As of summer 2010, herbicide resistance had been reported in 194 plant species (114 dicots and 80 monocots) in over 340,000 fields worldwide (Heap, 2010). Resistance has been reported towards 17 of the 22 Herbicide Resistance Action Committee's herbicide Mode of Action groups. In some species, for example Alopecurus myosuroides in the UK, resistance is so widespread that it is now described as being endemic. The number of cases of herbicide resistance will undoubtedly continue to rise. Indeed, resistance to active ingredients that have yet to be commercialised will already be present within existing weed populations. Presently the gene(s) conferring this resistance will offer little or no advantage to the individuals possessing them. However, once a new herbicide is commercialised and applied to weed populations the gene(s) will offer a distinct advantage, as only individuals possessing them will survive herbicide treatment and be in a position to grow and ultimately reproduce. As long as herbicides form a part of a weed management programme, then resistance will remain a problem, although integration of herbicide usage with non-chemical management practices can vastly reduce this risk.

Herbicide Resistance is the inheritable ability of a plant to survive a herbicide dose that would be lethal to a member of a normal population of that plant species. Inherent in this definition are three important points:

1 Herbicide resistance is inheritable, so it is a characteristic coded for in the plant genome. At least some of the progeny of that plant will either be resistant or will carry the resistance trait. This distinguishes herbicide resistance from other causes of poor herbicide efficacy, perhaps caused by environmental factors including their affect on spray effectiveness, plant physiology and biochemistry.

2 Herbicide - resistant plants survive herbicide treatment and can successfully complete their life cycle by flowering and producing seed. This does not mean that resistant individuals will not show symptoms of herbicide damage, but that they are not killed by herbicides. In many cases some herbicide damage may be observed but it does not lead to plant death.

3 A normal population is a population of the species that when treated with an optimum dose of a herbicide, under ideal conditions, all individuals within it are killed. This

Herbicides and Plant Physiology, Second Edition Andrew H. Cobb and John P.H. Reade © 2010 A.H. Cobb and J.P.H. Reade. ISBN: 978-1-405-12935-0

population will be one that has never been exposed to a herbicide. Such 'wild-type' populations are not always available to the researcher, so populations that have demonstrated 100% susceptibility are often used in research, regardless of their field history. In order to determine baseline sensitivity for a particular herbicide acting on a distinct species, several normal populations are ideally used.

Resistance should not be confused with tolerance, which is the term used to describe an individual species that is not controlled by a particular herbicide. For example, cleavers (Galium aparine) is described as tolerant to isoproturon because the species as a whole is naturally tolerant to this active ingredient.

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