By Jeffrey L. Gunsolus, Extension Agronomist - Weed Science
The wet weather pattern this spring and early summer has left a significant number of acres, especially in southeastern MN, unplanted. Current estimates in southeastern MN project 30% of the tillable acres have not been planted and on many of these acres weeds such as giant ragweed, common lambsquarters and waterhemp are thriving.
Although weeds are beneficial from an erosion control perspective their rapid growth will make seedbed preparation for planting cover crops very difficult and weed seed production potential will challenge even the best weed management tactics available in 2014.
Fields that have not been tilled this spring now have weeds that have been growing without crop competition and currently are several feet tall and growing rapidly. Therefore, weed management tactics must be implemented very soon. Fortunately our current string of dry and sunny days does provide a window of opportunity to manage some of the worst fields.
Based on weed size and rapid growing conditions I do not see herbicides as being a viable management option. Broad-spectrum herbicides such as glyphosate, glufosinate, and paraquat are logical choices but due to weed size, effective control is not likely due to incomplete coverage.
Tillage with a disk or field cultivator will also lose effectiveness as weed size increases; however, the disk is likely to perform better than a field cultivator for smaller weeds. At this stage in the growing season mowing or chopping the larger weeds appears to be the most effective recommendation because it will destroy the most plant biomass and it will not expose the soil to wind and water erosion. In some particularly weedy fields, if a cover crop is desired, mowing or chopping will still be necessary before seedbed preparation can begin.
If left untended and without crop competition giant ragweed can produce approximately 10,000; common waterhemp 70,000; and waterhemp 100,000 seeds, or more, per plant. Such large additions to the weed seed bank make next years weed management tactics less effective because as weed density increases herbicide effectiveness decreases.
Seed dormancy also contributes to long-term weed management problems. The estimated time to reduce the weed seed bank by 50% is 12 years for common lambsquarters and 3 years for common waterhemp. Giant ragweed populations tend to decline more rapidly, with estimates of 99% reduction within 2 years if seed is left near the soil surface. A confounding factor to consider is that many giant ragweed and waterhemp populations are likely resistant to glyphosate and/or ALS herbicides.
Attempting to inhibit weed seed viability by applications of 2,4-D or other systemic growth regulators is not recommended because the risks of off-target movement due to volatilization or drift far exceed their effectiveness in inhibiting seed viability. The extended flowering period and rapid seed maturation of weed seed would imply that multiple treatments would be necessary and at best only a small percentage of the seed would be affected.
This is a difficult weed management situation during a difficult growing season, however, action now will pay dividends in the years to follow. It is not realistic to think that all fields in need of weed control will receive treatment. It would be wise to focus your attention on the fields that contain weeds that will be the most difficult to control in next year's crop and have the highest weed densities. Mapping of field areas that you anticipate to be particularly challenging next year is strongly encouraged.