Ruminal degradation and in vitro gas production characteristics of foliage from Atriplex species and Cassia sturtii

 

 

W.A. van Niekerk^I, ; Abubeker Hassen^I; P.J. Vermaak^I; N.F.G. Rethman^II; R.J. Coertze^I

^IDepartment of Animal & Wildlife Sciences, University of Pretoria, Pretoria 0002, South Africa
^IIDepartment of Plant Production & Soil Science, University of Pretoria, Pretoria 0002, South Africa

 

 

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ABSTRACT

This study was undertaken to determine the degradation characteristics and in vitro gas production kinetics of drought tolerant shrubs (Cassia sturtii, Atriplex nummularia and three Atriplex canescens cultivars, viz. Santa Rita, Field Reserve 1 and Rincon) growing under South African conditions (Hatfield in the Gauteng Province and Mier and Lovedale, both in the Northern Cape Province). Edible forage was incubated for 72 h to record in sacco neutral detergent fibre (NDF) degradability parameters and in vitro gas production. No significant difference was found between locations in terms of the NDF degradation parameters, except the NDF effective degradation (ED). However, the species differed in terms of the slowly degradable NDF fraction (NDF b-value) and rate of the degradation of NDF (NDF c-vlaue) in the Hatfield samples. Species differed significantly in terms of NDF ED value for samples collected at Mier. Species also differed in the rate of gas production (c) the volume of effective gas produced (EGP) and the potential extent of gas production (b) values. Atriplex nummalaria was ranked better than A. canescens (Field Reserve 1) in terms of NDF ED-value in Mier and NDF b-value in Hatfield, though the latter had shown higher c-values for samples collected in Hatfield. A similar trend was recorded in the gas production study, where A. canescens (Field Reserve 1) ranked least in terms of effective gas production and the potential extent of gas production compared to the other Atriplex species and C. sturtii. This indicates a lower feeding value of this cultivar as a ruminant feed compared to other Atriplex cultivars and C. sturtii.

Keywords: Atriplex, Cassia sturtii, in sacco degradation, neutral detergent fibre, shrubs

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Introduction

Fibrous feeds are the most important diet ingredient for ruminants under extensive livestock production systems. Due to the low rainfall, especially in the Northern Cape Province of South Africa, the quantity of forage is often limiting. Atriplex species may have potential of increasing animal production in such areas because of their drought and salt tolerance (Kleinkopf et al., 1975; Le Houérou, 1992). The nutritive value of these feeds could be predicted from their ruminal degradation characteristics which are highly correlated to voluntary intake, compared to in vivo digestibility or chemical composition alone (Ahmed & El Hag, 2004). Nsahlai et al. (1994) also reported that fibre degradation and gas production using the in situ nylon bag (0rskov et al., 1980) and the in vitro gas production (Menke & Steingass, 1988) methods, respectively, are reliable techniques for screening fodder trees and shrubs for nutritive value. The aim of this study was to evaluate the feeding value of three Atriplex spp. on the basis of their NDF degradation and gas production characteristics, and to compare them with Cassia sturtii, reported previously by Le Houérou (1980) for its adaptability to low rainfall areas of Africa and other regions.

 

Materials and Methods

This study used Atriplex canescens (Santa Rita), A. canescens (Field Reserve 1 and Rincon) and A. nummularia. Edible foliage (leaves and stems of less than 5 mm diameter) of these species was collected from three different sites in South Africa during March 2002. The sites were Hatfield, Pretoria (Gauteng Province), Mier and Lovedale (both the latter in the arid regions of the Northern Cape Province) with average annual rainfalls of 600, 200 and less than 100 mm, respectively. The samples were dried at 60 °C for 48 hours. The samples destined for in situ degradation were ground to pass through a 4 mm screen, while those intended for in vitro gas production were ground to pass through a 1 mm screen. For the in sacco NDF degradation study samples of 5 g (AFRC, 1992) were incubated for 8, 16, 24 and 72 h in rumen fistulated sheep using nylon bags (10 x 20 cm bag, 12.5 mg/cm²). Bags of the zero hour interval as well as the incubated ones were washed under a running tap until the water ran clear. The NDF for the residue was analyzed according to the procedures of Van Soest et al. (1991). The values at different time intervals were used to calculate degradation constants of NDF using the nonlinear model: y = a + b (1- e^-ct) suggested by 0rskov & McDonald (1979), where y = the disappearance of NDF at time t; a the washing loss (g/kg DM) (rapidly soluble fraction); b the slowly degradable NDF fraction (g/kg DM), and c the rate (%/h) of degradation of fraction b. These degradation constants were used to estimate effective degradability (ED) following the model of Ørskov & McDonald (1979): ED = a + [bc/(k + c)] where k is the passage rate from the rumen, estimated to be 4%/h.

For the in vitro gas production analysis the procedure of Pienaar (1994) was used. Due to cost limitations, only samples from the Hatfield site were analysed. They included Cassia sturtii, A. canescens (Santa Rita), A. canescens (Field Reserve 1), A. nummularia and A. canescens (Rincon). Rumen fluid was obtained from rumen-fistulated sheep consuming a good quality lucerne hay. Volume of gas produced was recorded at 0, 8, 16, 24 and 72 h of incubation. The potential extent of gas production and the gas production rate (%/h) were calculated using the nonlinear model: y = b(1-e^-ct) suggested by 0rskov & McDonald (1979), as modified by Osuji et al. (1993), where y is the volume of gas produced (mL/g DM) with time (t) (h), b is the potential extent of gas production (mL/g DM) and c is the gas production rate (% of b/h). The intercept is not included in the model as there will be no gas production from unfermented feed, as explained by Osuji et al. (1993). Effective gas production (EGP) was calculated using the equation: EGP= (bc/(c+k)), where k is the outflow rate from the rumen, assumed to be 5%/h.

Analysis of variance was performed using the General Linear Model (SAS, 1994) to establish the effect of species and location on the measurements and means and standard errors (s.e.) were calculated. Means were separated using Bonferroni's test (Samuels, 1989) at P < 0.05 confidence limit.

 

Results and Discussion

The NDF degradation characteristics of Atriplex species collected at the three locations are presented in Table 1. The a-value was not affected by location, but within locations, species differences were observed for samples collected from Hatfield and Mier. Similarly the slowly degradable portion (b-value) was not affected (P > 0.05) by location. Differences between species were observed for samples collected from Hatfield, but not from the other locations. In Hatfield, A. nummularia had a higher (P < 0.05) b-value (511 g/kg DM) than A. canesecens (Field Reserve 1) (296 g/kg DM).

The rate of ruminal degradation ranged between 0.020 and 0.093%/h, but rates of corresponding species from different locations did not differ (P > 0.05). However, the individual species differed in the rate of degradation, e.g. at Hatfield, A. canescens (Field Reserve 1) had a higher (P < 0.05) rate of degradation than A. canescens (Santa Rita) and A. nummularia. The average rate of degradation of 0.053%/h of these Atriplex species is lower than average values of 0.099%/h, 0.074%/h and 0.064%/h reported for DM degradability by Kaitho (1998) for A. halimus, A. nummularia and A. rhagodiodes, respectively. El Hassan et al. (2000) also reported rates of degradation of DM values (/h) of between 0.032 to 0.110 for six browse species.

Effective degradability (ED) ranged between 247 to 389 g NDF/kg and the respective species from different locations differed in ED. The ED of NDF in A. canescens (Santa Rita) from the Hatfield sample was lower than that from Lovedale and the ED of NDF in A. nummularia from Hatfield was lower than that from Mier. The ED of NDF in A. nummularia from Mier was higher in than that from the other two species at Mier. These average NDF ED-values are lower than those reported by El Hassan et al. (2000) for some browse species such as Acacia angustissima (515 g/kg), Chamaecytisus palmensis (725 g/kg), Leucaena leucocephala (613 g/kg), Sesbania sesban (704-706 g/kg), Vernonia amygdalina (580 g/kg) and Medicago sativa (662 g/kg).

The in vitro gas production of C. sturtii and the four Atriplex species is presented in Table 2. The potential extent of gas production ranged between 125 to 164 mL/g DM. These values are very low as compared to values of 289.5 to 334 mL/g recorded for four browse leaves in Turkey (Kamalak et al., 2004). In the present study the potential extent of gas production was lower in A. canescens (Field Reserve 1) than in A. canescens (Santa Rita), A. nummularia, A. canescens (Rincon) and C. sturtii. The volume of effective gas production (EGP) ranged between 74 to 96 mL/g DM, which is lower than the 169-206 mL/g DM estimated for tree species (Glycrrhiza glabra, Arbutus andrachne, Juniperus communis and Pistica lentiscus) and a barley cultivars in Turkey (Kamalak et al., 2004; Colkesen et al., 2005). In the present study the EGP volume was lower in A. canescens (Field Reserve 1) than in the other Atriplex species and C. sturtii. The amount of gas released is closely related to the digestibility of the feed (Osuji et al., 1993), and a highly digestible feed will produce more gas than a feed with a low digestibility (Ouda et al., 2005). However, for those feeds with a similar digestibility, the one with lowest gas production could be regarded to have a higher nutritive value since more of its degraded fraction is likely to be incorporated into the microbial biomass or be absorbed directly by the host animal (Rymer, 1999). In this study C. sturtii had the highest average gas production rate, that was higher (P > 0.05) than the average rates measured for A. nummularia, A. canescens (Santa Rita) and A. canescens (Rincon). The rate of gas production is also a better predictor of in vitro organic matter digestibility and hence can be used to predict the feeding value of forages (Chenost et al., 2001). In general, the ranges of gas production rates (c-values) in this study were higher than values reported by Ammar et al. (2005) (0.04%/h) for shrub species in the Mediterranean and by Apori et al. (1998) for Ghanaian browse species (0.0361%/h to 0.0654%/h), but are within the range of c-values reported by Kamalak et al. (2004) for tree leaves in Turkey (0.070 to 0.087%/h).

 

Conclusion

In this study location did not have any effect on NDF degradability parameters. However, the species differed significantly within a location for some of the NDF degradability parameters. A. nummalaria was ranked higher than A. canescens (Field Reserve 1) in terms of NDF ED-value in Mier and NDF b-value in Hatfield, though the latter had a higher rate of degradation for samples collected in Hatfield. This indicates that the former cultivar has the best potential as a ruminant feed. The gas production study supported this pattern. A. canescens (Field Reserve 1) ranked least in terms of effective gas production and the potential extent of gas production compared to the other Atriplex species and C. sturtii. This indicates a lower feeding value for this cultivar as a ruminant feed compared to other Atriplex cultivars and C. sturtii.

 

Acknowledgement

This research was supported in part under Grant No. TA-MOU-99-C16-091 funded by the U.S.-Israel Cooperative Development Research Program, Bureau for Economic Growth, Agriculture and Trade, U.S. Agency for International Development.

 

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# Corresponding author. E-mail: willem.vanniekerk@up.ac.za