Science - May 18, 2006

Fewer weeds with fast-growing rice

Weeds are a big problem in dry rice cultivation in South-East Asia and Africa. Many harvests are lost entirely. Weed growth can be suppressed, however, by using rice cultivars that cover the ground quickly after being sowed. according to Dule Zhao’s research.

Dry rice needs only limited irrigation, but farmers have a lot of problems with weeds. Dry cultivation is a solution for growing water shortages, however. At present, almost half of the water taken out of rivers and groundwater basins in South-East Asia goes to irrigate wet rice, which is the most common form of rice growing. Much of this water is lost to evaporation.

Zhao examined forty rice cultivars that are suitable for dry cultivation and found wide-ranging differences in their resistance to weeds. The Indica cultivar proved to be the most resistant to weeds and gave the highest yields. Zhao concludes that it is important to use a rice cultivar that grows quickly immediately after it is sown, therefore covering the ground at an early stage. That reduces the chance of weeds taking over.

Zhao advises rice growers who have selected rice on the basis of the number of rice grains per plume to do further selection for early growth vigour. He expects that in this way, after a few years, they will have ‘perfect’ rice cultivars that are tailored to regional conditions, such as soil and climate characteristics and the weed types present. Zhao notes, however, that some rice cultivars that are relatively susceptible to weeds are preferred for their taste. An example is Japonica rice. But it is also possible to work against weeds by sowing at the right density, Zhao found out: three hundred seeds per square metre turned out to be optimal.

Rice farmers in South-East Asia and in Africa do not always have access to rice cultivars that are good at suppressing weeds. The International Rice Research Institute, where Zhao carried out his trials, does make seed material available free to national breeding institutes in developing countries, who are then expected to pass the ‘weed-resistant’ rice on to farmers.

According to Zhao’s promotor, chair of Crop Ecology, Professor Huub Spiertz, in countries like China and the Philippines there are good prospects for dry rice cultivation. ‘It provides a sustainable solution to weed control. At present farmers have to weed by hand a number of times each season or spray herbicides against the weeds. These methods cost time and money, and herbicides are bad for the environment. By using the right rice cultivars, farmers will only have to weed once a season, and will be able to use less or no herbicides. That is good for more eco-friendly rice growing.’ / HB

Zhao will receive his PhD on 23 May. His promotor is Professor Huub Spiertz, chair of Crop Ecology.