By Brian Hefty

In soybeans, Roundup-resistant weeds are becoming a bigger challenge each year.  Worse yet, most of the post-emerge weed control options not named Roundup or Liberty stink.  That’s right.  I said it.  They stink.  That’s the reason every farmer moved so quickly to Roundup Ready soybeans when they first came out years ago.  Unfortunately, for the most part we have no new post-emerge options since 20 years ago.  If you have resistant weeds, you are stuck with the same herbicides as back then.

The answer to Roundup-resistant weeds is not in adding tankmix partners to Roundup.  The key is to kill the weeds before they emerge.  Here’s how we would suggest you do it.

For WATERHEMP, PALMER PIGWEED, COMMON RAGWEED, LAMBSQUARTERS, KOCHIA, and WATERHEMP in soybeans:

  • In no-till fields with a soil pH less than 7.5 and a CEC above 10:  Use Prowl plus Authority MTZ plus Parazone (generic Gramoxone) at full rates.  This will give you 3 residual sites of action and 99.9% control on all the weeds I listed above.
  • In conventional-till fields with a soil pH less than 7.5 and a CEC above 10:  Use Sonalan plus Authority MTZ at full rates and incorporate while applying the herbicide.  This will be as good as or better than our no-till mix.
  • If your soil pH is 7.5 or higher:  Switch the Authority MTZ to 3 oz. of Valor plus a low rate of metribuzin, something like 1/8th to 1/6th of a pound.
  • If your CEC is 10 or less (in other words, you have sandy, low organic matter soil):  Switch the Authority MTZ to 3 oz. of Valor and use 1/8th of a pound of metribuzin or less.

For MARESTAIL in soybeans:

  • Marestail is a winter annual, so it will likely be big by the time you get around to planting soybeans this spring.  We do not advise using 2,4-D in the spring in front of soybeans, but the burndown is the key.  If you don’t kill marestail with your burndown, you won’t get it.  Also, if you use conventional tillage instead, just make sure that tillage is aggressive enough to actually kill the marestail and not just bend it over.
  • Use the same no-till program we listed above.  The key is the Parazone.  Authority and Valor have good activity in combination with metribuzin, but not quite as good as Parazone.  Add crop oil or methylated seed oil to further enhance your control.
  • Another key to marestail control is making sure you spray when the weather is warm.  If it’s 45 or 50 degrees outside, put the sprayer back in the shed.

WON’T THIS COST A LOT OF MONEY?

I know that commodity prices aren’t what they were a year ago, but I also know that weeds can rob yield in a big way.  If Roundup no longer works on some of the weeds on your farm, you don’t have good rescue options post-emerge.  Pre-emerge is your best bet.  Yes, it will take an investment if you want to use the programs we suggest above, but that investment correlates to 1.5 to 2 bushels of soybeans.  If you use just one or two sites of action in a pre program, you will spend the equivalent of one bushel of soybeans and get 90% to maybe 95% control if you’re lucky.  Wouldn’t you rather take the sure thing?  Wouldn’t you rather have the confidence that virtually all your Roundup-resistant weeds will be dead?  Ultimately, it’s your decision; it’s your farm.  I just know that on our farm, we’re using the programs above again this year, and I am expecting great weed control AND top yields.