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How can I avoid getting stuck in an imi herbicide cycle?

with Sarah Wilson, Senior Technical Services Specialist, BASF

Group B herbicides are very handy tools in the weed control toolkit, but weeds can evolve resistance relatively quickly to their mode of action. To keep these herbicides as an option, and to maximise the benefits of imi-tolerant crops, it is essential that they are used correctly within farming systems.

Senior Technical Services Specialist with BASF, Sarah Wilson says it is easy for growers to get caught up in the imi-cycle of using imi-tolerant crops to avoid plant-back issues with imi residuals in the soil.

Senior Technical Services Specialist with BASF, Sarah Wilson says it is easy for growers to get caught up in the imi-cycle of using imi-tolerant crops to avoid plant-back issues with imi residuals in the soil.

“The problem arises when an imi-tolerant crop is sown to avoid imi-residues from the previous crop or fallow, but then the grower also wants to use the imi chemistry in the crop,” she says. “This leads very quickly to over-use of the imi herbicides, and research has shown that as few as four applications of group B herbicides (to which imi herbicides belong) to the same population of weeds can result in the selection of resistant individuals, so resistance can evolve within a very short period of time.”

In Australia there are four imidazolinone or ‘imi-type’ active ingredients registered to control a variety of grass and broadleaf weeds in crops and fallow. These actives are imazamox (e.g. Intervix®*, Raptor®), imazapic (e.g. Bobcat I-Maxx®*, Flame®, Midas®*, OnDuty®*), imazapyr (Arsenal Xpress®*, Intervix®*, Lightning®*, Midas®*, OnDuty®*) and imazethapyr (Lightning®*, Spinnaker®). The other types of herbicides in Group B are the pyrimidinylthiobenzoates, sulfonylureas (SUs) and triazolopyrimidines herbicides. They all inhibit the plant’s production of specific essential proteins.

Use the WeedSmart Big 6 to develop an integrated weed control program that keeps Group B herbicides as a viable option well into the future.

*products that contain more than one active.

How do Group B herbicides work?

Short answer: The Group B herbicides, including the imis, interfere with the activity of the ALS enzyme that is used in the production of certain essential plant proteins.

Longer answer: The Group B mode of action is to inhibit the production of the acetolactate synthase (ALS) enzyme in the plant cells. This enzyme is needed to produce essential plant proteins. By inhibiting ALS production, a foliar herbicide application causes the plant to deplete its supplies of the essential proteins and the plant will slowly die, often taking about three weeks. Group B herbicides with residual activity inhibit the production of amino acids so the plant uses up the reserves in the seed as it germinates and is exhausted before it breaks through the surface of the soil.

Whether using Group B herbicides as a pre-emergent, or post-emergent application; consider the use of registered tank mixes with herbicides from other modes of action.

What conditions do Group B herbicides need to work best?

Short answer: Small weed size is critical for effective foliar application. Imis will not kill older weeds, so applying these herbicides to large weeds is a waste of money.

Longer answer: ALS concentration is highest in young plant tissue and so foliar Group B herbicides are most effective when plants are small and actively growing. When plants are moisture-stressed there will be reduced uptake and translocation of foliar-applied imis.

Foliar uptake of imis is very sensitive to high temperatures. In summer, temperatures in the 30s will require much more active ingredient for the same level of control achieved at lower temperatures. Follow the label instructions.

There is a wide range of soil characteristics and environmental conditions that affect the efficacy of soil-applied Group B herbicides.

What are recommended Group B use patterns?

Short answer: Apply no more than two (2) Group B herbicides in any four (4) year period on the same paddock, and choose the right product for the situation.

Longer answer: A Group B herbicide application in either a summer crop or summer fallow is equivalent to a winter crop pre-emergent application, so no further Group B applications should be made in that paddock, that year.

Use Group B herbicides strategically, if you use imazapic (Flame) in the summer fallow and Ally, Logran, Atlantis or Intervix (for imi-tolerant cereals) over the top of your cereal crop in the winter, you then need to wait three years before using any other Group B chemistry in that paddock.

If you are planting imi-tolerant varieties to get around an imi residue problem, do not use imi chemistry over the top – it’s not good practice for resistance management and you will get stuck in the imi-cycle!


What are my options if there’s sufficient planting rain but the plant-back requirements for the Group B herbicide haven’t been met?

Short answer: Consider planting a Clearfield or imi-tolerant crop, but try to avoid using imis or other Group Bs in-crop. Imazapic (e.g. Flame) applied in a summer fallow is cheap and effective, but it will have implications for crop rotation flexibility.

Longer answer: Imis have a broad range of soil binding characteristics and the period of residual decay varies markedly. Microbial activity is the primary mechanism for breakdown of soil-applied imis. Consequently, soil moisture and temperature play an extremely important role in how long the herbicide remains effective in the soil and when it is safe to plant a sensitive crop.

Source: Soil behaviour of pre-emergent herbicides in Australian farming systems (GRDC)

Even if the residual has not broken down sufficiently to safely plant sensitive crops, there may be poor weed control due to sub-lethal amounts of herbicide remaining in the soil. This scenario represents a serious risk of partially-resistant weeds setting seed. Other weed control options must be set in place to control weed escapes.

While Clearfield and imi-tolerant crops are the most tolerant crops available, there are several non-Clearfield crops, such as chickpea, field pea, mungbean, peanut and soybean that have a degree of natural tolerance to imi herbicides. Look for a safe option that also enables the use of non-Group B herbicides and or cultural methods to manage weeds in-crop. If you need to use a pre-emergent, be sure to choose from an alternative herbicide MOA group.

Other resources:

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