Nutritive value of tropical browse legumes in the dry season.

Abstract

NUTRITIVE VALUE OF TROPICAL BROWSE LEGUMES IN THE DRY SEASON A. BAMUALIM*, R.J. JONES** and R.M. MURRAY* SUMMARY Twenty-four species (27 accessions) of tropical browse legumes were sampled in the dry season. Determinations of nylon bag dry matter digestibility (NBDMD), fibre, lignin, nitrogen, phosphorus and ash revealed large differences between accessions and also between leaves and stems (<5 mm diameter). Leaf NBDMD ranged from 30.4 to 80.9% with 8 of the accessions having NBDMD above 60%. Nitrogen concentration ranged from 1.38 to 3.66% for leaves and 0.59 to 2.28% in stems. Phosphorus ranged from 0.10 to 0.31% in leaves and 0.07 to 0.38% in stems. Ash was low for all species, especially in stems. The NBDMD values were highly correlated with all the other variables. Lignin was most closely related (r = -0.924, RSD 6.54). Determinations of NBDMD or lignin could be useful as an early screening technique to reduce the numbers of accessions for further testing. INTRODUCTION In areas which experience a long dry season, the leaves of browse are often regarded as important for the nutrition of grazing animals. Browse provides supplementary protein and energy when grasses are mature and of low nutritive value and can also act as a reserve of feed in times of drought (Wilson 1969). As browse legumes can provide a significant source of protein for animals, comparative studies on their nutritive value are of great importance. The nutritive value of these plants can be estimated through their digestibility and chemical composition. It has been shown that the nylon bag technique (Lowrey 1970) can be used to predict the apparent digestibility of forages.. This paper reports a preliminary study on the estimated dry matter digestibility (DMD), fibre, lignin, nitrogen (N), phosphorus (P) and ash contents of the leaves and stems of 24 pecies of browse legumes and of the relation between dry matter digestibility determined by the nylon bag technique (NBDMD) and these other measurements. MATERIALS AND METHODS Samples from 24 species (27 accessions) of browse legumes (>3 years old) were collected in June 1979 from an unfertilized nursery block on a yellow podzolic soil at the CSIRO Field Station at Lansdown, 50 km south of Townsville. They consisted of small branches with stems no greater than 5 mm in diameter. The leaves were separated from the stems and samples of each dried in a forceddraught oven overnight at 80�C and then ground through a 1.0 mm screen. Dry matter digestibility was estimated by the nylon bag technique (Playne et al. 1978) using 5 g dry matter samples. Samples were replicated in each of four rumen-fistulated steers and were digested for 48 hours. Leaves of 24 species and stems of 18 species were measured. Neutral-detergent fibre (NDF) was determined by the method of van Soest and Wine (1967), acid-deterqent fibre (ADF) and lignin by the method of van Soest (1963), N by Kjeldahl digestion, P by the phosphomolybdate method (Williams and Twine 1967) and ash after incineration at 500�C for 5 hr. * James Cook University of North Queensland, Townsville, Qld 4811, ** CSIRO Davies Laboratory, Private Mail Bag, MSO, Townsville, Qld 4814. 229 . Animal production in Australia RESULTS NBDMD, NDF, ADF, lignin, N, P and ash concentration in the leaves and stems of representative species are presented in Table 1. The range of digestibility was 30.4 - 80.9 percent for leaves and 18.8 - 44.7 percent for stems. The species have been allocated to three groups on the basis of leaf digestibility in Table 2. TABLE1 NBDMD, NDF, ADF, lignin, N, P and ash content of leaves (L) and stems (S) of seven representative browse legumes TABLE 2 The ranking of 24 browse species (27 accessions) on the basis of leaf digestibility. In each of the three groups the species are ranked in order with those of highest digestibility at the top. The stem 18 species ~30 lebbek, Cajanus than 40%. The samples had lower values for dry matter digestibility (13 out of percent DMD), than did the leaves, although three species, Albizia cajan and Acacia sp. 21488 had dry matter digestibilities greater relation between leaf and stem digestibility was poor (r = 0.002). Nitrogen levels of leaves ranged between 1.38 and 3.66% (23 samples contained >2% N). The range in N levels of stems was 0.59 - 2.28% (15 samples contained >l.O% N). Most P values (22 leaf and 12 stem samples,) ranged between 230 Animal production in Australia 0.10 and 0.20%. Levels of ash were generally low; only sixspecies had more than 10% in their leaves, and stems had lower value than leaves. Negative correlations existed between NBDMD and lignin, NDF and ADF. Positive correlations were measured between NBDMD and both N and ash (Table 3). Of the variables measured, lignin was the best overall predictor of NBDMD. TABLE 3 Correlations between NBDMD and various chemical components of the leaves and stems of some leguminous browse plants DISCUSSION Shrub and tree legumes have received little attention compared with herbaceous legumes in the development of forage for tropical regions (National Academy of Sciences 1979). Consequently there are little data on which to assess their value and with which to compare our results. Although many species in tropical regions have been recorded as browse in natural situations, few species have aroused sufficient interest to be planted'specifically as fodder. The notable exception is Leucaena leucoephala (National Academy of Sciences 1979). Evaluation for suitability as animal feed of all tropical leguminous shrubs which are known to be browsed, would be an enormous task. If browse is to be planted specifically for dry season feed, as opposed to the use of natural stands of native species, it is imperative that considerations of quality be given high priority in the selection of species or cultivars. The use of digestibility and chemical measures appears to offer possibilities for rapid screening in the nursery phase. Newman and McLeod (1973) concluded that - vitro digestibility value&could in be used to predict - vivo digestibility for the shrub species they studied proin vided samples of known - vivo digestibility were used for correction. The unin availability of standards for shrubs fed to cattle, as opposed to sheep, precluded their use in this study. However, the very large range in digestibility values obtained in this preliminary study clearly indicates that numbers of browse accessions can be reduced for the next phase of evaluation. It is interesting to note that leaf NBDMD was less than the neutral-detergent soluble (ND? content for most accessions, indicating that some of the NDS apparently was not solubilized during digestion. This anomaly may have been clarified if standards could have been used. Nevertheless measured NBDMD were high for most species (17 accessions having greater than 50%). McLeod (1973) using in vitro methods to mstudy DMD of 26 samples of Australian trees and shrubs known to be browsed by cattle and sheep, found that only 10 species had DMD greater than 50%. The only 231 Animal hduction in Australia Australian species which we tested, Catharmion umbellatum, had the lowest digestibility of all species, 30.4%. The low ash contents and adequate N levels recorded were similar to the results reported in other studies with browse (McLeod 1973; McDonald and Ternouth 1979). If 1.5% N is regarded as necessary to maintain a zero nitrogen balance in ruminants (Milford and Haydock 1965) then the N concentrations recorded were generally adequate in the leaf samples obtained in the dry season. The phosphorus values recorded were much higher than those of 0.05 - 0.08% for fertilized spear grass and for S. humilis measured in the dryseason at Lansdown (Playne and Haydock 1972). This indicates that some of these shrubs could provide supplemental P as well as N when grazed in conjunction with native pasture. Estimation of the NBDMD from lignin, fibre fractions, nitrogen or ash was associated with high RSDs. Although lignin was the best predictor, the RSD of 6.5 units was larger than the RSD of 5 units relating lignin to in vivo DMD measured on a range of pasture samples (Minson et al. 1976). Digestibility is only one aspect to be considered in the evaluation program. Vigour, acceptability and intake by grazing animals, ability to withstand grazing, and the presence of toxins or anti-nutritive factors will also need to be studied. However, we believe that screening for digestibility is a logical first step in reducing the number of browse accessions to be studied. We have concentrated on differences between species in this study, but appreciate that differences in digestibility between accessions of the same species could also be very important, and can be studied in a similar way. ACKNOWLEDGEMENTS We thank Mr R. Reid, CSIRO Davies Laboratory, who established the browse nursery and T. Hall and M. Breen for helpful discussion and assistance. REFERENCES 1 . WILSON, A.D. (1969). J. Range-Manage. &23. 232