By Dipo Olowookere

An international team of researchers representing the Africa Rice Center (AfricaRice), the International Rice Research Institute (IRRI) and Wageningen University, has raised the alarm over the enormous economic impact of parasitic weeds on rice production in Africa, threatening the food security and livelihoods of millions of resource-poor rice farmers and consumers in the region.

Smallholder farmers in the continent are losing every year half a million tons of rice worth about US $200 million because of parasitic weeds.

This is roughly equivalent to the annual rice consumption of Liberia, a low-income country, which is highly dependent on rice imports.

If the rice lost due to the parasitic weeds had been saved, it would have been enough to feed the total population of Liberia (4.5 million people) for a whole year.

Parasitic weeds are among the most destructive and problematic weeds to control.

“When these plants invade food crops, they turn into ferocious weeds,” said Dr Jonne Rodenburg, Agronomist at AfricaRice.

The most important parasitic weed species in rice are Striga asiatica, S. aspera, S. hermonthica and Rhamphicarpa fistulosa. They are all endemic to Africa and can also parasitize other cereal crops like maize, sorghum and millet.

The team of researchers reveal that these parasitic weeds, which survive by siphoning off water and nutrients from host crops, have invaded 1.34 million hectares of rainfed rice in Africa, affecting an estimated 950,000 rural households. They are increasingly becoming severe due to an intensification of agricultural production and climate changes.

The areas affected by parasitic weeds are home to some of the world’s poorest farmers.

Studies by AfricaRice and partners have shown that parasitic weeds seem to predominantly affect women farmers in Africa as they are often forced to grow rice on the most marginal and parasitic weed-infested plots.

Parasitic weeds threaten rice production in at least 28 countries in Africa that have rainfed rice systems. The most affected countries are Burkina Faso, Cameroon, Côte d’Ivoire, Guinea, Madagascar, Mali, Nigeria, Sierra Leone Tanzania and Uganda.

The researchers warn that these parasites are spreading fast in the rainfed rice area and if nothing is done to stop them in their tracks, the damage will increase by about US $30 million a year.

These findings were revealed in a recent article by Rodenburg, Demont, Zwart and Bastiaans, entitled “Parasitic weed incidence and related economic losses in rice in Africa,” published in Agriculture, Ecosystems and Environment 235 (306-317).

Rice is the second most important source of calories in Africa. It is also critical for smallholder incomes. Demand for rice is growing at a rate of more than 6% per year – faster than for any other food staple in sub-Saharan Africa (SSA), because of changes in consumer preferences and urbanization. Rice production is increasing across SSA, but the continent still imports some 40% of its rice.

Until now, there has been little information on the regional spread and economic importance of parasitic weeds in rice in Africa. “We have presented in this article best-bet estimates on the distribution as well as the agronomic and economic impact of parasitic weeds in rice in Africa,” explained Dr Rodenburg. “In fact, this is the first multi-species, multi-country impact assessment of parasitic weeds in Africa.”

The article focuses on the four most important parasitic weeds in rice. Striga species – known under the common name “witchweed” – occur in at least 31 countries with rain-fed upland rice systems.  Rhamphicarpa fistulosa – known under the common name “rice vampireweed” – threatens rice production in at least 28 countries with rainfed lowland rice systems.

Dr Sander Zwart, AfricaRice Remote sensing and Geographic information systems specialist, explained that for this study, a map of rainfed rice production areas, compiled from different databases, was overlapped with parasitic weed observation data retrieved from public herbaria to visualize regional distribution of these four important parasitic weeds.

From this overlap, probabilities of actual infestation were estimated. These estimates together with secondary data on parasite-inflicted crop losses and efficacy of weed control were combined into a stochastic impact assessment model.

The knowledge acquired on the distribution as well as the agronomic and economic impact of parasitic weeds in rice in Africa underlines the importance of finding effective measures to control these pests through research.

AfricaRice and its partners have been investigating and developing efficient parasitic weed management strategies that are affordable and feasible for resource-poor rice farmers. “A range of high-yielding, short-cycle, farmer-preferred rice varieties have been identified with resistance or tolerance to different species and ecotypes of Striga, as well as varieties with good defense against R. fistulosa,” said Dr Rodenburg.

He explained that such varieties can be combined with different agronomic measures, such as late sowing (against R. fistulosa) or early sowing (against Striga), and the use of organic soil fertility amendments. Growing a leguminous cover crop such as Stylosanthes guianensisand following a zero-tillage approach also contribute to effective control of Striga, as demonstrated by agronomic experiments conducted by AfricaRice and its partners.

To study institutional and socio-economic constraints underlying the challenge posed by the parasitic weeds, and to raise awareness and improve communication on efficient management strategies, AfricaRice and its partners have brought together stakeholders, including national research institutes, extension services, crop protection services and private sector representatives in workshops in East and West Africa.

At a time where there is a decline in public sector investments in agricultural research, efficient targeting of resources is becoming increasingly important. “The results of our studies emphasize the importance of targeted investments in further research, the development and dissemination of control technologies and capacity building of farmers, extension agents and other stakeholders, to reverse the observed trend of increasing parasitic weeds in rice,” stated Dr Rodenburg.

AfricaRice.