Madagascar's useful and weedy Digitaria grasses

Grasslands are incredibly important. They act as carbon sinks and provide food and water. 

In Madagascar, they are a vital component, covering around 65% of the island.

Shaped by animals and fire over millions of years, they support the livelihoods of millions and the national economy, providing building materials and food for people and cattle. 

Kew has been working on the grasses of Madagascar for over ten years, building a solid understanding of the country’s grassy ecosystems and aiding conservation planning.

There are an estimated 541 species of grasses found in Madagascar and in our newest project launched earlier this year, we have been focussing on Digitaria species.

This genus contains species that can be nutritious forage plants, while others are noxious weeds.

Proper management of Digitaria species can improve the productivity and resilience of local smallholder farmers and help to reduce poverty.

However, being able to distinguish between species is vital and, currently, we lack data on this genus with superficially similar inflorescences.

Over the last few months, we have been carrying out fieldwork to build up our knowledge of Digitaria species and the ethnobotany of Malagasy grasses.

Second trip: Ambohidray

In August, we visited Ambohidray, an area not too far from Madagascar’s capital Antananarivo.

The forest, situated to the east of the village, is highly biodiverse and is a protected area managed by the University of Antananarivo.

The soil is ferralitic, meaning it is weathered and rich in iron.

The main crops grown here are irrigated or lowland rice in the peripheral valleys, and rainfed crops located around the village (cassava, maize, bean and legumes). 

We again interviewed local people, including farmers, to learn the local names of the main species, their behaviour, effects on crops, and how they manage weeds.

We found, in general, that farmers had a good understanding of the damage caused by weeds to crops and the importance of weed control in agricultural work.

However, they have difficulty in distinguishing which are grasses (members of the family Poaceae) and which are not because for them the concept of "ahitra" or "bozaka" includes all herbaceous plants.

The grasses mentioned most frequently are those which are the most common, useful, or evidently harmful to cultivation.

For Digitaria, annual species seem to be poorly known to most of the farmers: these species seem to be localized and mixed with other species in Ambohidray.

Farmers generally attribute different names to different species and names are usually based on the behaviour, morphological features, or use of the plants, although a name can refer to one or many species at the same time.

For example, the grasses Cynodon dactylon, Digitaria humbertii, Panicum luridum and Brachiaria umbellatum are all known by the name “Fandrotrarana”, from ‘’fandrotra’’ which can mean a girdle or belt, indicating their spreading, binding habit. 

Rhamphicarpa fistulosa (Orobanchaceae), a weed that attacks rice fields, damaging soil and drying out the rice until it’s easily flammable, is locally known as ‘Angamay’ from ‘angatra’, meaning evil power and ‘may’, meaning that burns and causes burns.

The main aim of this project is to accurately identify the Malagasy species of Digitaria so smallholders are able to apply the most efficient methods of weed control.

Different weeds control methods are currently used by local farmers depending on their financial capacity and the availability of equipment.

They practice either manual weeding, or a combination of manual weeding with a rotary hoe or herbicides. 

To avoid troublesome species, they also practice early ploughing known as “tombamboroka” before the rainy season and before the plants produce seed, to eradicate the rhizomatous species before sowing crops. 

In some cases, farmers have found that planting sweet potatoes before other crops can control weeds such as Imperata cylindrica, effectively by out-competing them. 

We also heard of some benefits of weeds, if they are controlled.

Some are used as fodder or compost, and some species have have medicinal uses, for example, Cynodon dactylon, is locally used as a diuretic and to treat sprains.