Australian Herbicide Resistance Initiative (AHRI)

Warm water

Palmer pigweed
Some things in life are best served warm.
It’s just the way it is. The evening meal, toast, custard, hollandaise sauce, herbicide spray water…
New research by Devkota and colleagues from Purdue University in the USA tells us that cold water may not be as good as warm water when spraying weeds.
They found that when they sprayed giant ragweed and morningglory with a glyphosate + dicamba mix, weed control was improved anywhere from 6% to 26% by using spray water that was 18°C to 39°C compared to 5°C.
This response didn’t hold true for all of the weeds in the study. For example, there was no effect of spray water temperature on fleabane (Conyza) control.
It’s too early to make firm recommendations to growers based on this information, but this research does raise a lot of questions.  Which weeds will respond this way? Is the response big enough to consider warming water or should we simply adjust the rate? Are there other benefits of warm water such as herbicide mixing?
Do we need to consider serving our herbicides warm?

AHRI insight blog and tweet cards

Once in a while a piece of research appears out of the blue that makes you stop and think. The research we’re talking about in this edition is one of those!

This research certainly raises a lot of questions that may well be worth exploring further (hats off to the American researchers for bringing this topic to our attention). For now, let’s take a look at this research and we can consider other possibilities later.

The weeds

Four weed species were tested – Giant ragweed, Horseweed (Fleabane), Palmer amaranth, Pitted morningglory.

The herbicides

A mix of glyphosate + dicamba at a low rate (275gai/ha + 137gai/ha) and a higher rate (550 gai/ha + 275gai/ha).

Water temperatures

Five water temperatures were tested. 5, 18, 31, 44 or 57°C. Herbicides were mixed with water at these temperatures then sprayed immediately.

The results

The low rate of glyphosate + dicamba applied in water at 31°C provided 14 and 26% improvement in the control of giant ragweed and pitted morningglory respectively. There was little effect of water temperature on Horseweed and Palmer amaranth control.

 

Table 1.  Percent control of four weed species sprayed with a low rate of glyphosate + dicamba in water at a range of water temperatures. Numbers in bold (green) are significantly different from the 5°C treatment.

69 Table 1

 

The higher rate of glyphosate + dicamba applied in water at 31°C provided 20% improvement in the control of giant ragweed. This herbicide mix applied in 44°C water improved morningglory control by 20%.

 

Table 2.  Percent control of four weed species sprayed with a higher rate of glyphosate + dicamba in water at a range of water temperatures. Numbers in bold (green) are significantly different from the 5°C treatment.

69 Table 2


Mixing issues

Anecdotally, many agronomists comment that certain herbicide mixes experience poor physical compatibility under cold conditions. We contacted a number of agronomists about this issue and they all agreed that it does happen, but they didn’t point the finger at a particular herbicide or adjuvant. Most agronomists agreed that mixing problems occurred with cold water when there were a number of products in the tank.

Practical solutions

We’re sure many farmers are reluctant to heat their spray water as it would be a difficult, energy consuming task. However, there may be some simple solutions such as storing water in plastic tanks instead of concrete tanks, not leaving spray water in the tank overnight, or perhaps adjusting the rate of certain herbicides when spray water temperature is low. We will need more research before we seriously consider adopting some of these practices, but it’s food for thought.

Conclusion

There’s no denying that spray water temperature has had an effect on weed control in this study.  The results are not convincing enough to go out and buy a water heater, but there’s certainly merit in more research.

Perhaps our herbicides are best served warm to our weeds!

 

 

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