With cotton planting just around the corner, some folks are interested in tankmixing a residual herbicide in with their burndown. The typical herbicides used for this purpose would be Sharpen (42 day plant back to cotton), Valor (30 days plant back to cotton) and Leadoff (30 days plant back to cotton) but these are no longer options as the plant back interval is too long.
One option this close to cotton planting would be diuron. I typically do not recommend diuron pre emergence in cotton due to injury concerns. However, applied 7 to 14 days before cotton planting it should be safe and would provide excellent and relatively inexpensive residual control of horseweed and pigweed.
The diuron rate would be 8 to 16 oz/A. Use the higher rate if you expect cotton planting for a particular field to be well into May. Unless cotton planting is delayed well into May the diuron should provide residual control until planting and therefore should reduce the likelihood that paraquat (Gramoxone) is needed pre emergence.
If you go with diuron as the residual in your burndown it will allow you to plant at anytime. However, be aware that diuron is in the same herbicide family as Cotoran and Caparol. Cotton injury could be severe if Cotoran and/or Caparol is/are used pre emergence following diuron applied in the burndown. This probability of injury would be higher if the diuron was not washed in with a rain prior to planting. Moreover injury would be more likely to occur if high rates of Cotoran and/or Caparol are used. The diuron label states to back off the Cotoran and Caparol rate if diuron was used early pre plant. This is good advice and should be followed. So if you were planning to use a 1.5 to 2 qts/A rate of Cotoran and/or Caparol consider backing the rate off to 24 to 32 oz/A if diuron was used pre plant.
The other option would be to forgo a residual in with the burndown. Then use Gramoxone at planting to remove any newly emerged weeds. This option provides the most flexibility if cotton planting gets late and one decides to go with another crop. However, if planting is delayed into late May there could be a real grown up mess to have to control before the crop emerges. As with all these type decisions hindsight will be 20-20.
Palmer amaranth started emerging last week. Of course this is just the vanguard of the Palmer that will be coming. Palmer amaranth that emerges in April will often flower at 2 to 3” tall. These plants are typically very hard to control. The most consistent burndown herbicide to control Palmer amaranth is Gramoxone. In our research over the past several years it has performed well in burning down Palmer amaranth as large as 6” tall. Adding a PSII inhibiting herbicide like Cotoran or Caparol in with the Gramoxone will provide more consistent control of Palmer.
