If you're able to be successful with glyphosate or Gramoxone Inteon as your burndown application ahead of soybeans this spring, then plant-back restrictions are likely not an issue. However, Glenn Nice, a Purdue University weed control specialist, notes that you shouldn't expect immediate results from glyphosate, the active ingredient in Roundup products and a whole host of generic glyphosate herbicides known by a myriad of names. It will take seven to 10 days for glyphosate to do its job in most cases, Nice notes.
The situation changes if you add 2,4-D to the mix, depending upon how much you add. If glyphosate does the job, why would you want to add 2,4-D, a product that carries a plantback restriction ahead of soybean planting, in the first place?
The reason is simple, Nice says. In some parts of Indiana, glyphosate-resistant marestail is becoming a big concern. Marestail is officially known by weed scientists universally as horseweed. However, in Indiana farmers for years called giant ragweed horseweed. So Nice prefers to use the name marestail to distinguish it from giant ragweed.
Marestail started becoming more of an issue when farmers switched to no-till soybeans more than two decades ago. For many years, glyphosate at normal rates took down the slender weed. However, resistant populations now exist. It's one of more than a dozen weeds worldwide with known resistance to glyphosate.
If you add one pint or less of 2,4-D (product, not active ingredient amount) to the mix, then the restriction time is only seven days. You can plant soybeans in that case according to the label seven days after application without expecting injury to germinating soybean seedlings. However, if you apply more than one pint per acre of 2,4-D, then the label on most 2,40-D products calls for a plantback restriction before planting soybeans of 30 days. Especially if you apply it now, in late April, that becomes an issue.
Here's the rub. Where the widespread occurrence of glyphosate-resistant marestail is an issue, most recommendations call for at least one pint per acre of 2,4-D ester to be applied with the burndown.
If you're thinking a little bit is good, so I'll add more, say 1.5 pints of 2,4-D per acre, then you've jumped into a much longer plantback restriction window according to most labels.
The bottom line is to make sure you understand what restrictions might apply if you spray or have a custom applicator spray certain herbicide mixtures as burndowns ahead of fields where you intend to plant soybeans.