The great weed control robbery

The great weed control robbery – Alberta Farmer Express (albertafarmexpress.ca)

In a surprising turn of events, Clint Jurke, the agronomy director for the Canola Council of Canada and a farmer from Lloydminster, Sask., discovered the significant influence of water quality on herbicide effectiveness. Although he had never considered testing the water used in his sprayer, Jurke’s curiosity was piqued when a colleague suggested it as an article topic. The results of the water sample from the local municipal well were astonishing – it was laden with cations, specifically calcium, magnesium, and sodium.

Cations, positively charged ions found in water, have the ability to bond with negatively charged herbicide molecules, rendering them useless for weed control. Glyphosate, the most sensitive chemical in this regard, is particularly affected. Other herbicides such as Group 10 glufosinate, Group 1 (“-dim”) chemistries, Group 4 (2,4-D amine), and Group 27 products can also be influenced by cations.

Water quality tests conducted by WinField United across the Prairies have revealed that water hardness is a prevalent issue. Hard water, characterized by high levels of calcium and magnesium cations, is a common problem. Additionally, water conductivity, which indicates salt content, including sodium cations, is particularly high in east-central Alberta and west-central Saskatchewan.

Understanding the chemistry behind cation-herbicide interactions is crucial. Take the example of calcium and glyphosate: with a plus-two charge, calcium can bond with two glyphosate molecules, creating calcium glyphosate, which is essentially inert and unusable by plants. Water with a conductivity exceeding 500 µS/cm could start to impact herbicide performance.

When water hardness reaches a range of 150 to 300 ppm (parts per million), pesticide tie-up becomes evident. The more cations present, the more pronounced the issue becomes. To address this, farmers can test water from different local sources to identify those with lower cation levels. While well water tends to be consistent over time, neighboring wells tapping into different water sources may have varying cation content. Dugout water quality can be highly variable, especially after spring runoff or summer evaporation.

To mitigate the effects of cations, using higher label rates of herbicide can offer some relief. However, relying solely on increased herbicide dosage is not sustainable in the long term and may contribute to resistance issues. Instead, it is advisable for farmers to adopt multiple modes of action to combat weeds effectively. One common recommendation is the use of ammonium sulphate, which dissolves in water, forming ammonium and sulphate ions. The negatively charged sulphate ions bind with cations, preventing their interaction with herbicide molecules. Simultaneously, the positively charged ammonium ions bond with glyphosate, creating ammonium-glyphosate, an effective herbicide that plants readily absorb.

However, an exception arises when using 2,4-D amine with hard water. In such cases, the Saskatchewan government recommends the addition of a non-ionic surfactant, such as Agral 90, AgSurf, or Companion, at a rate of one liter per 1,000 liters of water. Ammonium sulphate has not been found to be effective when used with 2,4-D amine.

Overall, understanding the impact of cations on water quality is vital for optimizing herbicide effectiveness and ensuring sustainable weed control. By testing and selecting water sources with lower cation content, employing multiple modes of action, and using appropriate additives like ammonium sulphate, farmers can enhance their spraying practices and achieve better weed management outcomes.

Treatment

The order in which conditioner and herbicide are mixed is crucial when treating sprayer water. The conditioner should be added first to bind the cations, followed by the herbicide.

Carr, a representative from WinField United, acknowledged that they test a significant amount of water because they sell a large quantity of water conditioner. They have a specialized product called Crimson NG, which is a proprietary ammonium sulfate.

Ammonium sulfate is commonly stocked by various input suppliers.

Agricultural laboratories that test sprayer water can provide recommendations on the amount of ammonium sulfate to add in order to counteract the cation content. If farmers already have water test results from a provincial lab, private agricultural labs can use those results to suggest appropriate conditioners.

Carr analyzed Jurke’s water sample using the WinField United calculator, which assigns an “antagonism coefficient” for each cation. The calculator recommended that Jurke add 4.55 liters of 34% solution of ammonium sulfate per 100 U.S. gallons of sprayer water.

In dusty conditions, Carr would suggest adding an additional 2.83 liters per 100 gallons (0.75% volume per volume) to compensate for potential weed control loss on the leaf surface. Dust on the leaves can contain cations that can bind with glyphosate molecules before they enter the leaf.

Carr stated, “Often, you’ll find fewer dead weeds right behind the sprayer, where it kicks up more dust.”

Taking into account the extra volume for dusty conditions, the total recommended amount is 7.38 liters of ammonium sulfate per 100 gallons, or nine 10-liter jugs for a 1,200-gallon sprayer tank. Using the suggested retail price of Crimson NG, the cost to condition each tank would be approximately $441.

Other Options

There are alternative methods to mitigate the impact of high-cation water, including reducing water volumes, using urea ammonium nitrate (UAN), and employing acidifiers. However, none of these methods are as effective as switching water sources or adding ammonium sulfate.

Lowering water volume reduces the ratio of water cations to glyphosate molecules, but Carr does not recommend this approach. He explained, “Spraying straight glyphosate is no longer effective due to glyphosate resistance. Reducing water volume is not considered acceptable for the tank mix partners.”

Most herbicides require adequate coverage to work effectively, and lower water volumes may compromise the effectiveness of the tank mix.

Urea ammonium nitrate (UAN) supplies ammonium to bind with glyphosate and maintain performance, but it does not contain the sulfate needed to bond with the cations.

Using acidifiers to lower water pH neutralizes the charge of the herbicide, reducing the likelihood of it binding with cations. However, this approach carries the risk of certain herbicides falling out of solution and potentially clogging the sprayer if the pH becomes too low.

After receiving his water test results, Jurke chose to use ammonium sulfate, although he may not use it in every tank until he sees clear benefits. He will likely start with fields infested with kochia weeds.

“It would be great if conditioners could help with kochia control,” Jurke expressed.

Conditioners might assist in this regard. Sub-lethal herbicide performance can contribute to the development of glyphosate resistance in kochia. However, water conditioners cannot resolve glyphosate resistance if kochia is already resistant.