In Africa, weed control takes aim at parasitic striga

The multiple hazards confronting crop and livestock production in Africa sometimes seem almost to be vying with one another to create the greatest havoc. Drought, locusts, mealybugs, rinderpest, tsetse flies - all have been targeted as major impediments to the continent's efforts to achieve greater food security.

Now add to that list striga, a parasitic weed that attaches itself to the roots of several of Africa's most important food crops, siphoning off moisture and nutrients, inhibiting growth, and sometimes completely destroying the host plant, especially under the stress of drought. At serious risk to its depredations are about two thirds of the 73 million hectares devoted to cereal crop production in Africa. Heaviest losses occur in savannah zones stretching from Cape Verde on the west coast through western, central, eastern, and southern regions of the continent, a sweep that also embraces major food legume producing areas in most of the affected countries. Yields of cowpea, maize, millet, sorghum, rice and sugar cane have all been considerably reduced by striga infestations.

Quantifying the actual economic loss is extremely difficult, but its general scale of magnitude may be judged from the range of estimates that reach from around $500 million dollars annually to as high as $7 billion, this latter calculation arising from an eight-year-long survey involving a number of scientific institutes concerned with suppressing the weed.

Whatever may be the difference in estimates of damages attributable to striga, there appears to be general agreement that these food losses are increasing at an alarming rate. There are also other equally disturbing elements in the situation:

- Control of striga is extremely difficult because the plant produces myriads (as many as 40 000 per plant) of minuscule, powder-like seeds that may remain dormant - but viable in the soil for as long as 20 years, depending on climatic conditions, germinating only under the stimulus exuded from potential host plants, or, in some cases, from certain non-host plants. Since the initial parasitic growth generates from the roots of the host plant, considerable damage may already have been caused before farmers become aware of striga's presence above ground.

- Because they are least able to afford conventional methods of striga control, small farmers are most vulnerable to its infestations.

- While plant scientists do not claim to understand fully the causes of the present more rapid spread of striga, they now acknowledge that there is a possible link with continuous cereal cultivation and consequent decreases in soil fertility.

To tackle these problems, FAO and the Organization of African Unity (OAU) have proposed an intensive programme aimed especially at helping small-scale farmers to control the weed. Over the past couple of years this programme has sponsored a technical workshop and an all-Africa government consultation that have succeeded in collecting, and in exchanging among scientists, planners, and potential donor agencies, a considerable body of information about the nature of the striga problem and various experiences in attempting to control it. The recently released report of the FAO/OAU Government Consultation on Striga Control, held in Maroua (Cameroon) last October, remarked that "Yield losses attributed to striga damage cannot be over-emphasized in view of the importance of the host crop in the daily diet of the people affected. The small-scale farmer, who must grow cereals to feed his family, is helpless since, in the past, there were no effective striga control methods available to him. In most African countries, the small scale farmer practice of hoeing to control weeds is not only ineffective for controlling striga, but it is also labour-intensive and results in further depletion of nutrients and moisture in the already impoverished soil. Until recently, governments and international agencies have failed to recognize the economic impact of striga let alone help the small-scale farmer in his predicament."

More practical help may, however, be on the way. Dr L. J. Matthews, a weed specialist with FAO for the past ten years, is optimistic about the potential of a package of cultural practices recently developed and field tested in the Gambia as a measure that could be adopted by small-scale farmers to control the weed. Funded by the Permanent Interstate Committee for Drought Control in the Sahel (CILSS) this research programme identified a number of control "packages" that were effective for both sorghum and millet. These included: the use of striga-tolerant varieties, side dressing of urea fertilizer four weeks after planting, spot spraying of emerging striga shoots with contact herbicide using a hand sprayer, and tethering cattle in the fields after harvest whenever possible. For millet, strip cropping has proved an additional useful practice.

The important point, in Dr Matthews' view, is that weed control measures should be compatible with sound soil conservation practices. He cites the results from comparisons of four differing sets of weed-control practices tested in the Gambia, two of which depended on minimum tillage techniques; the other two involved much greater amounts of cultivation by both humans and animals. Using a jab planter for seeding and restricting weed control to a 20-cm band on either side row reduced labour requirements by about 50 per cent in the case where the band was hand weeded, or roughly 90 per cent if a herbicide was used. And yields were from 25 to 50 per cent higher than in the more traditional cultivation methods that involved shifting some 500 to 1000 tons of soil per hectare.

The Government Consultation was sufficiently impressed by these results that it recommended that the applicability of these packages should be verified in various national programmes. Since CILSS funding for the Gambia programme ran out early this year, the Consultation also recommended a meeting of donors as a matter of urgency.

Peter Hendry