Mathematics of glyphosate | successful farming
AMES, IA — Determining the proper glyphosate level is likely more important this spring than in previous years. Between high herbicide prices, reduced supply, and possibly the need to use another glyphosate product, it’s worth checking that you’re applying the optimum rate for effective control.
Many farmers may have used a 32 fluid ounce rate of glyphosate in the past. Since glyphosate comes in formulations containing a range of concentrations, the amount of glyphosate in 32 fluid ounces can vary widely from product to product. Depending on the glyphosate product you purchased for this year, your application rate may need to be different from previous years to deliver the same amount of herbicide.
When comparing glyphosate formulations, we always want to compare them based on the amount of acid equivalent rather than active ingredient. An example of an ingredient declaration on the label of a glyphosate product shows where you can find this information (Figure 1).
Table 1 compares the amount of active glyphosate ingredient and acid equivalent found in several popular glyphosate products. The rightmost column shows the equivalent rate required by the products to provide 1 pound of glyphosate acid equivalent; this is calculated by dividing 128 (number of fluid ounces in a gallon) by the amount of acid equivalent per gallon for the product. If a grower used a 4.5 pound acid equivalent product and is switching to a 4 pound acid equivalent product, their application rate to deliver the same amount of herbicide will need to be increased this year.
Even if you are using the same glyphosate product as in previous years, it is worth evaluating if your rate is appropriate for the weeds found in your fields. For most farmers, glyphosate levels have been increased due to decreasing water hemp control. Unfortunately, water hemp has overtaken us and glyphosate is no longer effective against this weed in many Iowa fields. I encourage farmers to carefully check the label of the glyphosate product they have chosen and determine the suggested rate to manage the targeted weed species. This may mean a slight rate reduction for post-emergent glyphosate applications or perhaps an increase if you are managing particularly stubborn weeds like certain perennials.
This article has not addressed other considerations for glyphosate applications that are almost as important as choosing the right rate, including weed size, weather conditions, nozzle selection, spray adjuvants and tank-mix partners. Careful consideration of glyphosate formulation and label recommendations will help provide optimal weed control while protecting the wallet this spring.
Author: Meaghan Anderson, field agronomist at Iowa State University