Those of us working in weed control have a major advantage over our fungicide and insecticide colleagues when it comes to managing resistance - the spread of weed resistance is rarely as explosive as with fungi and insects. This means we are given time to develop management strategies which can prevent further spread of the problem and ensure the farmer can control his weeds in an effective and economic manner. The biochemistry and genetics of weed resistance is a complex subject on which books have been written, so I won't try to review the science here. However, there are a few fundamental points which help to appreciate the basis of our strategies to manage resistance. This leads to two results.
Enhanced metabolism allows the plant to breakdown the herbicide faster to non-phytotoxic components.
Uptake and translocation may be altered so that less herbicide reaches the target site. The converse is unfortunately also true, that plant biotypes may resist a wide range of apparently unrelated herbicides. In Australia there is more than 300,000 ha infested with types of annual ryegrass able to resist phenylurea, sulfonylurea and ACCase herbicides. This a is really tough situation for cereal farmers in affected areas - and the prediction is that up to 1m ha could be infested with this resistant form by the year 2000. This complexity means that a lot of practical field work is needed to understand resistance problems.