Ruminal digestion characteristics and effective degradability of cell wall of browse species from northeastern Mexico

Small Ruminant Research 36 (2000) 49±55

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 sativa hay was used for comparison. Twelve ruminally cannulated male sheep (45 kg BW) were used (four sheep per plant) to incubate nylon bags (5  10 cm and 53 mm 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 in A. rigidula (2.8). Only O. lindehimieri and C. pallida (8.5) had higher CW degradation rates than M. sativa (7.1). Effective degradability of CW (EDCW), at out¯ow rates of 2%, 5% and 8% hÿ1 values were higher in C. 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) and O. lindehimieri (67.8, 56.6 and 50.6) than in M. 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 *

Corresponding author.

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

0921-4488/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 4 8 8 ( 9 9 ) 0 0 1 1 3 - 3

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R.G. RamõÂrez et al. / Small Ruminant Research 36 (2000) 49±55

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 sativa hay has been used as a reference forage in evaluating the digestibility of diets containing native shrubs from New Mexico in goats (Holechek 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 effective degradability of cell wall from forage of 15 shrubby plants as well as M. sativa hay. 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 longitude 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 Table 1 Scienti®c name and family of evaluated plants Scientific name

Family

M. sativa L. A. rigidula Benth. C. macrum I.M. Johnst A. farnesiana (L) Willd. P. angustifolia Engelm. C. pallida Torr. A. berlandieri Benth. L. leucocephala L. L. texanum Berl. D. virgathus L. A. greggii Gray. C. boissieri A. DC. C. obovata Hook. Z. obtusifolia T. and G. P. glandulosa Torr. O. lindehimieri Engelm.

Leguminosae Leguminosae Leguminosae Leguminosae Zygophyllaceae Ulmaceae Leguminosae Leguminosae Scrophulariaceae Leguminosae Leguminosae Boraginaceae Rhamnaceae Rhamnaceae Leguminosae Cactaceae

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 dominant 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 characterized by high clay and calcium carbonate contents (pH: 7.5±8.5) and low organic matter content (Foroughbakhch, 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. sativa hay. For each of the 15 shrubs and the M. sativa hay, 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 (10  5 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

R.G. RamõÂrez et al. / Small Ruminant Research 36 (2000) 49±55

was calculated by: CW digestibility …%† ˆ

…initial NDF ÿ final NDF† : …initial NDF†  100

Digestion characteristics of CW were calculated using the equation of érskov and McDonald (1979): p ˆ a ‡ b(1 ÿ eÿct), where p is the disappearance at time t, a the intercept representing the portion of DM solubilized at beginning of incubation (time 0), b the portion of DM slowly degraded in the rumen, c the rate constant for disappearance of fraction b, 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)(e ÿ(ct)LT) were calculated 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. sativa hay was estimated assuming rumen solid out ¯ow rates of 2%, 5% and 8% hÿ1 which indicates low, medium and high intakes, respectively (ARC, 1984). The signi®cance of plant species effects on chemical 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 Pearson correlation analyzes was performed between chemical composition and cell wall digestion characteristics 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 as A. 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) and Z. obtusifolia (91.3) resulted with OM values higher than M. 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. sativa hay (11.8).

51

Crude protein percentage was signi®cantly different among plant species (Table 2). L. leucocephala had the highest value (25.2) and O. lindehimieri had the lowest percentage (4.2). Only plants such as A. rigidula (16.5), C. boisieri (14.4), D. virgathus (17.8), L. texanum (14.0), O. lindehimieri and Z. obtusifolia (15.7) had lower CP values than M. sativa hay (18.1). In general, CP content in browse plants is high compared with grasses, and it is relatively constant throughout the year (Norton and Poppi, 1995). Therefore, 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 of O. lindehimieri, all evaluated shrubs had CP percentages in excess of those proposed as the minimum requirement for lactation (12% CP in diet) andgrowth(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. texanum (44.5), O. lindehimieri and P. glandulosa (47.1), all plants had lower CW percentages than M. sativa hay (42.2). Browse plants with relatively low CW content have consequently higher nutritive value compared 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 abundant 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), but C. macrum had the lowest value (4.9). With exception of A. greggii (21.5), A. rigidula (17.9), C. boisieri (20.5), L. leucocephala, and P. glandulosa (19.4), all plants had CW content lower than M. sativa (17.0). O. lindehimieri resulted with the highest (P < 0.001) hemicellulose content (42.2%) and

R.G. RamõÂrez et al. / Small Ruminant Research 36 (2000) 49±55

52

Table 2 Chemical composition of forage from shrubs from northeastern Mexico, collected during spring of 1993 Species

M. sativa A. belandieri A. farnesiana A. greggii A. rigidula C. pallida C. macrum C. obovata C. boisieri D. virgathus L. leucocephala L. texanum O. lindehimieri P. angustifolia P. glandulosa Z. obtusifolia Standard error, n ˆ 4 P level a

Percentage of dry matter Organic matter

Ash

Crude protein

Cell wall

ADFa

Cellulose Hemicellulose

Lignin

Tannins

Insoluble ash

88.3 93.0 91.0 83.8 94.4 80.0 88.9 85.9 83.8 88.0 90.1 91.6 74.9 87.4 93.2 91.3

11.8 7.1 9.1 16.2 5.7 20.1 11.2 14.2 16.3 12.1 9.9 8.5 25.2 12.7 6.8 8.7

18.1 20.3 21.2 21.8 16.5 21.7 23.4 18.6 14.4 17.8 25.2 14.0 4.2 18.0 19.8 15.7

42.2 36.6 37.7 41.9 52.3 33.7 24.8 29.2 35.9 25.9 32.4 44.5 57.1 38.7 47.1 26.0

23.8 27.0 23.3 31.5 35.1 14.3 14.6 17.2 26.4 16.9 16.8 33.3 15.0 27.9 35.5 17.1

17.0 10.8 9.0 21.5 17.9 10.8 4.9 6.4 20.5 6.1 38.7 11.1 12.8 14.4 19.4 6.0

6.8 16.2 14.3 10.0 17.2 3.5 9.7 10.8 5.9 10.8 8.3 22.3 2.2 13.6 16.1 11.1

0.2 13.2 1.8 3.4 15.2 0.3 3.9 0.9 0.3 8.9 7.5 0.4 0.2 0.5 0.2 13.7

0.1 0.3 0.3 0.1 0.1 1.5 0.3 0.2 2.0 2.0 0.2 0.7 0.1 0.1 0.1 0.1

0.2 0.001

0.2 0.001

0.6 0.001

0.2 0.001

1.0 0.001

1.0 0.001

18.4 9.6 14.4 10.5 17.2 19.4 10.2 12.0 9.5 9.1 15.6 11.2 42.2 10.8 11.7 8.9 1.0 0.001

0.7 0.001

0.3 0.001

0.1 0.001

Acid detergent ®ber.

Z. obtusifolia had the lowest (P < 0.001) value (8.9%). Only O. lindehimieri and C. pallida (19.4%) resulted with higher hemicellulose content than M. sativa hay (18.4%). The lignin percentage was variable (P < 0.001) among plant species (Table 2). L. texanum had the highest value (22.3), but O. lindehimieri had the lowest value (2.2). With exception of C. pallida (3.5), C. boisieri (5.9) and O. 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 pallida had the highest value (42.0)

and A. berlandieri had the lowest value (2.9). With exception of C. pallida and O. lindehimieri (36.7) all plants had lower a fraction than that of M. sativa hay. It seems that the OM (r ˆ ÿ0.73; P < 0.001), lignin (r ˆ ÿ0.77; P < 0.001) and condensed tannins (r ˆ ÿ0.38; P < 0.05) content, negatively affected the a fraction of CW in plants (Table 4). Conversely, ash (r ˆ 0.73; P < 0.001) in forage of plants positively in¯uenced the a 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. pallida had the highest (P < 0.001) percentage of fraction b (46.7) of the CW, but fraction b of 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 ‡ b values higher than M. sativa hay (Table 3). The

R.G. RamõÂrez et al. / Small Ruminant Research 36 (2000) 49±55

53

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 A. belandieri A. farnesiana A. greggii A. rigidula C. pallida C. macrum C. obovata C. boisieri D. virgathus L. leucocephala L. texanum O. lindehimieri P. angustifolia P. glandulosa Z. obtusifolia

26.2 2.9 18.3 12.3 6.5 42.0 23.2 23.2 12.1 21.6 25.8 10.9 36.7 19.0 5.1 18.4

38.9 12.6 29.4 35.1 9.0 46.7 41.4 44.3 42.7 46.2 46.5 35.3 45.7 23.0 34.5 29.3

65.1 15.5 47.7 47.4 15.4 88.7 64.6 67.4 54.7 67.9 72.2 46.1 82.4 42.0 39.4 47.6

7.1 2.7 4.0 4.5 2.8 8.5 6.4 6.7 4.2 5.4 6.5 4.2 9.8 3.6 3.9 6.1

3.8 2.6 4.5 4.6 3.8 3.8 3.8 4.0 4.9 4.6 4.1 3.5 3.8 3.5 3.9 2.8

54.1 10.8 30.7 34.5 13.4 74.3 48.2 54.5 38.2 47.0 53.7 33.4 67.8 32.9 26.1 39.1

44.9 7.8 28.3 25.5 11.4 62.9 37.8 43.8 27.3 35.5 42.1 24.7 56.6 27.2 17.5 32.3

39.5 6.3 24.8 21.0 10.1 56.6 32.8 67.7 22.0 30.4 36.5 20.2 50.6 24.5 13.4 28.5

Standard error, n ˆ 4 Significance

***

0.7

0.2

0.2

0.4

0.6

***

***

***

***

1.8

***

0.6

***

0.6

***

a

Dry matter basis; a ˆ fraction of CW (%) lost during washing; b ˆ fraction of CW (%) degraded; a ‡ b ˆ fraction of DM (%) potentially degraded in the rumen of sheep; c ˆ rate of degradation of CW (% hÿ1); EDCW ˆ effective degradability of CW, considering a rumen solid out¯ow rates of 2%, 5% and 8% hÿ1 which indicates low, medium and high intakes, respectively (ARC, 1984). *** (P < 0.001).

rate of degradation (c, % hÿ1) of CW in forages was highest (P < 0.001) in O. lindehimieri (9.8), but was lowest (P < 0.001) in both A. berlandieri (2.7) and A. rigidula (2.7). Only C. pallida (8.5) and O. lindehimieri had CW degradation rate higher than M. sativa hay (7.1; Table 3). The OM (r ˆ ÿ0.73; P < 0.001),

lignin (r ˆ ÿ0.77; P < 0.001) and tannins (r ˆ ÿ0.36; P < 0.05) content in forage of plants affected the rate of degradation of CW (Table 4). However, the ash (r ˆ 0.73; P < 0.001) and hemicellulose (r ˆ 0.39; P < 0.005) content positively in¯uenced fraction c of CW (Table 4).

Table 4 Pearson correlation analyses between chemical analysis and degradability characeristics of cell wall in shrubsa

Organic matter Ash Crude protein Cell wall ADF Cellulose Hemicellolose Lignin Tannins Insoluble ash

a (%)

b (%)

a ‡ b (%)

c (% hÿ1)

EDCW (2% hÿ1)

EDCW (5% hÿ1)

EDCW (8% hÿ1)

ÿ0.73*** 0.73*** ÿ0.12 ÿ0.15 ÿ0.52** ÿ0.10 0.32 ÿ0.77*** ÿ0.38* 0.15

ÿ0.60*** 0.61*** ÿ0.03 ÿ0.29 0.29 0.07 0.00 ÿ0.64*** ÿ0.58*** 0.40*

ÿ0.73*** 0.73*** ÿ0.08 ÿ0.25 ÿ0.44** ÿ0.01 0.17 ÿ0.77*** ÿ0.53** 0.31

ÿ0.73*** 0.73*** ÿ0.28 ÿ0.06 ÿ0.51** ÿ0.09 0.39* ÿ0.77*** ÿ0.36* 0.05

ÿ0.75*** 0.75*** ÿ0.14 ÿ0.19 ÿ0.48** ÿ0.03 0.25 ÿ0.79*** ÿ0.49** 0.23

ÿ0.75*** 0.75*** ÿ0.14 ÿ0.16 ÿ0.51** ÿ0.07 0.30 0.79*** ÿ0.46** 0.18

ÿ0.74*** 0.74*** ÿ0.15 ÿ0.15 ÿ0.52** ÿ0.08 0.32 ÿ0.78*** ÿ0.44** 0.16

a ADF ˆ Acid detergent ®ber; a ˆ fraction of CW (%) lost during washing; b ˆ fraction of CW (%) degraded; a ‡ b ˆ fraction of DM (%) potentially degraded in the rumen of sheep; c ˆ rate of degradation of CW (% hÿ1); EDCW ˆ effective degradability of CW, considering rumen solid out¯ow rates of 2%, 5% and 8% hÿ1 which indicates low, medium and high intakes, respectively (ARC, 1984). * (P < 0.05), ** (P < 0.01), *** (P < 0.001).

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R.G. RamõÂrez et al. / Small Ruminant Research 36 (2000) 49±55

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. leucocephala (54.7, 45.1 and 39.8, respectively) and O. lindehimieri (67.8, 56.6 and 50.6, respectively) had higher EDCW values than M. sativa hay (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¯uenced 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 correlated with decreased digestibility but the responsible mechanism has not been established. There is increasing 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, condensed 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, information 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 as A. berlandieri, A. regidula, L. leucocephala, L. texanum and Z. obtusifolia may 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 D0 Mello, 1995). 4. Management implications With exception of O. 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 highcondensed tannins or lignin resulted with low effective degradability of CW and vice versa. However species such as C. pallida, C. obovata, L. leucocephala and O. lindehimieri had EDCW values comparable or higher than M sativa hay. 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 animal productivity, but the major factor limiting their wider use is the presence of antinutritional factors such as lignin and condensed tannins. The crosslinked 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 degradation. 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. References AOAC, 1990. Of®cial Methods of Analysis. Association of Agricultural Chemists, 15th ed. Washington, DC.

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