Mark VanGessel, Extension Weed Specialist; firstname.lastname@example.org
As we approach fall and planting wheat, it’s time to consider options for weed control. A few species have become more troublesome, and I attribute a lot of this to the way we have been approaching weed control. Most herbicides go out in the spring with nitrogen. While this might be acceptable for many fields, it is often not the most effective application timing. When splitting nitrogen applications in the spring, neither one are a great timing for herbicide application. The first nitrogen application is applied when the wheat is beginning to “green-up” after the winter, so this is before the wheat is actively growing (also means that the weeds are not actively growing). The second nitrogen application is after tillering and just before stem elongation (means weeds have grown considerably in the spring from first application of nitrogen and much taller than 3”, and wheat growth prevents good coverage; both reducing the effectiveness of the spray). This is contrary to all recommendations which are: spray small weeds that are actively growing and achieve good coverage.
UD Weed Science Program has been looking at herbicide applications to winter wheat in the fall. Typically these treatments are going out in middle to late November. Some of the reasons this approach have been successful are:
1. Weeds are more susceptible in the fall. As I pointed out they are smaller and actively growing. Furthermore, fields with heavy poultry litter applications or excess nitrogen tend to have lots of weed growth in the fall and this contributes to even worse control in the spring.
2. Fall applications match better with weed development. As noted weeds are smaller the earlier they are sprayed. Furthermore, most annual species will progress to flowering early in the spring which can reduce herbicide performance as well.
3. Weed emergence is primarily in the fall. Some emergence occurs in the spring, but most occurs in the early fall. Fall emerging weeds are more detrimental to yield than spring emerging weeds. And, if there is significant spring emergence, then a herbicide treatment with the second split of nitrogen will be beneficial for these small seedlings.
4. Fall herbicide applications are not influenced by temperature as much as spring applications. It is usually late into March that we start to get consistent temperatures above 55°F. In the fall, heavy frosts have occurred by mid November, but soil temperatures allow for active plant growth during the days. Therefore, we have better environmental conditions for control in the fall than early spring.
5. Coverage is better with fall applications. See notes above
6. Spreads out the workload. And if all the best planned strategies do not get implemented in the fall, you have a back-up plan for treating in the spring (albeit not as effective as fall).
I realize some spring herbicide applications are necessary such as: wild garlic control (but Harmony Extra can be applied with the second nitrogen applications); other perennial species that emerge in the spring (Canada thistle or bulbous oatgrass); or late planted fields when the crop may not reach the stage for safe application. But fields with a history of poor weed control, should be targeted with fall applications.
Next week will be a review of available herbicides for fall applications.