Pest Patrol: The importance of doing a ‘jar test’ with fertilizers and herbicides

#PestPatrol with Mike Cowbrough, OMAFRA

Each year I get more questions about mixing the liquid fertilizer, ammonium thiosulphate (ATS), with herbicides. With reductions in atmospheric sulphur deposits, Ontario research has shown positive yield responses in some crops when sulphur fertilizers are applied, so it makes sense that farmers are looking at ways to add sulphur to their crop in an efficient manner. However, my experience with mixing ATS with herbicides is that physical compatibility is an issue.

I’ve yet to have ATS mix well with herbicides when used exclusively as a carrier (Figure 1 at top, Figure 2 below). However, in some cases, ATS can successfully be added to a mix when water or urea ammonium nitrate (UAN) are the primary carriers, and ATS is added last after the pesticide and carrier have mixed thoroughly. Let’s go through the process of determining compatibility using a recent inquiry.

Figure 2. An attempted jar test of Liberty 200 SN with ATS as a carrier, resulting in unacceptable mixing.
photo: Supplied

Q: I would like to mix Blackhawk herbicide with water (10 gal./ac.) and ATS (five gal./ac.). Is it compatible, and what mixing order should be used?

A: Step 1: Refer to the product label for guidance on mixing. Searching the term “fertilizer” on an electronic version of the Blackhawk label is a logical starting point. Although a paragraph exists that provides some wording around mixing the herbicide with fertilizers, no real specific guidance is provided, other than a general warning that mixing the herbicide with a fertilizer can decrease the level of weed control and increase the risk of crop injury. You are warned that if you decide to mix the herbicide with a fertilizer, you are responsible for any outcomes associated with the mixture and that you should contact the manufacturer for more information.

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Step 2: Contact the manufacturer. The manufacturer put me in touch with a technical representative who recommended a jar test be conducted and suggested that the herbicide be mixed with water first and then ATS added last. The influence of this mixture on the level of weed control and crop safety was unknown.

Step 3: Conduct a jar test. I converted each component’s rate to one-thousandth of a hectare (for simple conversion to a jar size) and tried different orders of operation to see if it mattered.

Water at 10 gal./ac. (10 gal./ac. x 3.79 l/gal. = 37.9 l/ac. x 2.5 ac./ha = 94.8 l/ha x 1,000 ml/l x 1/1,000 ha = 8 ml of water). NOTE: Full disclosure, I rounded up to 100 ml of water in the jar to make measuring out a bit easier.

Blackhawk application rate = 440 ml/ac. or 1.1 l/ha (1.1 l/ha x 1,000 ml/l x 1/1,000 ha = 1 ml).

ATS at 5 gal./ac. (5 gal./ac. x 3.79 l/gal. = 19 l/ac. x 2.47 ac./ha = 46.9 l/ha x 1,000 ml/l x 1/1,000 ha = 9 ml of water)… again, I rounded up to 50 ml per jar test for simplicity.

Figure 3. Blackhawk herbicide mixed with water as a carrier, as per normal best practices.
photo: Supplied

Jar Test Results: Four jar tests were conducted. In the first test, the herbicide was mixed with 15 gal./ac. of water as the carrier (Figure 3 above). This becomes the standard by which the other mixtures involving ATS are compared to. In the second jar test, ATS was added first, followed by water and the herbicide last (Figure 4 below). In this test, the herbicide did not mix with the carrier, it separated out and formed a line along the surface of the mixture. The inability of the herbicide to mix with the carrier solution would most certainly result in performance issues and should not be used.

Figure 4. Blackhawk herbicide added to a mixture of water and ATS as the carrier. Note the thin line of herbicide product that separated out and went to the surface.
photo: Supplied

In the third jar test, water was added first, ATS second and the herbicide last. Once again, the herbicide failed to mix properly, and the results were no different than the previous test shown in Figure 4. In the final jar test, water was added first, the herbicide was added second and allowed to fully mix, then ATS was added last (Figure 5 below). This mixing order appeared to result in a successful mixture that was similar in appearance to our standard (Figure 1 at top). This jar test was invaluable for identifying mixing orders that resulted in unacceptable incompatibility.

Figure 5. Successful mixing of water with Blackhawk herbicide first and adding ATS last. This mixture looks good but should be tested on a small area first to identify any performance issues.
photo: Supplied

Step 4: Even following a successful jar test, unless the manufacturer is willing, in writing, to support the mixture that you’ve discussed, there is still the issue of whether the mixture will negatively affect crop safety, weed control and perhaps even off-target drift. The label still states that you are responsible for any outcomes of applying the mixture. Therefore, testing out the mixture on a small amount of acreage and in an isolated environment would be a wise move, so that you can evaluate how it performs, and the risks and benefits with its use.

(For further reading search “jar test” at sprayers101.com)

Have a question you want answered? Hashtag #PestPatrol on Twitter to @cowbrough or email Mike at [email protected].

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