A survey of seasonal macro-mineral status of soil, plants and goats in Siavonga, southern Zambia

T Lundu, K Choongo, K Munyinda^*, K Walubita^* and N J Siulapwa

The University of Zambia, School of Veterinary Medicine, P.O. Box 32379, Lusaka, Zambia.
tlundu@yahoo.com   and   t.lundu@unza.zm
* The University of Zambia, School of Agricultural Sciences, P.O. Box 32379, Lusaka, Zambia.

Abstract

The status of selected macro minerals in goats in Siavonga was evaluated in the rainy, cold dry and the hot dry seasons between November 2009 and June 2010 in terms of soil, plants and blood plasma. Samples were collected in November 2009 (hot dry season), February 2010 (rainy season) and June 2010 (cold dry season). The samples were analysed for comparative seasonal content of Phosphorus, Calcium, Potassium and Magnesium. In addition to the minerals, soil was also analysed for pH.

 

Mean plasma Phosphorus and Magnesium were significantly higher (p<0.05) in the hot dry season than in the cold dry and the wet seasons. Calcium concentrations were significantly higher (p<0.05) in the cold dry season than in the hot dry and the wet seasons. Potassium concentrations were significantly different in all the seasons (p<0.05), being highest in the hot dry season and lowest in the cold dry season. Thirteen plant species consumed by free ranging goats were collected and analysed for their comparative seasonal mineral content. Pooled plant results show that Phosphorus and Potassium concentrations were significantly higher (p<0.05) in the hot dry season than in the wet and cold dry seasons. Concentrations of Calcium were significantly higher (p<0.05) in the cold dry season than in the hot dry and wet seasons. Magnesium was significantly higher (p<0.05) in the wet and cold dry seasons than in the hot dry season. Soil minerals were significantly higher (p<0.05) in the hot dry season than in the cold dry and the wet seasons. Therefore, season has been shown to affect concentrations of Phosphorus, Calcium, Potassium and Magnesium in goat plasma, plant material and soil.

Key words: browse, climate, season

Introduction

Ruminants obtain most of their nutrients from natural grasslands; consequently, they become susceptible to seasonal changes in the availability and nutrient value of the natural grasslands. During the dry season, they may be unable to eat enough herbage to supply their nutrient requirements, which may result in impaired productivity (Hatendi 1991). Goats are able to utilize a broad range of forage species and to select forage material with high nutrient concentration (Lovelace et al 1993; Narjisse 1991). Browse leaves and pods form a natural part of the diet of goats, which meets over 60% of the forage requirements (Aganga et al 2000). Despite the fact that most of the mineral requirements of goats can be supplied by the browse plants, their mineral content is also affected by season and soil type. Supplementation is therefore recommended in many areas especially during the dry season (Aganga and Mesho 2008; McDowell 1997). Dietary analysis forms the basis for understanding the seasonal nature of mineral deficiencies in grazing goats. Measurement of minerals in blood and animal tissues potentially offer the best indicator of mineral nutrition in the goat, as they account for dietary selection and the variable uptake and availability of minerals. There is however, a lack of knowledge on the status of mineral nutrition in goats in Zambia. Thus, it is necessary to obtain information on the macro mineral status of goats to establish baseline data that is essential for formulating ways of supplementing minerals to improve goat production, enhance income generation and to assess whether the consumer of goat meat obtains the essential elements from the meat. The objective of the present study was to compare seasonally, the status of Calcium (Ca), Phosphorus (P), Potassium (K) and Magnesium (Mg) in goat plasma, plant material and soil in Siavonga District of Zambia’s Southern Province.

 

Materials and methods

 

Study area and animals of study

 

The study was carried out in Lusitu and Simamba of Siavonga district, situated approximately 152.4 km south of Lusaka, Zambia. These are rural areas with subsistence farming being the major activity. Animal husbandry is dominated by goat production based on free range management. Siavonga is one of the districts with the largest population of goats in Zambia. The goat population was estimated to be about 45,272 in 2008 (DVLD 2008). The study area has a mountainous landscape, lying about 950 meters above sea level. The climate is characterized by high temperatures of above 29°C and annual rainfall less than 800mm. A multistage sampling method was used to sample the goats. The primary sampling units were villages with night shelter and the secondary sampling units being adult female goats aged between 2 and 3 years. A total of 72 goats were recruited in the study. 36 of these goats were from Lusitu and the other 36 were from Simamba. To achieve this sample size, five goats were sampled per household in each season. The selected goats were identified using numbered ear tags. This was done in order that the same goats would be sampled at each subsequent visit.

 

Sample collection and preparation for analysis

 

Blood sampling

 

Blood samples were collected in heparinised Vacutainer tubes through jugular venipuncture and plasma was harvested by centrifugation at 1500 rpm for 10 minutes. The plasma was transported to the laboratory in a cool box. At the laboratory, plasma was kept frozen at -20°c prior to analysis.

 

Plant sampling

 

Plant samples were collected by cutting grass close to the ground and leaves from browse plants. The samples were dried in an oven for 48 hours at 60°c. The dried plant samples were then ground to pass a 1 mm screen mesh and stored in labelled paper bags.

 

Soil sampling

 

Soil samples were collected using a stainless steel sampling Auger, at a depth of approximately 20cm. Soil was collected from ten randomly selected sites at each of the selected villages. The soil was then pooled to come up with one composite sample for each village.

 

Sample analysis

 

Plasma and plant sample analysis

 

Plasma samples were deproteinised using 10%Trichloroacetic acid (TCA). The deproteinised plasma and plant samples were processed by wet digestion using Nitric acid (HNO₃) and Perchloric acid (HClO₄). K, Ca, and Mg were measured in the filtrate after diluting samples with Strontium chloride (SrCl₂.₆H₂O). The respective elements were read on an atomic absorption spectrophotometer (Perkin-Elmer, Model 2380). P was determined on a spectrophotometer at 882nm after development of molybdenum blue colour by reacting ammonium molybdate in acid solution with ascorbic acid.

 

Soil sample analysis

 

Ammonium acetate was used to extract Potassium, Calcium and Magnesium. K⁺, Ca²⁺, and Mg²⁺ were measured in the filtrate after diluting samples with Strontium Chloride (SrCl₂.₆H₂O).

Minerals were then determined using an atomic absorption spectrophotometer. The Bray 1 method was used (Bray and Kurtz, 1945) to determine soil P. Soil pH was determined using Calcium Chloride.

 

Statistical analysis

 

Data was analysed using Statistical Package for Social Scientists (SPSS).  The one way Analysis of Variance (ANOVA) was used to compare mineral concentrations among seasons. Benferroni t tests were conducted to test for significant differences between seasons. The Student’s t-test was used to compare differences in means between Lusitu and Simamba. Throughout significance was declared at p less than 0.05.

 

Results and discussion

 

Plasma

 

There were large variations in plasma mineral concentrations between seasons. This may have been because of the great variations in weather conditions among the seasons. During the hot dry season, the study area had completely no grass growing hence grass was not sampled. These variations in mineral content were also observed in plants and soil.

 

Plasma Phosphorus

 Plasma P levels are partly influenced by dietary selection and the variable uptake and availability of the element in the diet. In this study, levels of plasma P were highest when plant P was at its highest. Plasma P was adequate for optimum adult goat performance in the three seasons except during the cold dry season in Simamba. The observed deficiency may be attributed to low plant P levels observed during the same season. P was significantly higher (p<0.05) in the hot dry season than in the cold dry and wet seasons (Table 1). There was no significant difference (p>0.05) between the cold dry and the wet season.

 

Plasma Calcium

 The findings of the present study were similar to the findings of Khan et al (2009) who reported high Ca levels in plasma during winter in male goats. Plasma Ca exceeded the critical level of 105 mg/liter (Simesen 1980) in the three seasons in Simamba. In Lusitu however, Ca was below the recommended critical level during the wet and the hot dry seasons. This was despite having adequate Ca concentrations in plants as shown in Table 2. This may have been due to increased Ca demand during lactation as some of the goats were lactating.  Seasonal differences for plasma Ca were found to be significant (p<0.05), with concentration in the cold dry season being higher than in the wet and the hot dry seasons (Table 1). The wet and the hot dry seasons showed no significant difference (p>0.05) in Ca concentration.

 

Plasma Magnesium

Wet and cold dry season plasma Mg concentrations reported in this study (Table 1) were lower than those reported by Khan et al (2008) in non lactating goats. However in the hot dry season this study shows higher Mg levels than those reported by Khan et al (2008). Plasma Mg concentrations were sufficient to meet adult goat requirements in all the seasons. This may have been due to adequate Mg levels in plants as shown in Table 2 below.

 

Plasma Potassium

In the three seasons plasma K concentrations were sufficient for adult goat performance as the concentrations were above the recommended critical level of 200 mg/liter suggested by Kaneko (1980). This is despite K being deficient in some plants during the wet and the cold dry seasons. This may be as a result of different individual preferences for plants as the goats were feeding which may have resulted in goats selecting only those plant that were high in K. Plasma K was significantly higher (p<0.05) in the hot dry season than in the cold dry and wet seasons, in both localities (Table 1). This is in agreement with the findings in plants.

 

Plants

 

Thirteen plant species were collected and analysed. The plants were grouped according to location into Lusitu and Simamba because of variations in the types of plant species collected between the two locations. This study was designed to investigate the mineral status of plants commonly browsed by goats and not the mineral status of plants in Lusitu and Simamba per se, so samples were collected in areas commonly browsed by the goats regardless of the need for a balanced sample size. Only one type of grass (Paspalum dilatum) was collected and analysed during the wet season. This is because during the dry weather months of the year there is little to no grass growing for goats to graze. The following is a list of plants collected from Lusitu:

Balanites aegyptiaca, Eleusine coracana, Azadirachta indica, Amaranthus spinosum, Lonchocarpus capassa, Acacia geradii, Xanthocersis zambesiaca, Berchemia discolour, Tamarindus indica, Paspalum dilatum, Acacia tortilis.

The following is a list of plants collected from Simamba:

Paspalum dilatum, Maesopsis eminii, Acacia tortilis and Colophospermum Mopane.

 

Plant Phosphorus

During the hot dry season all the plants that were analysed (Table 2) had P levels within or above the critical limits of 1.6-3.8 g/kg (NRC 1981; Kessler 1991) for adult goat requirements, with a range of 1.75-6.15 g/kg, except for Acacia geradii collected from Lusitu that had a P concentration of 1.15g/kg. However in the cold dry season all the plants had P levels below the recommended critical level. A similar pattern was observed during the wet season in the browse plants as well as in Paspalum dilatum, a grass, from Simamba which was marginally deficient in P (1.60g/kg). Studies done in Ethiopia have shown that most grasses were marginal to deficient in P (Kabaija and Little 1988).
 

Plant Calcium

Despite the seasonal differences in Ca concentrations shown in Table 2, all browse plants had adequate levels of Ca (range 0.02-58.4g/kg) to meet adult goat requirements of 1.3–3.3 g /kg in the diet (NRC 1981; Kessler 1991) except Eleusine coracana which had Ca concentration below the required minimum during the hot dry season (0.02g/kg) and in the wet season (0.36g/kg).  Ca is not usually deficient, for optimal livestock performance, in foliage from browse plants that grow in tropical regions (Ramirez et al 2001). Like all grain crops, millet was low in calcium. Ca and P are both important in the development and maintenance of the animal's body; the recommended calcium to phosphorus ratio in the diet is a minimum of 2:1 and a deficiency of either or both in growing animals leads to poorly developed bones. However, in the present study, this ratio was not achieved in any of the plants. Range forages often contain high levels of Ca in relation to P (Norton 1994). However, goats are known to be tolerant to high Ca:P ratios (Cohen 1975).

 

Plant Magnesium

All tested plants in all seasons had adequate Mg concentrations to meet goat requirements (range 1.73-16.8g/kg). Ramirez et al (2001) reported that all tested shrubs in all seasons in Mexico had Mg levels that met adult goat requirements. A study on macro mineral status of forages and grazing goats in West Sumatra, Indonesia indicated that mineral concentrations of forages varied widely among species and seasons (Warly et al 2006). The study also showed that the average concentration of forage Mg in the dry season was above the critical level, while in the rainy season the Mg concentration was below the critical level. Mg content in leaves increases with age, the highest concentrations found in older leaves. Therefore feeding goats on mature leaves may be advantageous especially in areas where Mg is known to be deficient in forage. During the hot dry season plants showed the lowest Mg concentration (Table 2). Some plant species have a particular sensitivity to Mg such that they will become Mg deficient under temperature stress even though Mg may be at sufficient availability levels in the soil (Jones 1998).

 

Plant Potassium

 The findings on plant K with a range of (1.58-37.1g/kg) are similar to those recorded by Ramirez et al (2001) who found higher K concentrations in shrubs grazed by goats during summer than in other seasons. During the hot dry season all the plants had adequate levels of K to meet adult goat requirements (Table 2). The daily adult goat requirements are 1.8–2.5 g/kg in their diets (NRC 1981). With the exception of Azadirachta indica (1.75g/kg), all the plants collected from Lusitu had adequate levels of K in the cold dry season. In the same season, Acacia tortilis from Simamba had K concentrations of 1.70g/kg which was below the required minimum. During the wet season, deficiencies of K were recorded only in Simamba in Paspalun dilatum (1.58g/kg) and Maesopsis eminii (1.64g/kg). Variation of K in plants within different seasons may partially be attributed to different stages of plant maturity at the time of sampling. With maturity, mineral concentration declines due to a natural dilution process and the translocation of minerals to the root system (Jones 1998). Plant species such as Balanites aegyptiaca (37.1g/kg in the hot dry season) and Amaranthus spinosum (15.9g/kg) had K concentrations as much as 10 times the required levels. This may become a problem because of the relationship that exists among K, Mg and Ca. High K concentrations first result in a Mg deficiency; when K is in greater imbalance, they will cause a Ca deficiency (Jones 1998).