Early Spring Control Of Weeds In Thin Stands Of Wheat

Some areas of Kansas have unusually thin wheat stands because of dry conditions last fall and early winter. Thin, late-developing wheat is likely to have excessive weed problems because of the open space and canopy, but treatment decisions can be difficult. Residual herbicide products such as Finesse, Ally, Rave, or Amber probably will provide the best weed control because they have both foliar and soil residual activity. These types of products may be the best option is you are committed to keeping the wheat crop for grain harvest.

However, these herbicides all have crop rotation restrictions, so if there is a crop failure, you may not have a lot of recrop options. Make sure to check the recrop guidelines on all herbicides before planting a subsequent crop. If you want to keep your options open for planting a summer crop, you probably need to consider using a short-residual herbicide such as MCPA, 2,4-D, dicamba, or Affinity, but realize that these products will not provide much residual control of later-germinating weeds.

Winter annual weed control

Some wheat fields may have heavy winter annual weed infestations if moisture was received after planting last fall. There are several options available to control these weeds now. In general, the sooner this is done, and the smaller the weeds, the better the results will be. Spring applications generally are most effective on winter annual broadleaf weeds soon after green-up when weeds are still in the rosette stage of growth, and during periods of mild weather. Once weeds begin to bolt and wheat starts to develop more canopy, herbicide performance often decreases dramatically.

When applying herbicides at this time of year, near jointing, there are some timing issues related to the growth stage of wheat that producers will have to take into account.

Dicamba can be applied to wheat between the 2-leaf and jointing stages of wheat. Application of dicamba after wheat reaches the jointing stage of growth causes severe prostrate growth of wheat and significant risk of yield loss. Dicamba is effective for control of kochia, Russian thistle, and wild buckwheat, but is not good for control of mustard species. Kochia, Russian thistle, and wild buckwheat are summer annual weeds that may emerge before or after wheat starts to joint, so timing of dicamba for control of these weeds can sometimes be difficult. Fortunately, dicamba provides some residual control of these weeds following application. Herbicides containing dicamba include Banvel, Clarity, Rave, Pulsar, Agility SG, and several generic dicamba products.

Other herbicides that must be applied prior to jointing include Agility SG, Beyond (on Clearfield varieties only), Olympus, Olympus Flex, Orion, PowerFlex, Pulsar, Rage D-Tech, and Rave. MCPA and 2,4-D have different application guidelines. In general, MCPA is safer on wheat than 2,4- D, especially when applied prior to tillering. We recommend that 2,4-D not be applied to wheat until it is well-tillered in the spring. Application of 2,4-D prior to tillering hinders the tillering process, causes general stunting and can result in significant yield loss.

Stunting from an application of 2,4-D to wheat prior to tillering. Photo by Dallas Peterson, K-State Research and Extension.

2,4-D is labeled for application to wheat from the full-tiller stage until prior to the boot stage of growth, but is probably safest between full-tiller and jointing stages of growth. Wheat will sometimes exhibit prostrate growth from 2,4-D applications applied in the jointing stage of growth, but yields generally are not significantly affected if applied before the boot stage of growth.

MCPA is relatively safe on young wheat and can be applied after the wheat is in the three-leaf stage (may vary by product label) until it reaches the boot stage of growth. Consequently, MCPA would be preferred over 2,4-D if spraying before wheat is well-tillered. Neither herbicide should be applied once the wheat is near or reaches the boot stage of growth, as application at that time can result in

Malformed heads from an application of 2,4-D at boot stage. Photo by Dallas Peterson, K-State Research and Extension.

Both 2,4-D and MCPA are available in ester or amine formulations. Ester formulations generally provide a little better weed control than amine formulations at the same application rates, but are more susceptible to vapor drift. Ester formulations generally are compatible for use with fertilizer carriers, while amine formulations often have physical compatibility problems when mixed with liquid fertilizer.

Other herbicides used in the spring on wheat can be applied up to the time the flag leaf is visible, or later. Affinity BroadSpec, Affinity TankMix, Ally Extra SG, Express, Harmony + 2,4-D or MCPA, Harmony Extra, and Supremacy must be applied before the flag leaf is visible. Huskie and WideMatch can be applied through the flag leaf stage. Herbicides that can be applied later in the spring – prior to the boot stage -- include Ally + 2,4-D, Amber, Finesse, Starane Ultra, and Starane Plus Salvo.

Potential for summer annual weeds in wheat

Winter annual weeds are not the only concern where wheat stands are thin and the wheat is very late developing. Early-germinating summer annual weeds such as kochia, Russian thistle, and wild buckwheat may also become a problem, especially at harvest time. Many of these weeds may be controlled by residual herbicides applied earlier in the season. If not, postemergence treatments should be applied soon after weed emergence and before the wheat gets too large in order to get good spray coverage and achieve the best results.

Dicamba is one of the most effective herbicides for kochia control, but as mentioned earlier, if the wheat is starting to joint, it shouldn’t be applied. At that point, Starane Ultra or other herbicides containing fluroxypyr would be a safer option and could still provide good kochia control. Most other broadleaf herbicides in wheat can be sprayed from the time that wheat starts tillering until the early jointing stages of growth, but the label should always be consulted to confirm the recommended treatment stages before application.

Source : ksu.edu