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Why Use AMS With Glyphosate Herbicide?

Some crop consultants and chemical dealers always recommend adding ammonium sulfate to glyphosate to improve weed control. Rod Swoboda

Some farmers and chemical dealers always use, and always recommended, the use of AMS (ammonium sulfate) with glyphosate herbicide. That's the case whether the spray is applied through Rogators, floaters, or farmers' own sprayers. They admit they really don't know the reasons why, other than it helps the glyphosate control larger velvetleaf, and that is reason enough.

Clarke McGrath, field agronomist at the ISU Armstrong research farm in southwest Iowa, provides some answers to that burning question: Why do farmers need to spend more dollars per acre on AMS?

Why adding AMS can help weed control

The addition of AMS to glyphosate is not always necessary, but it tends to improve glyphosate effectiveness on certain weeds and in certain situations. The addition of AMS to glyphosate accomplishes the following:

It prevents the cations in hard water from binding to glyphosate, which reduces its effectiveness.
It improves control of certain weeds, such as velvetleaf and some perennials.
It helps maintain glyphosate effectiveness under cold or other adverse conditions.
It helps overcome the reduction in glyphosate effectiveness when mixed with other herbicides that cause antagonism.
It helps maintain glyphosate effectiveness when mixed with manganese for foliar feeding of soybeans.

"Although many of us tend to always add AMS at the rate of 17 pounds of dry material per 100 gallons of spray solution when applying glyphosate, this rate may be higher than is needed for some situations," says McGrath. "The amount of AMS can be adjusted based on water hardness if it is known, and a rate of 8 pounds per 100 gallons may be adequate for many sources of water."

Avoid problems mixing AMS with herbicide

AMS is available as a dry material, and also as a liquid. Problems with mixing the dry materials with water can be minimized by using spray-grade AMS, which is formulated specifically for use in herbicide applications. Other dry AMS materials may be available, but those not specifically formulated for use in herbicide applications may present more problems in mixing or clogging of sprayer parts.

Various liquid AMS-replacement products are also available, and these are frequently used by many applicators due to the convenience. It can be difficult if not impossible to know the rate of a replacement product required to provide the same benefit of 8 or 17 pounds of AMS.

Some manufacturers provide this information, but it may be found only in technical or promotional literature, and not on the product label. This does not mean that these products are less effective than dry or liquid AMS, just that information is lacking to know how they compare to true AMS in their capacity to negate hard water or overcome antagonism from manganese.

"So, now you know way more about why AMS works than you really needed to," jokes McGrath, who offers this good advice: "Plan on using some form of AMS, with your glyphosate program and work closely with your local dealer. He or she will help you get hold of the right AMS product for your situation."

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