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Temperature and translocation

Agriculture.com Staff 07/06/2010 @ 5:21pm

Air temperature at the time of a foliar herbicide application can have a dramatic impact on the level of weed control achieved. The exact response of a weed to a herbicide applied under cool conditions is difficult to predict but can include: no control; reduced control; slower herbicide activity but no change in final control; or complete control.

Variability in the efficacy of an herbicide when applied under cool temperatures depends on the specific herbicide or weed, herbicide rate, weed height, and the duration and magnitude of the cool temperatures.

In order to be effective, all foliar herbicides must pass through the outer leaf surface composed of the waxy cuticle and then through the membrane of plant cells. The waxy cuticle and cellular membranes have commonly been described as behaving like butter under different temperatures. Warm temperatures will result in a soft, fluid consistency which will allow herbicides to penetrate the leaf surface and enter plant cells. Cool conditions will harden these structures and make herbicide entry much more difficult. That means herbicides applied under cool conditions may get stuck in the leaf surface and not be able to move further into the plant cells to have activity.

After penetrating the cuticle and the cell membrane, the herbicide must move to its specific site of action within the plant. Non-systemic herbicides (also referred to as contact herbicides) such as paraquat are able to exert their herbicidal effect in close proximity to the point of entry into the plant.

However, systemic herbicides such as glyphosate need to be moved (translocated) from the point of entry to a specific site of action in the shoot and root growing points. Cool air temperatures can greatly reduce the amount and speed of translocation of the herbicide in the plant which can limit or at least slow the activity of a systemic herbicide.

Finally, all herbicides inhibit a metabolic process such as the activity of a particular enzyme or interference with photosynthesis. Thus, plants must be actively growing for the herbicide to inhibit the targeted process. If cool conditions limit plant growth and enzyme function, herbicide activity will be reduced. Likewise, extremely high air temperatures can lead to weed stress, resulting in a reduction in herbicide activity due to lack of metabolic activity.

The herbicide molecule may persist in a plant under cool air temperatures, but the herbicide may not move, may have herbicidal activity, or be metabolized until higher air temperatures occur.

In summary, non-systemic herbicides are less influenced by cool air temperatures because they do not require translocation which is temperature dependent. Conversely, systemic herbicides must be translocated and cool air temperatures will have a greater impact on final weed control.

Do you have a question about fall strip till vs. spring planter attachments? Email rich.fee@meredith.com. We'll send some of the most common questions to professionals in the industry and see what they say. Look for answers in upcoming Agro-Connect Ask the Experts columns.

Air temperature at the time of a foliar herbicide application can have a dramatic impact on the level of weed control achieved. The exact response of a weed to a herbicide applied under cool conditions is difficult to predict but can include: no control; reduced control; slower herbicide activity but no change in final control; or complete control.

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