Case Study 1: Herbicide microdosing to control Striga

Striga, a parasitic weed. Credit IITA

Striga, a parasitic weed. Credit IITA

Striga is a weed that infests up to 40 million hectares of farmland in sub-Saharan Africa. It leads to yield losses between 20% and 100% and affects 100 million livelihoods, causing US$1 billion in annual crop losses. Some of the solutions developed so far include the use of Imazypyr (a non-selective herbicide used for the control of a broad range of weeds), but this can kill or damage the crop.[1]

The roots of several legumes, such as Silverleaf desmodium, are effective in suppressing Striga and have been incorporated into push-pull, intercropping system. The desmodium neutralizes the Striga and Napier grass serves as a lure for pests such as the maize stalk borer. Whilst promising, desmodium can be difficult to establish as small, slow growing desmodium seedlings vulnerable to invasive weeds.

Recently a mutant gene in maize that provides Imazypyr resistance (IR) was developed by tissue culture and bred into local maize varieties such as IR breed KSTP 94 developed by the Kenya Agricultural Research Institute (KARI). The International Wheat and Maize Center (CIMMYT) developed a novel approach of coating these newly resistant maize seeds with the Imazypyr herbicide before distribution. When the non-resistant Striga seeds germinate, they attach to the maize roots and take up the herbicide from the seed coating. The Striga is killed and the maize grows with little or no impact from the herbicide.

On-farm use of IR maize enables Striga-affected farmers in Kenya to increase harvests from an average of 500kg per hectare to 1,500kg per hectare. If 20% of severely infested land in western Kenya is cultivated with IR maize, it is possible to produce an extra 60,000 tonnes of maize or enough to feed at least 100,000 households. Grace Lugongo, a farmer from Butula in western Kenya explains, “Until 2007, I had never known the meaning of harvesting a full sack of maize from my 0.5ha piece of land thanks to the ‘Striga’ weed. All my efforts would yield only 2 ‘gorogoros’ (a tin measuring about 2kg) of maize. I decided to try the IR maize and over the years my yields have increased to 10 bags from the same piece of land. From the harvest I am able to cater for my subsistence needs and also afford some surplus to sell to cater for my other needs such as school fees for my children.”

Dick Morgan, from Vihiga, a town in western Kenya explained, “Before a new maize variety was introduced to me I used to plant maize without success. This was very frustrating as maize is our main food. In 2005 my fortunes in maize farming started to change after I was introduced to the IR maize which I tried and saw a significant increase in maize yields and also in the reduction of the Striga weed on my farm.”[2]

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