Abstract: Native trees (2), shrubs (11), forbs (4), cacti (3) and flowers, fruits and pods (8) that are consumed by range goats in North Mexico were evaluated to determine their Organic Matter (OM), Crude Protein (CP) and the rate and extent of Dry Matter (DM) digestion. The effective degradability of DM (EDDM) was determined by incubating nylon bags (5x10 cm; 50 μm) at 0, 3, 6, 12, 24, 48, 72 and 96 h in the ventral part of the rumen of mature Suffolk x Rambouillet sheep fed alfalfa hay (75%) and a commercial concentrate (25%). The CP (trees mean = 8.0% DM, shrubs = 14, forbs = 13, cacti = 5, flowers, fruits and pods = 8) was significantly different among all species within each group of plants. Moreover, EDDM (34, 56, 58, 70 and 65%, respectively) was significantly different among plants within each group. Nutritional analyses may allow to rank range plants in low nutritional quality (trees), medium (shrubs and forbs) and high (cacti, flowers, fruits and pods).

INTRODUCTION

Range goats managed under semiarid conditions compose their diet of a wide variety of individual native forages. However, animals should be facing a great variability in availability of forage and nutrients throughout the year (Juarez et al., 2004). Under dry conditions, foliage of shrubs and trees are prominent sources of food for range small ruminants (Bhatta et al., 2004) and are even utilized as protein foods (Makkar, 2003). In addition, range animals readily consume immature pods of cacti, which are a good source of energy and water in harsh environments (Ramirez et al., 2000a). Forbs, on the contrary, are important during the wet season (Papachristou and Nastis, 1993), which may represent up to 30% of the goats diet (Papachristou, 1997). Fruits are also widely used for grazing cattle (Riveros, 1992).

Most laboratory techniques used in feed evaluation are still judged according to their ability to predict the nutritive value of feedstuffs. The degradability of DM and CP will directly influence the nutritional characteristics of feedstuffs for ruminants (Ørskov and Shand, 1997). Thus, the in situ digestion kinetics of forage consumed by livestock has been used to determine whether, degradation characteristics of individual vegetative species could provide a useful basis for the evaluation of their nutritive value (Apori et al., 1998). Results obtained with this technique are closely related to those in vivo procedures due to its capability to mimic the natural conditions, in which the chemical compounds of forages (DM, CP and NDF) are digested in the rumen and in other segments of ruminant digestive tract (Broderick and Cochran, 2000). Studies on in situ digestion kinetics of native shrubs from North and Northeastern Mexico have been reported (Ramirez et al., 1998; Ramirez-Orduna et al., 2003) and on native forbs as well (Ramirez and Nunez-Gonzalez, 2006); nonetheless, more research is needed concerning the rumen degradation characteristics of individual foliage of trees, shrubs, forbs, cacti and fruits, commonly selected by range goats, because this could provide a better nutritional characterization of such forages than that obtained by conventional chemical analyses. The objective of this study was to determine the organic matter, crude protein and in situ DM degradability of a wide variety of native foliage collected in the semiarid region of North Mexico.

MATERIALS AND METHODS

The plant species studied were obtained from 7 sites belonging to 4 counties (Durango, Guadalupe Victoria, Penon Blanco and Cuencame) of the state of Durango, Mexico, located at 23°57' and 26°26' LN and 103°48' and 104°43' LO. The sites have a dry climate with total annual rainfall ranging from 243-450 mm, annual mean temperature from 17-21°C and with altitude that varied from 1240-1910 m. The main soils types in the 4 counties are regosol, vertisol, rendzina, xerosol and litosol.

Foliage from trees such as Quercus grisea Liebm. and Quercus eduardii Trel. shrubs such as Acacia shaffneri (S. Watson) F.J. Herm, Prosopis leavigata (Willd.) M.C. Johnst, Atriplex canescens (Pursh) Nutt., Celtis pallida Torr., Flourencia cernua DC, Cassia wislizeni A.Gray, Larrea tridentata (Ses. et Mocex DC.) Felger and Lowe, Condalia lycioides (Gray) Weberb., Acacia constricta Standl., Mimosa biuncifera (Benth.) Britt. and Rose and Cordia parvifolia A. DC; forbs such as Jatropha dioica Cerv., Parthenium incanum Kunth., Coldenia greggii (Torr. and Gray) Gray and Dalea bicolor Humb. and Bonpl.; cacti such as Opuntia leucotricha DC, Opuntia leptocaulis DC and Opuntia imbricata (Haw.) DC; fruits and pods such as white prickly pears and red prickly pears (Opuntia leucotricha DC.), Opuntia leptocaulis DC, Opuntia imbricata (Haw.) DC, fruits Atriplex canescens (Pursh) Nutt., pods of Prosopis leavigata (Willd.) M. C. Johnst., pods of Acacia shaffneri (S.Watson) F.J. Herm. and flowers of Yucca sp. were collected for in situ digestibility analyses. Plants chosen for each species were selected at random, taking at least 10 plants of each species, considered as dominant in the range. Tree, shrub and forb samples were air dried until constant weight. Cacti samples were burned and air dried. Flowers, fruits and pods were dried in an air forced oven at 55°C until constant weight. All samples were ground in Wiley mill to pass a 2 mm screen for degradability measurements and 1 mm screen for chemical analyses.

Crude Protein (CP) in samples was determined by the micro Kjeldahl technique. The rate and Extent of Digestion of DM (EDDM) were estimated by incubating nylon bags (5x10 cm of size with a pore size of 50 μm; ANKOM Technology, Macedon NY, USA) containing 5 g of each species. Bags were incubated at 0, 3, 6, 12, 24, 48, 72 and 96 h in the ventral part of the rumen of three sheep fed alfalfa hay (75%) and a commercial concentrate (25%). During each incubation period, the bags were placed into the rumen at once. After withdrawal, bags were placed in polyethylene bags and washed several times until the rinsing water was clear (about 5 L of water for 3 bags; Juarez et al., 2004). The bags were dried in a convection oven during 24 h at 55°C and stored in plastic containers until chemical analyses were performed. The DM bag losses were estimated by weight change of nylon bags before and after washing and disappearance of DM for each incubation time was calculated by:

Digestion characteristics of DM were obtained by fitting data to the equation (Ørskov and Shand, 1997):

Effective degradability of DM (EDDM) was calculated by the following equation:

The EDDM of samples was estimated assuming a rumen out flow rate of 5% h^-1. Data were analyzed among species within each group of plants by one way analysis of variance using the General Linear Model (GLM) of SAS (Cody and Smith, 1997). Mean differences were separated using Tukey’s test.

RESULTS AND DISCUSSION

The OM content was significantly different among all species within each group of plants (Table 1). The OM content in trees varied from 93-95%, shrubs from 80- 94, forbs from 86-93, cacti from 68-71 and flowers, fruits and pods from 79-94. The CP content was significantly different among all species within each group of plants (Table 1). Trees were in a range from 7-9%, shrubs 10-17, forbs 10-16, cacti 5-6 and flowers, fruits and pods 4-19. Shrubs and forbs exhibited higher CP content than other groups. High CP values in forbs (Ramirez and Nunez-Gonzalez, 2006) have also been reported. However, while great proportion of nitrogen content, in species such as Atriplex canescens and most cacti, is highly soluble, in others such as Acacia shrubs, nitrogen solubility, ruminal degradation and intestinal digestion are low because of great proportion of N is binding to cell wall fraction (Ben Salem et al., 2002).

Table 1:	Organic matter and crude protein (dry matter basis) content of native plants selected by range goats in North Mexico
OM = Organic Matter; CP = Crude Protein; SEM = Standard Error of the Mean; ***(p<0.001)

The amount of N-NDF has been estimated being in a range of 21-56% in individual species (Apori et al., 1998; Ramirez-Orduna et al., 2003) and 50% in the diet selected by goats (containing a wide array of individual species; Ramirez et al., 1991) and as a consequence, this nitrogen will be partially metabolized in the gut (Ramirez-Orduna et al., 2003). In this study, with exception of cacti species and some fruits, most plants had CP content within a range of 8.0 and 14%, these values provide CP levels above the minimum required (7%) by rumen microorganisms to support optimum growth (Yousef and Rouzbehan, 2008).

Table 2:	In situ digestibility characteristics of dry matter of native plants selected by range goats in north Mexico
SEM = Standard Error of the Mean, ***(p<0.001), aIntercept representing the portion of DM solubilized at the beginning of incubation (time 0); bPortion of DM that is slowly degraded in the rumen; cRate constant of disappearance of fraction b; EDDM = Effective Degradability of DM calculated with a rumen outflow rate of 5.0% h-1

The soluble fraction of DM, the rumen degradable fraction, rate constant and EDDM were significantly different among all forages within each group of plants (Table 2). Higher EDDM values were observed in cacti group (range = 71-73%) followed by the group composed by flowers fruits and pods (46-90), forbs (55-61), shrubs (48-69; 28-69) and trees (31-36). Low values in tree foliage may be related to the presence of secondary plant metabolites such as lignin and tannins that may limit ruminal microbial activity because of their ability to bind with dietary proteins, carbohydrates and minerals (Jung and Casler, 1991; Getachew et al., 2002), then reducing the degradability of plant material.

In this study, EDDM of studied plant species may be ranked in three categories: Tree foliage (mean = 34%), shrubs and forbs (mean value >50%) and cacti and flowers, fruits and pods (mean value >64%). Plant species of the latter category showed the fastest ruminal digestion since the very beginning of forage degradation (fraction a; Dhanoa et al., 1999) and the fastest constant rate of degradation c that represents the rhythm of degradation (h^-1 (%)) of the plants studied (Huntington and Givens, 1995). The constant rate of degradation c, is a key variable that not only describe the digestion process of feed nutrients in the rumen, but also the rate of degradation of the insoluble but slowly degraded fraction b (Mertens, 1993). Thus, they have good nutritional potential for range ruminants. Similar findings were reported by Ramirez et al. (2000b), these researchers mentioned that flowers, fruits and pods produced in rangelands at northeastern Mexico, when available, are prominent energetic foods for range goats.

CONCLUSION

Among all groups of plants studied, shrubs may be considered as prominent protein sources and forbs, cacti, flowers, fruits and pods as important energy foods for maintaining productivity of range goats growing in semiarid regions of North Mexico. Range managers should practice management techniques, which provide a diversity of plant species, allowing grazing ruminants the opportunity to select the highest quality diet available. Common brush management technique such as creating a vegetational mosaic using root plowing or shredding will open the canopy. This would allow an increase in native plant diversity and density, with its associated increase in nutritional value.

ACKNOWLEDGEMENT

Funds provided by CONACYT for a Ph.D. scholarship (201282) are widely recognized. Support provided by Ankom Technology, Inc (Macedon, NY. USA) is fully appreciated.