Reduced spray option with GMs
Herbicide-resistant crops show management
advantages for growers but there are still
questions over their spread into wild non-GM plants.
CONVENTIONAL two-spray herbicide programmes in winter oilseed rape can be replaced by a single total herbicide spray once the rape has been genetically-modified with herbicide resistance.
Both grass and broad-leaved weeds can be taken out by a single of spray of glufosinate-ammonium, Dr Mike Read, of AgrEvo UK, told the Dundee Crop Protection Conference. The flexibility of application timing also meant growers could risk spraying later than usual without endangering their crops.
Either autumn or early spring timings can be used for weed control although Dr Read warned there is a risk of some yield loss if high populations of some weeds are not removed early. At the later timing, a higher application rate of 4 litres/ha may be giving better control of grass weeds.
Trials with spring oilseed rape genetically-modified to be resistant to glufosinate – AgrEvos Liberty Link – show similar results. It was also found possible to step up control of some less susceptible weeds, such as fumitory, by increasing the herbicide dose without affecting the crop plants.
Dr Read said possible reductions in pre-emergence herbicide applications would not only benefit growers financially but there were environmental advantages from the use of less active ingredient for the same weed control. Control of difficult weeds such as charlock or volunteer oilseed rape is now obtainable. Furthermore, trials in England with resistant blackgrass have shown that strains resistant to the fop herbicides are killed by glufosinate-ammonium.
The potential for pollen spread from GM varieties into nearby crops or feral populations of oilseed rape is being examined at the Scottish Crop Research Institute in Dundee which first set up a 500sq km study area in 1993. This is considering the distribution, persistence and gene flow of conventional rape varieties in arable Tayside.
Dr Geoff Squire, of the SCRI, said these feral, or wild, populations of volunteer plants on roadsides or field margins increased more than three-fold over the four years between 1993 and 1996 as a result of both new populations and the persistence of existing volunteer rape plants. Many cases of the same oilseed rape plant flowering for two or three years have been recorded, he said, and they are frequently found surrounded by small seedlings.
Earlier work showed that significant hybridisation of rape plants was taking place up to 360m from pollen-producing plants. However, there is uncertainty about the true extent of hybridisation in wild populations in areas with a high density of fields, and uncertainty about the role of insects rather than the wind in causing pollination.
This is now being checked using male sterile bait plants and bee colonies positioned in and around typical feral locations. Analysis of new hybrids and the pollen from bees will help confirm the sources of gene flow.
New genetic fingerprinting techniques make it possible to identify rape varieties and their progeny much more readily. This molecular technology has already been used to pinpoint wild populations of even varieties such as Rafal which have not been grown in the immediate area since the mid-1980s.