Herbicide resistance and farmland biodiversity: Winter rape results

More grasses, fewer flowering plants

The cultivation of herbicide-tolerant winter rape could lead to a decline in flowering weeds. The potential impact on biodiversity in arable areas was the subject of a large-scale study undertaken in the UK. Different methods of weed control in winter rape were compared.

Trial field. Trials focussing on the ecological effects of HT rape were conducted in Germany as well. (Photo: BBA trial field in Dahnsdorf / Brandenburg)

F ewer bees due to fewer flowering plants? Female wild bee (Lasioglossum sexnotatum) on a rape flower

It was the largest study ever undertaken to investigate the potential effects of genetically modified herbicide-tolerant plants on species diversity. Between 1999 and 2002 a total of 270 sites in every region of the UK were involved. Different methods of weed control in the cultivation of maize, sugar beet, spring and winter rape were compared:

Genetically modified herbicide-tolerant crop varieties were grown in one half of each field. Weeds were controlled using the complementary herbicide (containing glufosinate), which is tolerated by the winter rape plants (HT system).

On the remaining half of each field the farmers continued to use a range of conventional methods of weed control as usual. Several commercially available herbicides were generally used. This study, also known as a Farm Scale Evaluation (FSE), was commissioned by the UK government, which was keen to establish a broad database for assessing GM plants.

The results for maize, sugar beet and spring rape were published back in October, and now the results for winter rape are available. With an area of 450,000 hectares under cultivation in the UK, winter rape is the most significant crop in the FSE study in economic terms.

From 2000 to 2002 the impacts of the different approaches to weed control in winter rape were studied at 65 sites. The trial fields were then observed for a further two years. The results for winter rape do not differ significantly from those for spring rape. However, the timing of sowing – in autumn for winter rape - and the application of the herbicide do affect the species spectrum of the associated weeds.

Weed species and composition: More grasses, fewer flowering plants

The most striking finding of the FSE study is a shift in the weed species spectrum.

  • Germinable seeds in the soil: At the end of the three-year period of cultivation more weed seeds were present in the soil in the HT plots than in the conventional plots. However, during the follow-up period the numbers converged once more.
  • Weed biomass: The different methods of growing winter rape had no significant effect on the total weed biomass. Neither was seed production markedly different; seed traps were set up and the seeds which fell over a certain period of time were counted.
  • Ratio of grasses and flowering plants: Differences were found in the ratio of monocotyledons (plants with a single seed leaf) to dicotyledons (plants with two seed leaves). The first group comprises mainly grasses (e.g. meadow grass), whilst the second group is made up of flowering plants (e.g. bistort).
  • Initially the overall number of plant species on the HT fields was above that of conventional fields. Then the number of flowering plants on the HT fields fell significantly whilst the number of grasses increased. Seed production in the flowering plants (dicotyledons) on HT plots was only one third of that on conventional fields. With the grasses (monocotyledons) it was the other way round: Seed production on the HT fields was five times higher than on the reference areas. The seeds collected in the seed traps were evaluated in each case.

As in the first part of the FSE study, the field margins were also included in the winter rape trials, but the results showed no major differences in terms of seed.

Invertebrates: Fewer butterflies and bees

With the majority of invertebrates there was nothing remarkable to report. The number of beetles and spiders collected from the soil traps on the HT fields was similar to the conventional fields. However, there were noticeable differences in bee and butterfly populations.

  • Bees: In July the number of bee species on the HT plots was 42 percent of that on the comparison plots, mainly due to the low number of bumblebees. The low number of flowering plants on the HT fields could explain this.
  • Butterflies: The occurrence of small cabbage white butterflies on the HT plots was 59 percent of that on the conventionally managed land.
  • In the field margins of the HT plots the number of small cabbage whites was higher in May than on the margins of the conventional plots, yet in July it was just the reverse.
  • The number of springtails, which feed on dead plant material, was noticeably higher on the HT plots.

The cultivation of HT winter rape in conjunction with the application of a complementary broad-spectrum herbicide mainly affects flowering weeds (dicotyledons). The number of flowering weeds on the HT plots was well below that on fields where conventional herbicides appropriate to rape cultivation were used. The results of the FSE study indicate that a decline in flowering plants can be harmful to bees and butterflies, though the importance of dicotyledonous weeds in winter rape as an alternative food source for bees and butterflies is not yet fully understood.

However the authors of the study believe that large-scale cultivation of HT winter rape could well result in a long-term reduction in the dicotyledonous weed seed bank in the soil. This type of change in the weed spectrum could lead to a worsening of living conditions for certain animal species, such as pollinators and also birds which feed on dicotyledonous seed.

The FSE winter rape study confirms what the results from the main part of the study had already indicated: All approaches to weed control cause a reduction in plant densities on the fields. The more effective the method, the greater the impact on weed numbers and diversity.