By Mark Loux, Weed Scientist

Gramoxone SL (paraquat) is one of those herbicides that, in our opinion, really could have been used much more than it has in recent years to help with management of marestail and to interrupt the cycle of continuous glyphosate use. A relatively high price has been one of the obstacles to more widespread use, but the price was cut approximately in half this winter. 

One of the problems with the standard “glyphosate + 2,4-D + residual” burndown is that the 2,4-D is carrying the full load for control of emerged marestail, and this is especially a problem in fields not treated the previous fall where plants are harder to kill. Swapping in Gramoxone for glyphosate results in an additional herbicide that has activity on marestail. In our soybean research, Gramoxone alone rarely was adequate on marestail in the spring, except on very small plants that had not overwintered. Most effective control of overwintered and larger marestail usually required a combination of Gramoxone, 2,4-D and metribuzin, or some product that contained metribuzin (Gramoxone and PSII inhibitors such as metribuzin and atrazine work well together). The same could probably be said for any relatively weedy no-till burndown situation, although some dealers have gotten by earlier in spring with a combination of Gramozxone and 2,4-D without the metribuzin. 

The addition of a product containing chlorimuron can also help with burndown when using Gramoxone. So an ideal mix could be something like Gramoxone + 2,4-D (or dicamba if approved for Xtend soybeans) + Canopy DF + maybe a few more ounces of metribuzin. Other products such as Valor XLT, Authority XL, etc., can be used in place of Canopy DF, but will still need the addition of 6 or more ounces of metribuzin 75DF for most effective burndown. Treating a field the previous fall creates more flexibility in how Gramoxone could be used in the spring, since the primary weed targets until early May would likely be spring-emerging ragweeds and marestail and maybe a few winter annuals. Gramoxone applied at the appropriate rate would probably handle most of these without much help, although our suggestion would be to include at least metribuzin still.

The same thing could be said for corn burndown programs — swap Gramoxone in for glyphosate — and make sure to include an atrazine-containing product. The more “diverse” no-till situations could probably still benefit from the inclusion of a growth regulator herbicide, but the Gramoxone + atrazine should be effective on emerged marestail. One advantage that Gramoxone has over glyphosate in corn burndown programs is where 28% nitrogen (N) is used as part or all of the spray carrier. Doing so with glyphosate is not optimum, as it tends to reduce the herbicide’s activity on larger weeds, and biennials and perennials. Whereas Gramoxone + atrazine can be even more effective when mixed with 28% N due to the synergistic contact activity of all three ingredients.

There are certainly situations where glyphosate is still a better choice for burndown than Gramoxone, including control of many cover crops, perennial grasses and certain biennials and cool-season perennial broadleaf weeds that get an early start in spring. And there tends to be more flexibility in the application parameters for glyphosate vs. Gramoxone. Optimizing these parameters for Gramoxone SL includes:

  • Use rate of 3 pints per acre or more in most situations, or 2 pints per acre for the 3-pound-per-gallon formulations.
  • Nozzles that are not coarser in droplet size than the current “medium to coarse” nozzle category. Some applicators believe that twin-orifice nozzles are most effective.
  • Spray volume of 15 gallons per acre or more.
  • Use of crop oil concentrate (1% v/v), which is generally more effective than nonionic surfactant (0.25% v/v).

In addition, like glufosinate, Gramoxone will have most rapid and most effective activity in warmer sunny weather, although the choice of tank mix partners will certainly influence the overall effectiveness. This is not to say that glyphosate maintains effective activity in spring regardless of temperature, since slow kill of dense winter annuals with glyphosate + 2,4-D under cool conditions was one of the drivers for fall herbicide treatments. 

While we have mentioned primarily Gramoxone SL in this article, there are certainly generic equivalents that are as effective. The restricted use status of all of these products has also been one of the obstacles to more widespread use, along with real and perceived issues relative to product toxicity. It does have to be ingested to cause poisoning, and changes to the formulation over the years have resulted in a generally safer product. The EPA recently issued a Human Health Mitigation Decision pertaining to paraquat, which will put in place additional measures to promote safe use. These will apply to all paraquat products and include:

  • Label changes emphasizing paraquat toxicity and supplemental warning materials.
  • Targeted training materials for paraquat users.
  • Closed-system packaging for all non-bulk (less than 120 gallon) end-use product containers.
  • Restricting use to certified applicators only (so prohibiting use by uncertified persons working under supervision of a certified applicator).

Finally — unrelated to the rest of this article — you can find a 10-minute video update on the status of Palmer amaranth in Ohio on our website (u.osu.edu/osuweed – “palmer amaranth” section) and our YouTube page (search “Ohio State University weed science”).