Questions come up annually on differences in chemical structure, specifically related to metolachlor vs s-metolachlor products. In this article we will discuss how chemical structure affects pesticide activity, specifically related to metolachlor products.
Weed Control
As the growing season starts up, weed control is a major topic of conversation. Utilizing residual herbicides is an important component of weed control programs. The very long chain fatty acids (VLCFA)-inhibitors (HRAC 15) class being a cornerstone across many cropping systems.
Site of Action (SOA) vs. Mode of Action (MOA)
Many pesticides bind to a specific location in the target pest to disrupt cellular function, thus controlling the pest. This is known as the site of action (SOA), with metolachlor binding to an enzyme that creates very long chain fatty acids in germinating weeds.
The Herbicide Resistance Action Committee (HRAC) uses SOA to characterize products into specific HRAC numbers, also known as “group” numbers (Metolachlor: HRAC 15). A mode of action (MOA) refers to the method in which a pesticide controls a pest, which in the case of metolachlor is by seedling shoot growth inhibition.
Chemical Structure of R- and S-Metolachlor
An Isomer is a compound with the same molecular formula but a different physical configuration. During chemical production commonly a Racemic Mixture of Isomers is produced, meaning an equal proportion of mirror isomers.
For pesticides, generally one isomer will have a greater level of activity on the target pest. This is usually related to how well the isomer binds to the SOA. With refined chemical production processes, it is possible to manufacture greater quantities of the desired isomer.
What are the benefits of refinement?
Benefits of refinement include greater production capacity, reduced production costs, and may result in a lower use rate for equivalent activity as the Racemic mixture.
Metolachlor vs. S-Metolachlor Products
Any metolachlor product is a racemic mixture of four isomers equally split in R- and S- configurations. Sometimes the racemic product is referred to as R-metolachlor consisting of this mix of isomers.
In this mixture the S-configuration is considerably more active. In the late 90s, a manufacturing innovation allowed for a greater proportion of s-metolachlor to be produced increasing from it from 50% to 88% of the mixture.
This allowed a new formulation, commonly referred to as s-metolachlor, to perform at or above the metolachlor products at equivalent use rates.
Use Rate Comparison
For comparison, a use rate of 1.0 pints of Visor® CRN (metolachlor + safener) is comparable to 0.6 pints of Visor® S-MOC II (s-metolachlor + safener) when corrected for the content of the active S-isomer. In some of these formulations, a safener is added, but this is not related to the s designation and will be listed separately.
Residual activity is not only related to use rate though, so when comparing metolachlor to S-metolachlor formulations at labeled rates minor differences in residual activity can be observed when applied alone.
This will commonly be observed on some of the toughest to control weeds later in the residual window.
How change in isomer configuration effects weed control
*Waiting on graph to be ready*
Figures 2 and 3 demonstrate how this change in isomer configuration effects weed control.
A greater rate of the racemic mixture (metolachlor) was required to achieve the same level of control as s-metolachlor on creeping bentgrass. However, when corrected for s-metolachlor content weed control was similar (Figure 2). This discrepancy amounted to minor differences (approx. 10%) in control of barnyard grass and green foxtail. With metolachlor still performing well on these major grass weed species (Figure 3).
Integrated Weed Management Plan
When using either metolachlor or s-metolachlor formulations at labeled rates, you will observe only minor to no differences in efficacy (depending on weed species)
An Integrated Weed Management Plan with overlapping residual herbicide, proper cultural practices, and strong knockdown products, either of the Metolachlor formulations will perform well in your weed control program.
The Takeaway
Please consult all label requirements, restrictions, varietal herbicide tolerance, and develop a plan going forward to ensure a great spring crop. If you have any questions, please consult your Agronomy Sales Enablement Team or your local Simplot Grower Solutions Crop Advisor.
Authors
Darrin Roberts, Northern Plains
Brice Bunck, Midwest
Chris Carpenter, Great Plains
Kevin Caffrey, Eastern Cornbelt
