Will GM-foods mean less pesticide use?

The industry claim that genetically modified herbicide tolerant (GMHT) crops will  reduce herbicide use is proving difficult to confirm. A report by the Pesticides Safety Directorate (PSD), an Executive Agency of the Ministry of Agriculture, Fisheries and Food (MAFF) based on limited evidence from independent trials and published company data highlights concerns.

No commercial GMHT crops are grown in Britain yet, but the most likely to be introduced are winter and spring oilseed rape, sugar beet, fodder beet and forage maize. These crops have been modified to become tolerant to the herbicides glyphosate and glufosinate which means weeds can be sprayed with these herbicides after the crop has emerged, eliminating a pre-emergence spray.

Little information available 
There are very few results available from public funded research which allow detailed evaluation of the agronomic consequences of introducing GMHT crops in the UK. GMHT crops have been commercially available in North America for some time (see Box). Some results from trials in Europe by the companies introducing the crops have been published, but provide few details. Some results from independent trials on sugar beet in the Netherlands have been published recently.

Likely UK uptake of GMHT crops
The cropping pattern in the UK is in part dependent on the monetary support given to farmers under the Common Agricultural Policy. This is under review in proposals known as 'Agenda 2000'. Support in the future will influence the area and type of crops grown and is also likely to affect the area of GMHT crop grown. It is anticipated that cereals will remain the major arable crop in the UK, so for the foreseeable future GMHT crops will be grown in rotation with non GMHT cereals.
    It is not clear from available information whether programmes of only glyphosate or glufosinate sprays will be used or whether mixtures and sequences with other herbicides will be sprayed on to the GMHT crops. In the trials on sugar beet no application of a soil-acting herbicide was needed before the crop emerged, as is often the case in current programmes. The need for a residual herbicide to be included in the programme of glyphosate or glufosinate sprays used once the crop has emerged has not been fully investigated.

Official concerns
In North America, higher levels of weeds that are less susceptible to glyphosate or glufosinate started to be found when GMHT crops were introduced. This might happen in the UK, but unlike in North America, these will be grown in rotation with cereals and other crops where different herbicides will be used. However in the UK, glyphosate, and to a lesser extent glufosinate, is likely to be used in these other crops, before they are planted or shortly before harvest.
    To help prevent resistance occurring to glyphosate and glufosinate in the UK farmers will probably use different herbicides in GMHT crops or in other crops in the rotation.
    Currently, 50% of maize crops are grown year after year on the same field. If the same is true of GMHT maize, or if this practice becomes more common with GMHT maize, then there will be a greater risk of development of resistance.
    Weed beet and volunteer oilseed rape are significant weed problems, particularly in sugar beet. Volunteers (crops that become weeds) growing from seed shed by glyphosate tolerant crops cannot be controlled using glyphosate, the so called super-weeds. Similarly, glufosinate crop volunteers cannot be controlled using glufosinate. These 'super weeds' will be a particular problem in GMHT crops and must be controlled using cultivation or different herbicides to either glyphosate or glufosinate.
    If herbicide tolerance is transferred from GMHT, a possibility that is under investigation, other herbicides will have to be used. Prevention of transfer of the tolerance can be helped by killing related weeds in crops before they flower stopping the possibility of crossing with GMHT plants within the crop.

The differences in herbicide use
The answers to these questions draw on the limited evidence available from independent trials and that published by the companies developing GMHT crops. It also includes the experience from North America and the judgement of expert advisers employed by independent trials organisations.
    The numbers of times that GMHT maize and oilseed rape is treated with herbicides is likely to be twice, similar to conventional varieties of these crops. With sugar beet the number of applications of herbicide is likely to be reduced to three sprays from an average of nearly five in conventional crops.
    The number of types of herbicide contained within these sprays will decline in GMHT crops. Complex mixtures and sequences of herbicides are used in conventional crops, particularly in sugar beet. In GMHT crops it is possible that in many cases the only herbicides used will be either glyphosate or glufosinate. In these crops use of some other herbicides may be necessary to prevent build up of naturally less tolerant species or weeds which develop resistance to these herbicides. This will be particularly important in maize that is grown year after year, and in GMHT crops where herbicide tolerant volunteers are present.
    Glyphosate and glufosinate have greater activity on emerged weeds than the herbicides used on conventional varieties, this gives the possibility of changes in timing of weed removal in GMHT crops. It is likely that residual herbicides applied before the crop emerges will not be used and spraying can commence when weeds reach a larger size in GMHT crops than in conventional varieties. The level of weed control achieved will be similar or slightly better for many species, with improvements in control of some perennial species. (DB)

Scientific Review of the Impact of Herbicide Use on Genetically Modified Crops, Report Produced by the Pesticides Safety Directorate, 15 December 1998.

[This article first appeared in Pesticides News No. 43, March 1999, page 4]