Time to think about burn-down herbicides

Ryegrass can be a very difficult weed to control in a burn-down situation if you do not spray early. Before it reaches the reproductive stage, it can be controlled with glyphosate. Later on it gets tough.

The best recommendation is to use a high rate of glyphosate (32 to 48 ounces per acre) and live with what you miss.

I have had good success with spilt shots of glyphosate at about 1.5 pints per acre each time, but not many of you want to make sequential burn-down applications.

There is such a thing as glyphosate-resistant ryegrass, although it has not been confirmed in the United States. It has been documented in Australian wheat fields for several years. We know this species can develop resistance. If you suspect you have resistant ryegrass, contact your county agent.

Straight glyphosate is still a cheap and effective burn-down treatment for many producers. However, some situations may call for the addition of a tank-mix partner, a residual herbicide or the use of another herbicide such as Gramoxone Inteon.

Gramoxone Inteon is the same active ingredient as Gramoxone Max, but in a 2-pound-per-gallon formula. If the rate of Gramoxone Max was 32 ounces per acre, the rate of Gramoxone Inteon would be 48 ounces per acre. The company changed the formulation to be safer for those applying the product. In our research, the activity on weeds has been equal to the old formulation. The new formulation still bears the skull and crossbones symbol and should be treated with the respect due all poisonous materials.

Gramoxone Inteon is an excellent choice if you have seedling horseweed prior to planting. The horseweed must still be in the flat rosette stage prior to starting to grow upward. Once the plant has started upward, Gramoxone activity diminishes.

Using Gramoxone Inteon is also a good resistance management strategy if you can fit it into your program. It has a different mode of action than glyphosate.

In normal burn-down situations, flowering winter annual broadleaves, such as cut-leaf evening primrose or henbit, are difficult to control with glyphosate alone. The addition of 2,4-D will aid in the control of these weeds at an economical cost.

If horseweed is present, 2,4-D is likely not sufficient. Clarity or generic dicamba at 8 ounces per acre should be added to glyphosate for horseweed. Dicamba will also aid in the control of other broadleaf weeds.

When adding 2,4-D or Clarity as a tank-mix partner, be aware of the replant intervals for certain crops. Labels vary on this. In general, if you plant soybeans or cotton and have injury, according to the label, you have done something wrong. However, in university trials, we have found that you can plant soybeans as early as 14 days after an application of 2,4-D as long as you get around an inch of rain between application and planting. With that same inch of rain, cotton can be planted at 28 days after 2,4-D or 21 days after dicamba applications.

2,4-D and dicamba are good resistant management tools for use with glyphosate, but dicamba is better on horseweed.

In the case of glyphosate-resistant horseweed, start off clean. If horseweed has germinated between early burn-down and planting, you must remove the seedlings with Gramoxone Inteon or Ignite prior to planting. Otherwise seedling horseweed will likely become mature horseweed in your field, especially in cotton.

If you start clean, a residual in cotton at planting can prevent further infestations of horseweed. Direx, Cotoran, or Caparol all have good residual activity on horseweed.

In soybeans, a residual treatment of Valor will also control horseweed for around 30 days at the rate of 2 ounces per acre. Valor has a 30-day plant-back interval on most crops, except soybeans.

If you do not utilize a burn-down program in soybeans, horseweed can get away from you. Preliminary data suggests that an at-plant application of 1.5 ounces per acre of Synchrony XP or 1.0 ounces of Python followed by in-season glyphosate tank-mixes with FirstRate at 0.3 ounce per acre is a pretty good horseweed program. More research is needed in this area.

The best recommendation I can make is to start with an effective burndown in both soybeans and cotton.

The plant-back intervals for Harmony Extra and Harmony GT have been reduced this year to 14 and seven days, respectively for cotton. Harmony GT can be used immediately prior to planting soybeans. Both products are good on wild onion/garlic, most broadleaves and smartweed. These products have 45-day plant-back intervals for rice and grain sorghum.

Always read and follow label directions. Also, see page 24 of the Arkansas Extension MP-44 publication or your state’s weed control guide for more information.

Bob Scott is the University of Arkansas Extension weed specialist. e-mail: [email protected]

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