Abstract
Introduction
Plant introductions
Nitrogen fixation under Swaziland conditions
Inoculation of legume seeds
Response to fertilizers
Veld improvement with legumes
Introduction of legumes into veld through grazing animals
Research on temperate legumes
The future of forage legumes in Swaziland
Conclusions
References
B.H. Ogwang
University of Swaziland, P.O. Luyengo, Swaziland
Livestock production in Africa is often limited by inadequate supplies of good quality forage. The importance of forage legumes in overcoming feed shortage has long been recognised in Swaziland judging by the amount of research that was carried out during the 1960s and 1970s. Many areas of legume management were explored during that period, including plant introduction and testing, N fixation, legume inoculation, legume nutrition and the introduction of legumes into native veldt While much was learnt from these studies, this paper emphasises the need to intensify research on other areas of legume management to minimise dependence on pure grass pastures, which require large quantities of costly fertilizer N for maximum output.
Swaziland, bordered by the Republics of South Africa and Mozambique, is one of the smallest African countries, situated between the 25th and 27th parallels south of the equator. It has a total land area of 17,365 km, which is divided into four distinct topographical regions: the high-veld, middleveld, lowveld and lubombo, each running roughly north to south. Agriculture is the backbone of the country's economy, and current estimates put the livestock population at some 636,000 head of cattle, 320,300 goats, 31,800 sheep and 16,456 pigs (HTS, 1983).
The cattle are mainly Nguni, a local breed of the Sanga type. It is a dual-purpose animal, producing both meat and milk. Its milk production is estimated at between 400 and 500 litres per lactation, which is just sufficient to raise a calf. However, under good management and feeding it may produce an extra 250 litres which could be sold. Jerseys, Friesians, Canadian Holsteins, Simmentaler and other Bos taurus breeds are gaining popularity in both the highveld and middleveld where the climate is favourable. Crosses of these breeds with the Nguni are to be found scattered throughout the country. Although the cattle population is far higher than the human population, the demand for meat, milk and other livestock products still exceeds supply. Government efforts to achieve self sufficiency in these products are emphasized in the current 5-year development plan. Inadequate nutrition is recognised as the major constraint to introducing exotic breeds and on the productivity of the local Nguni.
Given the prevailing economic conditions, Swaziland will have to rely heavily on intensive use of pastures to improve the nutritional status of livestock. The cost of other feed sources are generally too high to be recommended for widespread use at current management levels. However, the productivity of both native pastures (or veld) and planted pastures is limited by nitrogen deficiency in most soils in the country. Moreover, many forage species grow very rapidly as soon as the rains commence in early summer (October). At this time leaf production is high and crude protein content and dry-matter digestibility are satisfactory. By the end of the wet season (March) the plants develop tall flowering stems which are low in protein and high in fibre. These conditions place severe restrictions on livestock production, particularly during the dry season, which can be as long as 7 months in a year.
In order to increase pasture productivity and quality significantly, the low N status of the soils could be supplemented either by applying fertilizer N or by growing forage legumes.
Due to the high input of fossil energy required for its manufacture, N fertilizer is generally expensive and this is reflected in its low consumption in Africa (IFDC, 1980). Moreover, current trends in global recession are likely to create further instability in the price and supply of N fertilizer. A cheaper and more effective way of increasing the N status of the soil would be to introduce legumes, which not only fix appreciable quantities of atmospheric N but also contain high levels of protein, minerals and vitamins. Furthermore, this high quality can be maintained over a long period and their inclusion in grass pastures may prolong the grazing season.
The importance of forage legumes has long been recognised in Swaziland judging by the amount of research that was carried out during the 1960s and 1970s. Although much of the work was not published, many areas of legume management were covered, examples of which are given below.
Between 1959 and 1968 a programme of plant introduction and screening was carried out in the major ecological zones of Swaziland (Whitmarsh, 1975). A world collection of over 100 legume species and cultivars was established in nurseries located at Malkerns Research Station (moist middleveld), Luvo (dry middleveld), Big Bend (lowveld) and Mangcongco (high-veld). All introductions were evaluated for productivity, seed yield, persistence and palatability. At the end of the 10-year period, the following species were scored as highly suitable for the different ecological zones:
|
Moist middleveld
|
Cajanus cajan |
|
Desmodium discolor |
|
|
Desmodium intortum |
|
|
Desmodium sandwicense |
|
|
Desmodium uncinatum |
|
|
Glycine javanica |
|
|
Leucaena javanica |
|
|
Trifolium repens |
|
|
Dry middleveld |
Cajanus cajan |
|
Desmodium discolor |
|
|
Desmodium intortus |
|
|
Leucaena leucocephala |
|
|
Lowveld (under irrigation) |
Desmodium uncinatum |
|
Glycine javanica |
|
|
Medicago sativa |
|
|
Highveld |
Lolononis bainesii |
|
Trifolium spp. |
Later trials were generally based on these recommendations.
The beneficial effect of pasture legumes in maintaining or increasing the N status of soils in the temperate zone has long been established. Data collected from the temperate regions of Australia, Europe and the USA show that pasture legumes can fix up to 515 kg N/ha (Bryan, 1962). However, doubts have been expressed regarding the capacity of tropical legumes to fix N. From a number of experiments on the African continent, it was concluded that tropical legumes were unlikely to improve soil fertility and animal production (Anon., 1967), although these misgivings were later abandoned (Keya, 1977; Ayanaba, 1980). Several trials to investigate the ability of perennial tropical legumes to replace N fertilizers applied to pure grass pastures were established at Malkerns Research Station in the Swaziland middleveld. Some of the results are summarised in Table 1.
Table 1. Yields of grass pastures fertilized with 112 kg N/ha and unfertilized grass/legume mixtures, Malkerns Research Station, Swaziland, 1967/68 - 1972/73.
|
Species |
Year |
Mean |
|||||
|
1967/68 |
68/69 |
69/70 |
70/71 |
71/72 |
72/73 |
||
|
Eragrostis/Paspalum + N |
4571 |
6730 |
6141 |
5003 |
5790 |
1045 |
4880 |
|
Eragrostis/Paspalum + Desmodium |
3942 |
5535 |
2074 |
8919 |
1500 |
1018 |
3831 |
|
Kikuyu + N |
3436 |
4467 |
3537 |
3944 |
7314 |
1744 |
4073 |
|
Kikuyu +Desmodium spp. |
2689 |
1179 |
1642 |
2494 |
1164 |
769 |
1656 |
|
Stargrass + N |
6424 |
6573 |
4358 |
4137 |
5522 |
3272 |
5047 |
|
Stargrass +Desmodium |
2748 |
3153 |
2164 |
3211 |
1402 |
1140 |
2303 |
Source: Whitmarsh (1975).
It is clear from these trials that Desmodium spp. were unable to substitute adequately for fertilizer N. For example, Kikuyu grass + 112 kg N/ha yielded a 6-year mean yield of 4073 kg dry matter/ha, whereas Kikuyu + Desmodium uncinatum gave only 1656 kg/ha. Similarly, Stargrass + 112 kg N/ha gave a 6-year mean yield of 5047 kg dry matter/ha while Stargrass + Desmodium gave only 2303 kg/ha. These results are disturbing when compared with figures from East and Central Africa. Trials in Uganda by Wendt (1970) showed that adequately fertilized pastures of Stylosanthes guianensis (=S. gracilis) could fix as much as 290 kg N/ha per year. At Kitale in Kenya, Desmodium uncinatum was shown to fix up to 178 kg N/ha per year (PRS, 1969). In Malawi, the inclusion of Stylosanthes guianensis and Desmodium intortum produced a dry-matter yield equivalent to that of a pure grass sward receiving 448 kg N/ha per year (Department of Agriculture, 1961).
The inability of the legumes to fix N under Swaziland condition may be attributed in part to the relatively short growing season typical of southern Africa. It would perhaps require several years before a response could be detected. Furthermore, Thomas (1973) pointed out that the poor performance of tropical legumes in some instances may not be due to any inherent inability to fix N but rather to a lack of understanding of their requirements for normal growth and development. Thus, if the true potential of legumes is to be fully realised it is essential to gain a knowledge of the environmental factors that may limit their growth, nodulation and N fixation. Some of these factors have been investigated in Swaziland.
The presence or absence in the soil of the appropriate Rhizobium sp. determines whether or not inoculation of the legume seed is required. Those species or varieties that do not require inoculation have an obvious advantage in countries such as Swaziland where pasture improvement is being undertaken by small-scale farmers. In a review article, Thomas (1973) cited many instances of natural nodulation in Stylosanthes spp. However, he also observed that such symbioses may not necessarily be the most effective in fixing N. since there are wide variations in the effectiveness of rhizobium strains isolated from different sites. Inconclusive results were obtained from a short-term study conducted in Swaziland using inoculated stylo oversown in unimproved veldt The study comprised three cultivars of Stylosanthes guianensis together with Stylosanthes hamata and S. humilis. In one treatment the legumes were inoculated and coated with peat, Mo and P. In another, the legumes were not inoculated but coated with peat, Mo and P. The control plots received no treatment. Fresh herbage yields from two cuts are shown in Table 2.
Table 2. Effect of seed treatment on fresh-weight yields of Stylosanthes spp. cultivars from two cuts.
|
Legume |
Seed treatment |
||
|
Inoc/treat1 |
Uninoc/treat |
Control |
|
|
|
Fresh weight (t/ha) |
||
|
S.guianensis |
|
|
|
|
(Schofield) |
7.1 |
7.1 |
7.1 |
|
S. guianensis |
|
|
|
|
(Endevour) |
7.8 |
8.1 |
6.4 |
|
S. guianensis |
|
|
|
|
(Cook) |
8.5 |
7.8 |
7.8 |
|
S. guianensis |
|
|
|
|
(Verano) |
9.5 |
6.8 |
6.8 |
|
S. humilis |
|
|
|
|
(Townsville) |
6.4 |
5.9 |
7.9 |
|
Mean |
7.9 |
7.1 |
7.2 |
|
SE ± 0.94 |
|
|
|
Source: Ministry of Agriculture and Cooperatives (1979).
1. Inoc. = inoculated with rhizobia
Uninoc. = uninoculated
treat. = peat coating + Mo + P.
Average yields were 7.9 t/ha for inoculated legumes, 7.1 t/ha for non-inoculated legumes and 7.2 t/ha for the control. Thus, inoculation of these Stylosanthes species did not give any obvious advantage. These results appear to indicate that under Swaziland conditions, stylosanthes may modulate freely in the field without inoculation. Similar results have been reported for desmodium in Uganda (Wends, 1971) and Kenya (Keya and van Eijnatten, 1975). This could be confirmed by conducting a more comprehensive trial.
Apart from the short-term need to modify the environment to give favourable conditions for the seedlings to establish, long-term modifications are also needed to enable legumes to thrive in competition with grasses. An experiment established in 1964 (I'ons, 1967) showed that lime was essential for the legume Desmodium pilosiusculum (=D. sandwicense) but that trace elements (Zn and Mo) produced no significant effects. In a later trial (I'ons, 1968), green-leaf desmodium and silverleaf desmodium were shown to respond to lime up to 500 kg lime/ha, higher rates depressing dry-matter yield. This low lime requirement of desmodium on a highly acid soil is in agreement with several Australian findings (Norris, 1966). In the same trial, applying up to 25 kg K/ha significantly increased the productivity of silverleaf desmodium. No further yield increment was recorded beyond 50 kg K/ha. The legumes did not respond to P. This is surprising because many crops have responded to P fertilizers on the red kaolinitic soils at Malkerns Research Station and research elsewhere has shown that Desmodium spp. should be no exception (Olsen and Moe, 1971). This finding appears to suggest that response of legumes to P in Swaziland is unlikely to be as spectacular as has been reported in other countries.
Small-plot trials in Swaziland have consistently shown that the introduction of legumes into native veld results in large increases in both the amount and quality of herbage produced. One can expect therefore that the carrying capacity will be increased and the performance of the animals enhanced as compared with animals grazing unimproved veldt This hypothesis was tested in one trial in which Stylosanthes guianensis + 20 kg P/ha was oversown on an unimproved veld in the moist middleveld. Table 3 shows the liveweight gains per unit area per year from 1966 to 1973.
Table 3. Mean liveweight gain (kg/ha per year) of cattle grazing unimproved veld and veld oversown with stylo, Swaziland middleveld, 1966/67 - 1972/73.
|
Year |
Control |
Stylo + P |
|
1966/67 |
126.6 |
170.2 |
|
1967/68 |
128.8 |
182.6 |
|
1968/69 |
94.1 |
131.0 |
|
1969/70 |
97.4 |
173.6 |
|
1970/71 |
61.6 |
191.5 |
|
1971/72 |
134.4 |
224.0 |
|
1972/73 |
114.2 |
159.0 |
Source: University of Botswana, Lesotho and Swaziland (1975).
Although stocking rate was somewhat arbitrarily adjusted to conform with level of production in the paddocks, there was an increase in livestock performance from the use of stylo + P in every year of the trial. These results indicate that livestock productivity and the carrying capacity of the moist middleveld can be increased significantly by applying P and introducing stylo. It is unfortunate that the effects of P and the legume were not separated during the trial period.
The need to keep pasture establishment and maintenance costs as low as possible has been a major consideration in Swaziland's. research programmes, which are aimed largely at improving peasant agriculture. One study investigated the possibility of oversowing the veld with seed dropped in the faeces of cattle fed a meal containing legume seed. The use of such a method of seed establishment presupposes that the legume species used does not require inoculation, because it is assumed that rhizobia would not survive the ruminant digestive process and inoculation of seed would therefore be pointless. Presumably the inoculant would have to be introduced separately into the soil. Workers in New Zealand (Suckling, 1952) and Japan (Yamada et al, 1972) have succeeded in establishing legumes by this means, particularly those species that produce a large proportion of hard seed. seed that is not hard or that has been scarified in mechanical harvesting is more likely to be digested by the animal.
In the trial in Swaziland, established in 1974 (Whitmarsh, 1975), steers were fed a weekly ration of 1.5 kg maize meal and 200 g of live seed of Desmodium intortum per animal. Unpublished data from the study indicated that the germination percentage and survival rate of desmodium were high enough to recommend this method of legume establishment for veld improvement in Swaziland.
Many temperate legumes, e.g. white clover and lucerne, have been grown successfully in the subtropics, although their reliability for year-round grazing has been questioned. The winter climate of Swaziland appears to be ideal for the production of temperate pasture species. Research has shown that a wide range of temperate legumes can be grown with irrigation during the winter, including lucerne, red clover, white clover, purple vetch and lupin. However, there are indications that in most cases the hot, subtropical summer conditions may be too severe to allow persistence of the temperate species (Whitmarsh, 1968). The effect of over-seeding established Eragrostis curvula pasture with white clover for irrigated winter grazing was studied in the middleveld (Whitmarsh, 1975). The hypothesis was that temperate legumes may stand a better chance of survival if grown in the partial shade of a tropical grass, provided that correct management is employed to ensure that competition from the grass is not too intense. In this region, most tropical grasses become dormant during winter, irrespective of water supply. Thus if it proves possible to maintain the legume throughout the summer, it may thrive on its own in winter with irrigation. As a result, tropical species could provide the main grazing in summer followed by temperate species in winter. This would allow the pasture to be used throughout the year. Moreover, the legume would provide extra N. which would benefit the grass in each succeeding summer. Unpublished yield data taken from the study clearly indicated the attractiveness of this strategy. In a small-scale dairy project at Malkerns (Whitmarsh, 1977) it was demonstrated that very high levels of milk production could be obtained throughout the year by combining tropical species such as Rhodes grass with a mixture of temperate species such as white clover and Italian ryegrass with irrigation in the winter time.
Research on forage legumes in Swaziland started in 1958 and an active programme was pursued until 1978. Reference has been made to some of these trials above. Since that time, only a few long-term trials have been maintained and most of these have now outlived their usefulness. From the initial period of active research much valuable information has been accumulated and the basic principles of veld and pasture management for local conditions are now well established. However, as the cost of N fertilizer continues to increase there is an urgent need to increase research on the potential of legume-based pastures. Any new programme of research will inevitably resolve itself into three basic activities:
1. Plant introduction and testing;
2. Legume nutrient requirements; and
3. Evaluation of management techniques.
Plant introduction and testing
Since 1958, when the first introduction nurseries were established, the legume species that grow best in the local environment have been determined. Any future work should be directed towards the selection of more productive cultivars within these species. Plant breeders throughout the world are constantly improving on the existing material and many of the improved lines have not been grown or tested in Swaziland. In addition to introduced cultivars, a collection of indigenous legumes will have to be built up for subsequent identification and testing.
Nutrient requirements of legumes
Much groundwork has already been done but, as pointed out earlier, some of the results were inconclusive, emphasizing the need for further studies. Moreover, with improvement in management techniques, it is likely that nutrient requirements will change and new recommendations will have to be formulated.
Allied to legume nutrition will be the question of rhizobial activity and its contribution to the production of protein.
Evaluation of management techniques
Both irrigated and rainfed pastures may be planted as pure stands or as mixtures of grasses and legumes. The components of the mixture need not be planted at the same time. The seeds may be broadcast, drilled, sown under a companion crop or interplanted in an annual cash crop such as maize. More information is needed as to which combination of such factors is best in a given situation. Furthermore, a thorough knowledge of the growth curves of new cultivars will be necessary in order to put them to maximum use in extending the productive period of the pastures.
The success of such research will depend on cooperating with other institutions, reviewing world literature relevant to the research programme, visiting research institutions in neighbouring countries and building up a team of workers and technicians together with the necessary research equipment and facilities.
Apart from the limited research currently being carried out at the University of Swaziland (Ogwang, 1985), pasture research in Swaziland has been at a virtual standstill since 1978. A project on pasture improvement is soon to be started by the Ministry of Agriculture with the financial backing of the International Development Research Centre, Ottawa, Canada. This move should renew interest in forage legumes.
Although a vast amount of knowledge has been accumulated on forage legumes, their popularity has remained relatively low among Swazi stockowners. This may be attributed to problems of bloat and management, lack of persistence and poor hay-making qualities, as well as the availability of cheap N fertilizer from neighbouring countries in the past. The farmers can only be encouraged to adopt large-scale use of forage legumes through effective extension services. Simple technological packages can be easily adopted by the average stockowner while sophisticated systems may be tested first in the Government-sponsored Rural Development Areas.
Although legume-based pastures without N fertilization are likely to produce less forage than a pure grass pasture receiving high levels of N. the numerous advantages associated with legumes should make them a more attractive feed source in the coming years.
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