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Forty years after the dawn of the pesticide age, Asia’s rice farmers
are realising that their love affair with insecticides has been an unnecessary,
expensive and sometimes dangerous mistake. But as agricultural experts
counsel farmers to cut back on spraying insecticides, or even stop altogether,
the use of chemical weedkillers in Asian rice fields is burgeoning.

This rise in herbicide use echoes the insecticide boom during the 1960s
and 1970s. ‘The issue is how to do it smarter this time,’ says Paul Ehrlich,
an ecologist at Stanford University, California. ‘Otherwise, there’s disaster
20 or 30 years down the road, and probably a lot less rice.’

Seeking this smarter path, more than 40 specialists in Asian rice production
gathered in March at Stanford to explore how herbicides should be used.
Their plan was to lay the foundation for a strategy of integrated weed management
(IWM), which would need minimal amounts of herbicides. Everyone agrees that
chemical weedkillers are too valuable to abandon, but farmers should use
them sparingly to minimise environmental risks and slow the evolution of
herbicide-resistant weeds.

IWM lags decades behind integrated pest management, its better-known
cousin, which is aimed at killing insects (see ‘Beyond the pestkillers’,
this issue). To close the gap, researchers say they need more nonchemical
alternatives in the weed-control tool kit and a clear, simple strategy to
give to farmers. Many fear that unless these are found quickly, aggressive
agrochemicals companies will lock Asian countries into herbicide-dominated
farming methods they may later regret.

Economic appeal

Throughout Asia, weeds are the rice farmer’s most damaging adversary.
According to studies carried out across the region by the International
Rice Research Institute (IRRI) in the Philippines, weeds reduce harvests
by an average of 10 to 15 per cent. This is despite farmers’ efforts. Left
unchecked, weeds could wipe out as much as 95 per cent of the yield – certainly
a strong argument for herbicides.

In addition, recent changes in farming methods have boosted the economic
appeal of herbicides still further. Traditionally, Asian farmers have sown
rice into nursery beds, then transplanted the partly grown seedlings into
paddies. This head start gives the rice seedlings an advantage that helps
to keep weed damage down. Transplanting – and subsequent hand weeding –
is backbreaking, laborious work, however, and labour is growing scarcer
and more expensive as the developing economies of most Asian nations siphon
rural workers off to better-paid jobs in cities.

Booming sales

As a result, more and more rice farmers are finding it much cheaper
to sow rice seed directly into their fields and turn to herbicides to control
weeds. ‘Direct seeding and herbicides go hand in hand,’ says Keith Moody,
a weed scientist at IRRI.

Because of this shift, herbicide use in Asia is booming. In the Philippines,
sales of herbicides are increasing by almost 20 per cent a year. In China
and India, sales are expected to grow from $67 million today to some
$550 million by the turn of the century.

But problems that dogged insecticide use are now surfacing for herbicides
as well. Herbicides tend to be much less toxic to humans than insecticides
are. Many Asian farmers have still been poisoning by weedkillers, however,
often as a result of misuse or because they failed to take proper precautions.
Widely used classes such as chlorophenoxy herbicides and acetamide compounds
can cause a variety of symptoms including eczema, eye problems and gastrointestinal
irritation. The jury is still out on possible long-term health effects.

Virtually all scientists agree that constant use of chemical weedkillers
will speed the evolution of resistant weeds. Only a few resistant weeds
have appeared so far in Asia, but as farmers rely increasingly on herbicides,
many more will probably appear in the next few years, says Moody.

To prevent such problems, agronomists, agricultural economists, and
policy makers around the world are anxious to develop an IWM system for
rice to complement the integrated pest management system, which is winning
wide acceptance. Their goal is to equip farmers with ecologically based
methods for controlling weeds that use a variety of nonchemical means. Herbicides
would be reserved for those times and places where there is no alternative.

Nobody believes the task will be easy. For one thing, Asia’s rapidly
growing population leaves agronomists no leeway to sacrifice even a little
yield. ‘Whatever weed control method we use, it must contribute to moving
average rice yields for-ward,’ says Ken Cassman, chief agronomist at IRRI.
Moreover, research on IWM is still in its infancy, and scientists have few
nonchemical tools on which to build their strategy.

But there are already some steps that farmers can take to reduce their
dependence on pesticides, says Moody. Most important, they can make sure
their seed grain does not contain weed seeds. Clean seed alone could boost
yields by as much as 10 per cent. Yet many farmers sow grain set aside from
the previous year’s harvest that falls far short of the ideal. Educational
programmes targeted at women – who tend the seed grain in most farm families
– should pay a big dividend, says Moody. Even commercial seed has plenty
of room for improvement in most of Asia. ‘I’ve seen some so-called certified
seed, and it’s atrocious,’ he says.

Two other strategies are already in use on many farms. By carefully
tilling and levelling their fields before sowing, farmers can eliminate
the unkempt patches where weeds thrive. And by choosing wisely when to flood
their paddies, they can suppress weed germination and drown growing weeds
while the more water-tolerant rice grows unharmed.

‘You know what’s the best herbicide for rice? Water!’ says Surajit DeDatta,
an agricultural scientist formerly with IRRI and now at Virginia Polytechnic
Institute, Blacksburg. ‘If you can afford it, and if you can manage it,
that’s the way to go.’ As rice cultivation spreads to areas of Asia where
farming in paddy fields is not the norm, however, and as a growing population
demands water for other uses, many farmers find they no longer have this
luxury.

Seed cleanliness, tillage, flooding and herbicides: ‘That’s the extent
of the weed management tool kit that we have,’ says Kenneth Fischer, deputy
director-general for research at IRRI. ‘The tool kit is bare in terms of
developing plants that can compete with weeds. It’s bare in using biological
(control), and it’s bare in mechanisation, except for some simple tools.’

Researchers at IRRI and elsewhere are scrambling to fill the gaps. Plant
breeders are designing rice cultivars that compete well against weeds. Nobody
considered competition a particularly valuable trait in the days of hand
weeding. But breeders have recently found that rice strains with early rapid
growth and spreading leaves, which cast wide shadows, can beat the weeds
and deliver high yields, says Fischer.

In Arkansas, Robert Dilday of the US Department of Agriculture has
identified several rice strains that apparently exude a chemical – as yet
unidentified – that keeps weeds at bay around their roots. Dilday’s research
is still in its early stages, however, and any commercial use of such innate
weed control is perhaps twenty years away, predicts one IRRI breeder.

Somewhat closer to fruition is biological control of weeds, which several
research teams are working on. A fledgling programme at IRRI has identified
a number of fungal diseases that infest important weeds of rice, and either
kill them or weaken them enough to give the rice a competitive edge. Several
have shown promise in small-scale experiments, says Fischer. Similarly,
some evidence suggests that doses of herbicide too small to kill the weeds
may nevertheless stunt them and increase their susceptibility to disease.

Even with today’s limited repertoire, some farmers have successfully
implemented IWM programmes. In 1989, Malaysia launched a major campaign
to explain the benefits of clean seed, proper tillage and careful timing
of dosing with herbicide to farmers in its rice heartland, the Muda district
in the north of the country. In the first four years, herbicide use fell
by nearly 40 per cent, weed problems declined and yields rose, says Ho Nai-Kin
of the Muda Agricultural Development Authority in Kedah. The Malaysian
programme worked, Ho says, because farmers were not overwhelmed by a complex
strategy. ‘We emphasise simple techniques. That is the crux of the whole
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Indeed, no IWM programme is likely to catch hold in Asia, Ho and other
experts say, unless it can be presented in a simple, straightforward way
to the millions of busy farmers, many of whom are poorly educated. Thus,
while scientists are busy expanding their weed-control tool kit, they are
also searching for a simple ‘take home’ message for weed management.

Pest control

This simplicity was the key ingredient in the success of integrated
pest management for insects. To begin with, ‘IPM was complex, it was knowledge-based,
and it had a lot of detailed implementation strategies,’ says Fischer. ‘We
were unable to get IPM into the hands of the farmers.’ He says the turning
point came ‘when we were able to simplify IPM down to a simple, ecologically
sound concept’. That concept, which distils much of the hard-won understanding
of insect pest and predator populations, is that farmers should wait at
least 40 days after sowing before spraying insecticides. Most insect problems,
ecologists have found, go away on their own within this period. Only if
problems persist is spraying needed.

A similar simple prescription is not ready yet for weeds, Fischer says.
‘But we haven’t done a lot of the ecosystem study. It didn’t come easily
for insects.’ Researchers need to look closely at the ecology of weed species
and develop a thorough understanding of their life cycles, he says. Only
then might simple management strategies emerge.

But some agronomists remain sceptical. Cassman cautions scientists not
to let the simplicity of the insect-management message seduce them into
insisting that the corresponding message for weeds should be equally uncomplicated.
‘The goal of simplicity is wonderful, but let’s be realistic and accept
that the situation for weeds may be more complex,’ says Cassman.

As researchers struggle with issues like these, some admit to an added
sense of urgency. They worry that agrochemicals companies will beat them
to the prize with a strategy based on rice plants that are genetically engineered
to be resistant to herbicides. Since farmers would need only to plant and
spray, such an approach would be appealing in the short term. But many scientists
fear that longer-term, it would hasten the evolution of resistance in weeds
and encourage massive overuse of herbicides.

People say it could not happen that way, says Walter Falcon, an agricultural
economist at Stanford. ‘I say, damn it, it’s probably going to happen that
way. And it worries me.’

Bob Holmes is a freelance science writer based in Santa Cruz, California.