THE HERBICIDE RESISTANCE BATTLE IS ON
One result of modern agriculture and the reliance on herbicides is the emergence of weed populations which are resistant to products designed to control them. All natural weed populations, regardless of the application of any weed killer, likely contain individual plants (bio-types) which are resistant to herbicides. Repeated use of a herbicide will expose the weed population to a “selection pressure,” which may lead to an increase in the number of surviving resistant individuals in the population.
Consequently, the resistant weed population may increase to the point that adequate weed control cannot be achieved by that herbicide. Despite this seemingly dramatic development , no herbicides have been lost to agriculture; they are today, and will remain, an integral part of food production through their effective use in combination with other weed control practices.
do i have it?
how did i get it?
At this point, it should be assumed that at least a small number of plants in any weed population is naturally resistant to a given herbicide and that repeated application of that herbicide will allow these plants to survive and set seed. However, there are several factors to consider when evaluating herbicide resistance risk. Some of these relate to the biology of the weed species in question, others relate to farming practices.
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.
BIOLOGY OF THE PLANT – 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. 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. It’s important to understand the weed species to know whether it could be resistant or not.
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.
Failure to achieve expected weed control levels does not in most cases mean that a farmer has resistance. A full analysis of the herbicide application, rate of use, weed type and stage of growth, climatic conditions and agronomic practice should be reviewed. If, after the initial investigation, resistance is still suspected, then consideration of historical information may point to factors leading to resistance development.
ask yourself…
Has the same herbicide, or herbicides with the same site of action, been used in the same field or in the general area for several years?
Has the uncontrolled species been successfully controlled in the past by the herbicide in question or by the current treatment?
Has a decline in the control been noticed in recent years?
Are there known cases of resistant weeds in adjacent fields, farms, roadsides, etc.?
Is the level of weed control generally good on the other susceptible species except the ones not controlled?
If the answer to any of these questions is “yes” and all other factors have been ruled out, then resistance should be strongly suspected. Steps should then be taken to leave a small area to collect a sample of whole plant or seed from the suspected resistant weed population for a resistance confirmation test.
IS HERBICIDE RESISTANCE REVERSIBLE?
NO. HERBICIDE-RESISTANT WEED SPECIES WILL MAINTAIN THEIR GENETIC SUPERIORITY OVER THOSE CHEMISTRIES AS LONG AS THEIR OFFSPRING CONTINUE TO ADD WEED SEEDS TO THE SOIL. STOPPING THE USE OF THOSE HERBICIDES WON’ T REVERSE THOSE GENETICS.