In vitro gas production parameters in cacti and tree species commonly consumed by grazing goats in a semiarid region of North Mexico

M A Cerrillo and R A S Juárez

Facultad de Medicina Veterinaria y Zootecnia. División de Estudios de Posgrado. Universidad Juárez del Estado de Durango. Carretera Durango-Mezquital km 11.5. Durango, Dgo. México
acerrillos@terra.com.mx

Abstract

The objective of this study was to characterize the in vitro gas production of Encino blanco (Quercus grisea), Encino colorado (Quercus eduardii), huisache (Acacia shaffneri), cardenche (Opuntia imbricata), tasajillo (Opuntia leptocaulis) and nopal (Opuntia, spp) with the aim of identifying relationships between in vitro gas production and chemical composition. Crude protein (CP), Neutral detergent fiber (NDF), Acid detergent fiber (ADF), cellulose, hemicellulose and lignin analyses of the species were performed before the determination of in vitro gas production. The samples (200 mg OM) were incubated in glass syringes with rumen fluid obtained from three goats fed alfalfa hay. The gas volume was recorded at 0, 3, 6, 9, 12, 24, 48, 72 and 96h post-inoculation and the data fitted to the model P = a + b (1 - e^-ct). The model parameters were estimated using PROC NLIN (SAS) and the resulting data analyzed using a completely randomized experimental design using ANOVA PROC GLM. Tukey´s test was used to determine differences between means.

Acacia shaffneri had the highest (P<0.05) CP level (117g/kg DM); the Quercus had intermediate values (72 and 64g/kg for Quercus grisea and Quercus eduardii, respectively), and the cacti species the lowest values (34g/kg CP). NDF content ranged from 337g/kg in the nopal to 631g/kg in the Quercus species. NDF in Opuntia imbricata (513g/kg) was characterized by a high hemicellulose content (365g/kg). The lignin was greater (P<0.05) in tree leaves (176g/kg), while the cacti generally had low lignin contents (11 to 44g/kg). The cumulative gas release at 96 h was nearly 53 ml/200 mg OM for Opuntia leptocaulis, whereas Quercus eduardii produced 34 ml/200 mg OM. No differences (P> 0.05) were registered for the gas derived from the (a) fraction. The highest gas volume originated from the (b) fraction was found with Opuntia leptocaulis (P<0.05) (45.6 ml/200 mg OM). Quercus eduardii produced the lowest value for this fraction (26.3 ml/200 mg OM). The highest constant rate of gas production (c) was registered in Opuntia spp (11.6 % h^-1) whereas the lowest constant rate was registered in Quercus grisea (1.97 % h^-1). The potential production (a + b) ranged from 31.6 to 52.6 ml/200 mg OM (P<0.05) in Quercus eduardii and Opuntia leptocaulis, respectively. The correlations between in vitro gas production and the chemical composition of the forages were mostly negative. Significant correlations were observed with the chemical fractions related to the cell wall content: NDF: r = -0.73; ADF: r = -0.90 and lignin: r = 0.96. The in vitro gas production data and the fermentation parameters indicated a potentially high energy available in cacti species which could explain the relevance of those species in the nutrition of grazing goats in semi-arid regions.

Keywords: cacti, fermentation parameters, gas production, goats, shrubs.

Introduction

The relevance of evaluating the nutritional value of shrubs, trees and cactus is evident (Ramirez et al 2000a; Nherera et al 1999; Topps 1992) as their foliage makes an important contribution to the protein and energy consumption of browsing ruminants . This is particularly important in arid and semi arid regions where forage availability and quality may be severely limited during the dry season (Papachristou 1996; Degen et al 1997). Native vegetative species such as huisache (Acacia shaffneri), encino blanco (Quercus grisea), encino colorado (Quercus eduardii), cardenche (Opuntia imbricata), tasajillo (Opuntia leptocaulis) and nopal (Opuntia spp) are widely distributed in the semi arid regions of North Mexico and constitute a significant part of the diet consumed by grazing goats (Juárez et al 1998). However, scarce information is available on their digestible and fermentative characteristics as an indication of their nutritional value. The in vitro gas production technique is a useful tool to determine the nutritional value of the forages consumed by ruminants (Blümmel and Becker 1997) assuming that the volume of gas produced reflects the end result of the fermentation of the substrate to VFA, microbial biomass and neutralization of the VFA. In addition, the application of models permits the fermentation kinetics of the soluble and readily degradable fraction of the feeds, and the more slowly degradable fraction to be described (Gatechew et al 1998). Moreover, the gas production parameters of shrubs, trees and cactuces might demonstrate differences in their nutritional value that may be closely related to their chemical composition.

The objective of this study was to estimate in vitro gas production and its correlation to the chemical composition of browse species consumed by grazing goats in a semiarid region of North Mexico.

Material and methods

The study was conducted at the Facultad de Medicina Veterinaria y Zootecnia de la Universidad Juarez del Estado de Durango, Mexico, located between 22° 22´and 26° 50´ North latitude and 102° 25´ 55´´ and 107° 88´ 55´´ West longitude. The geographic area is characterized by climate BS1 (w)(e) (dry temperate), with a mean annual rainfall of 450 mm and mean temperature of 17°C.

Leaves of encino blanco (Quercus grisea), encino colorado (Quercus eduardii), huisache (Acacia shaffneri), cardenche (Opuntia imbricata), tasajillo (Opuntia leptocaulis) and immature pods of nopal (Opuntia spp.) were collected. The samples were dried at ambient temperature. The cacti pods were burned and chopped which is a standard practice performed by the farmers in the field. The samples were ground to pass a 1 mm sieve.

Chemical analyses of forages

Neutral detergent fiber (NDF), acid detergent fiber (ADF) and lignin analysis of the substrates were conducted according to Van Soest et al (1991). Crude protein (CP) and ash were determined according to standard procedures (AOAC 1990).

In vitro gas production

In vitro gas production evaluation was determined following the method of Menke and Steingass (1988). About 200 mg of OM sample in three replicates were placed into 100 ml glass gas syringes (Haberle Labortechnik, Germany). Rumen fluid was obtained from three fistulated goats (Hecker 1969) fed alfalfa hay (2.5% BW) twice a day. The inoculum was mixed with a sodium and ammonia bicarbonate buffer (35 g NaHCO₃ plus 4 g NH₄HCO₃ per litre) in a ratio of 1:2 (v/v). Thirty ml buffered inoculum was added to each syringe and excess gas released. The syringes were positioned vertically in a water bath kept at 39°C. Two blank syringes containing 30 ml of the medium only were also included. All the syringes were gently shaken 30 min after the start of the incubation and thereafter four times daily. Gas production was recorded at 0, 3, 6, 9, 12, 24, 48, 72 and 96 hours of incubation. The data were fitted to the exponential equation: p = a + b (1 - e ^-ct) (Ørskov and McDonald 1979), where p represents gas volume at time t, a the gas produced from the soluble fraction, b the gas produced from the insoluble but fermentable fraction, a+b the potential gas production, and c the rate constant of gas production during incubation (% h^-1). The parameters were estimated using PROC NLIN (SAS 1997). The data were analyzed using analysis of variance for a completely randomized experimental design by PROC GLM (SAS,1997). Mean comparisons were performed using Tukey´s test (Hicks and Turner 1999).

Results and discussion

Chemical composition of forages

The highest CP content (P<0.05) was observed for A. Schaffnerii (117g/kg) (Table 1). Quercus showed intermediate values (72 and 64g/kg for Quercus grisea and Quercus eduardii, respectively), while the lowest CP content was found with cacti (33g/kg). The CP in Acacia shaffneri (117 g/kg) is lower than the CP content registered in other shrub areas in the semi-arid region of North Mexico (200 g/kg, Ramirez and Ledesma-Torres 1997; 167g/kg, INIP 1977) but similar to A. catechu (130g/kg) and A. nilotica (139g/kg) (Mandal 1997). In cacti, the CP content in this study (35g/kg) is similar to Nopalea cochinellifera (43g/kg) (INIP 1977) but different from O. leucotrichia (75g/kg) (INIF 1981). The CP content in leaves from shrubs and trees is high compared to grasses, thus such species are preferred by grazing goats and are a good source of protein and minerals (Ramirez et al 2000b).

The NDF content varies among species: 390g/kg for A. farnesiana (Ramirez and Ledesma-Torres 1997), 530g/kg for A. catechu (Mandal 1997) and 640 g/kg for Q. semecarpofilea (Singh et al 1999). In cacti species in the present study, the NDF values were lower (340 g/kg) than those reported earlier by Ramirez et al (2000a) (370 g/kg). Similarly, ADF values in Quercus from this study were lower than the ADF content reported by Singh et al (1999) (510 g/kg) and Mandal (1997) (350 g/kg); however, such variations are considered to be due to differences among species. The low cellulose content (137 g/kg) and lignin (11 g/kg) in Opuntia imbricata indicates a readily degradable energy source and a strategic vegetative species for animal survival in semi-arid areas. Other researchers have reported a high IVOMD (67%) and energy content (2.2 Mcal/kg DM) (Gutierrez and Garcia 1998; Ramirez et al 2000) in this kind of vegetative species, again suggesting the potential of this feed for range goats during the dry season (Ricardi and Shimada 1992).

Cumulative gas production

Differences were observed between species (Figure 1). The higher value (52.6 ml/200 mg OM) was registered for Opuntia leptocaulis while the lower gas release (33.1 ml/200 mg OM) was for Quercus eduardii. Results obtained in this study are similar to those reported by Keir et al 1997 and Ly et al 1997 in leaves from tropical trees and shrubs.

Figure 1. Cumulative gas release of the studied vegetative species.

Figure 2. Cumulative gas release of trees and cacti.

Fermentation parameters of forages

Fermentation parameters from the evaluated species derived by fitting the equation P= a+b (1 - e^-ct) are presented in Table 2. The gas volume obtained from the soluble fraction (a) was similar (P>0.05) for all the species. The fermentation of the insoluble but degradable fraction (b) produced more gas (P<0.05) in cacti species (45.6, 39.8 and 38.1 ml/200 mg OM) for Opuntia leptocaulis, Opuntia spp, and Opuntia imbricata, respectively) and the gas produced was statistically different among them (P<0.05). The lowest value was registered for Quercus eduardii (26.3 ml/200 mg OM). The rate of gas production (c) was higher (P<0.05) in the cacti species (Opuntia spp = 11 % h^-1, Opuntia imbricata = 6.87 % h^-1 and Opuntia leptocaulis = 5.40 % h^-1) whereas the lowest value for this fraction was observed in Quercus grisea (1.97 % h^-1).

Similarly, the extent (a + b) of gas volumes was higher for cacti than for trees.  Khazaal et al (1995) indicated that the intake of a feed is mostly explained by the rate of gas production (c) which affects the rate of passage of the feed through the rumen, whereas the potential gas production (a + b), is associated with the degradability of the feed. Thus, the higher values obtained for the (c) and (a + b) parameters in the cacti species, may indicate a better nutrient availability for rumen microorganisms in animals grazing such vegetative species in semi-arid areas.

Correlations between cumulative in vitro gas production and chemical composition of forages

There was a close negative relationship between the in vitro gas production parameters and chemical composition, except for hemicellulose which was positive (Table 3).

The relation between the variables studied was performed by a simple linear correlation procedure with cumulative gas production as dependent variable and chemical components as independent variables. Significant correlation coefficients were obtained for CP (r = -0.78), NDF (r = -0.73), ADF (r = -0.90) and lignin (r = -0.96) which explain in 60, 53, 81 and 92%, respectively, the gas produced. According to Wolin (1960), the gas is generated from the fermentation of substrate to acetate and butyrate, and to a lesser extent to propionate. These results indicate that the gas produced from forages is related to their chemical composition. The production of gas in cacti species (49 ml/200 mg OM; lignin < 45 g/kg) was higher (P<0.05) than the gas produced by tree species (35 ml/200 mg OM; lignin > 14 g/kg) (Figure 2), which may indicate significant differences in VFA production and energy value of these forages.

Conclusions

• In vitro gas production parameters, particularly the constant rate of gas production (c), is a potential way to evaluate the nutritional quality of forages consumed by grazing goats.
• The cumulative gas production was explained to the extent of 60, 53, 81, and 92%, by the CP, NDF, ADF and lignin contents of the forages, respectively.

Acknowledgements

This project was supported by the International Foundation for Science (B-2985-1). The help received from the Cerrillo-Soto family for forage collection is gratefully acknowledged. Appreciation is given to Dr. Fergus Mould as well as the reviewers from LRRD for comments to improve the manuscript.

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Received 20 July 2003: Accepted 10 December 2003

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