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Minimum Time Between Herbicide Application and Tillage (05/19/16)

Question: I am aware that the time interval between when glyphosate is applied until the ground is tilled is (ideally) 5 days.

Minimum Time Between Herbicide Application and Tillage

Question: I am aware that the time interval between when glyphosate is applied until the ground is tilled is (ideally) 5 days. There are some who say that one can till after three days, if the weather is warm, 60 degrees or warmer. My question is what would be the minimum time interval in days, after spraying, to harrow the field to scatter/break up straw and roll out rocks? Thank you for all your help.

Answer: There are two processes that must occur after spray droplets are retained on leaf surfaces:

1. Deposition of the active ingredient on the surface of the plant foliage.
2. Absorption of herbicide active ingredient across the cuticle and active translocation to sites of action.

A review of published, peer-reviewed literature on glyphosate absorption and translocation show a range of absorption between 3 and 49% of the applied amount. Generally, there is an average adsorption of 33% across most weed species. Glyphosate is a very hydrophilic (water loving) herbicide and is repulsed by lipophilic (oil loving) compounds found in oil adjuvants and the waxy cuticle on leaf surfaces. The cuticle is a strong barrier to glyphosate adsorption. Humidity plays an extremely important role in aiding glyphosate adsorption (See #6, #14 on page 71-72 in the ND Weed Control Guide for further discussion). Growers and researchers consistently see greater weed control from glyphosate when applied in humid conditions (eastern ND compared to western ND or application prior to a rain event). Most small annual weeds require less total amount of herbicide applied and less time for the adsorbed herbicide to translocate than biennial or perennial plants. Plant death from glyphosate is slow and may take from 10 to 20 days depending on temperature and environmental conditions.

Small annual weeds may require only 3 days following application for maximum glyphosate absorption and kill. However, large winter annual weeds that have bolted, biennial and perennial weeds may require the full 5 days before tillage should be performed. Plants that have bolted or have an extensive underground root system will require at least 5 days for the herbicide to translocate to stems and roots for optimal control. In summary, small weeds, warm temperature, high humidity will increase adsorption and possibly allow a shorter interval between herbicide application and tillage. Bolted winter annual weeds, biennial and perennial weeds will require the full 5 days to get maximum kill.

Rich Zollinger

Extension Weed Specialist

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