Oct 21, 2003

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Herbicide-tolerant plants and species diversity

Fewer weeds, fewer butterflies

The cultivation of herbicide-tolerant GM plants could have a negative impact on the species diversity of agricultural ecosystems. This core message in a study commissioned by the British government has triggered new discussions on the future of genetically modified plants in Europe.

The British government has spent around 8.5 million euros on the study. Sixty fields, spread throughout the entire United Kingdom, were planted with maize, rape, fodder beet and sugar beet: one half of each field was planted with conventional varieties and weeds were controlled using standard commercial herbicides.

Plants with genetically engineered herbicide tolerance were planted on the other half of each field. On this half, weeds were controlled by spraying with suitable companion herbicides. In contrast to the herbicide preparations used on the conventional plots, these are only taken up through the leaves and break down quickly in the soil. The LibertyLink system (active ingredient: glufosinate) was used on the maize and rape, whilst the RoundupReady system (active ingredient: glyphosate) was used on the beet.

The British government had commissioned the study to provide a basis of decision-making for handling GM plants.

After three years of investigations, the findings for maize, rape and beet were presented at a sensational press conference on the 16th of October 2003 in London. Eight studies in all appeared in Philosophical Transactions, a journal of the Royal Society.

HT (herbicide-tolerant) rape and beet: Effective, but fewer species

HT systems comprising herbicide-tolerant plants and suitable companion herbicides proved effective in rape and beet.

• At the end of the growing season there were fewer weeds on the fields with HT plants. Their biomass and also the number of fallen seeds were significantly lower than comparative figures for conventionally cultivated fields.
• The smaller numbers of weeds on the HT rape and beet fields reduces the availability of food for insects which eat these plants.
  This in turn had a knock-on effect on predators just above them in the food chain. For example fewer butterflies and bees (in beets) were observed in the HT fields.
  Springtails (Collembola), which feed on decaying plant material, were the exception. On HT fields they find a more abundant food supply.

Maize: Species diversity through reduced effect

The HT system appeared to be less effective than conventional herbicides in maize.

• Weed biomass and seed production on the HT maize fields were higher than on the conventional plots throughout the entire growing season.
• Certain species of insect benefit from this, for instance ground beetles which feed on fallen seed. Bees and butterflies were also more prevalent in the HT maize fields.

On the conventionally managed neighbouring fields and margins, also included in the study, the findings were similar to those of the trials fields.

Herbicide: it depends on the dosage

The particular strategies for weed control were not specified in advance, but chosen by the farmers who worked the fields. On the conventional plots both the number of active herbicide ingredients and the spraying practices varied. According to the authors of the study, the fields, which were scattered throughout Great Britain, represented the full spectrum of agricultural practice. On some plots with HT plants the companion herbicides were applied only once, on others several times.

Since the active ingredients glufosinate and glyphosate are only absorbed through the leaves, they do not have a long-term effect. But with beet and maize in particular, weeds continue to re-emerge for quite some time and compete with the crops. As a result farmers often make several herbicide applications.

In HT systems, however, the actual amount of herbicide and the number of applications has a significant impact on the species diversity of ecosystems. For example a broad study of herbicide-tolerant sugar beet (RoundupReady) in Denmark showed that the species diversity of plants and animals on the fields is considerably greater if the herbicides are dosed in such a way that not all weeds disappear. The timing of the herbicide application is also critical.

In this study conducted by NERI (National Environmental Research Institute) in Roskilde/Denmark, cultivation systems based on GM sugar beet and companion herbicides were found to have a positive effect on the biodiversity of agricultural ecosystems.

Michael Crawley, who conducted a 10-year long-term study of the ecological effects of transgenic plants starting in 1990, drew attention to the importance of herbicide use and weed management systems during the presentation of the latest study in London. “Will farmers use the HT system in such a way that all weeds are wiped out, or will they be prepared to tolerate weeds on their fields? If they apply lower herbicide doses, the species diversity of wild plants and animals will increase”.