By Brian Hefty

Everyone is talking about Roundup-resistant weeds.  Every meeting I went to this winter, every conference I attended, and every farm magazine I picked up, Roundup-resistant weeds were there.  The good news is these weeds are easy to control in corn.  Think about waterhemp, lambsquarters, common ragweed, giant ragweed, palmer pigweed, velvetleaf, and marestail.  The bad news with all of them is there is absolutely nothing I can recommend to you in soybeans that will control ALL those weeds season-long in one application.  Worse yet, if you don’t dramatically reduce their numbers pre-emerge, you may have an incredibly difficult time post-emerge controlling them.  Here’s why.

If Roundup no longer controls certain weeds post-emerge, what are your remaining post-emerge choices?  They are basically all the same products we had 15 years ago when Roundup soybeans first came out.  That means Pursuit, Flexstar, Resource, Harmony SG, FirstRate, Cadet, Cobra, etc.  Those herbicides are great if you spray early (4” tall weeds or less), have few weeds (so you can get great spray coverage), use the right spray adjuvants, and use the right product(s) for the weed(s) you are after.

Your safest bet, since Roundup-resistant weeds continue to spread quickly each year, is to start with a good pre-emerge product that has lots of residual.  Here are my top 4 suggestions:

  1. Sonalan (best), Treflan (cheapest), or Prowl (no incorporation needed).  The “yellows” have always been great on grass, but they also have good activity on most small-seeded broadleaves, including lambsquarters, waterhemp, pigweed, etc.  These products can also be mixed with any of the rest of the herbicides on my list.
  2. Valor.  This is the least expensive product that offers primarily broadleaf control.  It gets even less expensive when you factor in the $3 Roundup bundling program.  Valor is very good on small-seeded broadleaves, and carryover for next year is not an issue.  Valor has little activity on large-seeded broadleaves like sunflowers and cockleburs, but it can be used in any soil pH situation.
  3. Authority (Assist or MTZ, not First).  Authority has tremendous soil residual.  This is great when it comes to late-season weed flushes.  This concerns me when it comes to carryover, especially in high-salt or high-pH soils.  Carryover is rarely an issue, but I’m always still cautious, as Authority’s half-life is in excess of 200 days.  You won’t find straight Authority anymore, so you’ve got to get one of the premix options like Authority First, Authority Assist, or Authority MTZ.  I don’t typically recommend Authority First, as it contains FirstRate.  FirstRate can only be used once per season, and I prefer to use it post-emerge.  The Authority products I do like are Assist (Authority & Pursuit) and MTZ (Authority & Sencor).  Assist is better on nightshade and velvetleaf.  MTZ is superior at controlling lambsquarters, waterhemp, and other pigweeds.
  4. Metribuzin is the generic version of the old Sencor.  The 2 problems this herbicide has always had is it can leach (meaning less residual if you get lots of rain), and it has increased activity in high-pH soils.  The advantages it has include its very low cost, the fact that it is pretty good on most broadleaf weeds, and that if you have consistent high-pH soils, you can use a low rate and get the same performance you’ll see in low-pH soils at a higher use rate.  I get a charge out of the fact that when I was a kid, Sencor plus Treflan was the most popular pre-emerge soybean program.  Here we are more than 30 years later, and Treflan and Sencor (albeit generic versions) are commonly used again.

If you’ve got Roundup-resistant weeds, one pre-emerge herbicide probably won’t cut it.  Use 2 pre’s and base your post-emerge decisions on whatever is left for weeds.