New strategies needed for weed control

The latest thinking on weed control and news of new herbicides were discussed at the annual British Crop Protection Council's Conference in Brighton in November. Two concerns for the thousand scientists and agronomists from countries around the world - temperate and tropical - were that weeds appear to be slipping in the perceived priority of farmers' problems; and there is growing public concern about the use of herbicides, in common with other crop protection products, and also of the use of biotechnology to control weeds.

While agrochemical companies announced a number of new products undergoing development, as much time was spent on discussing problems as on solutions. Many of the latest infestations are a result of human activity: importing weed seeds in seed imports and changing habitats by land clearance and thereby removing natural plant competition from species, which then become rampant. Examples were given by Philip Bacon of the Oxford Forestry Institute, who suggested that in Morocco as many as eight out of ten new weeds had been imported with crop seeds. He also instanced the development of Merenia as a pernicious weed in the Solomon Islands following forest clearance: insignificant in natural forest, Merenia grows vigorously on cleared land, despite control attempts using cutting, burning and chemical herbicides.

Herbicide tolerance also appears to be increasing, as described by Stephen Moss of the Institute of Agricultural Crops Research-Rothamsted, UK. Using genetic analysis, he has shown that resistant strains of Lindernia micrantha, an annual weed in rice in Japan, have been found to be occurring simultaneously in different parts of the country. In Japan, as elsewhere, decades of intensive herbicide use has increased selection pressure on target species, which have responded by developing resistance to several chemical modes of herbicide action. "Farmers have had the attitude that there will be a new chemical coming along to solve any new problems", he observed, "but the number of new chemicals is decreasing and it is far better to avoid resistance developing than to have to deal with it once it is a problem".

A "systems" approach to weed management was suggested by Dr Stephan Wise of IITA (International Institute of Tropical Agriculture) as a more effective long-term solution. Dr Wise is working with smallholder farmers in Cameroon, yet his observation struck a chord with Doug Derkesen, a consultant advising farmers on the extensive prairies of western Canada, who commented that herbicide technology is best used in an integrated approach with cultural practices including rotations and varied sowing dates. Meanwhile a delegate from Costa Rica, Bernal Valverde, described how he had helped to find a solution to the highly invasive weed Rottboellia cochinchinensis, (itchgrass) by interplanting velvet bean with maize to smother the weed. And from Zimbabwe, the Dean of Agriculture at the University of Zimbabwe, Ostin Chivinge, who is also Professor of Weed Management, provided a summary statement for many when he said, that he would like to see more farmers practising weed management rather than just weed control.

Proceedings of the 'Weeds' conference are available in a three volume set and include the Bawden lecture presented by Professor Christine Bruhn, Director of the Centre for Consumer Research, University of California, USA entitled "Public communication on the food chain: the foundation of global progress".

The set costs £85 (UK) or £90 (elsewhere) ISBN 1 901396 453 3 http://www.bcpc.org

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