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Herbicide Activity from Soil and Post Herbicides (06/15/17)

Herbicide Activity from Soil and Post Herbicides

Herbicide Activity from Soil and Post Herbicides

  1. Pre herbicide activity: I have evaluated most of the weed control research that my project has established this summer. In preparation for the imminent rain and prior to POST herbicide application I have evaluated the many PRE herbicides and herbicide combinations. Amount of activating rainfall ranged greatly between locations and general weed control ranged from 40% to 99% control. The main point of the article is that pre herbicides will result in some level of weed control even under low activating rain conditions. Where almost no rain occurred before weed emergence I still observed 40% or greater weed control. Even where the pre herbicide was not activated by rain for some time, the herbicides are stable and do not degrade until activated. The activity and degradation clock starts when activation occurs. This will help prevent successive flushes of weeds from emerging until crop canopy occurs. There are some pre herbicides, like Zidua, that show greater weed control from multiple rain events than one single event.


2.    POST herbicide activity: Many growers applied their POST herbicides prior to these recent rain events. Humidity increases before the rain event happens. Weed control is always greater under high humidity conditions and growers may very well get better weed control from these timely applications. Herbicides can be generally classified as water soluble (Ex. glyphosate and Basagran) or oil soluble (most other POST herbicides). The water soluble herbicides may benefit the most from high humidity conditions but greater humidity can increase weed control from most all post herbicides. Dr. John Nalewaja, the leading weed scientist at NDSU until he retired in the late 1990s invented MSO adjuvants with support from the NDSU Weed Science group. A major function of MSO adjuvant was their capacity to quickly dissolve leaf cuticular wax and increase herbicide absorption. The enhancement of post herbicides from MSO adjuvants are greatest on:

1. Drought-stressed weeds

2. Weeds not actively growing due to some stress condition

3. Weeds larger than recommended on the label.

4. Weeds that are show some level of tolerance to the herbicide


  1.  Liberty herbicide: Several growers are growing Liberty Link soybean as part of diverse weed management   program. The following recommendations to increase the activity of Liberty are taken from paragraph B9 on page 77 in the ND Weed Control Guide:

Apply in 15 to 30 gpa of spray volume by ground applicator and a minimum of 10 gpa of spray volume by aerial applicator. Choose nozzles and spray pressure to deliver a medium spray droplet. Large spray droplets will reduce weed control. Liberty works best during warm, humid, and sunny conditions. Rate, weed height, humidity, sunlight/time of day application, and temperature affects Liberty activity. High relative humidity (RH) significantly increases weed control and weed control may decrease as RH decreases. Full sunlight greatly enhances Liberty - apply Liberty after dawn and 2 hours before dusk. Liberty should control most broadleaf weeds but may not control large grasses or lambsquarters. Liberty will desiccate top-growth of biennial and perennial weeds but regrowth will follow due to limited translocation. Allow a 4 hour rainfree period after application.  Apply Liberty at the highest recommend rate with AMS at 3.0 lb.

- Do NOT omit AMS.

- Do NOT substitute use AMS replacement adjuvants for AMS

- Do NOT reduce the rate AMS.


4.    I have received several calls about the potential for residual herbicides applied in 2017 to carryover into the 2018 growing season due the dry soil conditions. Page 100 in the ND Weed Guide gives a good discussion of factors that affect fate of herbicides in many classes. Most herbicides are broken down through microbial activity. It is true that microbial activity may slow during periods of dry soil but significant breakdown will occur throughout summer and fall until soil temperature declines to near freezing. ALS herbicides, atrazine, metribuzin are exceptions and are broken primarily through a chemical reaction (acid hydrolysis) and to minor extent through microbial activity. There is sufficient moisture in soil for chemical breakdown to occur even in dry periods. The rotation intervals listed on herbicide labels should be followed and will be accurate under most environmental conditions, including periods of dry soil during the summer months.


5.    Roundup + Flexstar: I have received calls about incompatibility from glyphosate-K plus Flexstar with white precipitate occurring resulting in plugged nozzles. Glyphosate in the form of the K salt and Flexstar which contains the active ingredient fomesafen in the form of the Na (sodium) salt can result in incompatibility in some instances. The K and Na salts may interact and form crystal complexes. This ‘salting out’ may occur with any Flexstar formulation including those generics. Adjuvants that may be included in the tankmixture will not overcome this incompatibility. This may only occur in limited situations and the exact condition when it will occur has not been determined. However, water quality (hard water) may be a factor. Syngenta solved this possible incompatibility with Flexstar GT which contains the mono-ammonium salt of glyphosate similar to the Touchdown IQ formulation which has been discontinued (unfortunately).


6.    Roundup + 2,4-D: Potassium (K) salt formulations of glyphosate may negatively interact with dma (dimethyl amine) salt formulations of 2,4-D in the spray tank resulting in precipitation. Conditions that increase the risk of precipitation are application in low gpa, using cold water, and using high herbicide rates. This is another example of two dissimilar salts causing physical incompatibility and possibility of reduced weed control.


Grass antagonism from mixtures of glyphosate-ipa (isopropyl amine) and 2,4-D-dma (dimethyl amine) has been observed from several researchers. This is another example of negative herbicide salt interaction when mixing two herbicides containing different salts in the formulation. Some may remember an old product called Landmaster BW, a mixture glyphosate-ipa and 2,4-D-ipa used for weed control in fallow/no-till cropping systems. The ipa salt on both the glyphosate and 2,4-D avoided this antagonism.

Rich Zollinger

Extension Weed Specialist

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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