The weeds of abandoned cotton fields in Mozambique

Based on a botanical survey of Chibuto, Chemba, Namapa, Nampula and Mutuali, a table of 120 of the most common weeds of cotton and fallow fields in Mozambique is presented. After considering the factors which affect the flora of these zones, the successional weed vegetation is described and the principal species are enumerated. Finally, the dangers of infestation (shrubby weeds in alluvial zones) and the utilization of the weed flora in other zones of the country are described.

Tropical East Africa and Flora West Tropical Africa.
Analysis of the distribution of the species in the study areas (see Table 1) shows a difference in botanical composition indicating two distinct weed floras on distinct ecological sites.Weeds marked with an asterisk are abundant.
2. In the Nyassan sub-littoral and subplanaltic study areas, at Namapa and Mampula, and the south-central study area at Mutuali there is an abundant indigenous ruderal weed flora.Its compo sition and distribution is largely influenced by climatic factors and the cotton monoculture.The principal weeds are as follows: Aspilia kotschyi var.kotschyi, Aspilia schimperi, Chrysanthellum americanum, Digitaria milanjensis, Digitaria setivalva*, Eragrostis cylindriflora*, Indigofera colutea var.

OBSERVATIONS
The cultivation of cotton has had an important influence on economic development in the central and northern regions of Mozambique.The control of weeds in cotton fields in these regions has been a major problem for many years.Enforcement of control measures was responsible for socio/political trouble during the late 1940's and early 1950's.
In 1960 a survey was made of the weed vegetation of cotton and fallow fields (Lemos, 1961;Balsinhas, 1962).In this survey a large number of herbarium specimens was collected, and duplicates were sent to the herbaria COI, LISC, KEW, SRGH and PRE.Field notes including common and local names were recorded.
The present author then published the first check list of plants growing in cotton and fallow fields in Mozambique (Balsinhas, 1963).Research work and experimentation on herbicides was initiated in the same year (Almeida, 1970).
However, this is a vast subject and as Mozambi que is traditionally an agricultural country it is essential that knowledge of the weed flora is improved.This is the motive for a further contribution to the subject using the existing information in the National Herbarium, Pretoria.1, Tribulus terrestris and Vernonia glabra occur in all the study areas.The former species is largely disseminated, as a weed, in tropical and temperate regions throughout the world and the second is widespread only in tropical Africa.Boerhavia diffusa, Corchorus tridens, Ocimum canum and Striga asiatica are common tropical weeds and occur in four of the zones.The weeds that occur in one, two or three zones only, are more restricted to the environmental conditions described in the text.

The notes on geographical distribution in
If the fields in the alluvial zones are left fallow too long or without crop rotation, there is a danger that thorny thickets will develop which will rarely succeed to open grassy savanna.Once thickets develop the recurrent use of chemical products, to keep large areas of land clear, is both difficult and expensive.mocambic. 4,4 : 253 -264. B alsinhas, A ., 1962

V
+ + + + + Widespread in tropical Africa Vernonia kirkii Oliv.& Hiern + Zambia, Zimbabwe and Mozambique Vigna sp.(Balsinhas & Macuacua 543, at PRE) Throughout tropical Africa from Transvaal to Egypt and Senegal; also in Arabia, Palestine and in Madagascar * Study areas: 1 ,Chibuto; 2 ,Chemba; 3, Namapa; 4, Nampula; 5, Mutuali .In the sub-plan altic zon es th e succession o n fa llo w a n d a b a n d o n e d fields is to w ard s a type o f grassland w ith w o o d y e le m e n ts.In th e first tw o years a th ic k , close sw ard is fo r m e d w h ic h , w ith correct m a n a g e m e n t, can be u sefu l fo r o th e r a g ric u ltu ra l activities such as g ra zin g fo r cattle.A C K N O W L D G E M E N T S I w ish to th a n k M rs K .J .M u s il fo r h e lp in g w ith the tr a n s la tio n o f the p a p e r.R E F E R E N C E S A lm e id a, F. S., 1970.Herbicides on cotton.II.Results of the trials carried out in Mozambique from 1963 to 1970.Agron.

TABLE 1 .
-The occurrence o f weed species in five study areas in Mozambique (see Fig. 1) + Botswana, Zimbabwe, S.W. Africa, Angola and South Africa (Transvaal); also in Kenya and Tanzania Hewitia sublobata (L.f.) 0. Ktze + Throughout tropical Africa, Asia, Malaysia and Polynesia, also in Jamaica as an escape + + + + + In tropical and temperate regions throughout the world Trichodesma zeylanicum ( L.) R. Br. + + + Distributed through India, Malaya, Australia and Africa Tricliceras longepedunculatum (Mast.)R. Fernades var.eratense R. Fernandes + Known only from Mozambique.Endemic Tridax procumbens L. + + + A native of central America, now widespread in tropical Africa, Madagascar, Asia and Australia.Also in South