It is getting late early with respect to burndown in Tennessee cotton fields. With the long, cold, snowy February and March there was very little opportunity to get early burndowns applied. The planting season is now here and a large majority of the burndowns have yet to be applied.
The most common question is from those looking for a residual component to tankmix with their burndown with cotton planting just around the corner.  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 an option in most cases as the plant back interval is too long.
One option 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 likely hood that 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 of Cotoran and/or Caparol consider backing the rate off to 24 to 32 oz/A if diuron was used pre plant.