Canada Farms.com, 09-17-08 Fall Control of Winter Annuals Possible OMAHA (DTN) -- The combination of minimum and no-till practices, reduced use of residual soil herbicide and overdependence on glyphosate has created the ideal environment for winter annual weeds. While these weeds can be a significant problem in no-till or direct seeding systems, they can be effectively and inexpensively controlled if caught early enough. Winter annuals germinate in the fall, overwinter as small rosettes, then bolt the following spring. The overwintering rosette hugs the ground, which means the snow insulates it from cold temperatures and drying winds. The rapid development of the flowering shoot (bolting) in the spring gives it a competitive advantage over planted crops and makes spring control problematic. In the past, repeat tillage passes and an application of a residual soil herbicide kept winter annuals under control, but that isn't the case any more. Now farmers have to make an extra effort to control them in the fall or very early spring. Fall control of perennials, such as dandelions, is more effective than spring control. In the fall, food reserves in these perennials are moving to the roots. When a systemic herbicide is applied, that herbicide moves with the food reserves to the roots. But fall control does come with some potential problems. "The earlier you apply herbicide, the sooner it runs out and that limits your options next spring. To optimize herbicide use, it has to coincide with crop growth and that means applying in the spring," said Mike Owen, weed scientist at Iowa State University. "A fall-applied herbicide locks a grower into a specific crop next year and if the weather throws a curve ball, that might limit options next spring," he said. For example, if a sulfonylurea herbicide is applied in the fall, it may limit the grower to planting soybeans next spring. However, there are growth such regulators, as 2,4-D or dicamba, that have activity but do not lock a grower into a single crop. While Owen believes it is more effective in the long run to apply herbicides in the spring, he acknowledges that it might be more convenient for growers to apply in the fall and control might be better, especially on winter perennials like dandelions.
For other weeds, Owen said spring application might more convenient as long as it's done before the weeds get too tall. If a grower does decide to go with a control program in the fall, he has three options available: First, apply herbicides with soil-residual activity before most of the winter annual weed species germinate. Second, apply a non-residual herbicide, such as glyphosate, gramoxone or 2,4-D, to emerged winter annual weeds while they are still small or in the rosette stage. Third, use a combination of the first two methods. ALS (acetolactate synthase) and PPO (protoporphyrinogen oxidase) inhibitor herbicides provide residual control while growth regulator herbicides such as 2,4D and dicamba are non-residual. Glyphosate and gramoxone are also effective, but are not residual. Before ...