4 June 1999

Herbicide cost-cutting is not only GMspin-off

GENETICALLY modified herbicide-tolerant sugar beet is likely to bring more than lower input costs and easier management.

This is the view of Alan Dewar of IACR-Brooms Barn who believes new GM varieties offer scope to use weeds as cover crops to deter pests.

"Herbicides are the biggest input so we are keen to see if the new technology offers scope for cost-cutting," says Dr Dewar. "But there may be other valuable benefits for growers and the environment."

Swiss research shows that weeds left in the crop can decrease colonisation and virus-yellows.

In a 1998 trial on an organic soil with a high weed seed burden, a GM crop received a conventional spray programme of one pre-emergence and four post-emergence treatments involving eight different chemicals.

This was compared with glyphosate alone applied at the 2-4 and 12-14 leaf stages of the beet. Other treatments involved later sprays with the same chemical.

"Early glyphosate with a follow-up second dose was not only cheaper than the conventional approach but was also easier to manage and as effective in controlling weeds. Because glyphosate has a better toxicological profile than many of the other herbicides it is also safer for the environment.

"There is also another important environmental and practical benefit. With glyphosate there is less need for fine sprayer nozzles and consequently a lower risk of drift."

This approach boosted sugar yield by 5% compared to the conventional system. But where the first treatments went on later, at the 8-10 or 12-14 leaf stages, the beet were hit by early weed competition and yields cut by at least 24%.

Where the crop was left untreated there was so much weed that it would have been impossible to harvest the beet.

There were more aphids in the early glyphosate treatments than where the normal sprays were used. This could, Dr Dewar believes, be because adjuvants in the herbicide mix may have deterred the pests.

"The concentration of virus-transmitting aphids was highest where there were fewer weeds. This confirms the Swiss work and is probably because there were fewer targets for the aphids to land on.

"What was unexpected was the high number of another non-pest species which appeared on the weeds."

In the untreated plots in mid-June there were 2000 leaf curling plum aphids per square metre. The species is not a sugar beet pest, feeding instead on speedwell and other weeds. Winged types are often found in beet, but they cannot survive without the right weed hosts.

This surprise colonisation was accompanied by large numbers of parasites and predators which eventually caused its downfall and may have attacked beet-colonising aphids as well.

No parasites were found where a conventional spray programme kept the crop weed-free, and there were only a few on the dying weeds after the early glyphosate treatment.

Now Dr Dewar thinks the chemical could be band-sprayed down the rows, leaving weeds between as habitat for natures aphid killers.

"As herbicides represent such a major chunk of the inputs, we are wondering whether weeds can be managed to encourage a build-up of beneficial insects. If so the threat of aphids spreading virus could be reduced, avoiding the need for Gaucho seed treatment or insecticide sprays."

As well as increasing bio-diversity, weeds could also help reduce or even prevent soil erosion on flat windswept fields and replace traditional barley cover.

This season Brooms Barn is examining band-spraying. Crops will look untidy until mid-season when the canopy closes over. But Dr Dewar believes this may be a price worth paying.


GM beet spin-offs

* Managed weed cover.

* Aphid predator habitat.

* Spray timing important.

* Band-spraying potential.