Nutritional uniformity of neutral detergent solubles in some tropical browse leaf and pod diets

https://doi.org/10.1016/S0377-8401(99)00098-X Get rights and content

Abstract

An experiment was conducted to study the nutritional uniformity of neutral detergent solubles (NDS) in tropical browse species comprising Faidherbia albida , Acacia tortilis , Delonix elata and Mulberry (Morus alba ) leaves and F. albida, A. tortilis, A. nilotica and Dichrostachys cinerea pods. The browse supplements were fed together with maize bran to individually caged goats at increasing levels in hay-based diets. Each diet had five levels of the browse feeds in duplicate which were randomly allocated to the goats in five periods of 14 days each. Chemical composition of the feeds consumed and faeces voided were determined. Nutritional uniformity (Lucas test) of NDS was determined for individual browse by regressing the digestible amount upon its content in the dry matter, in which the regression slope represents true digestibility and the y-intercept, endogenous excretions.
A. nilotica pods and A. tortilis and D. elata leaves were not well accepted by the animals, due to the high levels of phenolic compounds and mould. There was no significant (p > 0.05) relationship between the estimated amount of digested NDS and content of phenolic compounds in the browses. True digestibilities of NDS were significantly (p < 0.05) different between the species. The y-intercepts were not significantly (p > 0.05) different from zero for all species except F. albida leaves (−7.89). The true digestibility (±SD) of NDS for all the species combined in the Lucas test was 91.9 ± 0.03% and the metabolic matter was estimated at 3.38 ± 1.13 g/100 g of dry matter intake. The variable true NDS digestibility among the species was an indication of non-uniformity of this fraction, implying that the digestibility of the NDS fraction of tropical browse species may not be predicted from its content. Further studies to include more species are necessary to determine if this is a general phenomenon with tropical browse species.

Introduction

The use of simple laboratory methods that can predict nutritive value by the content of nutrients in the forages is of great practical and economic importance. According to Lucas (1964), a rational system of feed analysis could be developed if feed fractions could be found for which the indigestible or digestible amounts could be predicted from their composition. A reliable prediction of the digestible amount will depend on the uniform availability of the nutrient fraction in question. A feed fraction which has the same true digestibility irrespective of level and origin is considered nutritionally uniform (Van Soest, 1994) and may be estimated by the Lucas equation (Lucas, 1964), in which the digestible fraction per unit of the feed is regressed upon its concentration in the feed. The regression slope in the Lucas test represents true digestibility and the intercept, endogenous excretions.
The neutral detergent soluble fraction of feeds is considered to be completely available (Van Soest, 1994). However, the NDS fraction is comprised of sugars, soluble carbohydrates, pectins, non-protein nitrogen, proteins, lipids and other solubles (Van Soest, 1994), some of which (e.g. proteins and carbohydrates) may react with tannins to form unavailable complexes (Nastis and Malechek, 1981, Zucker, 1983). A high proportion of the phenolics is also soluble in neutral detergent reagent, and therefore may not be digested by the animal. Tannins occur in many browse species (Reed et al., 1985, Reed, 1986, Mueller-Harvey et al., 1987), and can result in non-uniformity of the NDS fraction. To confirm this, an in vivo study was conducted with some browse leaves and pods.

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Animals and feeds

The trial was conducted at the Livestock Production Research Institute, Mpwapwa, in central Tanzania using 16 male goats weighing between 14 and 32 kg (mean = 20.8 kg). Prior to commencement of the experiment, all goats were sprayed against ectoparasites and drenched against endoparasites. Initially, 14 days were allowed for the animals to become acclimatised to the test feeds. The diets were comprised of hay and supplements. The supplements consisted of maize bran (MB) and browse leaves or pods.

Conclusions

This study has shown that NDS in some tropical browse is not a nutritionally uniform entity, suggesting that digestibility of the NDS fraction in tropical browse species may not be predicted from its content. However, to confirm this, further work with more species is necessary to determine if this is a general phenomenon with tropical browse species.

Acknowledgements

The financial support of the Swedish Agency for Research Cooperation with Developing Countries (SAREC) is gratefully acknowledged. Messrs Anthony Mbeho, Rehema Kasiga, Helina Nhonya and Hamisi Bakari of the Livestock Production Research Institute, Mpwapwa, Tanzania, are thanked for their assistance in maintaining the animals and for laboratory analyses.

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