The 2019 growing season is now within sight and with it, another season’s worth of herbicide, fungicide and insecticide applications. Given the challenges that can arise with sprayers and their products, it’s worth revisiting the subject of water quality and the potential impact of factors such as temperature, hardness and pH on product performance.
With market volatility in the crop sector, the need for pesticide efficacy is only increasing. And with tank mixing now the standard, the importance of water quality is clear.
In other words, it pays to know your water.
It seems that every eight to 10 years, there’s a renaissance in the industry where the chemical sector and its retailers remind growers of the importance of pH or turbidity. It often coincides with a launch of various products designed to ease any challenges of one or more of those parameters. Years ago, it was citric acid that was introduced as an effective and inexpensive acidifier for many pesticides. Today, glyphosate and glufosinate are viewed as acidifiers.
The subject is of particular interest as growers realize the days of single-pass, single-mode-of-action weed control are over, at least for now.
That’s a concern of Dr. Jason Deveau, the application technology specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), who acknowledges pH as a significant factor yet points to turbidity, hardness and temperature as equally important.
“We just have to be aware that with complicated tank mixes, more time, more care, more attention will be required,” says Deveau, adding that all of the details concerning product usage can be found on its label. “Each product comes as a complete package. It’s only when we start adding all of these packages together that we get into ‘The Great Unknown’. But as a general rule, these products are designed to work with water over a pretty reasonable range. In fact, pH being what it is, you might suggest that it’s not as much the pH as it is the mineral content or the cloudiness of the water that can wreak havoc with a chemistry.”
In the past, pH has been cited with certain products as it can drastically reduce efficacy in the field. A fungicide active like captan, with an optimum pH of five has a half life (its efficacy is cut in half) of 32 hours: with a pH of seven, it’s eight hours but with a pH of eight, it’s down to 10 minutes. Dimethoate, an insecticide active has a half-life of 48 minutes with a pH of nine (at four, it’s 20 hours).
When it’s a problem, it’s a big one. Yet most other products, including most herbicide actives, lose little in efficacy across a wide range of pH values.
However, as Deveau repeats, it’s the grower’s quest for productivity that confounds the process, referring to a “witch’s brew” of products and adjuvants.
“They may feel this is justified because they’re also reducing water use and compaction, and they’re engaging multiple modes of action, all of which are praiseworthy practices,” he says. “But today’s chemistries are concentrated — to make container size smaller — and contain many adjuvants that aid in mixing, shelf stability and a laundry list of other purposes, including pH buffering.”
Social media have provided a wealth of images about product mixing gone wrong resulting in sprayers filled with so-called “pancake batter.” There are also unintended synergistic effects resulting in hotter mixes that can cause crop injury, the effects of which can appear in a matter of hours or a day. In such instances, pH is less of an issue. Instead, growers need to consult the product label.
“When it’s important, the label is king,” says Deveau. “When it’s a non-issue, it’s not that the company is keeping secrets, it’s that it hasn’t been an issue in the past. It could become an issue when you start mixing things that were never intended for mixing, and the rule of thumb is never more than three or four products in the tank.”
Still a concern
On the pH issue, Dave Bell maintains that some concern remains with the impact on product efficacy, and not just for growers but for the entire chemical and dealer sector.
“Failed applications must be repeated, adding unnecessary chemical loading to the crop and environment,” says Bell, an agronomist with United Agri Products (UAP). “Pests receiving a sub-lethal dose run a greater chance of developing resistance and increased rates may be used to overcome the losses in the tank adding to the chemical load.”
As for observations that pH is no longer an issue — thanks to acidifiers and other newer chemistries on the market — Bell suggests that it might be the product that a grower has been using for years that’s missed. That goes with the grower’s demand to know the best procedure to solve their problem and to get on with the other duties requiring their attention that day.
“Even if it’s not a major issue — as with something like captan — if the efficacy can be increased by getting the pH right and it reduces the chance of resistance developing or spray failures resulting in re-spraying, I’d consider that wise and responsible pesticide use,” says Bell.
It’s not to say that pH isn’t an issue, adds Deveau. He makes reference to it in one of his Sprayer 101 documents, where he acknowledges it as a complex parameter given its relationship to pesticide solubility, hard water antagonism and pesticide degradation. But growers who use something like captan likely know the half-life issue.
If pH is a concern, Deveau advises measuring it at the end, not the beginning. Water sources across much of southern Ontario tend to be slightly alkaline but they’re fairly flexible; it doesn’t take a lot to adjust the pH. And with glyphosate a known acidifier, waiting until mixing is complete is a wise precaution.
Although the number of active ingredients that have serious half-life reductions is limited under the pH issue, there’s also been very little third-party research on the subject; much of the available information is eight to 10 years old.
Among other factors that can affect product efficacy, Deveau adds water temperature, water cleanliness (turbidity) and water hardness. Temperature doesn’t have a large impact, unless it’s abnormally cold water. In one experiment, Deveau created an irreversible chemical reaction: the resulting mix couldn’t be filtered nor could it be re-dissolved.
Turbidity becomes a problem if suspended solids — like clay — are present in the water. Glyphosate and diquat will readily adsorb to soil particles, reducing their effectiveness. Water hardness refers to positively charged minerals, namely calcium and magnesium, as well as sodium and iron. Like suspended solids, these cations can bind to some herbicides, particularly glyphosate along with 2,4-D amine.
Filling time is another parameter than can have an unintended negative impact on spray or product efficacy, and Deveau and his colleague Dr. Tom Wolf have frequent discussions on the topic. Wolf favours faster fill times as a means of increasing efficiency, and the two have investigated systems that can easily pump 300 gallons per minute. According to mixing instructions, most recommend filling the tank halfway before adding product.
“Given how fast a handler can pump, you barely have time to get your tank-mix products in before you’ve filled your tank,” says Deveau. “It’s a physical impossibility as you inject these into the line. So the question becomes, ‘Have we gone too far (with tank mixing)?’ The answer is, ‘For certain tank mixes — yes — just because you can fill so fast doesn’t mean you should.’”