Factors To Consider When Assessing Risk Of Herbicide Resistance

How does a farmer or ag retailer establish that a herbicide resistance problem is developing or if their farming practices may lead to resistance appearing?


There are several factors to consider when evaluating herbicide resistance risk, according to the Herbicide Resistance Action Commitee’s “Guideline to the Management of Herbicide Resistance”. Some of these relate to the biology of the weed species in question, others relate to particular farming practices. Some examples are given below:

Biology and genetic makeup of the weed species in question

Number or density of weeds: As resistant plants are assumed to be present in all natural weed populations, the higher the density of weeds, the higher the chance that some resistant individuals will be present.

Natural frequency of resistant plants in the population: Some weed species have a higher propensity toward
resistance development; this relates to genetic diversity within the species and, in practical terms, refers to the frequency of resistant individuals within the natural population.

Seed soil dormancy potential: Plant species with a longer soil dormancy will tend to exhibit a slower resistance development under a selection pressure as the germination of new, susceptible, plants will tend to dilute the resistant population.

Crop management practices which may enhance resistance development

Frequent use of herbicides with a similar site of action: The combination of ‘frequent use’ and ‘similar site of action’ is the single most important factor in the development of herbicide resistance.

Cropping rotations with reliance primarily on herbicides for weed control: The crop rotation is important in that it will determine the frequency and type of herbicide able to be applied. It is also the major factor in the selection of non-chemical weed control options. Additionally, the cropping period for the various crops will have a strong impact on the
weed flora present.

Lack of non-chemical weed control practices: Cultural or non-chemical weed control techniques, incorporated into an integrated approach is essential to the development of a sustainable crop management system.

Download the HRAC’s “Guideline to the Management of Herbicide Resistance.”

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