Strategies for Managing Weeds in Wheat

Linda Benedict, Williams, Billy James, Twidwell, Edward K.  |  10/30/2006 10:56:55 PM

Bill Williams and Ed Twidwell

Weed management in wheat is usually relatively simple compared to other agronomic crops, but it does require planning. The most important aspect of managing weeds in wheat is establishing a good stand before weeds emerge. The key to this is timely planting and good seedbed preparation. The optimum time to plant wheat is between Oct. 15 and Nov. 15 in north Louisiana and Nov. 1 to Nov. 30 in central and south Louisiana. Earlier planting dates favor wheat over weeds.

When possible, seedbeds should be prepared far enough ahead of planting so rainfall can replenish moisture lost dur ing tillage operations. Fall tillage may not be required if a no-till drill is available. Regardless of the tillage system, it is extremely important to completely kill existing vegetation before wheat emerges by using an appropriate herbicide such as glyphosate or paraquat.

While using a grain drill is the preferred method of planting, wheat is occasionally planted by broadcasting on a prepared seedbed and then lightly disking or harrowing to cover the seed. This seeding method works well when drilling wheat is impractical. A common mistake with this method is the belief that the tillage and harrowing operations will destroy existing vegetation. In most cases, weeds are simply covered by soil and resume growth with the first rainfall.

Weeds reduce wheat profits by reducing yield, quality and harvest efficiency. Some weeds such as ryegrass reduce yield and quality. Others such as wild onion and wild garlic primarily reduce quality, resulting in substantial dockage – penalties for foreign material. Dockage losses are serious and can equal or exceed losses from reduced yields. Producers have several strategies for managing weeds in wheat.

Annual Grass Management  
Annual bluegrass and ryegrass are the two most common grasses infesting wheat fields in Louisiana. Of the two, ryegrass causes the most economic damage. Annual bluegrass is more widespread than ryegrass but is often suppressed by good wheat stands. Only extreme annual bluegrass densities reduce wheat yield, and because of its low growth, annual bluegrass has little impact on quality.

Sencor, Hoelon and Osprey are the primary herbicides recommended for grass control in wheat in Louisiana. Sencor does an excellent job of killing annual bluegrass and suppresses many of the other annual grasses. Not all wheat varieties are tolerant to Sencor, so producers should use the herbicide only on varieties identified as Sencor tolerant by Bayer CropScience, the Sencor manufacturer. Sencor at 3 ounces per acre applied after the 2-leaf wheat stage and before weeds emerge has been an effective method of controlling weeds in wheat. Rates can be increased to 5 ounces per acre when wheat has two to three tillers, or stalks, and 8 ounces per acre on wheat with four tillers. These later applications are often less active on emerged weeds and result in more wheat injury.

Hoelon has been the standard for ryegrass control in Louisiana for a number of years. Hoelon resistance in ryegrass is common in the neighboring states of Mississippi and Arkansas, and a few unconfirmed cases have been reported in Louisiana. It’s not uncommon to see ryegrass control failures with Hoelon even on susceptible populations. Tank-mixing Hoelon with broadleaf herbicides and application errors are the two leading causes of Hoelon failures. Hoelon cannot be tank-mixed with any other herbicide for broadleaf weed control. Hoelon rates, adjusted according to ryegrass size, should be applied at 1.33 pints per acre when ryegrass has one to three leaves, 2.0 pints per acre at three to four leaves, and 2.66 pints per acre when ryegrass has up to two tillers. Unfortunately, ryegrass usually goes unnoticed until it is well past the appropriate growth stages.

Osprey, a relatively new herbicide for weed management in wheat, can be used to control Hoelon-resistant ryegrass. Osprey also is effective at controlling annual bluegrass and has some broadleaf weed activity. Osprey can be tank-mixed with Harmony Extra for improved broadleaf weed control. Osprey can be applied at 4.75 ounces per acre to ryegrass with up two tillers. The best control, however, is observed before ryegrass begins tillering. Ryegrass control from either Hoelon or Osprey will be better and more consistent when applications are made in December or January. The bottom line is that ryegrass needs to be controlled before the fifth leaf emerges, and by February or March it is usually too large.

Winter Weed Management
While present in most fields, winter weeds like henbit, chickweed, swinecress, buttercup and geranium rarely require treatment when wheat is planted on time and in a clean seedbed. Clover and medic species as well as curly dock are occasional problems that can be easily controlled with 2,4-D and dicamba. Curly dock is also controlled by Harmony Extra.

Wild onion and wild garlic are more difficult to control because they often go unnoticed until late in the season when they are larger, and wheat has passed key growth stages limiting herbicide options. While wild onion and wild garlic may not result in large yield reductions, but they do reduce quality and result in significant dockage that reduces price.

2,4-D is usually the preferred and most-economical herbicide for suppressing both wild onion and wild garlic. Tank-mixes of 2,4-D and dicamba are generally required to kill or suppress severe infestations, but they can injure wheat. 2,4-D and dicamba can only be used on wheat that has finished tillering and not begun to joint (stem elongation). Harmony Extra can be applied to wheat between the two-leaf stage and the development of the third node, but it kills wild garlic only. Wild garlic and wild onion are similar in appearance but can be distinguished by looking at a cross section of the leaves. Wild garlic has round hollow leaves, whereas wild onion leaves tend to be flatter and are not hollow.

Cultural Control
The most effective and economical program for controlling weeds in wheat is cultural. When wheat is planted in a timely manner to well-prepared seedbeds, it will suppress the majority of weeds commonly found in Louisiana. Ryegrass, wild onion and wild garlic are occasional exceptions that may require additional control from herbicides. It is important to determine if herbicides are needed and to develop a plan early because many of the herbicides used on wheat cannot be used until wheat reaches certain growth stages, and herbicide options are severely limited after wheat joints. Overall, the best weed control is often observed when Sencor applications are made in early December and followed by an appropriate herbicide to control ryegrass, wild onion or wild garlic as needed. Sencor must be used with Sencor-tolerant varieties.
Acknowledgment: Louisiana Soybean and Grain Research and Promotion Board

(This article was published in the fall 2006 issue of Louisiana Agriculture.)
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