Weeds are a menace in agriculture and if the weed population is unchecked then they can result in huge economic losses.
There are various ways to control weeds but as the weeds are controlled, they may show weed shifts and weed resistances.
What is a WEED SHIFT?
A weed shift is the change in the composition or relative frequencies of weeds in a weed population or community in response to natural or human-made environmental changes in an agricultural system
Here populations refers to all individuals of a single species in a defined area
And community refers to all plant populations in a defined area
Weed shifts occur when weed management practices do not control an entire weed community or population
What are the management pracices that can bring about a change in weed species composition?
The management practice could be herbicide use or any other practice such as tillage, manure application, or harvest schedule that brings about a change in weed species composition
How does weed shift actually occur?
Some species or biotypes are killed by (or susceptible to) the weed management practice, others are not affected by the management practice (tolerant or resistant), and still others do not encounter the management practice (dormant at application)
Those species that are not controlled can grow, reproduce, and increase in the community; resulting in a weed shift
Any cultural, physiological, biological, or chemical practice that modifies the growing environment without controlling all species equally can result in a weed shift
In the case of chemical weed control, no single herbicide controls all weeds, as weeds differ in their susceptibility to an herbicide
Susceptible weeds are largely eliminated over time with continued use of the same herbicide
This allows inherently tolerant weed species to remain, which often thrive and proliferate with the reduced competition.
As a result, there is a gradual shift to tolerant weed species when practices are continuously used that are not effective against those species
A weed shift does not necessarily have to be a shift to a different species
For example, with a foliar herbicide without residual activity like glyphosate, there could also be a shift within a weed species to a late emerging biotype that emerges after application
What is WEED RESISTANCE?
In contrast to weed shift, weed resistance is a change in the population of weeds that were previously susceptible to an herbicide, turning them into a population of the same species that is no longer controlled by that herbicide
What is main difference in weed shift and weed resistance?
While weed shifts occur with any agronomic practice (crop rotation, tillage, frequent harvest or use of particular herbicide), the evolution of weed resistance is only the result of continued herbicide appplication
The use of a single class herbicide application continuously over time creates selection pressure so that resistant individuals of a species survive and reproduce, while susceptible ones are killed
What is more common out of the two?
A weed shift is far more common than weed resistance, and ordinarily take less time to develop
What are general misconceptions about weed resistance?
If an herbicide does not control all the weeds, the tendency is to quickly jump to the conclusion that resistance has occurred
A common misconception is that weed resistance is instrinsically linked to genetically engineered crops which is not correct
The occurrence of weed shifts and weed resistance is not unique to genetically engineered crops
Weed shifts and resistance are caused by the practices (for example repeated use of single herbicide) that may accompany a genetically engineered crop and not the GE crop itself
Similarly, there is another belief that resistance is transferred from Genetically modified crop to weed species
However, unless the crop is genetically very closely related to naturally occuring weed, weed resiatnce cannot be transferred from crop to weed
Transgenic herbicide resistance crops have greater potential to foester weed shifts and resistant weeds since a grower is more likely to use single herbicide in transgenic herbicide resistance crops
The increase in acrege of these crops could increase the potential for weed shifts and weed resistance in the cropping systems utilising transgenic herbicide resistance crops
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