and there are several possible reasons for this. Fir... - Agriculture Cultivation lost_to_the_weeds_sp_ipm_2

and there are several possible reasons for this. First, the repeated use of a herbicide powerfully selects for

tolerant species; this results, often in a short time, in population shifts to those that are most tolerant.

Secondly, the repeated use of a herbicide can lead to resistant biotypes evolving. Herbicide resistance in

Asia lags behind the level in other areas, probably because of the continued use of hand weeding and

cultural practices such as flooding, although about 40 instances of resistance in rice weeds have been

reported (weedscience.org 2009). This problem of resistance is exacerbated by the low numbers of new

herbicide molecules being released nowadays compared with 20 years ago. Flooding the soil to prevent the

growth of weeds is still the most important cultural practice used in rice and, in part, this accounts for the

sustainability of these systems. Flooding requires huge amounts of water. This practice may not be

sustainable because between one-quarter and one-third of all the world’s freshwater resources go for

irrigated rice (Bouman et al. 2007), and 15–20 million ha of irrigated rice will probably face some level of

water shortage by 2030.

About the authors

Casimero, D Janiya, J

Chauhan, B Johnson, D

The authors are weed scientists

by training and are in IRRI’s

research and outreach teams.

Donna Casimero is based in

Sulawesi, Indonesia and Joel

Janiya in the Philippines and

both work in outreach activities

to improve crop and weed

management. Bhagirath

Chauhan and David Johnson

are both based in the Philippines

and have activities to develop

integrated weed management

approaches for direct seeded

rice. David Johnson is also

coordinator of the Consortium

for the Unfavorable Rice

Environments.

email: d.johnson@cgiar.org

All is not gloom

“Storm clouds” in the form of reduced farm labor and water supplies, herbicide resistance, weed population

shifts, and the spread of weedy rice are gathering to threaten the effectiveness of single interventions against

weeds. A long-term view suggests an urgent need for more attention to the integration of control measures to

make the most of potential synergies, and a rotation of management systems with strategic interventions to

prevent the otherwise inevitable “shift” to pernicious weed problems. More information on the differential

tolerance of rice and different weed species to flooding can make better use of water resources. Newly

developed rice varieties, tolerant of submergence at

germination, open opportunities to flood at the earliest

crop stages. Good agronomic practices can encourage

weed-suppressive and competitive crops and help to

fully exploit weed control interventions. Greater

knowledge of weed seed biology will allow more

effective use of soil cultivation, fallow, and mulching

systems to limit weed spread. Mixtures or sequences

and rotations of herbicides will also become more

important to maintain treatment efficacy, but so too will

the combination of these with complementary cultural

measures, such as flooding.

Timely flooding has prevented growth of weeds.

– J. Janiya

To remain effective, weed management strategies will increasingly need to move toward integrated weed

management (IWM), which combines different components. In part, these are likely to be knowledge-

intensive, and IWM will comprise several carefully

selected practices. If IWM strategies are to be

implemented, farmers will need to be able to

obtain adequate information to guide the

application of the components and be able to

adapt IWM strategies to best fit field conditions,

climate, and farming systems. Rigid systems are

not appropriate, and farmers must also be

equipped to adapt and develop alternative

systems to cope with the vagaries of the weather

and the weeds.

A longer term and broader approach to weed

management is imperative. Many of the tools

required for such changes are available and the

Some farmers still “fall back” on hand weeding but for

many others this is not an option. – J. Janiya

time is right to make a difference.

This Technical Innovation Brief

is published by:

SP-IPM Secretariat

SP-IPM@cgiar.org

www.spipm.cgiar.org