Herbicide resistance now becomes a worldwide headache for farmers as well as scientists from the agriculture sector. The 1st herbicide resistance was reported in the USA in 1968 (Ryan, 1970), where a biotype of Common groundsel (Senecio vulgaris) reported resistance to triazines. Resistance biotype of Spreading dayflower (Commelina diffusa) was also reported in 1957 by Hilton towards 2,4-D. In India too, the resistance biotype of Phalaris minor against isoproturon was reported during 1992-93 by Malik and Singh. Herbicide resistance may be defined as the inherent ability of weed species or biotype or weed population to survive and reproduce following a herbicide application at a dose that is earlier found lethal to the vast majority of the weed population. Herbicide resistance is simply an altered response of a weed species to a herbicide that was earlier susceptible. Resistance is always dependent on the rate of application of pesticides. Resistance development is an evolutionary process where a rare resistant individual becomes the major one. Resistance is also induced by genetic engineering, mutation, and gene flow. 

Chemical weed control by herbicide is the easiest and effective weed management practice. Weeds cause nearly 34% crop loss but the share of herbicide is only 16% among total pesticide use. The consumption of herbicide in Japan is as high as 5000g/ha, while in India it is hardly 40g/ha. The world’s use of herbicide is growing rapidly, including in India. In contrast to the benefits, there are some inadvertent drawbacks such as herbicide resistance, the shift in weed flora, and environmental concerns. The most prominent among them is the development of herbicide resistance in targeted organisms. Farmers shifted to sulfosulfuron and mesosulfuron + iodosulfuron from tailor-made herbicide i.e. isoproturon to control weeds in wheat crop due to the development of isoproturon resistant biotype after its continuous use in North and North-Eastern India.

Types of herbicide resistance

There are several types of herbicide resistance observed based on the mechanism of resistance i.e. partial resistance, complete resistance, reverse resistance, co-resistance, multiple resistance, cross-resistance etc.

Partial resistance: Resistant biotype receives severe inhibition on their vegetative growth by the used herbicide but continued to grow insufficiently to reach the reproductive stage as well as produce some seeds.

Complete resistance: Resistance biotypes are not at all affected by the herbicide used and their growth and reproduction continue even after application of herbicide.

Reverse resistance: The weed biotype resistance to a herbicide previously fall susceptible to that herbicide when it is not used for several years.

Co-resistance or Compound resistance: When herbicide mixture is used for years, weed biotype show resistance to both the herbicides in this type of resistance.

Multiple resistance: Multiple resistance refers to the condition where resistance plants possess two or more distinct resistance mechanisms and due to this, that weed or biotype show resistance to two or more classes of herbicide.  e.g., Lolium rigidum is a species of annual grass showing resistance to almost ten to fifteen herbicides.

 Cross-resistance (CR): Cross-resistance refers to a condition or situation when weed biotype is resistant to two or more herbicides due to the presence of a single resistance mechanism (single genetic mutation). e.g., Isoproturon resistant weed, Phalaris minor found resistance to Pendimethalin 30% EC.

a. Target site CR: It refers to the situation when all the herbicides to which the target has shown or evolve resistance affect the same site or enzyme-like Cochia species is resistant to ALS inhibitor (sulfonylurea and imidazolinone herbicide).

b. Metabolic CR:  It refers to the situation when all the herbicides to which a weed species have shown resistance, their toxic product is degraded by the same mechanism or same pathway.

c.Negative CR: NCR is the situation when a weed or biotype is resistant to one herbicide or a family of herbicides but more susceptible to some other than its natural wild type susceptible population. Negative CR is a very effective tool to prevent resistance in weeds.

Factors affecting the development of resistance

There are different factors that determine the development of herbicide resistance in weeds;

Weed characteristics: Initial frequency of the herbicide-resistant individual, selection pressure by resistant population, ecological/biological fitness of weeds, weed biology, seed bank in the soil, and nature of inheritance of resistant genes.

Herbicide characteristics: Herbicides with a highly specific mode of action, herbicide metabolism, the long residual activity of herbicide, over-dependence on single herbicide, type of herbicide use, and its time and dose of application.

Cropping practice: Tillage and cropping system.

Mechanisms of herbicide resistance

Mechanisms of herbicide resistance can be broadly grouped into two categories:

1.     Target site resistance

2.     Non-target site resistance

Target-site resistance (TSR):

It is generally due to a single or several mutations in the gene encoding the herbicide target enzyme, which, in turn, decreases the affinity for herbicide binding to that enzyme. In many cases, resistance to herbicides like ALS inhibitors, dinitroaniline, triazine, etc. is due to alteration of the target action site. In addition, gene amplification is the most recently described mechanism, example, for example, in Amaranthus palmeri and Kochia scoparia, EPSPS gene amplification correlates with glyphosate resistance and induces resistance by increasing target enzyme production, effectively diluting the herbicide relative to the target site.

Non-target-site resistance (NTSR):

It is caused by mechanisms that reduce the amount of herbicidal active compounds before it can attack the plant. Reduced penetration and/or altered translocation, enhanced herbicide sequestration, and/or metabolism (detoxification) are the major causes of resistance. Active vacuolar or cell wall sequestration can prevent the herbicide to reach the site of action resulting in herbicide resistance. For example, glyphosate resistance in Conyza canadensis, Lolium sp. etc. is due to vacuolar herbicide sequestration. Finally, the biochemical reactions that detoxify herbicides can be grouped into four major categories: oxidation, reduction, hydrolysis, and conjugation.

Mechanism of resistance

1.     Differential herbicide uptake: In resistant biotypes uptake of herbicides seems to be difficult due to morphological barriers like overproduction of waxes, hairy epidermis, reduced leaf area, etc.

2.     Differential translocation: In resistant biotypes, the apoplastic (cell wall, xylem) and symplastic (plasmalemma, phloem) transport of herbicide is reduced due to different modifications.

3.     Enhanced metabolism: Rapid degradation and conjugation of herbicide into non-toxic or less toxic form are major mechanisms of resistance in several weed species. In a few cases, triazene resistance in weeds has shown resistance through detoxification.

4.     Sequestration and compartmentation: Some plants are capable of restricting the movement of foreign compounds (herbicides) within their cells or tissues to prevent the compounds from causing harmful effects. In this case, an herbicide may be inactivated either through binding (such as to a plant sugar molecule) or removed from metabolically active regions of the cell to inactive regions, the cell wall, for example, where it exerts no effect.

5.     Altered site of action: It is the modification in the binding site of action of a herbicide due to some genetic changes in biotype or weed species showing resistance to a particular herbicide or group of herbicides compared to susceptible one, and the result of this the resistant biotype remain unaffected by that same applied herbicide. In fact, resistance in different weed species is due to different mechanisms. The development of resistance in many weed species to most the triazine herbicides is due to the altered site of action.

Management of herbicide resistance

Herbicide resistance management is as complex as its mechanism of development. There are some alternatives to control and prevent the development of herbicide resistance such as;

1.     Quarantine Measures

2.     Cultural Techniques

3.     Herbicide rotation and herbicide mixtures

4.     Use of alternative herbicide with short residual life

5.     Rotation of Crops

6.     The use of bioherbicide to manage resistance

7.     Integrated Weed Management

Some other cultural practices followed to block the emergence of resistance is a selection of weed competitive crop cultivars, use of clean seeds, stale seedbed technique, closer row spacing, timely sowing and rate of seeding, good crop husbandry, and soil solarization.

Conclusion:

Herbicide resistance is a global phenomenon and an unprecedented level is growing in the number of resistant weed biotypes. Herbicide over-reliance should be reduced and herbicide combined with other methods should be used. Herbicide should be used in rotation or as a mixture. The best way to manage the herbicide resistance problem is to include the cultural, quarantine, and other practices in an integrated manner.

References:

[1]. Duary, B., (2008). Recent advances in herbicide resistance in weeds and its management. Indian Journal of Weed Science, 24:124-135.

[2]. Shaner, D. L., (2014). Lessons learned from the history of herbicide resistance. Weed Science, 62(2):427-431.Herbicide