More Is Better When it Comes to Pigweed Control in Wheat Stubble

Weed control in wheat stubble following harvest has become more difficult with the proliferation of glyphosate-resistant Palmer amaranth and waterhemp - which we are lumping together here as pigweeds.  A tour near Salina, Kansas, on August 3 demonstrated the effectiveness of a few common weed treatments after wheat harvest. 

There is some good news in the fight against pigweeds: There are options that provide good control of these nasty weeds. The bad news is this: Glyphosate, sadly, no longer appears to be one of them.
"We're finding more and more disasters," admits Dallas Peterson, weed control specialist at Kansas State University. "We cannot continue to keep doing the same weed control treatment we've always done."

Cooperator Taylor Currie, who farms near Gypsum, Kansas, worked with KSU Extension to develop five postharvest herbicide treatments on a field of clean wheat stubble harvested in June and sprayed July 20. He used Turbo Tee nozzles, triple-rinsing the sprayer after each treatment. Prior to wheat planting in fall 2014, the field was disked because glyphosate treatments weren't providing weed control. Currie also spread manure on the farm last year.

• Crop rotation and chemistry selection for pigweed control

These were the treatments, with approximate cost per acre (not counting cost of application) and the weed specialists' comments following:

1. 32 oz/acre Buccaneer 5 Plus (generic glyphosate)/13 pounds dry AMS. $7.44 per acre. Peterson comments: This provided about 50% control of pigweeds, but those that survived were healthy and producing seed. Each pigweed can produce nearly .5 million seeds.
   
2. 32 oz/acre Buccaneer 5 Plus/27.5 oz 2,4-D Amine 4/13 pounds dry AMS. $11.05 per acre. Peterson comments: This provided slightly better control of pigweeds than the first treatment, but still plenty of healthy pigweeds remained. Healthy pigweeds can grow 2 inches (or more) per day.
   
3. 32 oz/acre Buccaneer 5 Plus/27.5 oz 3,4-D Amine 4/1 ounce Sharpen/12 oz MSO/13 pounds dry AMS. $18.52 per acre. Peterson comments: The Sharpen dinged the pigweeds pretty well, but they were beginning to come back. Whether they produce as much viable seed as the other treatments remains to be seen. Sharpen is inconsistent. It burns down weeds fast, but two weeks later you may see new sprouts coming on. It works well on small pigweeds, but two ounces may be better. Two weeks after spraying, there are some weeds with green stems; these will survive.
   
4. 3 pts/acre Gramoxone SL/1 qt per 100 gallons non-Ionic Surfactant (NIS). $15.20 per acre. Peterson comments: The Gramoxone smoked the pigweeds, although there is still some green in the stem, and these weeds could regrow. Growers need to use 15 gallons of water per acre. Rate is important; 3 pints per acre works better than 2. This can be mixed with atrazine or metribuzin to heat it up a bit. It is a contact herbicide and is more dangerous to handle: Use gloves and a mask.
   
5. 3 pts/acre Gramoxone SL/2 oz Sharpen/1 qt per 100 gallons NIS/12 oz MSO. $30 per acre. Peterson comments: The results of Treatment 5 are very similar to Treatment 4. We hoped that by adding Sharpen we would get a bit of residual control. We did get a little additional effect, but not sure if it is worth nearly $15 more per acre. 

Peterson told producers in attendance that mixing modes of action is paramount when trying to control pigweeds. Prior to planting a spring crop, growers must use a preemerge herbicide, hit pigweeds when they are young - preferably 3 to 4 inches tall - and be sure to use a postemerge herbicide.