Mar 10, 2004

Archive

Send

FSE study: Herbicide-resistant plants and agro-biodiversity

“Radical plant protection is bad for self-regulation”

Regimes consisting of genetically modified plants and their complementary herbicides are effective strategies for controlling weeds, but in summer rape and sugar beet they harm the species diversity of the agro-ecosystems. This was the key finding of a three-year farm-scale evaluation in Britain. Since then, the conclusions have been the centre of heated discussions in Germany too. GMO Safety spoke to Gesine Schütte of the University of Hamburg.

GMO Safety: Last autumn the results of a three-year farm-scale trial in Britain were published. They had investigated how the cultivation of herbicide-resistant plants affects biodiversity. In the case of sugar beet and summer rape they found that species diversity among arable weeds fell compared with conventional methods of weed control. In maize, the situation appeared at first glance to be different. What do you make of the British study? Can the results be applied to German agriculture as well?

Gesine Schütte: In principle, these trials transfer very well. The method in particular would seem to indicate it: over 60 sites were selected, the trial lasted three years and what is particularly important: the control plots were relatively large. In addition, in the various investigations the researchers did not limit themselves to one method (each method identifies different key animals or aspects). Insects were captured using various methods and the researchers did not only document the occurrence of plants, but also the flower and seed formation or the seeds that entered the soil. That is exemplary. However, the use of atrazine on the non-GM plots means that the maize experiments cannot be transferred to Germany, because atrazine is banned in Germany.

In addition, publications on crop cultivation in other countries show that broad-spectrum herbicides used in the system with HR plants have a stronger effect than those used previously. And this effect with its consequences for other organisms, particularly beneficial organisms, has been demonstrated.

GMO Safety: Does this mean, in your view, that one should avoid using HR crops and their complementary herbicides? Or should one pay greater attention to biodiversity when using them and reduce the herbicide dose accordingly? For instance, in trials with herbicide-resistant sugar beet in Denmark (NERI), using less herbicide than the manufacturers recommend was found to have positive effects on species diversity.

Gesine Schütte: It is only with sugar beet and fodder beet that there is a choice of strategies for weed control. And successes with these crops are due more to very late weed control in beet than to lower doses. However, this is not practicable because leaving weed control so late incurs yield risks. In trials in England it was shown that success – greater species diversity – is achieved only very late in the year. Yet beneficial organisms are needed particularly in early summer, e.g. to combat aphids. And the trials from Denmark unfortunately demonstrate only that at a particular date in the year more weeds survive. What matters though is the effect over the whole season.

If you compare publications from Germany over the last two decades, the species diversity of arable weeds per field was between two and 42. The trend over recent decades has been a downward one. If you then start using herbicides with practically no gaps in their spectra, the trend can only continue.

GMO Safety: Isn’t it the case that the British study just confirms the obvious: the more effective the weed management regime, the “cleaner” the field, the lower the species diversity on the field? Shouldn’t public debate be focusing more on the conflict of aims between securing yields and biodiversity, rather than on basic issues of green genetic engineering?

Gesine Schütte: Genetic engineering has for years been promising an improvement in environmental impacts, including impacts on weeds and insects. Now it must be judged against those promises.

Yield increases for herbicide-resistant plants are the exception rather than the rule. In Europe, slightly higher yields can only be expected with sugar beet, but not with maize or oilseed rape. The reasons for the use of herbicide-resistant regimes lie elsewhere: people think they are on the safe side if all or nearly all weeds are controlled. In other countries the term “convenience crop” is sometimes used. People often don’t check as much which pests are on the field and how many there are because the herbicide is effective against almost all weeds anyway. This means that farmers can sometimes save time.

I would describe the conflict of aims you mention like this: it is about simplifying production, by means of genetic technology, at the expense of self-regulation – in terms of integrated plant protection that means controlling pests using beneficial organisms – and therefore also at the expense of biodiversity.

GMO Safety: Insect-resistant crops, like Bt maize were not included in the British study. What potential impacts on biodiversity do you see here? After all, researchers are putting a lot of effort into checking that the Bt toxin affects only the target pest, like the corn borer, and not other creatures that come into direct or indirect contact with the Bt maize.

Gesine Schütte: The relatively high accuracy of Bt toxin has been known for a long time. The problem lies elsewhere: in terms of integrated plant protection, natural antagonists need to be able to survive, and that can only happen if a few pests survive too. Low densities of pests, which do not cause damage, need to be maintained. And that won’t work if the crop varieties produce a 99% deadly toxin throughout the season.

We hear so often that Bt varieties save on insecticides. In reality though, the insecticide is in the plant. In Germany, insecticides are not normally used at all on silo maize. And in other crops and regions only a small proportion of insecticide is replaced by the Bt toxin in the plant. In the case of maize in the USA, experts are now arguing over whether insecticide use may actually have increased.

GMO Safety: Should Bt plants be judged differently with regard to biodiversity from conventional Bt preparations that have long since become established?

Gesine Schütte: The conventional Bt preparation is not as intense and is not effective over the whole season. Besides which, it can be used at short notice, or not at all, depending on whether the infestation is actually above the damage threshold. This has advantages for the environment.

GMO Safety: Perfect plant protection strategies that do not have any gaps in their spectra are bad for biodiversity. Do you agree with this simplification? And what does that mean, in your view, both for agriculture and agricultural research?

Gesine Schütte: Not only are they bad for biodiversity, they also prevent self-regulation. Weeds, for instance, hold the soil in place against wind and water erosion, they improve the structure of the soil and its ability to supply plant nutrients and they play a vital role in conserving beneficial organisms. Without these beneficial organisms more pesticide will be needed. Conserving a remnant of biodiversity is a gratifying side effect of these types of plant protection strategy.

Agricultural research recognised this long ago. There are many successful methods and strategies that can be used to promote or conserve self-regulation in agriculture, e.g.:

• The selective conservation of small, less competitive weeds using very selective herbicides or manual control with low penetrating power – this in particular often does not incur financial disadvantages;
• Providing flowering or weed strips and permanent fallow strips to conserve food plants, nectar and pollen supplies and overwintering possibilities for beneficial organisms;
• Using crop rotations with significantly more than two crop types.

In regions where the local government provides funds for crop farming programmes, these and similar methods are encouraged. However, in practice, acceptance and money are often lacking.