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Maximizing Herbicide Performance

We have amazing herbicides. So amazing that they generally overcome small things that can contribute to less than ideal application. But not always. 

Something in the Water 

Water makes up the bulk of any spray solution. For that reason, it’s often the first thing that’s cut when looking for application efficiencies. But lowered water volumes can also decrease herbicide performance. This is especially true for contact herbicides that depend on good coverage for efficacy. In general, water volumes between 10 and 20 gallons per acre are the minimum recommendation for contact herbicides like Liberty, Basagran, Group 14 products and diquat.  

Systemic herbicides are typically less affected by water volume due to their ability to move within a weed. Five to 10 gallons per acre water volume is typically adequate. Keep in mind though, increasing water volumes improve herbicide efficacy when weed pressure is high or the crop canopy is dense.  

Now we’ve got the right volume of water, what about the quality? In my opinion, any and all water being used for pesticide applications should be tested for quality, regardless of the source. Hardness and turbidity are two important water-quality characteristics to consider for herbicide application. 

Hardness refers to the concentration of dissolved minerals in water. Calcium and magnesium are the main causes of hard water, although minerals like iron and manganese are also culprits. The hardness of a water sample is often measured in milligrams of calcium carbonate equivalent per litre (mg/L = ppm). 

Although hardness scales vary, groundwater across the Prairies is largely classified as hard due to an abundance of calcium and magnesium supplied by our calcareous parent material. Prairies dwellers dependent on well water recognize the symptoms of hard water in corroded taps and lime scale. 

In addition to costly hot-water tank element replacements, hard water can negatively impact herbicide performance. The positively-charged minerals are known to interact with weakly acidic herbicides, notably glyphosate and Group 4 amine formulations, and decrease their activity on weeds. It’s recommended that only water with a hardness of 700 mg/L or lower be used with such herbicides. 

Hard water can still affect herbicide efficacy below this level but there are “fixes” that can reduce the impacts. Opting for a higher-labelled rate increases the amount of herbicide active ingredient in the tank, which can offset a potential decrease in performance. Conversely – and opposite to earlier advice – this is a case where decreasing water volume (to five GPA) improves herbicide activity, by increasing the proportion of active ingredient relative to the concentration of minerals. Adding ammonium sulphate (AMS) is yet another solution as negatively-charged sulphate ions preferentially react with calcium and magnesium, freeing the herbicide for weed activity. 

Turbidity refers to particulate soil and organic matter in water and is measured as total dissolved solids (TDS) or suspended solids. In plain language, turbidity characterizes the “dirtiness” of water. Greater turbidity is generally associated with surface water sources used to fill the spray tank. 

Unfortunately, unlike hardness, there are no guidelines for the minimum level of turbidity that can impact herbicide application. It’s suggested that water that is visibly hazy or dirty can tie up herbicides that bind strongly to soil and organic matter, such as glyphosate or diquat. And, also unlike hard water, there are no fixes for dirty water. Surface water used for spray applications should be clean and kept clean by filtration and algae control and by limiting mixing of the water column during filling.  

What about pH? While it’s true that pH can affect herbicide activity, crop protection companies usually recommend or provide adjuvants or other additives to address this. However, data from Saskatchewan shows that high-sodium bicarbonate levels in water, which cause high pH, can adversely affect herbicides like 2,4-D amine and the Group 1 “dims.” Bicarbonate levels of less than 300 mg/L water are recommended for application. 

The Time of Day 

Farmers have long been admonished to “spray in the heat of the day” to optimize weed burndown with Liberty. In fact, Liberty applied at intervals over a 24-hour period clearly demonstrates decreased efficacy outside of daytime hours. As a rule, contact herbicides work best when it’s hot and sunny. Diquat is the exception, with better penetration of green plant material when applied on cloudy days or evenings.   

Surprisingly, the 24-hour application experiment yields similar results for systemic herbicides. Although visual differences are less evident compared with contact products, the efficacy of systemic herbicides is also maximized during daytime hours, especially on broadleaf weeds. Herbicides that degrade readily in sunlight, like certain Group 1s, may be the exception in this case. However, since these herbicides are almost always applied as part of a tank mix, the recommendation is to apply during the day without leaving product sitting in the sun. 

Getting the big things of herbicide application spot on – right product, right rate, right time – is essential for weed control. But it’s been said that it’s the little things that make the difference. Paying attention to the little things in a herbicide application, like water quality and time of day, might just be the difference to help maximize herbicide performance.

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