Best practices for using glufosinate (Liberty) herbicide

Navjot Singh – Weed Science Graduate Student, UMN,
Debalin Sarangi – Extension Weed Scientist, UMN
Liz Stahl – Regional Extension Educator – Crops, UMN
Joe Ikley – Extension Weed Scientist, NDSU
Tom Peters – Sugarbeet Extension Agronomist, NDSU/UMN

Glufosinate is a non-selective, contact herbicide (site-of-action Group 10) used in glufosinate-resistant corn, soybean, and canola. Herbicides like Liberty 280 SL, Cheetah, Finale, Interline, and Sinate contain glufosinate, with Liberty 280 SL being the most commonly used product among them. In addition to burndown applications before planting, glufosinate is mostly used for over-the-top applications in LibertyLink crops. However, label restrictions for rate and crop stage should be followed. The herbicide-resistant trait choices available for row crops in Minnesota and North Dakota are summarized in this article.

Figure 1. A recent waterhemp population survey
from Minnesota shows the presence of glyphosate
-resistant populations. The color gradient from green
to red indicates the reduction in glyphosate sensitivity
 among the populations tested. The counties in white
 were not surveyed.

Glyphosate resistance in waterhemp has been documented to be widespread in Minnesota (Figure 1). As a result, farmers have been relying more frequently on glufosinate for effective management of this weed. Surveys conducted at Private Pesticide Applicator Recertification Workshops by UMN Extension indicated that control with glufosinate has been less than optimal for some farmers in Minnesota. 

Resistance to glufosinate has not been confirmed to date in any weed populations in Minnesota and North Dakota. However, conditions at application and/or application practices are likely playing a role where control has been less than optimal. The following are several considerations for achieving optimal weed control with glufosinate, using the Liberty 280 SL label as a reference (always refer to the herbicide label for the specific product you are using).

1. Dose and crop stage: Liberty application should be made at 32 to 43 fl oz/A from the emergence to R1 stage of LibertyLink or Enlist E3 Soybean with a maximum yearly application of 87 fl oz/A. For LibertyLink corn, Liberty application rate is 29 to 43 fl oz/A applied from emergence through V6 stage with a maximum yearly application of 87 fl oz/A. Do not apply Liberty within 70 days of grain harvest and 60 days of forage harvest.

2. 

   Figure 2. Waterhemp survived a 32 fl oz/A rate
   of Liberty application as the herbicide was applied
    when weed height was more than 4 inches. Regrowth
    is occurring from the lower growing points at 21
    days after application. (Photo: Debalin Sarangi)

   Weed height: Liberty is a postemergence herbicide, so only weeds that have emerged will be controlled. Weed height is critical when using glufosinate. The herbicide should be applied to small, preferably less than 3 inches tall actively growing weeds. If applications are made to taller plants, one can expect regrowth from the lower growing points (Figure 2). This time-lapse video shows the influence of waterhemp height on the efficacy of Liberty. The shoot regrowth is visible when the herbicide was applied to 6-inch and 1.5 feet tall waterhemp plants.
   

   
   

3. 

   Figure 3. Wild radish survival after glufosinate
    application impacted by the air temperature.
    (Source: Kumaratilake et al. 2005).

   Temperature: The Liberty label states that warm temperature, high humidity, and bright sunlight improves the performance of this herbicide. Figure 3 shows the impact of temperature on the efficacy of glufosinate on wild radish. Survival was 78% at 50/41 °F day/night temperatures, whereas no plants survived at 77/68 °F day/night temperatures. For reference, air temperatures for May and June at Rosemount, MN are plotted in Figure 4. This shows that farmers can expect better performance if glufosinate is sprayed in later May or June versus earlier in the season.
   

   

   Figure 4. Daily and 30-year average temperatures at Rosemount, MN, for May and June when Liberty application is expected.

   
   

4. 

   Figure 5. Higher Palmer amaranth, redroot pigweed, and
    waterhemp control when Liberty applied at 90%  relative
    humidity vs. 35%. (Source: Coetzer et al. 2001).

   Relative humidity (RH): Research has shown that humidity is an important factor for glufosinate performance. Lower humidity is suspected to result in the quick drying of spray droplets and the reduction in the amount of water in the pectin strands within the cuticle (the protective film on the outer surface of plant leaves), causing lower glufosinate absorption in the plants. A study conducted in Kansas showed higher control of Palmer amaranth, redroot pigweed, and waterhemp at 90% RH than that at 35% RH (Figure 5). Although relative humidity in Minnesota is generally not a concern during summer days, there are occasional days and areas where applying glufosinate under low humidity should be avoided.
5. Time of the day: Glufosinate requires sunlight for activity. Applications around dawn and dusk should be avoided and can result in a drastic reduction in weed control due to the lack of sunlight. Typically, the applications should be made between 3 to 4 hours after sunrise and 3 to 4 hours before sunset. Figure 6 shows better control of Palmer amaranth from midday Liberty applications compared to dusk applications.
   

   

   Figure 6. Higher Palmer amaranth injury resulted from Liberty application at midday (left picture) compared to dusk applications (right picture). Plants in the left row in each picture were the nontreated controls. (Source: Takano HK and Dayan FE 2021).

6. Carrier volume: A minimum of 15 gallons per acre of water should be used under any circumstances. If Liberty is applied to dense foliage or higher weed populations, increase spray volume to 20 gallons per acre for better coverage and weed control.
7. Nozzle selection: Since glufosinate is a contact herbicide, proper nozzle selection and carrier volume are key in achieving adequate spray coverage and herbicide absorption inside the plants. The Liberty label recommends using nozzles that produce medium to coarse spray droplets.
8. Adjuvant use: Ammonium sulfate (AMS) should be used at 1.5 to 3 lbs/A to improve performance. AMS improves herbicide movement across the plant tissue layers and helps overcome any hard water issues. The use of additional surfactants or crop oil can increase crop injury; therefore, herbicide and surfactant labels should be consulted.

Note: Application of Liberty on a hot and humid day with AMS can result in some bronzing in LibertyLink crops, but in most cases, the injury is temporary and unlikely to impact yield. 

Resistance management

 It is critical to note that although we have not detected glufosinate-resistant weeds in Minnesota to date, overreliance on any particular herbicide including glufosinate is NOT recommended as this can lead to the evolution of herbicide-resistant weeds. Glufosinate-resistant palmer amaranth has already been reported in Arkansas, Mississippi, Missouri, and North Carolina. We are already very limited in postemergence options for control of weeds like waterhemp in fields across the state, thus, appropriate resistance management strategies should be adopted to maintain the efficacy of this herbicide. Some of these strategies include: 

• Scouting and monitoring fields for any weed escape from 10 to 14 days after herbicide application. 
• Diversifying weed management practices with the inclusion of non-chemical weed management tools such as cultivation, use of a rotary hoe, hand weeding, cover crops, and narrow rows.
• Using preemergence herbicides to start clean and add diversity to the herbicide program. 
• Rotating crops and herbicide-resistant traits.
• Using multiple herbicide sites of action either in a tank mix or sequential application.
• Minimizing the weed seed bank in soil by adopting multiple years of integrated weed management strategies and preventing the replenishment of the weed seedbank by escaped weeds. 

References: 

Kumaratilake AR and Preston C (2005) Low temperature reduces glufosinate activity and translocation in wild radish (Raphanus raphanistrum). Weed Science 53:10-16 

Coetzer E, Al-Khatib K, Loughin TM (2001) Glufosinate efficacy, absorption, and translocation in amaranth as affected by relative humidity and temperature. Weed Science 49:8-13 

Takano HK and Dayan FE (2021) Biochemical basis for the time-of-day effect on glufosinate efficacy against Amaranthus palmeri. Plants 10:2021

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