Science: Weedkillers from outer space: Satellite signals will soon enable farmers to be more economical with herbicides. Mike Williams reports

They are gathering in the harvest, down in Suffolk. Like mechanical insects, the combine harvesters are marching up and down the fields reaping the crops and separating grain from straw.

But this year, the high technology which has long replaced the traditional activities of the harvest is even more hi-tech. Some of these combine harvesters are eavesdropping on signals from outer space.

The result is likely to be a reduction in the amount of weedkilling chemicals sprayed on our fields each year.

The 24 satellites which form the Global Positioning System (GPS) were originally funded by American taxpayers so that their armed forces would know, with pinpoint accuracy, exactly where they were anywhere in the world. Vehicles, ships and aircraft with equipment to receive and process the signals can identify their position with an accuracy of less than two metres, a valuable asset when the Americans and their allies were fighting among featureless sand dunes during the Desert Storm campaign in Kuwait.

If tanks can listen in to the satellites, then so too can combine harvesters. Outwardly, those fitted with satellite receivers look little different from normal, apart from an additional small console in the driver's cab. Most modern combine harvesters have equipment to measure grain yield continously as the crop is harvested. Adding a GPS receiver to a harvester allows the production and position to be correlated and so yields can be plotted on a map of the field, held in a computer. Yields often vary by a factor of two, and a threefold difference between the best and worst yields in one field is not uncommon.

The problem may be an area of compacted soil or poor drainage, but a frequent cause of grain loss is competition from weeds. Most problem weeds grow in patches, and these can be indicated on the field map.

The final stage, once this year's harvest has been brought in, is to go over the fields with a special crop sprayer also equipped with its own GPS link and a computer to 'read' the weed data on field maps. The map and the satellite signals trigger the control system to switch the sprayer on as it passes over a weed patch and off, or to a low precautionary dose level, for weed-free areas.

Compared with conventional spraying, which applies a uniform dose of weedkiller over the whole field, the selective technique can achieve a significant reduction in herbicide application. This is attractive to the farmer, because spray chemicals are expensive, and there is an environmental pay-off also.

Equipment for the map-making stage is already available. It was developed by Massey-Ferguson - one of the world's leading farm machinery companies - and their Danish partner, Dronningborg. A prototype version was demonstrated in 1991 and this year about 30 farms in Britain have combine harvesters with GPS equipment. This number is likely to increase rapidly as more farmers are attracted by the opportunity to increase production from low yielding areas and reduce expenditure on weed-control chemicals.

Equipment to use the maps for selective spraying is still being developed. The first experimental sprayer with its own satellite link, known as the 'patch sprayer', has been working at the Silsoe Research Institute near Bedford, a Biotechnology and Biological Sciences Research Council centre. It is one of the Institute's major projects, attracting finance from several sources including the Ministry of Agriculture and the EU.

Dr John Stafford and Mark Paice of the Chemical Application Group at Silsoe, say their tests show that patch spraying often reduces herbicide application levels by 30 to 50 per cent, and up to 80 per cent has been recorded.

A further refinement in GPS weed control is to identify the principal weed varieties in each patch and include this information on the map. This would allow the sprayer control system to select appropriate specific weedkillers instead of a more expensive herbicide cocktail.

One approach developed by RDS, a company specialising in electronic equipment for agriculture, is to fit a control unit in the combine harvester cab with a push button for each of the most important weed species. Pressing the appropriate buttons adds the information to the map while the harvester is working.

The Silsoe research team is also developing techniques to provide more detailed information for the sprayer. One method is for the farmer to walk across each field with a GPS receiver and a hand-held computer. Details of patches and species are entered on a map displayed on the computer screen while GPS signals record the location.

(Photograph omitted)