Across much of Kansas — due to multiple rain delays at peak planting time — the wheat crop was sown late and then soil moisture was used up quickly by the seeded wheat. That resulted in less than ideal fall growth. Wheat that has fewer tillers and leaves will be more susceptible to potential winter kill, which unfortunately is the situation for the majority of the Kansas wheat crop.
At this time, it is difficult to truly assess the extent of potential damage. In any case, we only will be able to assess the true extent of damage at spring greenup. Growers might be hesitant to put more money into a questionable wheat crop. If a fall applied herbicide was not put down for controlling winter annual broadleaf weeds. That decision will have to be made this spring. Oftentimes, I have seen and heard growers say that when they decided not to apply a herbicide on their wheat fields, later they wished they had — due to having more weed pressure than expected.
There are several herbicide options for controlling winter annual broadleaf weeds in wheat. Generally, fall applications will provide the best control of winter annual weeds with any herbicide, as long as the weeds have emerged. The majority of winter annual weeds usually will emerge in the fall, although you still can have some emergence in the spring, especially if precipitation after planting is limited in the fall. However, winter annual weeds that emerge in the spring often are not competitive with the crop, at least in years when there is a decent crop.
Some herbicides can work well even when applied during the dormant part of the season, while others perform best if the crop and weeds are actively growing. The key difference relates to the degree of soil activity provided by the herbicide. Herbicides that have good residual activity, such as Glean, Finesse, Amber and Rave, generally can be applied in January and February when plants aren’t actively growing and still provide good weed control, assuming you have proper conditions for the application. Most other herbicides, which depend more on foliar uptake, will not work nearly as well during the mid-winter months, when the wheat and weeds aren’t actively growing, as compared to a fall or early spring application. This might be especially true this year due to the colder temperatures and dieback of foliage this winter.
Spring herbicide applications can be effective for winter annual broadleaf weed control as well, but timing and weather conditions are critical to achieve good control. 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.
Spring-germinating summer annual weeds often are not a serious problem for a good healthy stand of wheat coming out of the winter. However, if wheat stands are thin and the wheat is late developing, early germinating summer annual weeds such as kochia, Russian thistle, and wild buckwheat might be a problem, especially at harvest time. Many of these weeds can be controlled by residual herbicides applied earlier in the season. If not, post-emergence 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.
Another important consideration with herbicide application timing is crop tolerance at different application timings. For example, 2,4-D should not be applied in the fall or until wheat is fully tillered in the spring. On the other hand, any herbicide containing dicamba can be applied after wheat has two leaves, but should not be applied once the wheat gets close to jointing in the spring. Herbicides containing dicamba include Banvel, Clarity, Rave, Pulsar, Agility SG and several generic dicamba products. Dicamba is one of the most effective herbicides for kochia control, but 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 still could 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.
The best advice regarding crop safety with herbicide-fertilizer combinations and application timing is to follow the label guidelines. We generally see minimal crop injury and no yield loss from topdress fertilizer/residual herbicide applications during the winter months. However, these combinations often can cause considerable burn to the wheat if applied when the crop is actively growing and with warmer weather. The foliar burn is generally temporary in nature, and the wheat usually will recover if good growing conditions persist. But the risk of serious injury increases after wheat starts to joint.
• Information provided by Dallas Peterson, Extension weed management specialist.
Stacy Campbell is a Kansas State Research and Extension agent in Hays for the Cottonwood Extension District Office.