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Farm Scale Evaluations published today 16 October 2003
The publication today 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 growing among crop plants in sustaining natural communities within and adjacent to farmer's 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 commercial practice for weed control, 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 led 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.
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