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Ruminal digestion characteristics and effective degradability of cell

wall of browse species from northeastern Mexico

R.G. RamõÂrez

*

, R.R. Neira-Morales, R.A. Ledezma-Torres, C.A. Garibaldi-GonzaÂlez

Departamento de NutricioÂn y Metabolismo, Facultad de Medicina Veterinaria y Zootecnia, Universidad AutoÂnoma de Nuevo LeoÂn. Ave. LaÂzaro Cardenas 4600, Unidad Mederos, Monterrey, NL 64930, Mexico

Accepted 23 August 1999

Abstract

Foliage from 15 shrub species was used to estimate the extent and rate of cell wall (CW) degradation in the rumen of ®stulated Pelibuey sheep. Branches from the browse species:Acacia berlandieri,Acacia farnesiana,Acacia greggii,Acacia rigidula, Celtis pallida, Cercidium macrum, Condalia obovata, Cordia boisieri, Desmanthus virgathus, Leucaena leucocephala, Leucophyllum texanum, Opuntia lindehimieri, Porlieria angustifolia, Prosopis glandulosa, and Ziziphus obtusifolia, were collected during the spring of 1993 in MarõÂn, County, Nuevo LeoÂn, MeÂxico.Medicago sativahay was used for comparison. Twelve ruminally cannulated male sheep (45 kg BW) were used (four sheep per plant) to incubate nylon bags (510 cm and 53mm pore size); containing 4 g of ground material (1 mm screen) of each plant. Bags were incubated at 0, 4, 8, 12, 24 and 48 h. The rate of CW degradation (c, %) was highest (P< 0.001) in O. lindehimieri(9.8) and lowest inA. rigidula(2.8). Only O. lindehimieriand C. pallida(8.5) had higher CW degradation rates thanM. sativa (7.1). Effective degradability of CW (EDCW), at out¯ow rates of 2%, 5% and 8% hÿ1values were higher inC. pallida(74.3, 62.9 and 56.6),

C. obovata(54.5, 45.8 and 39.7),L. leucocephala(54.7, 45.1 and 39.8) andO. lindehimieri(67.8, 56.6 and 50.6) than inM. sativa(54.1, 44.9 and 39.5). High levels of lignin and condensed tannins affected the EDCW in some plants. Species such as

C. pallida, C. obovata, L. Leucocephala and O. lindehimieri may be considered as good available forages for grazing ruminants in northern Mexico.#2000 Elsevier Science B.V. All rights reserved.

Keywords:Cell wall digestion; Rumen ®stulated Pelibuey sheep; Browse; Northeastern Mexico

1. Introduction

The value of leaves from shrubs and trees as protein rich fodder for improving animal production has been underestimated. In northeastern Mexico, foliage from browse species represents a signi®cant proportion of available food for grazing small ruminants, and in

many situations is the only source of nutrients (RamõÂrez, 1996). In developing countries planting browse shrubs and trees for animal feed is a common practice that has been growing in recent years.

It has been accepted that the utility of forage in the diet of an animal is a function of its intake and digestibility. Moreover, forage intake is related to ®ber digestibility because intake is reduced when ®ber is increased in the digestive tract (Mertens, 1993). Important ruminal digestion characteristics of forages *Corresponding author.

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are: effective degradability, lag time, rate of digestion and the amount of digestible ®ber (Singh et al., 1989). However, only a few studies have reported the ®ber digestion characteristics of native shrubs, even though such information can be used to predict the nutritive value more accurately.

Medicago sativahay has been used as a reference

forage in evaluating the digestibility of diets contain-ing native shrubs from New Mexico in goats (Hole-chek et al., 1990), and in sheep feeding diets containing native shrubs from northeastern Mexico (RamõÂrez and Ledezma-Torres, 1997). Thus, this study was carried out with the objectives to compare, using sheep, the rumen digestion characteristics and effec-tive degradability of cell wall from forage of 15 shrubby plants as well asM. sativahay.

2. Material and methods

2.1. Species and collection area

During spring of 1993 all plants in Table 1 were collected in MarõÂn, County, NL, MeÂxico. MarõÂn has an elevation of 393 m (latitude 258430 North and longi-tude 1008020 West). The area is covered by a plant community dominated by blackbrush acacia (Acacia

rigidula) similar to the Eastern Coastal Plainshrub.

Shrubby vegetation is characterized by plants varying

from 1 to 3 m in height. The climate is typically semiarid with annual mean temperature of 218C with approximately 500 mm of precipitation. The domi-nant soils of the region are rocky type of upper cretaceous calcite and dolomite. Dominants are deep, dark gray, lime-clay vertisols which are the result of alluvial and colluvial processes. They are character-ized by high clay and calcium carbonate contents (pH: 7.5±8.5) and low organic matter content (Forough-bakhch, 1992).

Branches from each browse species were collected and dried under shade for a period of 15 days. Plants chosen for each species were selected at random, taking at least 10 plants of each species, considered as dominant in the range. Then were pooled in one sample for each species. Alfalfa hay in this study was used as a reference legume, and was obtained from a commercial store at MarõÂn, NL.

Leaves from branches were removed manually and partial dry matter (DM) was registered. Leaves were then ground in a Wiley mill (2 mm screen). Four samples of each species were separated for chemical and in situ digestibility analyzes. Dry matter, crude protein (CP) ash (AOAC, 1990), cell wall (CW), and acid detergent ®ber (ADF; Goering and Van Soest, 1970), hemicellulose was estimated by difference between CW and ADF, acid detergent lignin (AOAC, 1990), and condensed tannins using the vanillin±HCl procedure (Burns, 1971) as modi®ed by Price et al. (1978) were determined in leaves of shrubs and

M. sativahay.

For each of the 15 shrubs and theM. sativahay, the rate and extent of CW loss from nylon bags was evaluated using 12 rumen cannulated Pelibuey sheep (average weight, 45 kg). To evaluate each plant, four sheep were used. Sheep were fed alfalfa ad libitum. 4 g of each sample (2 mm grind) were placed in nylon bags (105 cm, 53 mm, pore size) and suspended in the ventral part of the rumen of each sheep. Bags were incubated for 4, 8, 12, 24, 36 and 48 h. Upon removal from the rumen, bags were washed in cold water, in a washer machine, ®ve times for a period of 5 min each time. Zero time disappearance was obtained by washing unincubated bags in similar fashion. Bags were dried in 608C oven; weight loss of DM was recorded. In the remaining DM of each bag, NDF was estimated (Goering and Van Soest, 1970). Disappearance of CW for each incubation time

Table 1

Scienti®c name and family of evaluated plants

Scientific name Family

M. sativaL. Leguminosae

A. rigidulaBenth. Leguminosae

C. macrumI.M. Johnst Leguminosae

A. farnesiana(L) Willd. Leguminosae

P. angustifoliaEngelm. Zygophyllaceae

C. boissieriA. DC. Boraginaceae

C. obovataHook. Rhamnaceae

Z. obtusifoliaT. and G. Rhamnaceae

P. glandulosaTorr. Leguminosae

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was calculated by:

CW digestibility…%† ˆ…initial NDFÿfinal NDF†

…initial NDF† 100 :

Digestion characteristics of CW were calculated using the equation of érskov and McDonald (1979):pˆa‡b(1ÿeÿct), wherepis the

disappear-ance at timet,athe intercept representing the portion of DM solubilized at beginning of incubation (time 0), bthe portion of DM slowly degraded in the rumen,c the rate constant for disappearance of fractionb, and t the time of incubation. The non-linear parameters a, b and c and the effective degradability of CW (EDCWM)ˆ(a‡b)c/ (c‡k)(eÿ(ct)LT) were calcu-lated using the Neway computer program (McDonald, 1981); k is the estimated rate of out ¯ow from the rumen and LT the time lag. The EDCW of leaves and

M. sativahay was estimated assuming rumen solid out

¯ow rates of 2%, 5% and 8% hÿ1which indicates low, medium and high intakes, respectively (ARC, 1984). The signi®cance of plant species effects on chemi-cal composition, non-linear parameters of digestibility and EDCW were determined by one way analysis of variance design. Comparisons were made among plants, and the signi®cance was reported. Also Pear-son correlation analyzes was performed between chemical composition and cell wall digestion char-acteristics of plants (Steel and Torrie, 1980).

3. Results and discussion

3.1. Chemical composition of plants

Organic matter (OM, %) content in plants was highest (P< 0.001) in A. rigidula (94.4) and was lowest (P< 0.001) in O. lindehimieri (74.9). Plants such asA. berlandieri(93.0),A. farnesiana(91.0),A.

rigidula,C. macrum(88.4),L. leucocephala(90.1),L.

texanum(91.6),P. glandulosa(93.2) andZ. obtusifolia

(91.3) resulted with OM values higher thanM. sativa hay (88.3; Table 2). The ash content was different (P< 0.001) among plants (Table 2). The highest value was for O. lindehimieri (25.2) and the lowest

for A. rigidula(5.7). Only plants such as A. greggii

(16.2),C. pallida(20.1),C. obovata(14.2),C. boisieri (16.3) and D. virgathus (12.1), O. lindehimieri and

P. angustifolia (12.7) had higher ash content than

M. sativahay (11.8).

Crude protein percentage was signi®cantly different among plant species (Table 2). L. leucocephalahad the highest value (25.2) and O. lindehimieri had the lowest percentage (4.2). Only plants such asA. rigi-dula (16.5), C. boisieri (14.4), D. virgathus (17.8),

L. texanum (14.0),O. lindehimieriandZ. obtusifolia

(15.7) had lower CP values thanM. sativahay (18.1). In general, CP content in browse plants is high com-pared with grasses, and it is relatively constant throughout the year (Norton and Poppi, 1995). There-fore, browse is often referred as a protein supplement for livestock. However, our data and those reported by RamõÂrez (1996) shows a wide range in CP content among browse species. The mean of 277 browse species reviewed from 22 literature reports showed a value of 17% and they were within a range of 2.0± 42.0% CP (RamõÂrez, 1996). Moreover, 44% of the browse species had CP values from 13% to 19%, 26% were within 5±12%, 24% were within 20±26% and only 6% were between 27% and 42%. In this study, with exception ofO. lindehimieri, all evaluated shrubs had CP percentages in excess of those proposed as the minimum requirement for lactation (12% CP in diet) and growth(11.3%CPindiet) inruminants (ARC,1984). Cell wall (CW) percentage was different (P< 0.001) among evaluated plant species (Table

2). O. lindehimieri resulted with the highest value

(57.1) and C. macrum had the lowest percentage (24.8). With exception of A. rigidula (52.3), L. tex-anum(44.5),O. lindehimieriandP. glandulosa(47.1), all plants had lower CW percentages thanM. sativa hay (42.2). Browse plants with relatively low CW content have consequently higher nutritive value com-pared with grasses (Lowry et al., 1992). Immature growth has lower CW contents than mature growth and pasture legumes leaf is generally more digestible than stem (Minson, 1990).

Cellulose is the most widely distributed and abun-dant polysaccharide in nature (Van Soest, 1994). In this study, cellulose content (%) was variable (P< 0.001) among species (Table 2).L. leucocephala had the highest percentage (38.7), butC. macrumhad the lowest value (4.9). With exception of A. greggii (21.5),A. rigidula(17.9),C. boisieri(20.5),L.

leuco-cephala, andP. glandulosa(19.4), all plants had CW

content lower thanM. sativa(17.0).

O. lindehimieri resulted with the highest

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Z. obtusifoliahad the lowest (P< 0.001) value (8.9%).

OnlyO. lindehimieriandC. pallida(19.4%) resulted

with higher hemicellulose content thanM. sativahay (18.4%). The lignin percentage was variable (P< 0.001) among plant species (Table 2).L. texanum had the highest value (22.3), butO. lindehimierihad the lowest value (2.2). With exception of C. pallida (3.5), C. boisieri (5.9) andO. lindehimieri (2.2), all plants had higher lignin content than M. sativa hay (6.9). In general, shrubs with high lignin content had low ash content and vice versa (Table 2). This ®nding is in agreement with that reported by Lohan et al. (1980) and Singh et al. (1989). Condensed tannins were not uniform (P< 0.001) among plant species (Table 2). A. rigidula had the highest percentage (15.2), but both O. lindehimieri and P. glandulosa had the lowest value (0.2%). In this study, all plants resulted with very low insoluble ash content (Table 2).

3.2. Cell wall degradability parameters

The fraction of CW lost during wash of nylon bags (a, %) was variable (P< 0.001) among species (Table 3).Celtis pallidahad the highest value (42.0)

and A. berlandieri had the lowest value (2.9). With

exception ofC. pallidaandO. lindehimieri(36.7) all plants had lowerafraction than that ofM. sativahay. It seems that the OM (rˆ ÿ0.73; P< 0.001), lignin (rˆ ÿ0.77; P< 0.001) and condensed tannins (rˆ ÿ0.38; P< 0.05) content, negatively affected theafraction of CW in plants (Table 4). Conversely, ash (rˆ0.73;P< 0.001) in forage of plants positively in¯uenced thea fraction of the CW (Table 4). Those plants that had high ash content (Table 2) resulted with high soluble fraction of CW (Table 3). This effect is explained by the fact that most of ash content in the CW of plants was in the soluble form (Table 2).

C. pallidahad the highest (P< 0.001) percentage of

fractionb(46.7) of the CW, but fractionbof the CW in

A. rigidula(9.0) was the lowest degraded in the rumen

of sheep (Table 3). The fraction of CW potentially degraded in the rumen of sheep (a‡b, %) was not uniform (P< 0.001) among plants (Table 3). This fraction was highest in C. pallida (88.7), but was lowest in A. rigidula (15.4). Only plants such as C.

pallida, C. obovata (67.4), D. virgathus (67.9), L.

leucocephala (72.2) and O. lindehimieri (82.4) had

a‡bvalues higher thanM. sativahay (Table 3). The

Table 2

Chemical composition of forage from shrubs from northeastern Mexico, collected during spring of 1993

Species Percentage of dry matter

Organic matter

Ash Crude

protein Cell wall

ADFa Cellulose Hemi-cellulose

Lignin Tannins Insoluble ash

M. sativa 88.3 11.8 18.1 42.2 23.8 17.0 18.4 6.8 0.2 0.1

A. belandieri 93.0 7.1 20.3 36.6 27.0 10.8 9.6 16.2 13.2 0.3

A. farnesiana 91.0 9.1 21.2 37.7 23.3 9.0 14.4 14.3 1.8 0.3

A. greggii 83.8 16.2 21.8 41.9 31.5 21.5 10.5 10.0 3.4 0.1

A. rigidula 94.4 5.7 16.5 52.3 35.1 17.9 17.2 17.2 15.2 0.1

C. pallida 80.0 20.1 21.7 33.7 14.3 10.8 19.4 3.5 0.3 1.5

C. macrum 88.9 11.2 23.4 24.8 14.6 4.9 10.2 9.7 3.9 0.3

C. obovata 85.9 14.2 18.6 29.2 17.2 6.4 12.0 10.8 0.9 0.2

C. boisieri 83.8 16.3 14.4 35.9 26.4 20.5 9.5 5.9 0.3 2.0

D. virgathus 88.0 12.1 17.8 25.9 16.9 6.1 9.1 10.8 8.9 2.0

L. leucocephala 90.1 9.9 25.2 32.4 16.8 38.7 15.6 8.3 7.5 0.2

L. texanum 91.6 8.5 14.0 44.5 33.3 11.1 11.2 22.3 0.4 0.7

O. lindehimieri 74.9 25.2 4.2 57.1 15.0 12.8 42.2 2.2 0.2 0.1

P. angustifolia 87.4 12.7 18.0 38.7 27.9 14.4 10.8 13.6 0.5 0.1

P. glandulosa 93.2 6.8 19.8 47.1 35.5 19.4 11.7 16.1 0.2 0.1

Z. obtusifolia 91.3 8.7 15.7 26.0 17.1 6.0 8.9 11.1 13.7 0.1

Standard error,nˆ4 0.2 0.2 0.6 0.2 1.0 1.0 1.0 0.7 0.3 0.1

Plevel 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001

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rate of degradation (c, % hÿ1) of CW in forages was highest (P< 0.001) inO. lindehimieri(9.8), but was lowest (P< 0.001) in bothA. berlandieri(2.7) andA.

rigidula (2.7). OnlyC. pallida(8.5) and O.

lindehi-mierihad CW degradation rate higher thanM. sativa

hay (7.1; Table 3). The OM (rˆ ÿ0.73; P< 0.001),

lignin (rˆ ÿ0.77;P< 0.001) and tannins (rˆ ÿ0.36; P< 0.05) content in forage of plants affected the rate of degradation of CW (Table 4). However, the ash (rˆ0.73; P< 0.001) and hemicellulose (rˆ0.39; P< 0.005) content positively in¯uenced fraction c of CW (Table 4).

Table 3

Non-linear parameters of digestibility and effective degradability of cell wall in shrubs of northeastern Mexicoa

Species a(%) b(%) a‡b(%) c(% hÿ1) Lag time

(h)

EDCW (2% hÿ1)

EDCW (5% hÿ1)

EDCW (8% hÿ1)

M. sativa 26.2 38.9 65.1 7.1 3.8 54.1 44.9 39.5

A. belandieri 2.9 12.6 15.5 2.7 2.6 10.8 7.8 6.3

A. farnesiana 18.3 29.4 47.7 4.0 4.5 30.7 28.3 24.8

A. greggii 12.3 35.1 47.4 4.5 4.6 34.5 25.5 21.0

A. rigidula 6.5 9.0 15.4 2.8 3.8 13.4 11.4 10.1

C. pallida 42.0 46.7 88.7 8.5 3.8 74.3 62.9 56.6

C. macrum 23.2 41.4 64.6 6.4 3.8 48.2 37.8 32.8

C. obovata 23.2 44.3 67.4 6.7 4.0 54.5 43.8 67.7

C. boisieri 12.1 42.7 54.7 4.2 4.9 38.2 27.3 22.0

D. virgathus 21.6 46.2 67.9 5.4 4.6 47.0 35.5 30.4

L. leucocephala 25.8 46.5 72.2 6.5 4.1 53.7 42.1 36.5

L. texanum 10.9 35.3 46.1 4.2 3.5 33.4 24.7 20.2

O. lindehimieri 36.7 45.7 82.4 9.8 3.8 67.8 56.6 50.6

P. angustifolia 19.0 23.0 42.0 3.6 3.5 32.9 27.2 24.5

P. glandulosa 5.1 34.5 39.4 3.9 3.9 26.1 17.5 13.4

Z. obtusifolia 18.4 29.3 47.6 6.1 2.8 39.1 32.3 28.5

Standard error,nˆ4 0.4 0.6 0.7 0.2 0.2 1.8 0.6 0.6

Significance *** *** *** *** *** *** *** ***

aDry matter basis;aˆfraction of CW (%) lost during washing; bˆfraction of CW (%) degraded; a‡bˆfraction of DM (%)

potentially degraded in the rumen of sheep;cˆrate of degradation of CW (% hÿ1); EDCWˆeffective degradability of CW, considering a

rumen solid out¯ow rates of 2%, 5% and 8% hÿ1which indicates low, medium and high intakes, respectively (ARC, 1984). ***(P< 0.001).

Table 4

Pearson correlation analyses between chemical analysis and degradability characeristics of cell wall in shrubsa

a(%) b(%) a‡b(%) c(% hÿ1) EDCW (2% hÿ1) EDCW (5% hÿ1) EDCW (8% hÿ1)

Organic matter ÿ0.73*** ÿ0.60*** ÿ0.73*** ÿ0.73*** ÿ0.75*** ÿ0.75*** ÿ0.74***

Ash 0.73*** 0.61*** 0.73*** 0.73*** 0.75*** 0.75*** 0.74***

Crude protein ÿ0.12 ÿ0.03 ÿ0.08 ÿ0.28 ÿ0.14 ÿ0.14 ÿ0.15

Cell wall ÿ0.15 ÿ0.29 ÿ0.25 ÿ0.06 ÿ0.19 ÿ0.16 ÿ0.15

ADF ÿ0.52** 0.29 ÿ0.44** ÿ0.51** ÿ0.48** ÿ0.51** ÿ0.52**

Cellulose ÿ0.10 0.07 ÿ0.01 ÿ0.09 ÿ0.03 ÿ0.07 ÿ0.08

Hemicellolose 0.32 0.00 0.17 0.39* 0.25 0.30 0.32

Lignin ÿ0.77*** ÿ0.64*** ÿ0.77*** ÿ0.77*** ÿ0.79*** 0.79*** ÿ0.78***

Tannins ÿ0.38* ÿ0.58*** ÿ0.53** ÿ0.36* ÿ0.49** ÿ0.46** ÿ0.44**

Insoluble ash 0.15 0.40* 0.31 0.05 0.23 0.18 0.16

aADFˆAcid detergent ®ber;aˆfraction of CW (%) lost during washing;bˆfraction of CW (%) degraded;a‡bˆfraction of DM

(%) potentially degraded in the rumen of sheep;cˆrate of degradation of CW (% hÿ1); EDCWˆeffective degradability of CW, considering

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Effective degradability of CW (%) at assumed rumen out ¯ow rates of 2%, 5% or 8% hÿ1 were different (P< 0.001) among browse species (Table 3). The shrubs C. pallida had the highest EDCW (74.3, 62.9 and 56.6, respectively), A. berlandieri had the lowest value of EDCW (10.8, 7.8 and 6.3, respectively). Only plants such as C. pallida, C.

obovata(54.5, 45.8 and 39.7, respectively),L.

leuco-cephala (54.7, 45.1 and 39.8, respectively) and O.

lindehimieri (67.8, 56.6 and 50.6, respectively) had

higher EDCW values thanM. sativahay (54.1, 44.9 and 39.5, respectively). In this study, lignin content negatively in¯uenced the EDCW. This ®nding was also observed by Singh et al. (1989) who reported that high levels of lignin in browse plants from India reduced EDCW. Also, Foroughbakhch et al. (1998) reported that high levels of lignin affected the EDCW in forage from shrub plants from northeastern Mexico. In this study high levels of lignin negatively in¯u-enced EDCW. This effect could indicate that when lignin and tannins increased, the solubility of CW constituents decreased (Hat®eld, 1993). Moreover, ligni®cation of plant cell walls has long been corre-lated with decreased digestibility but the responsible mechanism has not been established. There is increas-ing speculation that the utilization of forage cell wall components as an energy source is regulated by the cross-linked nature of cell wall components (Jung and Deetz, 1993). High levels of condensed tannins also limited EDCW.

It has been reviewed that the presence of tannins in many nutritionally important browse leaves reduces their utilization as ruminant feed. In this study, con-densed tannins negatively affected the non-linear parameters of digestibility and EDCW (Table 3). This effect may be explained by the fact that the presence of condensed tannins in the CW and ADF indicate that tannins are strongly bound with the ®ber (Van Soest et al., 1986). The toxic effects of condensed tannins present in various feeds and fodders has been reviewed (Kumar and Vaithiyanathan, 1990). However, infor-mation about browse and pasture legumes in scanty. In this study, the ®ber bound with tannins in leaves of plants containing high levels of condensed tannins, such asA. berlandieri,A. regidula,L. leucocephala,L.

texanumandZ. obtusifoliamay resist its degradation

by the rumen microbes and also free tannins would inactivate microorganisms and ®ber enzymes,

conse-quentially fermentation would be inhibited in the rumen. (Kumar and D0Mello, 1995).

4. Management implications

With exception ofO. lindehimieri all species had high CP content and most plants had low CW content. In general, plants with high lignin had low ash and vice versa. Furthermore, those plants that had high-condensed tannins or lignin resulted with low effective degradability of CW and vice versa. However species such asC. pallida,C. obovata,L. leucocephalaandO.

lindehimierihad EDCW values comparable or higher

than M sativahay. High EDCW might indicate that

these browse plants have great potential as forages for ruminants. Forages with low CW and high effective degradability are highly consumed by ruminants, because high CW levels limits the rate and degree of rumen degradation of plant forages (Norton and Poppi, 1995). Thus, it is concluded that the evaluated plants offer considerable potential for improving ani-mal productivity, but the major factor limiting their wider use is the presence of antinutritional factors such as lignin and condensed tannins. The cross-linked nature of cell wall components and the amount of lignin may be the key limitation to CW degradation; however, the organization of the wall matrix regulate the extent of lignin in¯uence on degradation of the wall polysaccharides. This effect would not appear to be similar in legumes as in grasses (Jung and Deetz, 1993). The stearic hindrance would appear to be the major mechanism-limiting forage cell wall degrada-tion. This seems to apply in native shrubs from arid zones that identi®cation of speci®c limiting-factors will contribute to enhance the utilization of forage cell wall energy.

Acknowledgements

Research was funded by Consejo Nacional de Ciencia y TecnologõÂa (CONACYT), Project No. 1129P-B and Universidad AutoÂnoma de Nuevo LeoÂn (PAICYT) Project CT-1999.

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