Multiple herbicide-resistant weeds and the challenges ahead

The evolution and widespread distribution of herbicide-resistant weeds and their management is a challenge for crop producers and land managers. 

 

By 2014, 29 weed species have evolved resistance to glyphosate worldwide, including 15 weed species in the United States. Six weed species in Nebraska have been confirmed resistant to glyphosate and some of them, such as common waterhemp (Amaranthus rudis), also are resistant to ALS-inhibiting herbicides (Classic, Pursuit, Raptor, etc.). They are known as multiple herbicide-resistant weeds.

Confirmed cases of multiple herbicide-resistant weeds in Nebraska
Weed Species	Type of Resistance	Example Herbicides	Distribution in Nebraska
Common waterhemp	ALS	Pursuit, Classic	Throughout the northeast, southeast and south central Nebraska
	Glycine	Roundup, glyphosate
Palmer amaranth	Triazine	AAtrex	A corn seed production field in south central Nebraska
	HPPD	Callisto, Laudis, Impact, Armezon
Kochia	Glycine	Roundup, glyphosate	Isolated fields in southwest Nebraska
	ALS	Express

To combat the issue of glyphosate-resistant weeds, while maintaining the benefits of glyphosate-tolerant crop technology, seed and agrochemical industries have developed multiple herbicide-tolerant crops by stacking tolerance traits for additional herbicide chemistries. In stacked herbicide-tolerant crops, the herbicide-tolerant traits are combined to allow use of diverse herbicides and herbicide mixtures. For example, Roundup Ready 2 Xtend™ soybean (glyphosate- and dicamba-tolerant) and Enlist Duo™ (glyphosate- and 2,4-D choline-tolerant), Balance GT soybean (BalanceFlexx-tolerant). Using diverse herbicides or herbicide mixtures with different modes of action will reduce overreliance on any single herbicide and minimize the likelihood of selection pressure for resistance. This technology is expected to add a new tool to the weed management toolbox for control of resistant and difficult to control weeds. However, indiscriminate use or overreliance on this technology may result in more complex weed issues, including multiple herbicide-resistant weeds.

 

To avoid this, use an integrated weed management approach:

• Use weed-free crop seeds for planting.

• Prevent weed seed production by scouting the fields to control missed weed plants.

• Use residual herbicides.

• Select herbicides based on multiple modes of action.

• Apply herbicides at recommended rates and proper weed stage.

• Adopt cultural practices like manipulating row spacing.

• Use competitive cultivars, tillage, crop rotation, and use of mechanical management practices if feasible.

• Clean farm machinery to prevent seed movement within and between farms.

• Maintain borders to avoid weed seed influx.

• Take measures to avoid build-up of weed seed bank in collaboration and coordination with neighbors.