Problems caused by unfavorable weather and related conditions during the 2019 planting and growing seasons have resulted in a greater-than-normal number of weed escapes in corn and soybean fields across Iowa. These weeds may reduce crop yields, and they definitely will contribute to future weed problems, as they are producing new seed.
While it is too late now to protect crop yields, a common question is whether herbicides can be used late in the growing season to reduce the quantity of viable weed seed produced.
While there is no simple answer due to the many different scenarios across the state, in most situations late-season applications of herbicide are not warranted.
The potential to limit weed seed production is affected by two main factors:
- susceptibility of the weed to the herbicide
- stage of seed development at time of application
It’s important to recognize that herbicides that are effective early in the season will be much less, if at all, effective on the mature weeds in fields in late August.
Impact on seed production varies
Waterhemp is unlikely to be killed by any labeled herbicides at this time of the year. While other species may be killed with herbicides, the impact on seed production will be highly variable.
Research in the early 1980s looked at the effect of late-season 2,4-D applications in corn on seed production by velvetleaf and cocklebur (the 1980s equivalent of today’s waterhemp infestations). While 2,4-D applications made at the brown-silk stage of corn growth were able to kill both of these weed species, the treatments were much more effective at preventing seed production by cocklebur than velvetleaf.
Much of the velvetleaf seed had filled by the time of the 2,4-D application, whereas cocklebur was still in early flowering stage. Seeds that had filled prior to 2,4-D application retained their viability even if the parent plant was killed prematurely. Before committing to any late-season treatments, you need to examine weeds to determine stage of seed development. Both waterhemp and giant ragweed present in fields in central Iowa this year had fully developed seed during the week of Aug. 26.
Reduce weed seed production
The 2,4-D label for late-season applications was changed in the 1980s from after brown-silk to after the dent stage of corn, and this change will reduce the effectiveness of the treatments since it provides more time for weed seeds to mature.
Reducing weed seed production is essential to minimize the seed bank. By reducing the size of the seed bank, future weed management will be simplified and the risk of new herbicide-resistant biotypes will be reduced. However, late-season herbicide applications are unlikely to provide significant benefits for most fields.
Hand pulling weeds is an alternative, but this late in the growing season, the plants would need to be removed from the field. In situations where a patch of a weed is present that is suspected to possess a new resistant trait for that field, removal of the weeds probably would be worth the effort.
Rather than spending money on a questionable treatment, spend time determining why this year’s program failed and develop an effective weed management plan for 2020.
Hartzler is an ISU Extension weed management specialist. Contact him at firstname.lastname@example.org.