Herbicide Resistant Weeds in Small Grains

Herbicide-resistant weeds are an ever increasing problem.

When most people hear herbicide-resistant weeds these days they think of glyphosate resistance. However, I think Group 2 resistance (or ALS-resistance) is just as bad, if not worse, for our area. Small grains is a crop that relies heavily on ALS herbicides for weed control, and over the years the products have been used heavily. We now have three weed species that are resistant to this mechanism of action. ALS-resistant common chickweed, annual (Italian) ryegrass, and horseweed (or marestail) are becoming more common. Before I talk about some options to control them, let’s look at how we got here. First, these three species are winter annuals whose emergence period is from late summer throughout the fall and into the spring. They infest small grain crops and are common in fallow fields over the winter. They are present when fields are sprayed prior to planting no-till corn or soybeans. Secondly, ALS-inhibiting herbicides include a large number of products labeled in many crops and for many uses (see table below for a list of Group 2 herbicides).

Many fields have been treated two to three times a year with this mode of action either when small grains were planted, or as part of the management of winter annuals for no-till. In a hypothetical situation, a field is planted to no-till wheat and either Finesse or Peak is used pre-plant. In the early spring, Osprey or PowerFlex is used for annual ryegrass and a later application of Harmony Extra is used to control wild garlic. Double-cropped soybeans are planted and if horseweed is present a product like Canopy or FirstRate might be used. In the fall after harvest, Canopy EX or Autumn is used to keep the field clean, but it does not control annual ryegrass. The following spring, no-till corn is planted but because there is annual ryegrass present, Resolve or Basis is used with the burndown. After corn harvest, maybe Canopy EX or Autumn is used again. Prior to planting soybeans, Envive or Valor XLT is used with a low rate of 2,4-D and glyphosate to burndown (but because the horseweed is glyphosate resistant and the rate of 2,4-D is less than 1 qt; the ALS herbicides in Envive or Valor XLT are doing most of the work to control horseweed). While this is not a specific field, this is not unrealistic from what some people are doing.

So with ALS-resistant weeds, what can we do to control them, or help prevent developing resistant weeds?

• DO NOT use ALS herbicides for fallow no till fields in the fall or early spring.
• Do not let these species go to seed; in the case of common chickweed it flowers and produces seeds in late March and April, annual ryegrass produces late April and into May, and horseweed produces seeds in August.
• Use a competitive cover crop to prevent weed growth in the fall and early spring.
• Avoid use of ALS herbicides preplant for small grains; postemergence applications of ALS herbicides provide much more consistent control.

ALS-resistant annual ryegrass can be treated with Axial XL in the fall or early spring. Be aware that there are biotypes of annual ryegrass that are resistant to the mechanism of action of Axial XL (Group 1); Hoelon resistant ryegrass (also a Group 1) was an issue in some parts of the region. There is not enough local data to know how effective Zidua might be on annual ryegrass, but since it will not control emerged ryegrass plants and Zidua needs to be applied after crop emergence, early emerging ryegrass plants will not be controlled. To lessen selection pressure on Group 1 herbicides, avoid use of Select or Poast (both Group 1 herbicides) to control annual ryegrass in the spring. Furthermore, glyphosate-resistant ryegrass has been reported in Mississippi, Tennessee and North Carolina. So be sure to use glyphosate in a manner that maximizes its effectiveness (be sure ryegrass is actively growing when it is treated and avoiding tankmix partners that reduce glyphosate activity), and when appropriate, include use of paraquat as a burndown to provide a different herbicide mode of actions.

ALS-resistant common chickweed can be treated with Starane Ultra, but the level of control is not as good as what we are used to with ALS-inhibiting herbicides. The full rate of Starane Ultra of 0.4 pts/A is needed. Glory is very good on common chickweed, but crop safety could be an issue if applied to sensitive varieties.

ALS-resistant horseweed is quite challenging. Currently our best option is 2,4-D, but timing is very limited, and there is a narrow rate range to balance control and crop safety. On sandy soils, I do not like to use more than 8 oz/A, and it has to be applied prior to jointing

Fields with ALS-resistant biotypes of all three species will have to rely on three different herbicides for what previously was treated with one or sometimes two. And the level of control will likely not be as good. For vegetable growers who like to double-crop vegetables after small grain harvest, use of Axial XL and Starane Ultra requires 90 and 120 days prior to planting, respectively. Starane Ultra at 120 days requires a fall application or very early spring.

We have a number of trials out this year for resistant weeds in small grains, but none of the products we are testing have the effectiveness AND flexibility of ALS herbicides. If you do not have ALS-resistance, evaluate your weed management programs and take steps to reduce the selection pressure for ALS-resistance. Be sure you can continue to use products like Harmony Extra, PowerFlex, or Osprey in small grains where they are really necessary by avoiding overuse of ALS products at other times.

The following table is a partial list of ALS-inhibiting herbicides (Group 2). The first two columns are products that contain a single active ingredient. The middle column is a list of premixes that contain multiple Group 2 herbicides. And the last two columns contain at least one Group 2 herbicide plus another herbicide chemistry. The effectiveness for resistance management of the premixes in the last two columns depends on the premix partners and their rates (not all premixes provide rates that are adequate for effective control).

Partial List of ALS-Inhibiting (Group 2) Herbicides