accession 2384 (V. narbonensis) and 2565 (V. villosa) at Holetta and Ginchi respectively (Table 8). The content of cellulose for different feeds is influenced by harvesting stage (Adane 2003) and morphological fractions (Fekede 2004). The presence of cellulose limits the digestion of intact cell walls. While cellulose is composed of simple linear chains of glucose, the individual chains are very tightly packed into large fiber bundles which results in slower cellulose digestion by rumen microbes than digestion rates observed for hemicellulose or pectin (Weimer 1996). However, all cell wall polysaccharides are completely degradable if non-lignified. Lignification of cell walls dramatically reduces the extent of cellulose and hemicellulose digestion but has less impact on pectin digestion, particularly of legumes (Jung and Engels 2002). The lesser impact of lignin on pectin digestion in legumes is because of tissues which contain large amounts of pectin in legumes never lignify, whereas the pectin-rich primary walls of most grass tissues do incorporate at least some lignin (Jung and Engels 2002).  

The hemicellulose content varied from 10.2 to 19.8% with a mean of 15.3% and from 7.4 to 30.5% with a mean of 15.3% at Holetta and Ginchi respectively (Table 7). The result showed that Vicia sativa and Vicia narbonensis had the highest hemicellulose content at Holetta and Ginchi respectively, whereas Vicia dasycarpa had the lowest hemicellulose content at both locations. The highest hemicellulose content was recorded for accession 2387 (V. narbonensis) at both locations, while the lowest for Namoi (V. dasycarpa) at Holetta and accession 61212 (V. sativa) at Ginchi (Table 8). The composition and content of cell walls are the key factors affecting herbage digestibility. Cell walls are predominately composed of cellulose, hemicellulose, and lignin. This experiment confirms that, among the cell wall constituents, cellulose is the dominant followed by hemicellulose and lignin at both locations and findings reported by Diriba et al (2003) and Fekede (2004) in different feeds also support this result. The higher hemicellulose content in the feed limits forage intake and digestibility and its content in the feed vary among morphological fractions (Fekede 2004) and increased with advancing age (Adane 2003).

In-vitro dry matter digestibility  

Legumes can make a major contribution to improvement in the diet of quality and productivity of large ruminants in the tropics and subtropics, because of desirable nutritional attributes of tropical legumes (Coates 1995). The result revealed that the in-vitro dry matter digestibility (IVDMD) ranged from 60.47 to 73.39% with a mean of 66.47% and from 60.33 to 73.22% with a mean of 66.31% at Holetta and Ginchi respectively (Table 7). At both locations, IVDMD of Vicia dasycarpa was the highest, while Vicia sativa was the lowest. The IVDMD values greater than 65% indicates good feeding value (Mugeriw et al 1973) and values below this threshold level result in reduced intake due to lowered digestibility. The IVDMD values observed in this study were above this threshold level for all vetch species except Vicia sativa at both locations, which result in higher voluntary intake and digestibility of vetch species and this result also supported by Getnet and Ledin (2001). On the other hand, accessions of vetch also showed difference at both locations (Table 8). The highest and lowest IVDMD was recorded for Lana (V. dasycarpa) and accession 61212 (V. sativa) at both locations respectively. It was generally observed that early maturing vetch species had lower IVDMD compared to intermediate to late maturing vetch accessions. This could be due to the presence of higher fiber and cell wall constituents, and lower CP content in the early maturing vetch accessions than the intermediate to late maturing accessions.

IVDMD of any forage crop varied with harvesting stage (Adane 2003); fiber and cell wall constituents (Mustafa et al 2000); proportions of morphological fractions (Fekede 2004); soil, plant species and climate (McDowell 2003). Most forage legumes contain highly soluble protein which is easily fermented in the rumen. Supplementation with legumes results in an increased digestion and feed intake simulated largely by the provision of additional rumen degradable nitrogen. Low cell wall content is an indicator of high potential digestibility, immature growth has lower cell wall contents than mature growth and legume leaf is generally more digestible than stem (Minson 1990). On the other hand, changes in leaf to stem ratio are less marked in legumes during progress to maturity, and nutritive value declines at a slower rate than grasses over a similar period. As forage mature, there is a point at which the accumulation of digestible DM declines despite increasing forage DM yields. Minson (1990) reported that forage legumes have a greater DM digestibility than grasses. As grasses and legume forage matures, the nitrogen content drops and digestibility of fibrous feeds decreases. The poor digestibility of fibrous feeds is reflected in very low intakes by livestock.

Relationships between nutritional traits 

The linear correlation coefficients among nutritional parameters are shown in Table 9. The ash content showed a significant (P<0.001) positive correlation with CP content (r= 0.86) and IVDMD (r= 0.91). But, it was weakly and positively correlated (P>0.05) with CP yield (r= 0.11), NDF content (r= 0.07), ADL content (r= 0.23), and hemicellulose content (r= 0.09). According to Diriba et al (2003), ash was positively correlated with CP, NDF and ADF, but poorly and negatively associated with lignin, cellulose and hemicellulose contents. The IVDMD is positively correlated to the CP content and inversely related to the fiber content (NDF and ADF) and cell walls constituents (ADL, cellulose and hemicellulose) for most vetch species. The CP content showed a significant (P<0.001) positive correlation with IVDMD (r= 0.96), but non-significant positive correlation with CP yield (r= 0.13), and ADL content (r= 0.18). It was not significantly and inversely correlated with NDF content (r= -0.11), ADF content (r= -0.12), cellulose content (r= -0.25), and hemicellulose content (r= -0.05). Significant but negative correlations were found between IVDMD and cell wall components, and IVDMD and CP were significantly and positively correlated (Tessema et al 2002). Tessema et al (2002) also reported that CP showed high positive correlations with IVDMD, whereas NDF, ADF, ADL and cellulose showed negative correlations with IVDMD in Napier grass harvested at different heights.  

The NDF content was significantly (P<0.001) and positively correlated with hemicellulose (r= 0.90), but had very weak and non significant negative correlation with ADF (r= -0.03), ADL (r= -0.01), cellulose (r= -0.04) and IVDMD (r= -0.09) contents. Paterson et al (1994) also reported that NDF content is negatively correlated with voluntary intake of forage DM. The ADF content showed a significant positive correlation with ADL content (r= 0.69; P<0.01), cellulose content (r= 0.91; P<0.001) and IVDMD (r= 0.08), but significantly (P<0.05) and negatively correlated with hemicellulose content (r= -0.47). Hassan and Osman (1984) also reported that ADF showed positive correlations with ADL, cellulose and negative correlations with cell wall components and hemicellulose. Both cellulose and hemicellulose contents had a non-significant negative correlation coefficients of r= -0.05 and r= -0.11 with IVDMD, respectively. Cellulose content also inversely related with hemicellulose content (r= -0.43). Fekede (2004) also reported that oats varieties had negative but non-significant correlation between cellulose and hemicellulose content. 

Conclusion

• Twenty accessions of different vetch species were evaluated for their nutritional differences in a randomized complete block design with three replications at Holetta and Ginchi in the central highlands of Ethiopia. The forage nutritive values for vetch species and their accessions varied across testing sites at forage harvesting stage. Intermediate maturing and erect growth habit vetch species had comparatively better ash, CP, CP yield and IVDMD contents, but lower fiber and cell wall constituents than early maturing and erect growth habit vetch species. Generally, Vicia dasycarpa had the highest ash content, CP content, CP yield, and IVDMD than the remaining vetch species at both testing sites. Among the nutritional parameters, CP was positively correlated with ash, CP yield and IVDMD, but negatively correlated with NDF, ADF, cellulose and hemicellulose contents. 

Acknowledgments

The first author is grateful for the financial support provided by the livestock process of Holetta Agricultural Research Center (HARC) to undertake the experiment. His special gratitude goes to forage and pasture research colleagues at HARC for their technical and material support throughout the entire work. He is also grateful to feeds and nutrition research staff members at HARC for their assistance in feed quality analysis using Near Infra Red Spectroscopy /NIRS/ techniques.

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Received 1 December 2013; Accepted 17 December 2013; Published 1 January 2014

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