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Which came first: herbicides or herbicide resistance? Recent findings suggest the latter

Recent analysis of herbarium specimen collections, some over 200 years old, suggests that the risk of herbicide resistance may be greater than previously thought: now more than ever, the necessity to diversify weed control practices has come to the fore. The findings, obtained by INRA researchers in Dijon’s Agroecology Research Unit, were published in the October 2013 issue of the scientific journal Plos One.

Botanical conservatory and garden of Geneva © Bruno Chauvel
Updated on 04/03/2014
Published on 02/28/2014

Gene frequency and resistance

Herbicides are synthetic organic molecules that act by disrupting vital functions of targeted plants. Since the end of WWII, the use of herbicides has been the base strategy to keep weeds under control in most cropping systems. Yet, all herbicides are only effective in the field for a limited time. This is due to the selection by the herbicide of weed plants that naturally have one or more resistance genes. Herbicides kill only the sensitive plants, thereby favouring the resistant plants. That is why, as herbicides are applied year after year, resistant plants multiply much better than sensitive plants and increase in frequency in the field’s weed population. Herbicide resistance has now become widespread, with cases reported in some 234 weed species throughout the world.

The frequency of resistance among weed populations prior to the application of herbicides (initial frequency) is a recurring theme in weed science. From one weed generation to the next, mutations spontaneously appear and disappear in field weed populations. By default, the frequency of spontaneous mutation in a given gene is estimated at one in a billion. If mutations that endow resistance to herbicides are naturally present in higher frequencies in weed populations before herbicides are marketed, resistance will develop more quickly than expected.

Black-grass and herbicides: a matter of Darwinian selection

To address this issue, scientists studied mutations in the Acetyl-Coenzyme A carboxylase (ACCase) gene, which endows black-grass with resistance to herbicides. Herbicides targeting ACCase have been widely used against grass weeds in France. Black-grass has been considered a major weed in winter crops in France since the 1960s. The idea was to study black-grass plants that had never been sprayed with herbicides, in other words, plants whose inclusion in herbarium specimen collections pre-dates the use of herbicides.

DNA analyses of 734 plants collected between 1788 and 1975 and kept in herbariums in Dijon, Geneva and Montpellier, led to the discovery of a mutation in a plant collected in 1888. This mutation is currently the most widespread in black-grass populations where resistance has developed. This discovery confirms that resistance is a process of Darwinian selection, and also suggests that the initial frequency of certain mutations in weed populations can be greater than the “mutation frequency”. That is, the initial frequency of resistant plants in the field may be greater than generally accepted. Accordingly, the risk of resistance, or the ease with which herbicides select resistant plants, may well be higher than previously thought.

Weed control must be diversified and integrated

These findings do not mean that the rapid development of resistance is inevitable, nor that an informed and well thought-out use of herbicides in order to slow down the evolution of resistance is no longer timely. On the contrary, these results support the implementation of an integrated weed control strategy including both carefully considered applications of herbicides with a diversity of modes of action and a diversity of non-chemical weed control practices, in order to hamper the selection of resistances. Diversity that must be the base strategy in weed control is even easier to achieve when long rotations are used that contain a variety of crops. This type of rotation allows to use non-chemical agricultural practices (alternating planting dates, false seed beds, etc.) and a wider variety of herbicide modes of action, which in turn allows to keep weed density to a level that is compatible with a viable agricultural production in the long term. Now more than ever before, it is in farmers’ best interests to diversify their weed control practices to ensure their continued efficacy in the long term.

Conservatoire et jardin botaniques de la Ville de Genève © Bruno Chauvel

Find out more

In English:
Délye C.*, Deulvot C., Chauvel B.** 2013. DNA Analysis of Herbarium Specimens of the Grass Weed Alopecurus myosuroides Reveals Herbicide Resistance Pre-Dated Herbicides. PLOS ONE, 8, (10), e75117.

>> Go to Plos One

In French:
Deulvot C., Boucansaud K., Michel S., Pernin F., Chauvel B.**, Délye C.* 2013. Herbicides : la résistance existait avant eux... la preuve. Phytoma – LdV 669, 30-33.