What is the difference between Enlist and Roundup Ready 2 Xtend soybeans, and how good are they at controlling glyphosate-resistant weeds?

The key selling point of each new seed system is that they have tolerance to specific “auxinic” herbicides that traditionally have been used to control weeds in corn and cereals, and in theory should control glyphosate-resistant weeds. The Enlist herbicide system has resistance to 2,4-D from the “phenoxy” class of auxinic herbicides, while the Xtend herbicide system has resistance to dicamba from the “benzoic acid” class of auxinic herbicides. Therefore, Enlist Duo (glyphosate/2,4-D choline) can only be applied to Enlist soybeans, while either Engenia (dicamba) or Roundup Xtend (glyphosate/dicamba) can only be applied to Roundup Ready 2 Xtend soybeans. The Enlist system also has resistance to glyphosate and Liberty while the Roundup Ready 2 Xtend system has tolerance also to glyphosate. This can become confusing, so Table 1 (at top) summarizes what each seed system is resistant to and shows that when the wrong herbicide is applied, either severe injury or plant death will occur.

Activity on glyphosate-resistant and other problem weeds

Table 2 (below) summarizes efficacy from public trials conducted in Ontario for the glyphosate-resistant weed species and tufted vetch, a perennial weed that is naturally quite tolerant to glyphosate. With the exception of giant ragweed, all weeds are most sensitive when these herbicides are applied at the highest labelled rate. Three of the weed species are best controlled when these herbicides are applied to seedlings which have emerged after crop emergence. Postemergence applications of auxinic herbicides can increase the risk of off-target drift injury, so in the next #PestPatrol, I’ve asked Jason Deveau, our ministry’s application technology specialist, to run through the best management practices for drift mitigation.

Have a question you want answered? Hashtag #PestPatrol on Twitter to @cowbrough or email Mike at [email protected].