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What is
Resistance?

When a particular weed proliferates extensively inspite of the use of the herbicide that was effective in controlling it earlier, this phenomenon is called resistance.

What is seen on the farm?

Farmers see that a particular herbicide that used to eliminate most of the weeds is slowly turning ineffective. The number of weeds that survive is increasing in number with every crop cycle, crop loss is steadily increasing. Within a decade, the particular herbicide is of no use against the weeds at all and most crop is lost.

Why

resistance happens?

Weed characteristics

Genetic pre-disposition:

The weed species itself have a few individual weeds that are genetically resistant to the herbicide being used.

Unique growth opportunity:

These resistant individuals are usually less competitive than their susceptible brethren. However, as the susceptible weeds succumb to herbicides, these resistant weeds survive and proliferate in number.

Weed Species Evolution:

Eventually, due to natural selection the entire weed population evolves to the genetic composition of the original resistant weed individual.

Herbicide selection and usage patterns

Lack of rotation of herbicides:

Continuous application of the same herbicide or different herbicides with the same mode of action only leads to proliferation of resistant weeds as they remain unaffected by these herbicides and continue to grow.

Herbicides with long residue period:

Herbicides with long residue periods ensure susceptible weeds do not re-grow for a long time. However, this also provides resistant weeds a longer period for unchallenged proliferation, thus allowing them to flourish.

Herbicides with highly specific mode of action:

If an herbicide has only one site of action, it is easier for resistant weeds to grow as resistance to only one site of action is required. Weeds find it harder to grow resistant to multi-site action herbicides.

Cultural practices

Mono-cropping (a shift away from multi-crop rotations)

Use of herbicide only as weed control program e.g. No tillage program

Failure to eliminate weeds which escape the herbicide treatment

Continuous or repeated use of the same herbicide or those which have the same mode of action

Both under-dose or overdose of herbicide than recommended dose rate for target weed

Types

of resistance

Herbicides work via a ‘mode of action’ at the ‘site of action’.

Resistant weeds can impair this mechanism in multiple ways.

Early signs

of resistance

Be alert to determine
the development of
resistant weeds
on your farm

Initial frequency of the resistant individuals

Check the number of weeds that survive post herbicide application. If this initial number is high, then resistance will surface more quickly than in cases where the initial number is low (if the same herbicide is continuously applied).

Low weed seed residue in the soil seed bank

It’s a fact that nature chooses proliferation of susceptible weeds over resistant weeds. Hence, only after major portion of the susceptible weed seeds have been exhausted from the soil, do resistant weeds propagate. For this very reason, key an eye on weed species that germinate readily from its propagules, for these will develop resistance more quickly than those species whose propagules remain dormant in the soil.

Hypersensitivity of weeds to a particular herbicide

Observe whether the weed species is displaying hypersensitivity to the applied herbicide. Hypersentivity can lead to the elimination of almost all susceptible weeds in a single herbicide application. This allows resistant species to evolve rapidly due to high natural selection pressure.