I recently received a phone call from a south Georgia corn farmer who told me that someone was going around saying that a Roundup + Atrazine tank-mix, applied POST in field corn, will reduce yields. My response was show me the data. Here is my data from 3 on-farm field trials conducted in a high yield environment (250+ Bu/A) from 2014-2016. Bottom line, yields from plots treated with Roundup + Atrazine + AMS applied POST were NOT statistically different from the untreated check in all trials.
As long as growers follow the label (i.e. rates and timing), I am not concerned at all. A quick look at the Roundup PowerMax3 label:
USE INSTRUCTIONS: This product may be applied alone or in a tank-mix over the top of field corn hybrids with Roundup Ready 2 Technology from emergence through the V8 stage (8 leaves with collars), or until corn plant height reaches 30 inches (freestanding), whichever comes first, unless otherwise directed. Use drop nozzles for optimum spray coverage and weed control when corn plant height is 24 to 30 inches. When corn plants are 30 to 48 inches tall (freestanding), apply this product using only ground application equipment fitted 16 with drop nozzles aligned to avoid spraying into the whorls of the corn plants. Maximum single in-crop application rate of this product up to 48-inch field corn is 30 fluid ounces per acre. Total in-crop application of this product from corn plant emergence through 48 inches in height must not exceed 60 fluid ounces per acre.
If a grower uses higher rates than listed above and/or applies Roundup later than suggested, yield losses can occur. This is what what happens to corn ears when glyphosate is applied at tassel (i.e. way off-label).
As a reminder, atrazine is only labeled for POST use in field corn up to 12″ tall.