No. 1 Article 8/March 16, 2005

Price Comparisons for Glyphosate Products

The past several years have witnessed a tremendous growth in the number of glyphosate-containing products commercially available for postemergence application in glyphosate-resistant soybean varieties and corn hybrids. Currently, more than 40 glyphosate-containing products are registered for use in Illinois agronomic crops, and that number is expected to continue increasing over time. Keeping track of product names and formulations can be a daunting challenge.

When selecting one of these products to use for weed control, several important considerations should be kept in mind: How much acid equivalent (ae) does the formulation contain? Should a spray additive such as nonionic surfactant be added to the tank, or does the formulation contain a "built-in" additive system? Are factors such as rain-free interval and toxicity category similar between or among the products you are considering? Once these questions have been answered and you have narrowed down the list of products you're interested in purchasing, how should price comparisons be done among products? Should price comparisons be based simply on cost per gallon of formulated product? Similar to determining equivalent application rates among various products, producers should compare prices on an acid equivalent basis.

In 2000, we began to discuss the distinctions between herbicide active ingredient and herbicide acid equivalent. The primary goal was to inform our clientele that equivalent application rates of various glyphosate-containing products should be calculated on an acid equivalent basis and not on an active ingredient basis. This is because various salt formulations of gly-phosate (isopropylamine, potassium, diammonium, etc.) are available, and calculating equivalent application rates of products formulated as different salts using active ingredient can produce erroneous results. If you would like to review previous newsletter articles that discussed acid equivalent, please see "Herbicide Formulations and Calculations: Active Ingredient or Acid Equivalent?" (issue 2, 2000); "A Roundup of Glyphosates--What Are the Differences?" (issue 1, 2001); or download a PDF fact sheet discussing this topic from the University of Illinois Integrated Pest Management Web site.

Essentially, to compare prices among glyphosate-containing products, we need to do a few, simple calculations. First, we need to determine what application rate we want to use. For well-timed applications, a rate of 0.75 lb. ae/acre can be very effective on many broadleaf and grass species. Once our application rate is determined, we then need to calculate how many fluid ounces of each product are needed for this rate (see the previous references for these calculations or Table 4). Next, we convert the price per gallon for each product to price per ounce. Finally, we multiply the number of fluid ounces needed to achieve the 0.75 lb. ae/acre rate for each product by the cost per fluid ounce. Let's work our way through an example to illustrate these calculations. Please note that we make no assumptions or references to manufacturer support programs, such as respray policy, in this discussion.

We have decided to apply our glyphosate-containing product at 0.75 lb. acid equivalent per acre when the majority of broadleaf weeds are 4 to 6 inches in height. We are contemplating purchasing one of two glyphosate-containing products and want to know which product offers the lowest per acre cost (we also assume additive requirements, if any are required by label, are identical for each product). Specifics for these two glyphosate-containing products follow:

So, while the cost per gallon of Glyfo A is $1.25 more than Glyfo B, calculating costs on an acid equivalent basis illustrates that the per acre cost is $1.13 less with Glyfo A compared with Glyfo B.

Determining how many pounds of acid equivalent are contained in a given formulation may seem the most daunting part of this exercise, but several references are available that list the amount of acid equivalent contained in many of the commercially available glyphosate formulations. Those of you who have access to the 2005 Illinois Agricultural Pest Management Handbook can consult Table 8 ("Glyphosate formulations and product equivalents") in Chapter 2 for such a list. Alternatively, please see Table 4 accompanying this newsletter article, or visit the University of Illinois Weed Science Web site.--Aaron Hager and Dawn Nordby

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