Growers in the upper Southeast continue to battle back against the growing problem of weed resistance to herbicides. Knowing herbicide mode of action is critical to making management decision geared to reducing the resistance threat.

The most common families of herbicides typically have a number of different products sold under different trade names and used in different crops. These materials look different on target and non-target crops, but the mode of action is the same.

Speaking at the recent Northeast Ag Expo in Tyner, N.C., North Carolina State University Peanut Specialist David Jordan showed attendees a series of test plots sprayed with different herbicides. Planning the use of herbicide families in crops this season should affect what is used in the same field on a different crop next season, he says.

Knowing the mode of action of these herbicides is a key piece of the puzzle in reducing the risk of herbicide resistance Jordan adds.

The first group of materials Jordan showed the crowd was photosynthetic inhibitors, Basagran being the particular herbicide used in the test. These materials have a fairly narrow spectrum of activity. It is especially good on yellow nutsedge, prickly sida and cocklebur.

Bromoxynil is another family of herbicides, and one that is not widely used on row crops. Buctril is the product used in the North Carolina tests. Jordan pointed out that it, like Basagran, has a distinctively different mode of action than glyphosate or ALS inhibitors, both of which have been plagued by resistance problems in recent years.

Bromynil herbicides are commonly used in tank-mixes with one of several triazine herbicides. The combination controls over 40 of the most common weed pests found in row crops and can be an economically attractive combination in fields with a wide spectrum of weed problems.

PPO (protoporphyrinogen oxidase) inhibitors, including such products as Blazer, and Cobra, play a key role in fighting herbicide resistance. So far, none of herbicides from this family have shown resistance problems in the Southeast.

Jordan stresses that the PPO inhibitors are commonly used in fields in which plants have developed resistance to glyphosate, the ALS inhibitors, or in some cases both in the same plant. Over-use of the PPO inhibitors he warns could put added stress on these materials and lead to resistance problems.

For peanut growers who have lost the use of ALS inhibitors because of resistance, losing the PPOs would be a serious blow. Jordan stresses that avoiding resistance problems is critical to avoid having to come in with the PPOs and clean up these fields.

Glufosinate is a family of herbicides that builds up ammonia toxicity in weeds. It is a good option for managing pigweed resistance in cotton. It is not a salvage type material, and won’t kill pigweed that are much larger than those controlled by the PPO herbicides.

Diclofop, commonly sold as Hoelon, has shown some problems with ryegrass resistance in wheat. Jordan points out that in many fields in North Carolina growers turned to ALS inhibitors to deal with ryegrass resistance to Hoelon. Now they are seeing resistance to both families of herbicides.

Clethodim is used extensively in peanuts, cotton and soybeans. There have been pockets of johnsongrass resistance, but in general few resistance problems with this family of herbicides. Poast, Assure, and other herbicides in this family have been around a long time, and losing these to resistance problems would be a serious blow to grass control in the Southeast.

The phenoxy herbicides, primarily 2, 4-DB and 2,4-D are widely and successfully used in peanuts. These materials are frequently mixed with glyphosate to help reduce the risk of resistance. So far, no resistance has been reported. The two materials are different in how they interact with plants.

There are some cotton varieties coming along that are phenoxy resistant varieties that will be important in the fight against glyphosate and ALS resistant weeds in cotton. The phenoxy herbicides will kill pigweed, but not when they get much bigger than 3-4 inches.

Dicamba, which is a benzoic herbicide has similar mode of action to 2,4-D. Dicamba resistant soybean varieties look promising and may be available to growers in the Southeast in the next two or three years.

If the gene trait for tolerance of dicamba in soybeans becomes a reality, this will be a significant boost to growers in the Southeast in their quest to fight glyphosate resistant Palmer pigweed. If high yielding bean varieties are developed to carry both the glyphosate and dicamba tolerant genes, weed control and herbicide resistance management will take a giant step forward.

The ALS (acetolactate synthase) inhibitors are among the most commonly used herbicides. Most scientists say it takes about six times applying it to a field to get resistance. Resistance to ALS inhibitors is widespread in the Southeast. In some fields ALS and glyphosate resistance shows up in the same seed.

Regardless of which of the 25 or so ALS inhibitors on the market is used the risk of resistance is basically the same. If there is ALS resistance in one crop to one ALS product, it is a good bet the resistance will show up in a different ALS product in a different crop in the next year in the same field.

Triazine herbicides have been around a long time. Atrazine is the most often-used product of this family of herbicides. It is a staple herbicide in corn, but it can be highly toxic to other crops, such as cotton. There have been some resistance to the triazines, especially in the Midwest, where it has been used at high rates in consecutive years in the same field.

Cotoran is another of the triazine herbicides that is commonly used in cotton in the Southeast. Again, changing the trade name doesn’t change the mode of action and growers should be careful to know the mode of action and family of herbicides rather than the trade name.

e-mail: rroberson@farmpress.com