Invasive and prevalent weeds pose frequent challenges for crop management in modern Agriculture. The herbicide resistance in several crop species has executed new challenges in cropping system of Pakistan.
There are many herbicide-resistant biotypes of weeds which include grasses and sedges in wheat- rice cropping systems and glyphosate also found resistant in some broadleaf and grasses.
Globally there are 499 unique cases of herbicide resistance with 255 weed species (148 di-cots and 107 mono-cots). Worldwide there are 23 out of 26 well- known herbicide sites of action and 167 different herbicides which have evolved resistant against herbicides. In 70 countries of world weed species become resistant in more than 92 crops.
How do weeds become resistant to herbicides?
Herbicide resistance is defined as” genetic ability of a weed plant to remain alive and imitate with concentration of herbicide that is lethal for less tolerant type of weed species”
Cross resistance occur when weeds population become resistant to one or more herbicides with same mode of actions.
Environmental dynamics and production practices affects crop species physiologically at any time, at any place, a phenomenon well-known as selection pressure. Under continuous conditions, the weed density will fully dominated by weed species that remains under those conditions.
The shifting of weeds can be due to agronomic or horticultural- practices (tillage, fertility, irrigation, etc.) and by the excessive use of herbicides, with very strong active- agents.
Some weed species will be more susceptible (less tolerant) than others weed species to many managing practice, and continual use of the same active ingredient can shift weed species to become dominated by many tolerant species.
Herbicide resistance, indicates that a genomic change has caused a formerly less tolerant weed species to become more resistant to any one of herbicides.
Herbicide resistance emerge from the way of adaptive evolution, in which alterations changes the physiology of weed plants in such a manner that the selective agent is not more effective to kill weed species.
In this continuous selection pressure exerted by the selective agent of herbicides, tolerant weed species with the new genotypic characters are not under-controlled and their progeny grow up with more damage.
Herbicide Resistance; Current scenario
In some agronomic crops during in- season and off- season the use of tillage and along with the excessive use of pre-emergence and post-emergence herbicides, herbicide resistance is less widely spread in contrast to other areas of the country where no-tillage and minimum-tillage systems have been commonly adopted.
Mechanism of herbicide resistance
For an active ingredient to cause harm to a weed plant, it is necessary that the herbicide must be taken into weed specie and must transfer (in toxic concentration) to a definite area where it has site of action.
When this lethal concentration of the herbicide reaches its target area it will cause damage to plant. However many physiological and metabolic activities can interfere with herbicide to reaching at its site of action. These deviations are stated as herbicide resistance mechanism in plants.
Mechanisms through which herbicide become resistant in weed plants include an alterations in site of action, over-production of site of action, improved metabolism of herbicide, less herbicide absorption, rate of translocation and herbicide sequestration.
The effect of active ingredient of herbicide starts when it binds with site of action. Variations in the mode and site of the action prevents the selective agent from binding are the utmost tools of herbicide resistance.
It is the superior ability of tolerant weed plants that they convert the molecules of active ingredient to a form that is nontoxic, whereas susceptible (less tolerant) species are not capable of detoxifying the herbicide at the same dose of herbicide.
How herbicide resistance can be assessed
The farmers can evaluate the resistance of weeds by following factors:
- By density of weeds in the field: As the density of weeds higher there will be the more chances of presence of resistance individuals of weeds.
- By observing the number and nature of weeds (broad leaf or narrow leaf) species which remains alive after application of herbicide.
Management and control:
- Herbicide mixtures are superior to herbicide resistance in prevention and management of weeds population.
- Crop Rotation is better option to avoid or to control herbicide resistance.
- Inspection of weed escapes after herbicide application.
- Use of herbicides with short residual lives.
- Use of certified seed.
- Herbicide Rotation.
- Application instruments should be clean before application of herbicides.
- Check for weedy patches after application of herbicide and their removal by hand hoeing or tillage.
- Diversification of crops to disrupt the growth season of weeds.
- Use of herbicides with different modes of action.
- Ploughing or cultivation before sowing of crop to manage emerged weeds plant and incorporation of non- germinated seeds.
- Delay planting for control of initial weeds population with a non-selective herbicide.
- Integrated weed management
Authors: Muhammad Shafique, Muhammad Rehan Saleem, Zubair Majeed, Zain Ul Abidin