COLUMBUS, OHIO - The first half of November is usually an effective time to control winter annual weeds and dandelions in wheat. A number of wheat herbicides can be applied in the fall after the crop has emerged. Application of a premix product or combination of two herbicides is usually the most effective strategy to obtain a broad enough spectrum of activity. Some of the more effective treatments for winter annual broadleaf weeds include: 1) Huskie; and 2) combinations of dicamba (2 to 4 oz/A) or metribuzin (1 to 2 oz/A) with either tribenuron (Express, Nuance, etc), a tribenuron/thifensulfuron premix (Harmony Extra, Nimble, TNT Broadleaf, etc), or Peak.

In our research, the combination of tribenuron and dicamba has been one of the most effective treatments for control of dandelions. It may be possible to use a combination of dicamba and tribenuron/thifensulfuron premix instead, but thifensulfuron has little activity on dandelion. Using the premix results in a lower rate of tribenuron, which can reduce dandelion control.

While the mixtures listed above provide broad-spectrum control, control of ALS-resistant marestail is one of the probable weaknesses. The low rates of dicamba or metribuzin that are labeled for use in these mixtures are not likely to be consistently effective for control of ALS-resistant marestail, although the 4 oz/a rate of dicamba may control very small marestail plants.

Huskie may therefore be one of the most effective fall-applied options for control of emerged ALS-resistant marestail populations, and it evidently provides some residual activity also.

We receive many questions about the safety of 2,4-D applications, alone or in combination with other herbicides, to emerged wheat in the fall. We recommend avoiding this use of 2,4-D based on the following:

• we have observed yield reduction from use of 2,4-D on emerged wheat in the fall in three of the four studies we have conducted on this topic. We applied 2,4-D alone in one of these three studies, and a premix of 2,4-D and dicamba in the other two studies. The yield loss from 2,4-D treatments ranged from 11 to 20% compared with a range of other treatments, which included dicamba applied alone or in combination with Express, Harmony Extra, Starane, and Stinger.

• Fall application of a mixture of Peak and 2,4-D was recommended by a manufacturer and used several years ago in northwest Ohio and southern Michigan. This treatment cause considerable injury and yield loss in some wheat fields.

• Labels for 2,4-D products, including premixes of dicamba and 2,4-D (Brash, Weedmaster, etc), either do not recommend application in the fall, or indicate that the user must be willing to accept the risk of crop injury (manufacturer has no liability).

The labels for the 2,4-D/dicamba premix products state that application in the fall should occur after wheat has started to tiller, and some growers using 2,4-D in the fall have told us that they do wait until after tillering to apply. The stage of the wheat plant may have some influence on the potential for injury from fall-applied 2,4-D, but we do not have research-based information with which to make an informed recommendation. What’s interesting about this is that 2,4-D is usually extremely safe on wheat when applied in the spring. We have been unable to cause yield loss in spring when applied before the early boot stage (note – some 2,4-D labels specify that application must occur before jointing).

The risk of an 11 to 20% yield loss is enough for us to recommend avoiding use of 2,4-D in the fall, but it might be helpful to apply some simple “weed science” economics. First, wheat is currently worth a lot - $7 per bushel or so, which makes an average Ohio wheat yield (60 bu/A) worth $420. We’ll assume an average of 15% yield loss where 2,4-D is used in the fall, which is a loss of 9 bu/A or $63/A. Assuming you are lucky enough to get 90 bu/A wheat, this loss jumps to about 13 bu/A or $94. The difference in cost between 2,4-D (0.5 lb/A) and either tribenuron + dicamba (0.4 oz + 4 oz) or Huskie (13.5 oz) is about $9/A. There is essentially no difference in cost between using 2,4-D (1 pt) and either dicamba (4 oz) or metribuzin (2 oz) in a mixture with tribenuron. To summarize, using 2,4-D instead of dicamba means trying to save somewhere between nothing and $9/A, at the risk of losing about $60 to $90 per acre in income at current wheat prices.

SOURCE: Ohio State University Extension