Over the last several days, I have received numerous inquiries about the reduced performance of preplant burndown herbicide applications. Several factors can help contribute to reduced performance of burndown herbicides including application rate (too low?), weed size (too big?), environmental conditions (too cold?), small grain cover crop stage of growth (generally more susceptible prior to boot and after heading), droplet size (too big?), and time (too soon after application?).
For today, let's just focus on air temperatures. I like to use a cold temperature threshold of <50 F as a benchmark for when herbicides start to not work like they should, If you check out Table 1, there have been many days this month below that temperature threshold. On average, 73% of the days in March at these Georgia locations had low temperatures below 50 F. In my opinion, this may be one of the main factors that has influenced the performance of some burndown herbicides. Good news is that the low temperatures forecasted for next week will be above 60 F so that should help improve burndown efficacy. Although I usually am not a huge fan of using AMS with glyphosate, unless hard water has recently been confirmed with a lab test, I think AMS might help improve glyphosate performance when air temperatures dip below 50 F.
