Herbicide resistance is one of the most significant challenges in modern agriculture, threatening the effectiveness of crop protection and productivity.
Among the various forms of herbicide resistance, ALS (Acetolactate Synthase) and ACCase (Acetyl-CoA Carboxylase) inhibitor resistance are two of the most widespread and impactful. Both types of resistance affect the ability of farmers to control weed populations, leading to reduced crop yields and increased production costs.
Understanding these resistance mechanisms is crucial for developing effective weed management strategies.
Molecular understanding of ALS and ACCase inhibitor resistance:
Mechanism of ALS resistance
The broad application of ALS inhibitors in agriculture is due to their high efficacy against a wide spectrum of weed species. The ALS inhibitor effectively blocks the action of the enzyme acetolactate synthase, which plants use for synthesizing branched-chain amino acids such as leucine, isoleucine, and valine. Without these amino acids, the plant cannot survive and will eventually die.
However, due to broad and constant reliance on ALS inhibitors, many weed species have developed resistance to these herbicides. The most prevalent form of resistance to ALS inhibitors is a single mutation in the ALS gene that confers an alteration in the binding site of the herbicide to the enzyme. Thus, the enzyme cannot be adequately inhibited by the herbicide, allowing the survival of the weed.
ALS gene mutations may occur at several different sites within the gene conferring a range of resistance levels. Some confer high levels of resistance to specific ALS-inhibiting herbicides, while others confer cross-resistance to several herbicides within the same chemical class. Evolved resistance due to ALS gene mutations is becoming increasingly difficult to manage weed populations especially in those crops having heavy reliance on ALS inhibitors for their weed control.
Mechanism of ACCase resistance
The ACCase inhibitors act on an enzyme called acetyl-CoA carboxylase, which is very crucial for the synthesis of fatty acids and is required in plant cells for growth and development. ACCase herbicides are active, especially against grass weeds; therefore, they are an important option to aid farmers in controlling grassy weeds in crops such as wheat, barley, and canola.
ACCase resistance is analogous to the example of ALS resistance and involves mutation of the ACCase gene such that the herbicide can no longer bind to its target enzyme. This type of resistance is common in most grass species including wild oats and ryegrass. Inhibitors of ACCase may also develop resistance through gene amplification where plants may synthesize multiple copies of the gene for ACCase thus swamping the effect of the herbicide.
Over-reliance on a single site of action has contributed to the rapid spread of resistant populations in both ALS and ACCase resistance. Resistant weeds can monopolize fields, making it increasingly difficult to effectively use conventional herbicides.
Implications for weed control
The development of ALS and ACCase-inhibitor-resistant weeds has great implications for weed management. Once the weed population becomes resistant to these herbicides, alternative methods have to be used; these are often more expensive and less effective in managing weed populations. More often than not, herbicide resistance results in increased applications of herbicides, which means higher input costs for farmers and potential environmental damage.
On the other hand, resistant weeds will compete with the grown crop for water and light, hence lowering the yields of such crops. This has been a great concern for farmers across the continent, especially in Canada, where herbicide-resistant weeds have become quite common; examples include wild oats and kochia.
Weed resistance to ALS and ACCase inhibitors further contributes to the reduction of herbicides available for effective weed management. This is simply because as resistance develops, fewer herbicides are available to effectively protect crops. This situation shows the importance of diversified strategies for weed control, and this calls for the adoption of integrated weed management practices.
Strategies for mitigating ALS and ACCase-inhibitor resistance
Overall, the prevention and management of ALS and ACCase-inhibitor resistance are followed by an integrated approach, which is a combination of chemical, cultural, and mechanical means of weed control. The strategies that might reduce the impact of herbicide resistance include the following:
1. Diversify herbicide modes of action
Rotating herbicides with dissimilar modes of action is one of the most efficient ways of managing herbicide resistance. Farmers apply mixtures of residual and contact herbicides in the same spray solution to target weeds at a range of different growth stages; this may lower the likelihood of the development of resistance. A successful application of pre-emergent herbicide in Canada frequently catches weed species that emerge early enough to get established before the crop does. The use of herbicides with multiple modes of action within a season contributes to preventing weeds from developing resistance to any single herbicide class.
2. Tank Mixing and Herbicide Stacking
Another tactic involves the application of tank mixes comprising herbicides with different modes of action. This will increase the spectrum of weed control and reduce the selection pressure towards resistance. For example, an effective ACCase-inhibiting herbicide tank mixed with an effective broad-spectrum herbicide will be active against both grassy and broadleaf weeds and reduce the likelihood of resistance. The advantage of including a herbicide stack in a tank mix is insurance in case one of them is resistant to weeds, as even if one herbicide were to not work as well, there would be other active ingredients available to control the weed population. If possible, always try to use two or more effective modes of action to slow the development of herbicide resistance.
3. Crop Rotation
The rotation of crops is still an essential component of weed management. Often, various crops are subjected to various herbicides. For instance, the rotation of cereal crops and pulses brings about new requirements for herbicides and thus, decreases the selection pressure on any one weed species.
4. Mechanical Weed Control
Incorporating mechanical weed control methods, such as tillage or mowing, can also help manage herbicide-resistant weeds. Mechanical control disrupts weed growth, prevents seed production, reducing the weed seed bank in the soil. Although mechanical control may not completely negate the use of herbicides, it minimizes reliance on chemical control and serves as a useful component in integrated weed management systems.
5. Cultural Practices
Some of the major cultural practices that can suppress weeds by creating a competitive environment include cover cropping, increased crop seeding rates, and narrow-row planting. In fact, cover crops have the potential to reduce the number of weeds emerging in the spring. This also possesses additional benefits relative to soil health and erosion control. These are some of the major components of a diversified weed management program.
6. Herbicide Resistance Monitoring
Monitoring of the weed population for resistance is of utmost importance, as early detection and management of herbicidally resistant species allow for prompt action against such cases. In fact, confirmation of such suspicions by a laboratory test of resistant weeds helps farmers to create the necessary adjustments in their weed management strategy. Early detection provides farmers with the opportunity to make informed decisions to reduce the spread of resistant weed populations.
The phenomenon of herbicide resistance, especially ALS and ACCase-inhibitor resistance,is a real threat to modern agriculture. The emergence of such resistance mechanisms might render weed control ineffective, leading to reduced crop yields and increased costs of production. However, farmers can minimize this resistance impact by diversifying their weed management practices and crop protection products, hence remaining effective in their management.