Content of macro and micro minerals of deferred forages in silvo-pastoral traditional fodder banks (Ngitiri) of Meatu district of central north-western Tanzania

C D K Rubanza^a,c, M N Shem^b, S S Bakengesa^c, T Ichinohe^d and T Fujihara^d*

^a United Graduate School of Agricultural Sciences, Faculty of Agriculture,Tottori University, Tottori 680-8550, Japan
^b Department of Animal Science and Production, Sokoine University of Agriculture, PO Box 3004, Morogoro, Tanzania
^c Tanzania Forestry Research Institute (TAFORI), PO Box 1257, Shinyanga, Tanzania
^d Laboratory of Animal Science, Faculty of Life and Environmental Science, Shimane University, Matsue, Shimane 690-8504, Japan
fujihara@life.shimane-u.ac.jp

Abstract

Information is lacking on mineral nutritive potential of native pastures in pastoral rangelands in the tropics including Tanzania. An experiment, using wet nitric acid (HNO₃) digestion in a micro wave digester followed by an induced coupled plasma spectroscopy (ICPS), was carried out to quantify levels of macro and micro minerals of in situ conserved herbage forages of six grazing lands of Meatu district of north-western Tanzania.

The deferred forages had variable levels of Ca, P, Mg and S (ranges of 6.1-7.8, 2.7-5.7, 1.8-2.6 and 1.7-2.7 g kg^-1 of dry matter (DM), respectively, and were slightly higher than the minimum recommended levels in the diets of ruminants. Content of micro minerals varied among the grazing land forages. The forages had lower levels of Cu (4.0-6.1 mg kg^-1 DM) and slightly low to moderate contents of Zn (27.4-33.7 mg kg^-1 DM) compared to recommended dietary requirements of ruminants. The feeds had moderate to high levels of Mn (47.7-198 mg kg^-1 DM), and relatively high levels of Co (0.13-0.53 mg kg^-1 DM). The deferred feeds had high levels of Mo (51.1-61.3 mg kg^-1 DM) that could be detrimental through depressed Cu utilization efficiency due to its negative interactions with Cu and S, and could even cause Mo toxicity in ruminants. The forages had levels of Fe (205-803 mg kg^-1 DM) considerably in excess of requirements.

In order to optimise livestock productivity in ruminants fed on the deferred forages, specially during dry seasons, there is a need to supplement with micro mineral sources.

Keywords: Fodder banks, herbage, minerals, natural pasture, Tanzania

Introduction

Ruminant production during dry seasons in the tropics including semi-arid areas of central and north-western Tanzania is limited by low productivity of rangeland forages, which often contain too low crude protein (CP) to meet the minimum requirement for optimal rumen microbial activity (Annison and Bryden 1998). Utilization of low quality roughages such as hays, straws and stover could also be further limited by their low contents of macro and micro minerals due to their effects on rumen microbial growth and activity, leading  to lowered feed digestibility (Leng 1990). To alleviate feed shortages during dry seasons, the Sukuma agro-pastoral tribe of central and north-western Tanzania preserve portions of pastures during wet seasons as standing hay (deferred feed) for dry season livestock feeding . This entire vegetation conservation system is locally known as "Ngitiri". Ngitiri is an in situ Sukuma traditional vegetation conservation technology to promote regeneration of both herbaceous shrubs and tree vegetation (Brandstrom 1985) for enhanced supply of fodder for livestock and wild ungulates, poles for construction and fuel wood resources, restoration of soil fertility and environmental recreational goals. Conserved herbage forages in traditional fodder banks, crop residues and browse fodder constitute the sole source of feeds for ruminants in central and north-western Tanzania during dry seasons, which are characterized by low animal productivity (Rubanza 1999).

Interventions made to enhance livestock productivity in semi-arid areas of Tanzania include planting of promising exotic fodder trees such as Leucaena spp., Gliricidia, Sesbania spp. and indigenous species such as Acacia spp. and Dichrostachys sp. in traditional fodder banks, and supplementation of browse tree foliages, a cheap, affordable and locally available protein source comparable to expensive oil-seed cakes. For example, supplementation of grazing steers with Leucaena leucocephala dried leaves resulted in improved weight gains (Rubanza et al 2005). Supplementation of stall-fed goats with Acacia nilotica and L. leucocephala leaf meal improved growth rates, while feeding of A. polyacantha leaf meal could only overcome weight losses (Rubanza et al 2004). Available data suggest limited use of deferred feed under Ngitiri traditional fodder banks due to their low contents of CP (20-50 g/kg DM) and poor digestibility (Kakengi et al 2001; Rubanza et al 2005). Information is also limited on mineral nutritive potential of these dry season feed resources. Therefore, there is a need to establish levels of mineral of conventional feed resources in order to predict ruminant production responses in these semi-arid areas of Tanzania.

Minerals are vital for normal growth, reproduction, health and proper functioning of the animal's body (McDowell 1992). Minerals protect and maintain the structural components of the body, organs and tissues, and are constituents of body fluids and tissues as electrolytes. Minerals catalyze several enzymatic processes and hormone systems (Underwood and Suttle 1999), maintain acid-base balance, water balance and osmotic pressure in the blood and cerebral spinal fluids.

There is scanty information on the mineral nutritive potential of forages in the tropics including Tanzania. Also, despite the importance of conserved forages in Ngitiri traditional fodder banks as source of feed for livestock during the critical dry seasons, little has been done to establish levels of minerals of these feed resources. Knowledge on mineral composition of  deferred forages would form base-line data on mineral status of available feed resources for enhanced nutrition of grazing ruminants in semi-arid areas of north-western Tanzania. Content of minerals of indigenous browse fodder species such as Acacia spp. and Dichrostachys sp. that grow naturally in traditional fodder banks has been established (Rubanza 2005). A study was therefore conducted to assess the mineral nutritive potential of conserved herbage forages of selected traditional fodder banks in Meatu district of north-western Tanzania.

Materials and methods

Study area and climate

This study was conducted in Meatu district of Shinyanga region of Tanzania (3^o-4^o S; 34^o-35^o E), south of Lake Victoria (1000-1500 m above sea level). Meatu district is located within a semi-arid zone that receives low uni-modal rainfall of 600-800 mm per annum between mid November through mid May. Minimum and maximum temperatures vary annually from 26.8 to 33.6^oC, respectively. Meatu district is characterised by small hills with black clay loam soils on the lower valleys, and with sandy loamy soils on the uplands. Vegetation is purely savanna, characterised by short grasses with scattered shrubs and trees that are dominated by Acacia spp. and Dichrostachys spp.

Selection of grazing lands and transects layout

Meatu district was selected for herbage vegetation sampling and study of forage nutritive value based on its high population of cattle, sheep and goats, and due to its characteristic dry season fodder shortages (Ngendello et al 1996). A total of six grazing lands (Mwanhuzi, Mwamishali A, Mwamishali B, Mwamishali C, Mwambegwa A and Mwambegwa B) of Mwanhuzi division, Meatu district, were selected and demarcated forthe forage nutritive study to represent the rest of the seven districts of Shinyanga region of Tanzania. The grazing lands were selected to represent both communal and privately owned rangelands; and included one demonstration grazing land which was used as a positive control.

Forage sampling

Forage samples that were used for the mineral study were harvested from three-plots (20 m x 20 m) in each of randomly selected six grazing lands of Mwanhuzi division of Meatu district, in Shinyanga region in early June (i.e., early dry season) in 2002. A total of 270 samples, (i.e., 45 samples per grazing land; 15 samples per plot), were harvested from the six grazing lands using 0.25 m² quadrats thrown at random at five paces in each of three transects: 2-diagonals and a bisector in each plot. The herbage samples (grasses and forb species) were clipped in situ at 2 cm above the ground using hand sickles. The harvested forage samples were handled carefully to avoid soil contamination. The collected samples from each grazing land were weighed instantly in the field for fresh weight determination, and then dried in a forced-air oven at 60^oC for 48 h to determine dry matter (DM). Chemical composition, feed degradability characteristics and performance of grazing cattle during the dry season have been reported elsewhere (Rubanza et al 2005).

Analysis of minerals

Mineral contents of in situ herbage forages were determined by wet ashing using a microwave oven as described by Mullis et al (2003). Forage samples (0.5 g) were weighed into Teflon-lined digestion vessels to which 5 ml of HNO₃ was added, and were allowed to digest for 25 min in a CEM MDS-2000 Microwave digester (Model No. 9240020, CEM Corporation, Mathews NC., USA). The microwave oven was set at four times and pressure phases of 40 PSI for 5 min (step 1); 80 PSI for 5 min (step 2); 120 PSI for 5 min (step 3) and 160 PSI for 10 min (step 4). After cooling, the samples were filtered through an ash-free filter paper, and diluted in ion exchanged distilled water to a final volume of 50 ml. Mineral contents were detected by Induced Coupled Plasma Spectroscopy (ICPS) (SPS 7700 Plasma Spectrometer, SII, Seiko Instruments Inc., Japan) fitted with an automatic sampler (400 Auto Sampler, SII, Seiko Instruments Inc., Japan). The samples were analyzed for macro minerals: calcium (Ca), phosphorus (P), magnesium (Mg), and sulphur (S); and micro minerals: iron (Fe), zinc (Zn), manganese (Mn), molybdenum (Mo), and cobalt (Co) and copper (Cu).

Statistical analysis

Data on content of minerals were subjected to the General Linear Model (GLM) procedure of SAS Statistical package (SAS/Statview 1999) based on the following statistical model:

where

Y_i is the response of a factor under investigation (mineral composition),
µ is the general mean peculiar to each observation,
R_i is the i^th effect on the observed parameter due to grazing land, and
e_i is the random error term.

Results

Content of macro minerals

Content of macro minerals of grazing land forages is shown in Table 1. Forages harvested from grazing land 1 (Mwanhuzi) had the lowest (P<0.05) Ca (6.1 g kg^-1 DM) compared to forages harvested from Mwamishali C (7.8 g kg^-1 DM). The content of P ranged from 2.7 (Mwambegwa A) to 5.7 g kg^-1 DM (Mwambegwa B). There were no differences (P>0.05) in the content of P among the grazing land forages except for forages harvested from Mwambegwa B, which had  the highest (P<0.05) P content. The grazing land forages showed variable Ca:P ratios that ranged from 1.5 (Mwanhuzi) to 3.2 (Mwambegwa A). Other grazing land forages showed intermediate Ca:P ratios (Table 1).

The grazing land forages had variable (P<0.05) levels of Mg that ranged from 1.8 (Mwambegwa A) to 2.7 g kg^-1 DM in forages harvested from Mwamishali A, which had (P<0.05) highest levels of Mg (Table 1). There was no difference in the content of Mg between forages harvested from Mwanhuzi and Mwamishali A, compared to the rest of the grazing land forages. Forages harvested from Mwanhuzi contained the lowest level of S (1.7 g kg^-1 DM) compared to those harvested from Mwamishali A. With exception of Mwamishali A, all the grazing land forages had lower levels of S (1.7-2.2 g kg^-1 DM).

Content of micro minerals

Contents of micro minerals of selected grazing land forages are indicated in Table 2. There was no significant difference (P>0.05) in the content of Cu among the grazing land forages (Table 2). Copper content ranged from 4.0 to 6.1 mg kg^-1 DM (Mwamishali B and Mwambegwa B, respectively). Content of Mo ranged from 51.1 (Mwanhuzi) to 61.3 mg kg^-1 DM in forages harvested from Mwambegwa B. Forages harvested from Mwamishali C had the lowest Fe (205.0 mg kg^-1 DM) compared to Mwanhuzi, which had the highest (P<0.05) level of Fe (802.8 mg kg^-1 DM). There was no difference (P>0.05) in the content of Fe among grazing land forages.

Forages harvested from Mwamishali C had the lowest levels of Mn (47.7 mg kg^-1 DM) compared to forages harvested from Mwamishali A (198 mg kg^-1 DM). There was no difference in Mn among forages harvested from Mwanhuzi, Mwamishali A and Mwambegwa B (Table 2). Also, there was no difference in Mn among forages harvested from Mwamishali B, Mwamishali C and Mwambegwa A. Content of Zn varied from 27.4 (Mwamishali B) to 33.7 mg kg^-1 DM (Mwambegwa B). Content of Co ranged from 0.01 (Mwamishali C) to 0.53 mg kg^-1 DM in forages harvested from Mwamishali A. There was no difference in Co among the different grazing land forages.

Discussion

Macro minerals

The studied grazing land forages had slightly higher levels of Ca than the lower range of Ca (3.3-4.7 g  kg^-1 DM) of most tropical grasses (Minson 1990). Variations in the levels of Ca between findings in the current work with values reported in the literature could be partly explained by botanical species composition, stage of growth and season, and variations in soil characteristics due to location of the different grazing lands. Higher contents of macro minerals, for example Ca, could probably be explained by proportion of forb species. Herbage forages that were used in this study were harvested to include a mixture of grasses and forbs species that simulates the diet consumed by grazing ruminants. It could be assumed that forbs contain more minerals than would be in grass species or their mixture. Forbs also retain most of their leafy portions even during dry seasons, which are richer in minerals, including Ca (Minson 1990) than stem fractions (Underwood and Suttle 1999).

High levels of Ca of these conserved forages would meet the theoretical Ca requirements of 4.0 g Ca kg^-1 DM diet needed for all forms of production in ruminants (ARC 1980). These forages had higher levels of Ca than dietary requirement of growing cattle (1.2-4.4 g Ca kg^-1 DM), lactating dairy cows (1.6-4.2 g Ca kg^-1 DM), growing lambs (0.9-5.3 g Ca kg^-1 DM) and lactating ewes (1.2-3.7 g Ca kg^-1 DM). However, efficiency of Ca utilisation from these forages, and therefore its bioavailability in ruminants, would depend on presence of adequate level of P, active form of vitamin D, and calcitonin and parathyroid hormone (PTH). Calcitonin and PTH mobilise conversion of vitamin D to its active form.

The grazing land forages in the current study had slightly higher levels of P than the mean level of 2.9 g P kg^-1 DM of most tropical grasses (Minson 1990). Higher levels of P assayed in the current work were comparable to higher levels of P (1.6 g kg DM^-1) of traditional fodder banks' herbage in the same study area in Meatu district of Tanzania (Rubanza 1999). Variations in the content of P observed in the current study with those reported in the literature could be partly explained by both species' and intra-species' variations. Variable contents of P could be due to differences between varieties and cultivars in the factors that control accumulation of P in forages. The differences could be also due to variability in the available soil P and soil pH, forages' growth stage and proportions of leaf and stem fractions harvested for mineral analyses, and sampling season (Minson 1990). Contents of minerals in forages including P decrease with plant maturity (McDowell 1996).

The grazing land forages had lower levels of Mg than most tropical grasses (3.6 vs. 2.8 g kg^-1 DM) (Minson 1990). However, differences in the content of Mg in this study with those in the literature could be partly explained by differences between forage species, level of Mg in the soil, influences of locality and climate, growth stage, proportion of leaf and stem fractions collected for mineral analysis, and season when herbage sampling was done. The grazing land forages had slightly higher levels of Mg than the recommended requirement of 2 g kg^-1 DM in the diet of cattle (ARC 1980). These forages would therefore meet the theoretical requirement of Mg for beef cattle (0.2-1.2 g kg^-1 DM), (NRC 1996) and for lactating cows (1.2-2.1 g kg^-1 DM), NRC (2001). These forages had also higher levels of Mg than the recommended requirements for growing lambs (0.8-1.5 g kg^-1 DM) and lactating ewes (0.9-1.8 g Mg kg^-1 DM), (INRA 1989), and goats (Meschy 2000).

The grazing land forages had relatively high levels of S that were comparable to most forages and feeds (Underwood and Suttle 1999). Variations in the content of S among the grazing land forages could be mostly explained by differences between forage species, variability in soil fertility and available levels of S, N and P in the soil, and stage of maturity (Minson 1990). Observed results on differences in the level of S were comparable to the variable nature of content of S of forages (Underwood and Suttle 1999). Differences in the content of S among the grazing lands forages could also be due to the proportion of grasses and forbs in the sample that was harvested for mineral analyses. These forages had higher levels of S than the minimum recommended requirement (1.25 g S kg^-1 DM) of cattle (MAFF 1990; Underwood and Suttle 1999).

Micro minerals

All grazing land forages except Mwambegwa B had lower levels of Cu than the mean content of 6.1 mg Cu kg^-1 DM of most tropical grasses (Minson 1990). Differences in Cu observed in the current work with literature values could be partly explained by genotypic differences, vegetative parts, stage of maturity, levels of available Cu in the soil and soil pH. Forages' Cu content declines with herbage maturity, and is higher in leaf vs. stem fractions (McDowell 1996). The deferred forages had lower levels of Cu than the minimum recommended requirements of ruminants for different production purposes (Spears 2003a,b). Ruminants that depend on deferred feed as their sole feeds, for example, cattle especially during dry seasons, could be deprived of Cu (McDowell 1996). Copper-deficiency of grazing cattle, sheep and goats in these pastoral communities in the study area could be further intensified by its reported low bioavailability coefficient of 0.01-0.04 (ARC 1980).

High contents of Fe of the grazing land forages were comparable to high levels of Fe of most forages (100-700 mg kg^-1 DM) reported by McDowell (1992). Variations in the contents of Fe among grazing land forages (Table 2) could be partly explained by forage species' differences and the influence of grazing lands on the level of Fe in the soil. All the grazing land forages had higher levels of Fe than the critical content of Fe in animal tissues (30-50 mg kg^-1 DM). Differences in the contents of Fe between the grazing land forages and literature values could be partly explained by variations in the content of Fe in the soil, and climatic conditions between localities. Forage Fe content is a function of forage species, soil Fe content, nature and type of soil on which forages are grown (McDowell 1992). These feeds had higher levels of Fe than the normal requirements of 30-60 mg Fe kg^-1 DM of ruminants (NRC 1989; McDowell 1992). However, Fe bioavailability in ruminants would depend on feed mixture fed together and form of Fe in these feeds.

Forages harvested from Mwanhuzi, Mwamishali A and Mwambegwa B had higher levels of Mn than mean content of 86 mg kg^-1 DM of tropical forages (Minson 1990). Forages harvested from Mwamishali B, Mwamishali C and Mwambegwa A had lower levels of Mn (Table 2) than the mean content of Mn of most tropical grasses (Minson 1990). Differences in the content of Mn among the grazing lands' forages could be partly explained by the herbage species, level of available Mn in the soil, soil pH, and influence of soil pH on Mn uptake by forages. The grazing land forages had higher levels of Mn than the recommended level of 20 mg Mn kg^-1 DM diet (ARC 1980), and had even higher levels of Mn than tabulated requirements of 40 mg Mn kg^-1 DM of NRC (2001) of ruminants.

The deferred herbage had slightly lower levels of Zn than mean content of 36 mg kg^-1 DM of most tropical grasses (Minson 1990). The Zn content in these deferred forages could be sufficient for recommended requirement for sheep (24-51 mg Zn kg^-1 DM). However, efficiency of Zn utilization of these forages would depend on zinc bioavailability, and its interaction with other mineral elements.

Cobalt is essential in carbohydrate metabolism. Cobalt is a constituent of vitamin B₁₂, the anti-pernicious anaemia factor, and is also a constituent of enzymes in the Krebs or tricarboxylic acid (TCA) cycle. Content of Co observed in this study was comparable to that in most tropical grasses (<0.01 to 1.26 mg kg^-1 DM) reported by Minson (1990). The deferred feeds in traditional fodder banks had higher levels of Co than the dietary recommended levels for cattle (0.06-0.07 mg kg^-1 DM), (ARC 1980), sheep and goats (0.11 mg kg^-1 DM) (ARC 1980; INRA 1989; Meschy 2000)

Conclusions

• Forages harvested from selected grazing lands in traditional fodder banks of Meatu district of western Tanzania had almost similar levels of macro and micro minerals.

• The in situ conserved herbage forages had adequate levels of macro minerals: Ca, P, Mg and S. However, these deferred forages contained lower contents of Cu and Zn than the dietary requirements for most ruminants.

• Productivity of grazing ruminants in these semi-arid areas could be improved through supplementation with micro mineral sources.

Acknowledgments

This work was funded by the Japanese Government, Ministry of Higher Education and Culture (Monbukagakusho) through a postgraduate fellowship to the first author at Shimane and Tottori Universities, Japan. This work was carried out in collaboration with the International Centre for Research in Agroforestry (ICRAF) Tanzania agroforestry project.

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Receved 25 February 2005; Accepted 19 October 2005; Published 1 December 2005

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