Short communication
Chemical composition, DM and NDF degradation
kinetics in rumen of seven legume straws
A.M. Bruno-Soares
a,*, J.M.F. Abreu
a, C.V.M. Guedes
b,
A.A. Dias-da-Silva
baInstituto Superior de Agronomia, Tapada da Ajuda, 1399 Lisbon Codex, Portugal bUniversidade de TraÂs-os-Montes e Alto Douro, 5000 Vila Real, Portugal
Received 22 January 1999; received in revised form 25 June 1999; accepted 4 October 1999
Abstract
Seven legume straws (Cicer arietinum,Vicia benghalensis,Vicia sativa,Vicia villosa,Vicia faba,
Lens culinarisandPisum sativum) were studied in rumen-cannulated rams.
The chemical composition of the studied legume straws varied from 4.3 to 10.1% for ash, 6.1 to 11.4% for CP, 58.0 to 76.5% for NDF, 40.3 to 59.6% for ADF and 8.2 to 14.2% for ADL.
Extent of degradation for DM and NDF by the nylon-bag technique varied between 45.4 and 63.2%, and 36.6 and 57.1%, respectively.The lowest DM degradability was seen in chickpeas and the lowest NDF degradability in horse beans. The highest values for NDF and DM were seen in peas.
Significant differences between DM and NDF degradation kinetics were observed in most of the straws using the Gallant test. The differences may be explained by the variation in cell-wall content. NDF potential degradability was negatively, and significantly, correlated with NDF, ADF, and ADL contents (r ÿ0.829,p0.05;r ÿ0.826,p0.05; andr ÿ0.917,p0.01; respectively).
The use of the aforementioned legume straws by ruminants may have some advantages relative to some cereal straws, due to their higher DM degradability and degradation rate in the rumen. #2000 Elsevier Science B.V. All rights reserved.
Keywords:Legume straws; Chemical composition; In situ degradability; Ruminants
1. Introduction
In the extensive Mediterranean production systems, fibrous feeds, particularly cereal straws and stubbles, are the most important diet ingredients for ruminants. Although
83 (2000) 75±80
*Corresponding author. Tel:351-1363-8161; fax:351-1363-5031.
quantitatively less important, legume straws can represent a valuable feed resource during summer for those animals having access to the site of grains threshing.
Since cell-wall carbohydrates are the most important components of the straws, an efficient microbial digestion in the rumen is crucial for their utilization in ruminant feeding. In recent years, a number of studies has suggested that degradation characteristics of this type of feeds in the rumen will provide a useful basis for the evaluation of their nutritive value (érskov et al., 1988; Shem et al., 1995). However, the available information on the nutritive value of legume straws is scarcer in case of cereal straws or grass hays. Therefore, a study was undertaken to measure chemical composition and degradation kinetics in the rumen of seven different legume straws.
2. Material and methods
2.1. Legume straws and animals
Seven legume straws were used in this study: chickpea (Cicer arietinum L.) cv. Ch K283, common vetch (Vicia sativa L.) cv. Caia, hairy vetch (Vicia villosa Roth.) cv. Amoreiras, horse bean (Vicia fabaL. vsr minor) cv. Beja, lentil (Lens culinarisMiedicus) cv. L 214, pea (Pisum sativumL.) cv. Gp950, and purple vetch (Vicia benghalensisL.) cv. Fontainhas.
Three rumen-cannulated rams (58 kg average live weight) were used, each receiving (on a DM basis) 700 g of chopped hay (Avena sativa±Vicia sativa) and 300 g of a concentrate meal (50% barley grain, 25% soybean meal, 21% soybean hulls and 4% minerals and vitamins), in two identical meals, at 8.30 and 16.30 h. The CP and NDF contents were 9 and 59.1%, 33 and 30.3%, for hay and concentrate, respectively.
2.2. DM and NDF degradabilities
DM and NDF losses in the rumen were assessed by the nylon (42mm pore) bag
technique (érskov et al., 1980). Samples were milled to pass through a 2.5 mm mesh. For each feed, one nylon bag (size 12 cm7 cm) was used per animal and per incubation time, for two consecutive incubation periods (3 rams2 periods1 bag). Bags were introduced in the rumen before the 8.30 h meal and removed 3, 6, 16, 24, 48, 72, 96 and 120 h after. Once removed, the bags were mechanically washed for 30 min at 378C, dried at 658C for 24 h, and the residual DM and NDF were determined. Washing losses (Wl) at 0 h (t0) were determined using two bags for each straw. The water-soluble losses (Ws) were measured by filtering the samples (1 g ) through a Wattman paper No. 11, after soaking in water for 1 h.
Kinetics of DM and NDF degradation were described by the Dhanoa (1988) model pab[1ÿexp (ÿc(tÿL))], where a is the immediately soluble fraction, b the insoluble but slowly degradable fraction, c the constant rate for the degradation of fraction b, L the lag time, and p the DM and NDF degradation after time t. The (ab)DM and (ab)NDF are defined as the DM and NDF potential degradability, respectively.
2.3. Chemical analysis
The straw samples were ground to pass through a 1 mm screen. Ash was determined by combustion at 5508C (18 h) and crude protein (CP) by the Kjeldahl method (N6.25). Cell wall components were determined according to Robertson and Van Soest (1981).
2.4. Statistical analysis
Data on DM and NDF losses were submited to the Gallant test (Gallant, 1975) to compare the degradation kinetics of the legume straws. The null hypothesis was tested at a confidance level, 95% ifT>c*. T is the statistical value of the test and c* the critical value.
A stepwise analysis was used to establish which among the studied variables (Ash, CP, NDF, ADF and ADL) gave the best explanation of the DM and NDF potential degradability variation.
3. Results and discussion
3.1. Chemical composition, DM and NDF degradabilities
The chemical composition (Table 1) shows the higher CP (ca. 10%) and lower NDF (ca. 60%) and ADL (8.2%) for vetches and peas, respectively, and the highest NDF (ca. 77%) and ADL (14.2%) for chickpea. Hadjipanayiotou et al. (1985) observed different values, namely CP for hairy vetch, horse bean and chickpea. However, similar values for CP were mentioned by Sundstùt (1988) for the pea and horse bean. In general, the NDF content showed higher values than those referred by Hadjipanayiotou et al. (1985) for hairy vetch, horse bean and chickpea.
The degradation parameters, water-soluble and particulate matter losses, and potential degradability of DM and NDF fractions are presented in Table 2.
The (ab)NDF value was lower than the (ab)DM value, which agrees with the results reported by Varga and Hoover (1983) for different feedstuffs. The (ab)DMvalue showed a range from 45.4 (chickpea) to 63.2% (pea), whilst the (ab)NDFvaried from 36.6 to 57.1% between horse bean and pea straw, respectively. The degradation rate,c, varied from 3.8% hÿ1
Common vetch 8.7 7.7 64.7 45.8 10.1
Hairy vetch 7.7 10.9 64.2 49.4 11.4
Horse bean 9.2 6.8 69.1 52.3 12.8
Lentil 7.3 8.4 72.7 51.3 13.3
Pea 10.1 9.6 58.0 40.3 8.2
Purple vetch 8.3 11.4 64.5 46.9 13.1
from 2.8% hÿ1
(chickpea and purple vetch straws) to 4.7% hÿ1
(lentil straw) for NDF fraction. The range of the NDF fraction was lower than the one observed for DM, whilst in case ofbfraction it was slightly higher (32.7±52.5% vs. 28.7±42.6%).
The (ab)NDF and b fraction were negatively correlated to the NDF contents (r ÿ0.829, p0.05; and r ÿ0.930; p0.01, respectively), while no significant relationships between the (ab)NDF and c were found. In addition, the rate of NDF degradation was not correlated to any chemical (CP, ADF and ADL) or digestion parameters of the straws. Conversely, the strongest relationship was found between the extent of DM and NDF degradations (r0.942, p0.001). In fact, the degradation curves of DM and NDF fractions showed a parallelism, wich is similar to that observed by Varga and Hoover (1983), and Flachowsky et al. (1993) for different feeds.
The lack of any correlation (p> 0.05) between cell-wall contents and DM lag time was also observed by Van Eys (1982) in tall fescue and fescue-red clover. On the other hand, NDF lag time was negatively correlated to ADF (r0.79, p0.05), though no relationship (p> 0.05) was found betweenLand NDF, andcorL, and thebfraction and (ab)NDF.
The degradation ratecand thebfraction of the NDF fraction of the straws under study were not shown to be correlated (p> 0.05). A similar observation was made by Varga and Hoover (1983) for a collection of feeds with NDF contents varying from 6.0 to 92.0%. Another aspect to be underlined was the lack of any relationship between the degradation rate of either DM and NDF and the concentration of ADL (p> 0.05). In the case of the lentil and purple vetch straws, with similar ADL contents (13.3 and 13.1%, respectively), Table 2
Degradation parameters (a,b,c), lag time (L) obtained by the Dhanoa model, potential degradability (ab), water-soluble (Ws) and particulate matter losses (Wp) of dry matter and neutral detergent fiber fractions
Chickpea Common Hairy Horse Lentil Pea Purple
vetch vetch bean vetch
aa, Soluble fraction;b, slowly degradable fraction;c, fractional degradation rate of fractionb; RSD, residual
standard deviation of the fitted model.
NDF rates differed considerably (4.7 and 2.8% hÿ1
, respectively). According to Mertens (1993), some factors of physical nature, crystallinity and degree of polymerization of the polysaccharides of the cell wall, may have a greater effect on the digestion rate than the lignification itself.
3.2. Comparative analysis of the DM and NDF degradation kinetics
The application of the Gallant test allowed for the finding of significant differences (c*2.599;p0.05) in DM and NDF degradation kinetics among most of the straws, except between common vetch vs. pea (T1.62; p0.05), hairy vetch vs. common vetch (T2.31, p0.05), hairy vetch vs. purple vetch (T2.27,p0.05), lentil vs. purple vetch (T1.83,p0.05) and lentil vs. hairy vetch (T2.34,p0.05) for DM fraction and common vetch vs. hairy vetch (T11.76,p0.05) for NDF fraction. Their significant differences in DM degradation reflect the differences in the levels of cell walls. There was a strong correlation between, both theaandb fractions and the NDF contents (r ÿ0.824, p0.05; r ÿ0.812,p0.05, respectively). About 82% of the variation of the (ab)DM of legume straws could be explained by the NDF contents (abDM126.9ÿ1.05NDF; p0.01).
The significant differences among (ab)NDFof legume straws may have been related to the amount of cell-wall constituents. Strong correlations among thebfraction and the NDF, ADF and ADL contents were observed (r ÿ0.930, p0.01; r ÿ0.926, p0.01 andr ÿ0.978,p0.001, respectively). In fact, 93% of the variation of theb fraction could be explained by the regression upon ADL (b76.5ÿ3.09ADL; p0.001).The fit of the model was slightly improved when the NDF (r20.96) was added to the former (b92.18ÿ0.397NDFÿ2.089ADL;p0.001). In contrast, thea fraction was independent (p> 0.05) from cell-wall contents.
Stepwise regression analysis described the sources of variation of the (ab)NDF of legume straws more accurately when the ADL contents was used (abNDF85.2ÿ 3.31ADL;r20.81 andp0.01).
4. Conclusions
The studied legume straws showed CP contents above 6.0%. These feeds showed high values for the extent and degradation rates of DM and NDF in the rumen. High values of soluble DM fraction were found in most straws (>14%).
The content and the composition of the cell walls, namely NDF and ADL, were shown to be the best variables to explain the variation of (ab)DM and (ab)NDF of these feeds.
The NDF degradation kinetics differed more among the legume straws than did the DM ones.
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