ABSTRACT:

1. Liberty® and glyphosate are considered weak acid herbicides. Water hardness will greatly influence the performance of a weak acid herbicide.

2. Weak acid herbicides are more likely to become tied up by cations, in turn reducing herbicide absorption into the plant.

3. Liberty® alone or Liberty + glyphosate requires 2 to 3 lb. of AMS. High heat, humidity, and tank-mix partners can impact crop response.

WHY IS AMS SO IMPORTANT?

Liberty® and glyphosate are considered to be weak acid herbicides. Water hardness will greatly influence the performance of a weak acid herbicide. Much of the spray water many applicators use comes from wells and is considered to be “hard water.” Hard water is caused by high levels of cations, primarily calcium (Ca), magnesium (Mg), and sodium (Na). Not all herbicides are affected by hard water, but weak acid herbicides are more likely to become tied up with these and other cations, creating insoluble complexes, in turn reducing herbicide absorption into the plant. AMS can be used to reduce the concentration of cations in the water. As glyphosate-resistance (GR) continues to affect a larger number of weed species, many post-emergence programs rely heavily on Liberty, and AMS is critical to Liberty performance.

AMS IMPACT ON WEED CONTROL

The graph below shows how AMS reduces the amount of herbicide tied up from hard water. Beck’s 2019 Practical Farm Research (PFR)® data shows that when adding AMS to Liberty® + glyphosate, waterhemp control increased by 16% 14 days after treatment (DAT). For grass control, fall panicum control increased by 20% when 2 lb. was added and 23% when 3 lb. of AMS was added. When spraying Liberty or glyphosate alone, AMS is still needed to avoid tie up of the herbicide. Liberty + glyphosate was effective when we sprayed weeds less than 4 in. and used 2 to 3 lb. of AMS. However, Liberty + glyphosate with no AMS resulted in an 18% reduction in waterhemp control vs. Liberty + 3 lb. of AMS. This demonstrates that the higher surfactant load from the glyphosate was not enough to overcome not having AMS. 

WHAT ABOUT CROP RESPONSE?

AMS is critical for effective Liberty or glyphosate applications, but in high heat and humidity, we can see slight crop response with higher loads of AMS. Using 3 lb. of AMS will result in greater weed control vs. 2 lb. of AMS, but more crop response may occur in hotter weather with the greater load of AMS. This response is typically more cosmetic, and, when managing problematic weeds, the higher AMS load may be necessary. More extreme cases of crop response can be seen if any overlap occurs. The addition of foliar feeds and other surfactants will also increase the crop response from AMS. 

SUGGESTIONS FOR MAXIMIZING LIBERTY® PERFORMANCE WITH AMS

• Test your water — measure water hardness and pH
• Always add AMS to the spray solution prior to Liberty® or glyphosate
• Ensure AMS is completely dissolved before adding any herbicide
• Based on the water test, supply an adequate volume of ammonium and sulfate to the spray carrier
• Sulfate (SO4 -2) ions bind with hard water ions (Ca, Mg, Na,etc.)
• Ammonium (NH4 +) binds with Liberty to increase herbicide absorption
• Utilize higher AMS rates when targeting tough-to-control weeds
• Mix the spray solution as close to application timing as possible 

CONCLUSION:

ANTAGONISM IS MAGNIFIED WHEN HERBICIDE APPLICATIONS ARE MADE TO LARGE WEEDS. NO MATTER IF THE PRODUCT IS CONTACT OR SYSTEMIC, IT’S VITAL TO TARGET SMALL WEEDS. LIBERTY IS VERY EFFECTIVE, BUT WE MUST ENSURE PROPER COVERAGE.