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Last Minute Burndowns and PRE Herbicides
Author: Tom Barber, Extension Weed Scientist

Judging by the phone calls lately, growers in Arkansas have been able to farm the last 2-3 days at least in a few select areas.  April is here and the feeling of urgency is in the air.  Many growers are scrambling to make final burndown applications and plant something.  Most of the questions I get lately are what can be used on rice, corn and grain sorghum acres to burndown winter annuals, volunteer beans, and still plant the crop immediately or at least within a week.   Many options exist for burndown and the most recent updates on pre-plant intervals can be found in the MP-519 on this website.  There is also some confusion around plant back intervals, especially with Sharpen.  Evidently there are some misprints in the MP-44 regarding Sharpen plant-back intervals to rice and other crops.

Sharpen at 2 oz/A mixed with glyphosate and 1% MSO (methylated seed oil) provides good control of most broadleaf winter annuals and volunteer beans as well as provides some residual control of broadleaf weeds such as glyphosate-resistant pigweed.  There is no plant-back interval for rice, corn or grain sorghum following a 2oz/A Sharpen application.  Sharpen can effectively be applied as a preemerge herbicide to all of those crops.  Soybean and cotton are different. Soybean should not be planted prior to 30 days following a Sharpen 1.0-1.5 oz/A rate on sandy soils with less than 2.0% organic matter, for all other soils the plant-back is immediate for 1.0oz/A or 14 days for 1.5 oz/A.  These plant-back intervals are extended to 14 and 30 days respectively if Sharpen is tankmixed with another group 14 herbicide such as Valor or sulfentrazone (Authority).  The plant-back interval to cotton following a Sharpen application is 1.5 months.  Always read and follow labeled instructions regarding herbicide pre-plant intervals or crop injury may occur.

As I have mentioned before Verdict is a pre-mix application of Sharpen and Outlook.  Verdict at 5 oz/A contains 1 oz/A of Sharpen and 4 oz/A Outlook, 7.5 oz/A Verdict contains 1.5 oz/A Sharpen and 6 oz/A Outlook.  These are the only 2 rates recommended prior to planting beans due to the plant-back restrictions mentioned above.  Verdict can be applied as a PRE herbicide to Corn and Grain Sorghum at rates as high as 10 oz/A which is equivalent to 2 oz/A Sharpen and 8 oz/A Outlook.  Glyphosate + Verdict 10 oz/A and 1% MSO will provide good control of most winter annuals including glyphosate-resistant horseweed as well as decent residual control for grass and broadleaf weeds in corn and grain sorghum.  Verdict cannot be applied once the corn and grain sorghum have emerged.  

Corn and especially grain sorghum weed control starts with a preemerge herbicide.  Many growers are looking to cut corners this year to save money.  The preemerge herbicide is not a corner that needs to be cut in any crop that we grow.  Most of the yield that is lost to weeds can be accounted for in early season weed competition. In other words the critical weed free period for most crops ranges from emergence to 4 weeks after emergence.  In our weed control plots over the years we have seen a 10-15 bu/A average yield increase from a two shot program in corn where a PRE is used followed by a POST around V3-V4.  It can be as simple as Dual II Magnum PRE followed by Glyophosate + Dual II Magnum + Atrazine POST, depending on which weeds are present.  The PRE is even more critical in grain sorghum.  Every acre of grain sorghum should have a PRE residual herbicide program for grasses and pigweed. Dual Magnum or Verdict fit well as a PRE for grain sorghum.  This can be followed by Dual or Atrazine POST again around the V3-V4 stage.  Remember the atrazine cutoff for over the top applications is 12 inches for both corn and grain sorghum.

Preemerge herbicides buy valuable time and conserve yield in all crops.  Timeliness is very important, especially in wet springs, where we can’t get back into the field to spray weeds while they are small.  It appears this spring is shaping up to be a wet one, so don’t forget the PRE.


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