The value of indigenous browseable tree species in livestock production in semi-arid communal grazing areas of Zimbabwe

H M Sibanda ¹ L R Ndlovu ²

¹ GTZ/ADA-PPU (German Agency for Technical Cooperation/Agricultural Development Authority-Provincial Planning Unit)
PO Box 357, Masvingo, Zimbabwe

² Department of Animal Science University of Zimbabwe
PO Box MP 167, Mount Pleasant, Harare, Zimbabwe

ABSTRACT

A study was carried out to evaluate the nutritive value of indigenous browseable trees and to determine the patterns of utilisation of these trees by cattle and goats in two semi-arid areas. These trees are of good nutritive value: both leaves and pods have a high content of crude protein (7-28%) and low to medium content of neutral detergent fibre (11-64%). Livestock eat leaves in the wet months of November to February and pods in the dry months of May to September. Tree species that produce both leaves and pods thus contribute substantially to the diets of livestock in the areas studied. The results indicate that the current recommended stocking rates for these areas need to be reappraised as they were calculated excluding the contribution of trees/browses. More research on the propagation and utilisation of these species is required.

RESUME

Valeur nutritive des espèces de ligneux fourragers en animale dans les pâturages communaux des zones semi-arides du Zimbabwe

Une étude a été effectuée en vue de déterminer la valeur nutritive des espèces de ligneux fourragers rencontrées dans les deux zones semi-arides du Zimbabwe ainsi que leur mode d'utilisation par les bovins et les caprins dans ces régions. Ces ligneux ont une bonne valeur nutritive dans la mesure où leurs feuilles et leurs gousses sont riches en protéines brutes (7-28%) et pauvres ou moyennement riches en fibres NDF (11-64%). Les animaux consommaient les feuilles pendant l'hivernage (novembre-février) et les gousses en saison sèche (mai-septembre). Par conséquent, les espèces ligneuses produisant à la fois des feuilles et des gousses entrent pour une part importante dans l'alimentation des animaux dans ces régions. Au vu de ces résultats, les charges recommandées pour ces zones doivent être révisées dans la mesure où elles ont été calculées sans tenir compte de l'importance des ligneux et des arbres fourragers dans l'alimentation des animaux. Des études supplémentaires s'avèrent nécessaires sur la dissémination et l'utilisation de ces espèces végétales.

INTRODUCTION

The semi-arid communal areas of Zimbabwe, Natural Regions IV and V (Vincent and Hack, 1960), are characterised by low rainfall and poor sandy soils. Cropping consistently fails in these areas, and so livestock production is the dominant agricultural activity. Livestock management is rudimentary and consists mainly of herding the livestock in the veldt Grass is limited and only available in the rainy months of December to March Indigenous browseable species play an important role in the survival of livestock in these regions, particularly during those months when grass is unavailable.

However, most research on livestock feeds and feeding in Zimbabwe has concentrated on cereal crop grains (Topps and Oliver, 1978), agricultural byproducts (Ndlovu and Manyame, 1988; Smith et al, 1990) and protein supplementation (Sibanda, 1990), all of which are either unavailable or available only in very limited quantities in semi-arid areas. Very little is known about the nutritive value of indigenous browseable tree species. This study therefore aimed to evaluate the nutritive quality of such trees and to establish patterns of their utilisation by cattle and goats throughout the year.

MATERIALS AND METHODS

Study sites

The study was carried out at two sites in the Gwanda District of Matebeleland South Province in Zimbabwe (29°E, 21°S).

Site 1 was Wenlock Communal Area in Natural Region IV, at an altitude of 900-1200 m. Annual rainfall is 450-650 mm; average temperatures are 22-30°C in summer and 10-16°C in winter. The vegetation of the area is the dry Miombo woodland (FAO/UNESCO, 1978) and is dominated by Terminalia sericea and Julbernardia globiflora trees. Other tree species, such as Colophospermum mopane, Combretum spp, Kirkia acuminata, Sclerocarya caffra and Acacia, Albizia and Grewia species dominate in the dark gneiss-derived soils.

Site 2 was the Gwaranyamba-Gwanda-Dibilishaba Communal Areas in Natural Region V, at an altitude of 600-900 m. Annual rainfall here ranges from 300 to 500 mm, and average temperatures are 25-34°C in summer and 14-22°C in winter. The vegetation of the area is the Mopane Tree Savanna, dominated by Colophospermum mopane on the alkaline soils and Combretum apiculatum, Kirkia acuminata, Adonsonia digitata and Sclerocarya caffra on dolerite-derive of soils. Acacia, Albizia and Grewia species are found on dark gneiss-derived soils as at Site 1. The only grasses found at both sites are annuals, such as Boscia, Aristida and Panicum spp.

In both areas cattle and goats are the dominant livestock species and on average each farmer owns 10-17 head of cattle and 10-20 goats. The normal stocking rate is 3-4 ha/tropical livestock unit (TLU: 1 TLU = 250 kg liveweight), which is considered excessive based on the recommended stocking rate of 5-6 ha/TLU.

Data collection

A total area coverage approach was used in order to have a spatial representation of the areas. Key informants, identified among farmers, agricultural extension workers and part-time farmers, were given formal questionnaires on utilisation of indigenous tree species. Hand-clipped samples of leaves, twigs and pods of browseable tree species were collected throughout one calendar year: samples were analysed for crude protein (CP) and neutral detergent fibre (NDF) using AOAC (1985) methods. Patterns of utilisation of different parts of browseable tree species were determined from visual observations by the researchers with the assistance of the key informants. Distribution (density and occurrence) of the tree species was assessed in selected representative locations within each site.

RESULTS

Many of the tree species were common to both sites, but site 1 had a higher overall tree density than site 2 (Table 1). At both sites, leaves were the tree parts most utilised by livestock.

Chemical analysis of the tree parts eaten by the animals (Table 1) showed high CP and low to medium NDF contents. In the two species in which leaves were analysed at different stages, NDF content increased and CP content decreased with maturity. For A. nilotica and D. cinerea, leaves had a higher CP and lower NDF content than pods.

Patterns of utilisation of browseable trees by cattle and goats are shown in Table 2. Cattle fed mainly on leaves between October and January while goats fed on leaves between December and March. Pods were preferred by both cattle and goats in the months of June to September. Cattle ate species like C. mopane and D. cinerea almost all year round while goats mostly ate Acacia spp and D. cinerea all year round. Leaves of Grewia spp were eaten in the wet months of December and January, while leaves of A. quanzensis, K. acuminata and P. thonningii were eaten in the dry months of June to October.

Table 1. Densities, occurrences and crude protein (CP) and neutral detergent fibre (NDF) contents (of the parts utilised by livestock) of browseable tree species

Species	Approximate tree density (trees/m²)		Occurrence (fraction of locations visited)		Parts used	Chemical content of parts used (%)
	Site 1	Site 2	Site 1	Site 2		CP	NDF
Acacia nilotica	0.2	-	2/4	-	Leaves	11.25	13.20
Acacia tortilis	1	2-3	¾	5/5	Leaves	19.12	-
Afzelia quanzensis	-	0.002	-	1/5	Leaves	19.74	50.96
					Early leaves	18.46	44.16
Colophospermum mopane	3	1	2/4	3/5	Mature leaves	15.42	47.58
					Dry leaves	14.86	56.90
Combretum apiculatum	1	0.01	3/4	5/5	Leaves	21.50	18.34
Combretum hereroense	1	0.007	2/4	1/5	Leaves	8.51	19.27
Combretum imberbe	-	0.001	-	1/5	Leaves	13.00	28.20
Commiphora africana	-	0.003	-	2/5	Leaves	17.64	-
Dichrostachys cinerea	1	0.01	4/4		Pods	28.54	45.93
Grewia bicolor	1	1	3/4	5/5	Leaves	21.12	60.71
Grewia flavescens	-	1	-	1/5	Leaves	11.45	32.40
Kirkia acuminata	-	0.003	-	3/5	Leaves	8.11	11.80
Lonchocarpus capassa	0.2	-	1/4	-	Early leaves	22.29	59.08
					Mature leaves	12.72	63.83
Piliostigma thonningii	0.4	-	3/4	-	Pods	7.87	55.37
Terminalia sericea	1	-	2/4	-	Leaves	9.15	20.23

DISCUSSION

Tree density seems to be affected by rainfall; site 2 had a lower tree density than site 1. While tree density is important in indicating the amount of biomass available to livestock, CP and NDF contents indicate the quality of the biomass; in general, feed quality increases as CP content increases and/or NDF content decreases. On this basis, the Combretum and Acacia species and K. acuminata are good quality browses. The role of C. apiculatum in livestock production is limited by its low density. There is a need for research to study the possibility of seeding and propagating C. apiculatum. Unavailability notwithstanding, cattle eat C. apiculatum leaves for five months of the year, indicating that the leaves are quite palatable to them.

Table 2. Tree parts eaten by cattle and goats during the year

Of the tree species that serve as livestock feed throughout the year, C. mopane is the most interesting. In October-December, C. mopane produces reddish brown young leaves that livestock eat avidly, but only after they have been cut and wilted for a day or two; mature green leaves are not eaten at all. The dry leaves are often covered with sweet-tasting mucilage, produced by insects, which apparently enhances their palatability. No information is available on the chemical composition of the mucilage. Research is needed to establish optimal utilisation of C. mopane which is highly drought-resistant and has a high CP and moderate NDF content.

The other tree species eaten by livestock throughout the year is D. cinerea. Cattle browse on the leaves from December to April, and on pods from May to September. The pods are not processed in any way, and this leek of processing could reduce their effective utilisation.

Acacia species are thorny and only goats, which have pointed snouts and mobile lower lips, are able to avoid the thorns and browse on the leaves. Cattle only eat the pods from Acacia species.

The abundance of pods in the dry months of June to September partly accounts for the survival of livestock during this time, even though, according to official stocking rates, these areas are overstocked. Official stocking rates were calculated without taking into account the contribution of browseable trees.

It is interesting that cattle at both sites eat tree parts for 10 months of the year. Bovines, as a species, are grazers, but the cattle indigenous to these areas have adapted to the shortage of grazing and have become facultative browsers.

There is need for more intensive laboratory evaluation of these browseable tree species, not only on their CP and NDF contents but also on potential anti-nutritional factors. At present very little information is available on the presence of polyphenolics in these species, even though these compounds are known to affect digestibility and utilisation of forages (Barry and Manley, 1986).

CONCLUSIONS

The results of this study show that browseable trees are highly nutritious and contribute substantially to the diet of livestock in these communal areas. However, stocking rates were calculated excluding the contribution of trees. Incorporating this contribution in the calculations could clear up some of the contradictions between the people's need for livestock and policy-makers' fears of environmental degradation if livestock numbers increase.

More research is needed on the propagation of these browseable tree species and on technologies to increase their productivity and utilisation. The current trend of introducing non-indigenous tree species at the apparent expense of the indigenous ones should be re-evaluated.

REFERENCES

AOAC (Association of Official Analytical Chemists). 1985. Official methods of analysis. 14th edition. AOAC, Washington, DC, USA. 250 pp.

Barry T N and Manley T R. 1986. Interrelationships between concentrations of total condensed tannin, free condensed tannin and lignin in Lotus sp and their possible consequences in ruminant nutrition. Journal of the Science of Food and Agriculture 37:248 254.

FAO/UNESCO (Food and Agriculture Organization of the United Nations/United Nations Educational, Scientific and Cultural Organization). 1978. Report on the agro-ecological zones project. Volume 1. Methodology and results for Africa. World Soil Resources Bulletin 48. FAO, Rome, Italy. 168 pp.

Ndlovu L R and Manyame Z. 1988. Hydration as a means of improving utilization of maize stover fed to steers. In: Said A N and Dzowela B H (eds), Overcoming the constraints to the efficient utilization of agricultural by-products as animal feed. Proceedings of the Fourth Annual Workshop held at the Institute of Animal Research, Mankon Station, Bamenda, Cameroun, 20 27 October 1987. ARNAB (African Research Network for Agricultural By-products). ILCA (International Livestock Centre for Africa), Addis Ababa, Ethiopia. pp. 343-353.

Sibanda S. 1990. Protein supplementation of steers on range. DPhil Thesis. University of Zimbabwe, Harare, Zimbabwe. 240 pp.

Smith T. Manyuchi B and Mikayiri S. 1990. Legume supplementation of maize stover. In: Dzowela B H. Said AN, Asrat Wendem-Agenehu and Kategile J A (eds), Utilization of research results on forage and agricultural by-product materials as animal feed resources in Africa. Proceedings of the First Joint Workshop held in Lilongwe, Malawi, 5-9 December 1988. PANESA/ARNAB (Pasture Network for Eastern and Southern Africa/African Research Network for Agricultural By-products). ILCA (International Livestock Centre for Africa), Addis Ababa, Ethiopia. pp. 302-320.

Topps l H and Oliver J. 1978. Animal foods of central Africa. Rhodesia Agricultural Journal Technical Handbook No 2. Government Printer, Salisbury, Rhodesia [now Harare, Zimbabwe]. 122 pp.

Vincent V and Hack H R. 1960. Land use and recommended farming systems for different land capability classes in different areas of Southern Rhodesia. Annual Conference of the Professional Officers of the Department of Research and Specialist Services. Government of Southern Rhodesia, Salisbury, Southern Rhodesia [now Harare, Zimbabwe]. pp. 16-27. [ILCA library accession (microfiche) number 56017]