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The following is a post on burndowns in row crops from Dr. Eric Prostko, Extension Agronomist and Weed Scientist.

Many growers are now in the process of spraying pre-plant burndown herbicides.  Low temperatures over most of Georgia are expected to drop below 40 F over the next three days.  These temperatures are not very favorable for optimum weed control with herbicides such as Roundup (glyphosate) or Gramoxone (paraquat).  Thus, you might want to advise your growers to hold off for a few days until temperatures warm up again (above 40 F but I would prefer warmer).  Here is some data to chew on:

1) From some older air temperature research (47 F, 56 F, 65 F, 75 F, and 87 F) conducted by my colleague at Michigan State University, Dr. Christy Sprague:

a) Air temperatures had very little effect on the control of common chickweed with Roundup.  However, common chickweed control at lower temperatures was compromised with Gramoxone (only 70% control at 47 F).

b) Air temperature had a significant effect on the control of henbit with both Roundup and Gramoxone.  When applied at 47 F, henbit control was < 30% with both herbicides.

2) From Harre et al. (2021) in Weed Technology 35:279-288:

a) Shoot biomass reduction of horseweed and purple deadnettle was 34% and 28% greater, respectively, when paraquat was applied at day/night temperatures of 81/61 F than when applied at 64/55 F.

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