Jean Diamouangana
Dipection Generale de la Recherche
Scientifique et Technique
B.P. 2499
Brazzaville, Republic Populaire du Congo
Abstract
Introduction
Study methodology
Results and discussion
General conclusions
References
Almost thirty years ago the government of the Congo established five large ranches covering a total area of 80,364 ha in the Niari valley. In December, 1986 the estimated livestock population was 32,200 head constituting 45% of the total cattle population in the Peoples Republic of the Congo. Over the past few years changes in the vegetation cover of the ranches, probably due to mineral deficiencies, have been observed. Consequently, tests were conducted on the vegetation cover of the ranches between 1975 and 1987 to collect scientific data with a view to improving the management of these important forage resources and increasing animal growth.
The tests revealed that - the mean above-ground biomass was 20 kg/DM0.01 ha (2000 kg DM/ha) during the month of December. It increased rapidly between January and May, reaching a maximum yield in June-July.
- Optimum mean above-ground biomass yield varied between 80 and 135 kg DM0.01 ha (8000 and 13500 kg DM/ha). There was a significant difference between the mean biomass yield of Louboula ranch (P<0.001) and the other four ranches at Massangui, Louamba, Lolila and Dihesse.- the relation between volume (x) and biomass (y) is expressed by the equation.
y = 0.51x + 25.19 (r = 0.56, P<0.05)
These results were particularly useful in the production of forage potential maps to be used by ranch managers.
Tests conducted on mineral nutrient contents of forages, mineral block licks, and maize bran in the valley showed that:
- the iron and management contents, of forages were satisfactory and the cobalt contents were within the desired limits. Copper and zinc contents however, were well below the desired limits.- the copper, zinc and manganese contents of the maize bran were high. Consequently the distribution of maize bran and mineral block licks as remedial measures against copper and zinc deficiencies was recommended.
There was a need, identified, that micro-elements contents of the blood plasma and hair of animals should be tested.
The Niari valley, located on Scisto calcareous rock outcrops is part of the Niari river basin, which covers some 15,000 km2 extending from 12°30' and 14°00'E longitude and 2°30' and 4°30'S latitude (Figure 1). The valley receives 1100 to 1400mm rainfall between October and May and experiences a marked drop in the amount of rainfall in January and February. Between May and October, there is a long dry season.
Mean annual temperature is 24°C and mean relative humidity is 83%. Using the French soil classification system, soils of the study areas may be classified as:
- ferralitic soils
- poorly evolved mineral soils;
- hydromorphic mineral soils.
Soil analytical tests done on these soils in the study ranches revealed, a soil texture ranging from fine to very fine, organic matter content of 3.5%, and mean pH of 5.0.
The vegetation consists of intermediate forest (Koechlin, 1961) but savanna grasses, Hyparrhenia diplandra, Schizachyrium platyphyllum and Annona arenaria are predominant species.
Changes in the vegetation cover were observed and mineral deficiencies in the cattle herds were observed for several years. Consequently, tests were conducted on the herbaceous cover during the 1975 and 1987 period to collect basic data with a view to monitoring these forage resources, ensuring more efficient management, diagnosing mineral deficiencies and identifying ways of alleviating these deficiencies. The tests focussed on:
-herbage above ground biomass and volume
-micro-element contents of forages, mineral block licks and maize bran produced in the region
Biomass is the weight of living organisms per unit area (Touffet, 1982). Estimates of biomass were made on 406 stations covering the five ranches (Massangui, Louamba, Louboulou and Dihesse). The study was preceeded by observations of above ground biomass variation in several stations.
Figure 1: Location of the ranches in the Niari Valley
The above-ground biomass of the grass cover was measured using the cut-and-carry method (Faye et al, 1986). Dry matter vegetation was harvested from two 1m2 quadrats and samples were cut at a 2.5cm stubble height. Fresh weight was recorded and dry weights, after oven-drying at 85°C for 24 hrs were also recorded. A mean biomass (in kilograms of dry matter) per hectare for each station was then calculated.
Volume was expressed as the product of size of biomass in m3/ha for each station. Forage samples were obtained from three topographical locations in the Dihesse ranch (slope, depression, flat terrain). Samples of mineral salt (block licks) were obtained from Dihesse ranch and from stocks of the Direction General de Office du Gros Betail (OGB) in Brazzaville.
Analyses of soil samples were carried out at the ORSTOM Centre in Brazzaville. The macro-elements contents were determined by the Soils and Vegetation Laboratoire of the Centre de Cooperation International en Recherche Agronomic pour le Development (CIRAD) in Montpellier, France. Determinations of micro-elements contents of forages, mineral block licks and maize bran samples were done at the Institut d'Elevage et de Medecine Veterinaire des pays Tropicaux (IEMVT) in Maison Alfort, France.
Statistical analyses of biomass and volume data were done on the Apple Microcomputer with softwares developed by Professor Cusset of the laboratoire de Phytologie Quantitative, at the Pierre and Marie Curie University in Paris. Correlations between biomass and volume were calculated on the Boss microcomputer with softwares developed by Moukoko and Dimi of Mathematics Department of Marven Ngouabi University, Brazzaville.
Mineral composition of forages
Chemical composition of mineral block licks
Figure 2 shows the growth cycle of forages at Dihesse ranch. The mean December biomass was estimated at 20 kg DM/0.01 or 2000 kg DM/ha and rapidly increased between January and May. It reached a maximum growth in June-July when mean biomass ranged between 80 to 95 kg DM/0.01ha depending on the year. Although not shown in this graph, it was observed that biomass decreased considerably from the latter half of July. Consequently, it appeared that the optimal biomass correspond with the flowering/fruiting periods of the dominant perennial grass species.
Optimum biomass for all the ranches are given in Table 1. A comparison of the means using the poisson test (Table 2) shows a significant difference between above-ground biomass of the grass cover at Louboulou ranch and biomass of the other four ranches. Based on these data it is clear that the optimum biomass differences could be used for estimating the carrying capacities of the ranches (Boudet, 1978). The carrying capacity of a unit area of pasture is the maximum stocking rate possible which the unit can support without irreversible deterioration. Within the framework of management and rotational utilisation of pastoral resources this definition of capacity is of real practical interest. Consequently, with the assistance of the Institut d'Elevage et de Medecine Veterinaire de pays Tropicaux potential maps of each ranch were produced (Diamouangana, 1988; Diamouangana and Kiyidou, 1983; Diamouangana et al 1984). If allowance is made of biomass losses during the dry season, due to trampling and the need to maintain some ground cover to protect soil from various agents of erosion, it could be estimated that about one-third of the palatable above-ground biomass is consumed annually. Also if we adopt the tropical livestock unit (TLU) of 250 kg at maintenance with a daily consumption rate of 6.25 kg of dry matter, then the carrying capacity estimates vary between 1 and 2 TLU/ha. So far estimates of carrying capacity have been theoretical and several tests will have to be done. Boudet (1978) suggested that these tests should focus on the condition of the herds which should not suffer excessive weight losses during the dry season and should have the capacity for rapid compensatory weight gain once the rainy season starts.
Table 1. Analysis of variance in dry matter yields (kg DM/ha) of the grass cover of the ranches
|
Ranches |
Massangui |
Louamba |
Louila |
Louboulou |
Dihesse |
|
No of stations |
69 |
80 |
80 |
79 |
94 |
|
Mean yield |
93.7 |
89.0 |
80.0 |
134.8 |
94.9 |
|
Variance |
1543.9 |
1206.6 |
513.0 |
2168.3 |
1488.8 |
|
Standard deviation |
32.29 |
34.73 |
22.65 |
46.56 |
38.58 |
|
|
95% |
+77.0 |
+44.39 |
+91.25 |
+75.61 |
|
Confidence interval |
99% |
+101.36 |
+58.44 |
+120.12 |
+99.63 |
Table 2. Comparison of mean dry matter yields in kg DM/ha of the herbage in the ranches (Polssons test)
|
Ranches |
Massangui |
Louamba |
Louila |
Louboulou |
Dihesse |
|
Massangui |
1.00 |
0.77NS |
2.55* |
5.82*** |
0.19NS |
|
Louamba |
0.77 ns |
1.00 |
1.94 NS |
7.02*** |
1.19NS |
|
Louila |
2.55* |
1.94 NS |
1.00 |
2.55* |
3.16** |
|
Louboulou |
5.82*** |
7.02*** |
9.42*** |
1.00 |
6.06*** |
|
Dihesse |
0.19NS |
1.19NS |
3.16** |
6.06*** |
1.00 |
Descoing (1976) noted that volume, the function of area covered by mass could be calculated quite easily in the field, and that the major constraint in its calculation was inadequate knowledge of the vegetation. By establishing coefficients of correlation between volume and biomass, the biomass can be correlation between volume and biomass, the biomass can be calculated from the volume which is more easily obtained. Figures 3 gives the line of regression of volume and standing biomass and the coefficients of correlation. The relation between volume and biomass was studied by trying out various permitations of simple mathematical models. The linear function, by its very simplicity proved to be most appropriate.
The mean mineral contents of forages at Dihesse ranch are given in Table 2. Several authors (Faye and Grillet, 1984; Faye et al, 1986; Lamand, 1972; Lamand, 1979) have considered such analyses of forages to be correct. The deficiency limits for forages and other fodder crops were established by Conrad et al (1985), Faye and Grillet (1984), Faye et al (1986) and Lamand (1972, 1979) as follows:
|
Cobalt: |
0.07 ppm of DM |
|
Copper: |
7.00 ppm of DM |
|
Zinc: |
45.00 ppm of DM |
|
Manganese: |
45.00 ppm of DM |
|
Iron: |
50.00 ppm of DM |
The iron and manganese contents in the current study were satisfactory. The cobalt contents were within the accepted deficiency limits but those of copper and zinc were well below the deficiency limits.
Block licks distributed in the Niari valley ranches were in the form of rock salts obtained from the Tchitondi (House) potash deposits (Table 3). The sodium content of both samples was between 30% and 40% of dry matter. They were extremely deficient in calcium, potassium, magnesium and phosphorus. The iron contents were satisfactory, but cobalt, copper, zinc and manganese contents were well below the deficiency limits.
Figure 3. Relationship between volume and above-ground herbaceous biomass.
***: Highly significant at P>0.001
n: Number of samples
(Data in kgDm/0.001
Table 3. Cobalt, copper, manganese and iron contents of forages in Dihese ranch during the 1977-78 vegetation cycle in ppm (Serre, 16.17).
|
Sample locations |
Cobalt |
Copper |
Zinc |
Manganese |
Zinc | |
|
|
1 |
0.04 |
3.5 |
22.5 |
179.4 |
96.4 |
|
|
2 |
0.11 |
3.7 |
20.1 |
181.5 |
101.8 |
|
|
3 |
0.03 |
3.6 |
19.7 |
258.3 |
68.7 |
|
Mean and standard deviation |
0.06 |
3.6 |
20.7 |
206.4 |
88.9 | |
|
|
+0.04 |
+0.39 |
+1.51 |
+44.95 |
+17.75 | |
|
In depression | ||||||
|
|
1 |
0.04 |
2.7 |
19.7 |
169.9 |
54.9 |
|
|
2 |
0.03 |
2.7 |
20.6 |
191.6 |
62.7 |
|
|
3 |
0.10 |
2.1 |
47.7 |
211.6 |
96.7 |
|
Mean and standard deviation |
0.056 |
2.5 |
29.3 |
191 |
71.4 | |
|
|
+0.03 |
+0.34 |
+15.91 |
+20.86 |
+22.22 | |
|
On flat terrain | ||||||
|
|
1 |
0.08 |
3.6 |
23.1 |
185.7 |
324.7 |
|
|
2 |
0.15 |
3.3 |
23.4 |
213.8 |
468.0 |
|
|
3 |
0.06 |
2.4 |
15.8 |
167.0 |
149.5 |
|
Mean and standard deviation |
0.09 |
3.1 |
20.7 |
188.8 |
314.0 | |
|
|
+0.05 |
+0.62 |
|
+23.55 |
+159.51 | |
|
Instory laterit zone | ||||||
|
|
1 |
0.09 |
3.5 |
25.8 |
134.2 |
443.4 |
|
|
2 |
0.07 |
3.5 |
23.0 |
201.0 |
290.0 |
|
|
3 |
0.06 |
3.7 |
19.5 |
374.9 |
294.0 |
|
Mean and standard deviation |
0.07 |
3.6 |
22.7 |
236.7 |
443.4 | |
|
|
+0.01 |
+0.12 |
+3.15 |
+124.25 |
+87.43 | |
The mineral composition of maize bran produced by the Nyaki cattle feeds mill and popularly used in the Niari valley ranches is shown in Table 4. These compositions can be considered as satisfactory for calcium, phosphorus, magnesium, potassium, copper, zinc, manganese and iron. The content of cobalt was unsatisfactory.
Table 4. Chemical composition of maize bran produced by the Nkayi caste feed mill
|
Elements on % DM basis |
Fine maize bran |
Remulled maize bran |
|
% of dry matter |
|
|
|
Organic matter |
93.77 |
94.33 |
|
Total protein content (N × 6.25) |
18.98 |
17.64 |
|
Row cellulose |
11.88 |
11.51 |
|
Fat content (ether extract) |
3.56 |
2.59 |
|
Nitrogen free extracts |
59.35 |
62.59 |
|
Total mineral matter (ashes) |
6.23 |
5.67 |
|
Insoluble hydrogen chloride |
0.07 |
0.07 |
|
Calcium |
0.13 |
0.12 |
|
Phosphorous |
1.39 |
1.24 |
|
Magnesium |
0.45 |
0.38 |
|
Potassium |
1.38 |
1.24 |
|
Trace-elements (zn ppm) |
11.70 |
12.10 |
|
Copper |
0.03 |
0.03 |
|
Cobalt |
90.50 |
86.10 |
|
Zinc |
65.40 |
65.40 |
|
Iron |
101.00 |
|
Table 5. Chemical composition of block licks
|
Elements on % DM basis |
Sample 1 |
Sample 2 |
|
% 100 dry matter organic matter |
-(*) |
0.76 |
|
Minerals |
-(*) |
99.24 |
|
(Ashes) | ||
|
Insoluble hydrogen chlorides |
-(*) |
0.03 |
|
Calcium |
0.11 |
0.02 |
|
Phosphorus |
0.00 |
0.06 |
|
Magnesium |
0.04 |
36.91 |
|
Potassium |
0.11 |
|
|
Sodium |
39.00 |
|
|
Trace-elements (in ppm) | ||
|
Cobalt |
-(*) |
0.02 |
|
Copper |
1.50 |
11.80 |
|
Zinc |
1.30 |
3.50 |
|
Manganese |
13.80 |
5.90 |
|
Iron |
69.00 |
211.00 |
The above ground biomass of forages in the ranches varied between 80 and 135 kg DM/0.01 ha, which production is comparable to that of the savanna grassland of Lamto in the Ivory Coast (Cesar, 1981; Fournier, 1987). The theory of correlation between volume and biomass may be considered proven and biomass can therefore be estimated using the equation derived.
Y = 0.51x + 25.19
where y is biomass (kg DM/ha)
and x is volume (M3 DM/ha)
Tests on micro-elements contents of forages in the Niari valley although in their early stages show that cobalt, copper and zinc contents are below the desired limits. Research on trace element deficiencies should be more comprehensive and involve the animal itself by evaluating the amount of micro-elements such as molybedenum and selemium in the blood plasma an hair. This study is relevant in identifying problems in forages and feed resources in real world animal production systems.
The chemical composition of the mineral block licks from the Tchitondi region was considerably varied and appeared to be extremely deficient in cobalt, copper, zinc and manganese. The macro-element contents (Calcium, phosphorus, magnesium and potassium) and micro-element contents (Copper, zinc and manganese) of the maize bran produced by the cattle feeds mill were extremely high. This makes maize bran as a mineral supplement product of major importance to livestock production in the Peoples Republic of the Congo.
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