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United States Department of Agriculture

Agricultural Research Service

Title: Organic herbicide update

Authors
item Webber, Charles
item Shrefler, James - OSU, LANE, OK
item Brandenberger, Lynn - OSU, LANE, OK
item Taylor, Merritt - OSU, LANE, OK
item Boydston, Rick - OSU, STILLWATER, OK

Submitted to: Proceedings of Horticultural Industry Show
Publication Type: Proceedings
Publication Acceptance Date: January 10, 2009
Publication Date: April 23, 2009
Citation: Webber III, C.L., Shrefler, J.W., Brandenberger, L.P., Taylor, M.J., Boydston, R.A. 2009. Organic herbicide update. In: Horticultural Industries Show, January 16-17, 2009, Ft. Smith, Arkansas. p. 237-239.

Interpretive Summary: Very few chemical weed control options are approved for organic use (corn gluten meal, vinegar, clove oil, and most recently ammonium pelargonate), but additional compounds are under investigation and pursuing organic approval. Weed research is the top research priority among organic producers. Corn gluten meal (CGM), a by-product of the wet-milling process of corn, and mustard meal (MM) are phytotoxic. As non-selective herbicides CGM and MM inhibit root development, decrease shoot length, and reduce plant survival of weed or crop seedlings. CGM can be effectively used for weed control with established turf, transplanted vegetables, and, if a non-treated strip is provided, for direct-seeded vegetables. Mustard meal the similar application and weed control potential as CGM. There are organically approved products that contain vinegar (i.e. 5%, 10%, and 20% acetic acid), a non-selective contact herbicide. Weed control increases as the acetic acid content and the application volume increase (i.e., 20, 40, 80, and 100 gpa). Vinegar is less effective in controlling grasses than broadleaf weeds and more effective on annual species than perennials. Weed control is also dependent on the weed size and species. Clove oil is the active ingredient in a number of organically approved postemergent non-selective herbicides. Clove oil applied at lower application volumes provides weed control efficacy as good or better than acetic acid herbicides. Broadleaf weed control, in general, is greater than grass weed control. Adding certain organically approved adjuvants will increase weed control with clove oil. Ammonium nonanoate/ammonium pelargonate is another non-selective contact postemergent herbicide has to have shown excellent weed control activity and has just recently received clearance as an organic herbicide. Ammonium nonanoate occurs in nature and is formed from the biodegradation of higher fatty acids. Ammonium nonanoate is more affective on broadleaf weeds than grasses, and smaller/younger weeds than larger/more mature weeds. Ammonium nonanoate can be affective at lower application volumes than the acetic acid products. Additional active ingredients and variations in formulations are being developed, entering initial greenhouse screenings, and progressing to field evaluations. Even if all these active ingredients, and their commercial formulations are registered by EPA, and approved for organic use, the application technology and timing will play an essential element in their successful integration into certified organic systems. Research with post-directed applications of non-selective contact herbicides are showing promise. Height and plant maturity differences between the crop and target weeds are important factors in controlling weeds and protecting crops from herbicide damage. Post-directed application technique is especially effective when used in combination with either preemergence applications or transplanted crops. Producers should also integrate as many non-chemical methods of weed control (i.e., stale seedbed, cultivation, cover crops, and plastic mulches) as possible into production systems. Producers will need to continue to monitor weed populations and control weeds early in their growth cycle.

Technical Abstract: Weed research is the top research priority among organic producers. Very few chemical weed control options are approved for organic use (corn gluten meal, vinegar, clove oil, and most recently ammonium pelargonate ), but additional compounds are under investigation and pursuing organic approval. Corn gluten meal (CGM), a by-product of the wet-milling process of corn, and mustard meal (MM) are phytotoxic. As non-selective herbicides CGM and MM inhibit root development, decrease shoot length, and reduce plant survival of weed or crop seedlings. CGM can be effectively used for weed control with established turf, transplanted vegetables, and, if a non-treated strip is provided, for direct-seeded vegetables. Mustard meal the similar application and weed control potential as CGM. There are organically approved products that contain vinegar (i.e. 5%, 10%, and 20% acetic acid), a non-selective contact herbicide. Weed control increases as the acetic acid content and the application volume increase (i.e., 20, 40, 80, and 100 gpa). Vinegar is less effective in controlling grasses than broadleaf weeds and more effective on annual species than perennials. Weed control is also dependent on the weed size and species. Clove oil is the active ingredient in a number of organically approved postemergent non-selective herbicides. Clove oil applied at lower application volumes provides weed control efficacy as good or better than acetic acid herbicides. Broadleaf weed control, in general, is greater than grass weed control. Adding certain organically approved adjuvants will increase weed control with clove oil. Ammonium nonanoate/ammonium pelargonate is another non-selective contact postemergent herbicide has to have shown excellent weed control activity and has just recently received clearance as an organic herbicide. Ammonium nonanoate occurs in nature and is formed from the biodegradation of higher fatty acids. Ammonium nonanoate is more affective on broadleaf weeds than grasses, and smaller/younger weeds than larger/more mature weeds. Ammonium nonanoate can be affective at lower application volumes than the acetic acid products. Additional active ingredients and variations in formulations are being developed, entering initial greenhouse screenings, and progressing to field evaluations. Even if all these active ingredients, and their commercial formulations are registered by EPA, and approved for organic use, the application technology and timing will play an essential element in their successful integration into certified organic systems. Research with post-directed applications of non-selective contact herbicides are showing promise. Height and plant maturity differences between the crop and target weeds are important factors in controlling weeds and protecting crops from herbicide damage. Post-directed application technique is especially effective when used in combination with either preemergence applications or transplanted crops. Producers should also integrate as many non-chemical methods of weed control (i.e., stale seedbed, cultivation, cover crops, and plastic mulches) as possible into production systems. Producers will need to continue to monitor weed populations and control weeds early in their growth cycle.

Last Modified: 4/16/2014
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