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Corvus - A new 'one-pass' herbicide from Bayer   
by
Bob Hartzler

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February 20, 2009Corvus is a combination product for corn containing isoxaflutole, thiencarbazone-methyl, and a safener. It provides broad-spectrum residual control, and also has burndown activity on small, annual weeds in no-till situations. Bayer recommends the addition of atrazine to improve performance consistency. Most people are familiar with isoxaflutole, the active ingredient in Balance Pro/Flexx. Isoxaflutole is a bleaching herbicide whose utility has been limited somewhat by a narrow margin of crop safety, resulting in the use of sub-optimal rates that compromise weed control. The new safener (cyprosulfamide) present in Corvus and Balance Flexx minimizes the injury risk, therefore allowing more effective rates. Corvus is labeled at rates of 3.3 to 5.6 oz/A; a 5 oz rate will provide the equivalent of 2.3 oz of Balance Pro or 4.6 oz of Balance Flexx.

The second active ingredient in Corvus is thiencarbazone-methyl, a new ALS-inhibiting herbicide. Although it inhibits the same enzyme as sulfonylurea and imidazolinone herbicides, it is from a distinct chemical family. Corvus has performed well in University trials across the North Central region, and it appears expectations for the product are high among potential users.

In addition to field performance, the chemical characteristics are useful in evaluating the risks and benefits of a new product. The KOC (soil organic carbon-water coefficient) describes the ratio of herbicide bound to organic carbon to that dissolved in soil water (Table 1). As the Koc increases, the percentage of herbicide bound to soil colloids increases. This coefficient is useful in comparing the response of different chemicals to soil type and rainfall. Thiencarbazone’s K value of 100 suggests it will require slightly less rain to activate than competing amide products.

Table 1. Chemical properties of several herbicides used in corn. (See discussion of
half-lives at end of article)
.

Herbicide

KOC*

Half-Life* * (days)

Half-Life* (days)

GUS
Leaching Index*

thiencarbazone-methyl

100

NI

17

2.46

acetochlor (Harness)

156

6.3

14

2.07

S-metolachlor (Dual Magnum)

226

13.7

15

1.94

dimethenamid (Frontier)
108
7.3
13
2.19

atrazine

100

NI

75

3.75

glyphosate (Roundup)

21699

NI

12

-

* Data from IUPAC Pesticide Properties Database: http://sitem.herts.ac.uk/aeru/iupac/index.htm
**Data from Mueller et al. 1999. Weed Technol. 13:341-346.
NI = Not included in the study.

The half-life for thiencarbazone is 17 days, which is sufficient to control weeds until the corn canopy develops under most situations. Carryover risk is determined by both the half-life of the herbicide and the susceptibility of the rotational crop, and one would also need to consider the sensitivity of rotational crops to isoxaflutole when evaluating carryover risks with Corvus. There is a 9 month plant back interval for soybean following Corvus application, and several crops have a 17 month rotation restriction.

The GUS Leaching Index is calculated using the K and half-life values. Chemicals with a value greater than 2.8 are considered to be leachers. Thiencarbazone would be expected to move deeper into the profile than acetochlor or metolachlor due to its longer persistence and lower adsorption. However, its GUS value is much lower than atrazine due to thiencarbazone’s shorter half-life.

So what should be expected of Corvus? Although many products have been promoted as a one-pass weed control option, this is asking a lot from any herbicide. Full-season weed control can be accomplished with a single application, but it requires selection of fields with susceptible weed infestations and favorable environment throughout the growing season. Weed species that are likely to be controlled inconsistently by Corvus alone include cocklebur, giant ragweed, morningglory and woolly cupgrass. Regardless of the weed infestation, a systematic scouting program should be implemented throughout the early growing season to detect any escapes in time to allow timely implementation of effective post options and to prevent yield losses due to competition.

Half-life: The half-life describes the time required for 50% of the chemical to be degraded. Since the degradation rate varies upon the environment the herbicide is placed, large differences in reported half-lives can be found. For example, the half-life of atrazine in a Minnesota soil with a pH of 7.9 was found to be 261 days, whereas in a Georgia soil with a pH of 6.8 the half-life was 39 days. It is unclear whether the half-lives in the IUPAC database were determined under similar conditions, whereas the half-lives in the Mueller (1999) article were determined under identical environments.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
http://www.weeds.iastate.edu
Submit questions or comments here.  

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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.