Resistance

Help

Resistance in weeds to various biochemical mode of actions of herbicides (MOA) occurs in 2 different ways:

• as target-site resistance
• as unspecific resistance

Target site resistance: A biochemical target site may often be an enzyme, which catalyze a natural chemical process in plants, which a specific herbicide molecule can be inhibit and thereby block, so important life-process in plants will stop. The level of inibition depend on dose rates, and often dose rates can be found, which are tolerated by some plant species, e.g. crops (and some weeds, too), while more susceptible weed species, will be controlled more or less efficiently.Target-site resistance will often affect all herbicides, which contain this MOA, and all herbicide products containing this MOA will be affected. Target-site resistance is characterized by:

• sudden appearance
• fast spread, if the MOA is used continously and unilaterally)
• requiring 10-20 times higher dose rates, why options for compensation by increasing dose rates, will often not be feasible in practice

Unspecific resistance: Unspecific resistance means resistance across many (or all) MOA. It appears often as metabolic resistance, which means that weeds can degredade or inactivate the efficacy of herbicides in general.Unspecific resistance is characterized by:

• slow development, enabling compensation by graducal increases of dose rates
• determination difficulties, especially in early stages/li>

So far, specific resistance is the most common type, however, research has not yet provided a clear overview of possible interactions between specific and unspecific resistance.

In Denmark, specific resistance have been detected for mainy 2 MOAs:

• ACCase, which in HRAC classification is Group A. ACCase is the name of an enzyme in plant cells, which is blocked. Such herbicide products are also called fops and dims, which refer to the end of the names of the active ingredients.
• ALS, which in HRAC classification is Group B. ALS is the name of an enzyme, which can be blocked. These herbicides are also called sulfonylureas, or just SU-herbicides. Group B includes also floralulam, even though this is not a sulfonylurea.

There is widespread consensus on the perception that herbicide resistance arise due to mainly the following 2 conditions:

• natural mutations in genomes of weeds
• unilateral control by a MOA, against which resistance have already developed, why such biotypes will be propagated effectively and quickly, according to the usual biology of such species

Unfortunately, only sparse documentation is available on how resistance can be prevented, and it is not realistic to wait several years on results from research in different strategies and frequencies for use of different MOA. Consequently, it cannot be estimated, how safe it may be to change in different systeamatic ways between different MOA, however such alternative strategies will most likely be more expensive.

In IPMwise, strategies for prevention are based mainly on common sense and results from monitoring of resistance development. In the Nordic region, monitoring is conducted by the organisation NORDBARAG.

In IPMwise, adjustments of decision algorithms will be made dynamically in line with current development in new incidents.

IPMwise recommends for Danish conditions that MOAs, for which several incidents of resistance have already been detected, shall be recommended for use maximum in every second generation of weeds. However, until more clear documentation can be provided from research, IPMwise will leave it up to You to determine, how strict this rule (or alternative rules) shall be applied in your place.

IPMwise will assist some decisions including your preferences. For example, if you decide to limit prevention measures to fields, where weed species occur, which NORDBARAG estimates to be particularly subjected to risk.This allows you to use risky MOAs as often as you like, as long as such threatned species have not been found in your place.

IPMwise will also assist your preference that mixing with safe MOAs should allow any risky MOA. This may include pre-formulated mixtures as well as tank-mixtures. However, this preference must be considered to be relatively risky, as fields may easily occur, where the actual composition of weeds will only/mainly be controlled by a risky MOA, why only poor prevetion will be achieved. In the tool Consultation the list of treatment options also show, whether the reported weeds are treatned or not (in Denmark, from Group A or B).

Show prevention of herbicide resistance
When you tag this box, options for prevention of herbicide resistance are presented, according to your preferences or, from the strategy in IPMwise

Exclude MOAs
Select MOAs, which you wish to exclude from the list of recommendations. The list of recommendations shows, which MOA that has large risk of resistance development. In the link Mode-of-action to the right side, you can see the MOAs included in different herbicide products. Locate the herbicides you used in the previous crop (generation of weeds) and note the MOAs they contained.

Avoid only when
You can allow use in certain circumstances as listed below. If you tag both boxes, both conditions will simultaneously be met.

Mode-of-action(s) are used alone
When risky MOA are used in combination with other MOA, a continuous use of risky MOAs are allowed despite the unsertainties explained above.

The field has threathened weed species
If you have species threatened to resistance development, i.e. several incidents have been found in the Nordic countries, you may tag here, and no recommendations wil be made which include this MOA.

---

---