Peanut Weed Control: Success or Failure?

Check out this picture I took yesterday from one of my weed control studies at the UGA Ponder Farm.  I think you would agree that it looks pretty good at 83 DAP.

Figure 1.  Peanut weed control plots at UGA Ponder Farm (Ty Ty, GA) on July 24, 2023. 83 days after peanut planting (DAP).  No rainfall or irrigation until 8 DAP and 17.92" of total rainfall from May 1 - July 24 (6.86" above long-term average). PRE – Applied 1 DAP; POST – Applied on May 25, 2023 - 22 DAP (Cadre + 2,4-DB + Cobra + Dual Magnum).  Weed-free at planting/twin rows. 

How did I make this happen?

1) I started clean and planted in twin rows.

2) Since I did not get to apply any irrigation until 8 DAP due to an irrigation pump problem, I pulled the trigger early on my POST treatment (22 DAP) and also added Cobra to that treatment. 

3) Spraying weeds when they are smaller in size, works almost 100% of time!  In my opinion, timeliness trumps (or desantiss?) GPA, nozzle type, tractor speed, and boom height.  The effects of these other factors on spray coverage are more critical when herbicides are applied to bigger weeds.  

4) Take a quick look at the Cadre (imazapic) label (https://s3-us-west-1.amazonaws.com/agrian-cg-fs1-production/pdfs/CadreR_Herbicide_Labelp.pdf). With the exception of cocklebur and rhizome johnsongrass, maximum labeled heights for all weeds listed are 4" or less.  A recent growth chamber study conducted on sicklepod control with Cadre indicated 16% less control of sicklepod when Cadre was applied at 6" vs 3" (remember that herbicides always work better in the greenhouse or growth chamber).

5) POST treatments in all my peanut tests are applied in 15 GPA, 3.5 MPH, 20" boom height, and with AIXR11002 nozzles (coarse droplets, 402 VMD50). 

6) It has been well documented that lower GPA's (10 GPA), faster operating speeds (>10 MPH), higher boom heights (>24"), and extremely coarse/ultra coarse spray droplets (i.e. dicamba tips) provide less spray coverage.   

What does this mean for the grower, especially when behind, treating larger weeds, and needing to get better spray coverage?

a) Regardless of any application tweaks, weed control expectations should be lower.  I have never been able to consistently control large weeds in my 37 year weed science career.  Every once in awhile, there are a few exceptions (i.e. bristly starbur/goathead/Texas sandspur and Strongarm).

b) Apply herbicides in 15 GPA.

c) Avoid using nozzle/pressure combos that produce extremely coarse or ultra coarse droplets (VMD50 > 502).

d) Reduce tractor speeds and boom heights as much as possible/practical.

e) Use full labeled rates.

Tank-Mixes, Perennial Weeds, and Pink Purslane (Prostko)

 A few things for you to chew on:

1)  The following are some pictures of a few peanut tank-mixes that I observed over the last few weeks that were not included in previous slides I sent you.  Nothing too scary here.

2)  Last week, I received numerous inquiries about controlling broadleaf perennial weeds in peanut (dogfennel, horsenettle, maypop passionflower, heliotrope, etc.).  None of the herbicides we have in our peanut arsenal work well on these types of weeds.  Growers who have perennial weeds in their peanut fields might consider the following:

a) If perennial weeds are in small spots and growers are willing to sacrifice the peanuts in those spots, they can be spot treated with a high rate of glyphosate (i.e. Roundup PowerMax3 5.88SL @ 44 oz/A or equivalent).  

b) In the fall after harvest, allow perennial weeds to regrow then treat fields with a high rate of glyphosate (same rate as above).  This fall application of glyphosate must be be applied at least 2 weeks before first frost.  Then in the spring of next year, plant RR-field corn or Xtend soybeans/cotton and plan on using a POST combination of glyphosate + dicamba in those crops.

c) It will take multiple years of fall + spring applications of glyphosate to get perennial weeds under control.  It took more than 1 year for these weeds to become a problem and will take more than 1 year to get them under control.

d) Avoid planting peanuts in fields with known populations of perennial weeds.

3) Pink purslane has been a bur under my saddle for many years.  Consequently, my PhD graduate student (Nick Shay) is conducting POST control studies as part of his dissertation.  Nick already completed 2 trials in the greenhouse last fall and 1 field trial this summer.  The following are the recent results of the field trial that he conducted.  Surprisingly, only Aatrex (atrazine) and Cobra (lactofen) were effective in this POST trial.  This field trial will be repeated again later this summer.