Running wild

credit: Dr David Johnson, IRRI

With its resemblance to domestic rice, but the potential to wreak havoc in the crop, wild rice is like a wolf in sheep's clothing. In the early stages of growth, domestic and wild rices look so similar that farmers have difficulty telling them apart. They share the same environment and have a similar growth habit. Add to that vigorous growth and, because of its similarity with domestic rice, the inability to use selective herbicides, it's easy to see why control is difficult. But when left unchallenged, wild rice out-competes the crop, often smothering it entirely before quickly shedding its seed and leaving its legacy for the next season's crop. In the worst cases, as in Mali or Tanzania, large areas of land are abandoned.

But despite the challenges, control is possible, says David Johnson a weed scientist based at the International Rice Research Institute (IRRI), who until recently was working on rice in Africa. The troublemakers there are two annual species (Oryza barthii and O. punctata) and one perennial species (O. longistaminata) which are all African in origin. The starting point, he says, should be prevention, which means looking for sources of contamination.

credit: Soungalo Sarra, Mali

For example, in large irrigation schemes, fields may become contaminated by seed being washed down with irrigation water. The seed may come from other fields that are being drained into the canals, or from wild rice infesting the canals. Dr Johnson gave the example of farmers in Mali - along the Niger River where large areas of rice are grown - who are currently working together to clear the canals, with different farmers groups taking responsibility for particular stretches of canal.

Another potential source of contamination is from rice seed. "As with other seeds, it is important for farmers to make sure that they are getting their seed from known sources that are free from wild rices." For larger farms, machinery can also be a source, particularly where contractors or government hire schemes are operating and moving regularly between fields.

However, despite vigilant preventative methods, infestation can still occur, and initially selective weeding or pulling the wild rices by hand can help. Once infestation has become established, Dr Johnson stressed that it takes an integrated approach over a number of seasons to tackle the problem. Stale seedbeds are one approach. Seeds of the wild rice that are in the soil are encouraged to germinate, before the crop is planted and germinating wild rices can be killed either by the use of a non-selective herbicide such as glyphosate, or through cultivation. Roguing may also be required. Control of the perennial species (O. longistaminata) requires action before the crop is established. "In this case, land cultivation can be used to kill the rhizomes in the soil, or herbicides such as glyphosate can be used to control the wild rice before the crop is established."

So preventing these weeds from running wild in rice crops is possible even without the option of selective herbicides. The key is an integrated, long-term approach with a focus on preventative measures.