The publication of the results of the Farm Scale Evaluations (FSEs) in a journal of the Royal Society, reveals significant differences in the effect on biodiversity when managing genetically modified herbicide-tolerant (GMHT) crops as compared to conventional crops.

The publication of the results of the Farm Scale Evaluations (FSEs) in Philosophical Transactions: Biological Sciences, a journal of the Royal Society, reveals significant differences in the effect on biodiversity when managing genetically modified herbicide-tolerant (GMHT) crops as compared to conventional varieties. The study emphasises the importance of the weeds within crops in sustaining natural communities within and adjacent to farmers fields.

About 60 fields each were sown with beet, maize and spring oilseed rape. Each field was split, one half being sown with a conventional variety managed according to the farmer’s normal practice, the other half being sown with a GMHT variety, with weeds controlled by a broad-spectrum herbicide (glufosinate-ammonium in maize and spring oilseed rape, and glyphosate in beet). Comparisons in biodiversity were made by looking at the levels of weeds and invertebrates, such as beetles, butterflies and bees, in both the fields and the field margins immediately surrounding them.

A total of eight papers are published - two looking at the effects on weeds in the fields, two looking at the effects on invertebrates in the fields, one looking at weeds and invertebrates in the field margins and one looking at the effect of the contrasting herbicide regimes on both weeds and invertebrates as a whole. Another looks at the background to the study and the rationale for its design and interpretation and a final paper compares the management of the crops in the study with current conventional commercial practice to provide readers with contextual information against which the results should be considered.

Effects on weeds in fields (1,2)

The study showed significant and variable impacts of GMHT cropping in beet, maize and spring oilseed rape on the arable weeds when compared to current commercial practices. In GMHT beet and oilseed rape crops more effective weed control lead to the decline of the number of weed seeds left in the soil at the end of each growing season (known as the seedbank). Although this has been going on in cropped fields in Britain for many decades it could be accelerated by the management associated with these particular crops. In contrast, GMHT maize showed the opposite effect. Typically conventional maize has lower weed burdens because of the widespread use of persistent herbicides - the herbicide regimes used on the GMHT maize were not as effective at controlling the weeds.

In beet and oilseed rape, the densities of weeds shortly after sowing were higher in the GMHT treatment. This effect was reversed after the first application of broad-spectrum herbicide in the GMHT treatments. By the end of the season, the weight of weeds collected from a fixed area (biomass) and number of weed seeds falling to the soil (seed rain) in these GMHT crops were between one-third and one-sixth those of conventional treatments. The changes in seed rain affected the seedbank, resulting in seed densities about 20% lower in the GMHT treatments.

In maize the effect was different. Weed density was higher throughout the season in the GMHT treatment. Biomass was 82% higher and seed rain was 87% higher than in conventional treatment. However, this had no detectable effect on the seedbank as total seed return was low after both treatments.

Twelve of the most common weed species in the UK were examined. The biomass of six species in beet, eight in maize and five in oilseed rape was significantly affected. Generally, biomass was lower in GMHT beet and oilseed rape and higher in GMHT maize. Significant effects on seedbank change were found for four species of weed. However, for many species in beet and oilseed rape (19 out of 24 cases), seed densities were lower in the seedbank after GMHT cropping. These differences, if compounded over time, could result in large decreases in population densities of arable weeds. In maize, populations may increase.

Effects on invertebrates in fields (3,4)
Differences in GMHT and conventional crop herbicide regimes had a significant effect on the capture of most surface-active invertebrate species and larger groupings (higher taxa) in at least one crop, with most increases occurring in GMHT maize and most decreases occurring in GMHT beet and oilseed rape. One species of carabid beetle that feeds on weed seeds was less frequent in GMHT beet and oilseed rape, but more frequent in GMHT maize, showing how the numbers in some invertebrates tracked the amounts of food available to them.

Most higher taxa of invertebrates active on weeds and in the litter layer were little affected by the treatment. However, smaller numbers of butterflies were recorded in GMHT oilseed rape and smaller numbers of bees, butterflies and Heteroptera (‘true bugs’) were found in GMHT beet.

However, in all crops under GMHT management there were significantly more Collembola, a type of detritivore known as a ‘springtail’, which feeds on dead and decaying weeds. This is because the herbicides were applied later in the GMHT crops, and so weeds tended to be larger when killed, providing more food for these insects.

Effects on weeds and invertebrates in field margins (5)
Three components of field margins were sampled: the uncropped tilled area, the field verge (the grassy strip between the tilled land and the fence or hedgerow that forms the actual field boundary) and the boundary itself. In oilseed rape, the cover, flowering and seeding of plants were 25%, 44% and 39% lower, respectively, in the GMHT tilled margin. For beet, flowering and seeding were 34% and 39% lower in the GMHT margins. For maize, the effects were reversed, with plant cover and flowering 28% and 67% greater in the GMHT half. These results corresponded to the effects on weeds within the crops, because these plants had also been affected by the herbicide. Fewer, smaller effects were found in the verges and boundaries, and levels of herbicide damage were low.

24% fewer butterflies were counted in margins of GMHT oilseed rape, reflecting differences in the amount of flowers available. Few differences were found for bees, slugs and snails, or other invertebrates sampled in the field margins.

Effects on plants and invertebrate trophic groups (6)
The effect of GMHT cropping on the interaction between invertebrates with different feeding habits was studied by examining the relations between plants and the abundance of insects grouped according to their feeding preferences (trophic groups). The negative effect of GMHT cropping on weeds in beet and spring oilseed rape, and the positive effect in maize, resulted in similar changes higher up the food chain.

Where the weed flora was less abundant, there were fewer herbivores, pollinators and natural enemies (the insects which prey on the herbivores). Detritivores increased under GMHT management across all crops due to the greater input, later in the season, of dead weeds on which they feed. Compared to large differences through the season and between crop species, GMHT management imposed relatively small (less than twofold), but consistent, differences in the abundance of most trophic groups. The direction of change depended on how effective the herbicide was compared to conventional management.

Rationale and interpretation (7)
This paper provides the background information that was analysed to guide and interpret the FSEs. Previous surveys of soil, vegetation and field management were used to ensure that the chosen fields were typical and representative of commercial practice. Knowledge of the plants and invertebrates, and their sensitivity to the GMHT crop and herbicide, was used to guide the sampling plans applied to each field-half. Historical and recent changes in the buried, living weed seeds - the seedbank - were used to assess the initial diversity of sites and the longer term trends that might result from growing GMHT crops. Re-interpreting field experiments from the 1990s indicated that changes in management practice may cause large differences in biodiversity (e.g. a 50% difference). The experiment was designed to ensure that such differences between conventional and GMHT management would be detectable.

Crop management and wider UK context (8)
It was important that the crop management systems on the studied sites reflected the activities of farmers in the UK countryside. The locations of field sites and intensities of cropping had to represent the range found in the UK and this was found to be the case.

The amounts of herbicide used, and when it was applied, were recorded and compared well with current commercial practice for conventional crops, and the industry-recommended guidelines for application to GMHT crops.

Comparison of the amounts of herbicide applied with the density of weeds showed that farmers applied more herbicide when the density increased in beet and maize. Generally GMHT crops were found to receive less herbicide, later in the season, than the conventional crops.

Commenting on the results, Dr Les Firbank, Centre for Hyrdrology and Ecology, Merlewood, and co-ordinator of the project that submitted the papers, said:

“The results of these Farm Scale Evaluations reveal significant differences in the effect on biodiversity when managing genetically herbicide-tolerant (GMHT) crops as compared to conventional varieties. The study emphasises the importance of the weeds growing among crop plants in sustaining natural communities within, and adjacent to, farmer’s fields.”

“One of the key points to remember is that the results are only applicable to the three crops studied, and only under the regimes of herbicide usage which were employed. Each new application of GM crop technology must be looked at on a case-by-case basis, using a rational evidence-based approach.”

References:

1. Weeds in fields with contrasting conventional and genetically modified herbicide-tolerant crops. I. Effects on abundance and diversity
2. Weeds in fields with contrasting conventional and genetically modified herbicide-tolerant crops. II. The effects on individual species
3. Invertebrate responses to the management of genetically modified herbicide-tolerant and conventional spring crops. I. Soil-surface-active invertebrates
4. Invertebrate responses to the management of genetically modified herbicide-tolerant and conventional spring crops. II. Within-field epigeal and aerial arthropods
5. Invertebrates and vegetation of field margins adjacent to crops subject to contrasting herbicide regimes in the Farm Scale Evaluations of genetically modified herbicide-tolerant crops
6. Responses of plants and invertebrate trophic groups to contrasting herbicide regimes in the Farm Scale Evaluations of genetically modified herbicide-tolerant crops
7. On the rationale and interpretation of the Farm Scale Evaluations of genetically modified herbicide-tolerant crops
8. Crop management and agronomic context of the Farm Scale Evaluations of genetically modified herbicide-tolerant crops