By Dwight Lingenfelter, Extension Associate, Weed Science
Now is the time of year we start to get questions about the possibility of controlling weeds after silage harvest. The answers depend on what kind of weeds are being targeted.
Dealing with Perennial Weeds
If perennial weeds (such as pokeweed, horsenettle, smooth groundcherry, Canada thistle, quackgrass, Johnsongrass, and dandelion) or biennial weeds (such as common burdock, wild carrot, and bull thistle) are present after silage harvest, then fall can be an excellent time to manage these.
However, make sure that the foliage on the weeds appears relatively healthy and capable of absorbing the herbicide spray. In general, allow several days for the weeds to recover from the harvesting process and be actively growing. Plants that have been damaged by insect feeding, drought, harvest equipment, frost, or autumn leaf senescence/drop are not good candidates for fall applications.
So, if that pokeweed you have been dealing with during season is still standing and the leaves and stems are not too tattered after harvest, then there is still a great chance to control it yet this fall. Make sure to use adequate herbicide rates, high spray volumes, and get good spray coverage over the plant for effective kill. Favorable air temperatures should be a consideration immediately before, during, and after application. In general, the warmer the better, with daytime high temperatures in the mid-50s at a minimum. Cold nights and cool, cloudy days will reduce and slow the effectiveness of the applications. The more active the weeds are growing, the better the herbicide performance.
If annuals are the target, the primary goal right now would be to prevent seed production. Of extreme importance, monitor the status of Palmer amaranth and waterhemp once the crop is removed. If the plants are not cut low enough new shoots can develop at the nodes and produce viable seed yet this fall. Hopefully, no new Palmer germination flushes or much growth will occur in the coming weeks, but it still could happen if the weather stays mild. Palmer plants do not need to reach a few feet tall to develop a seed head. Even if a plant is less than a foot tall it can still set seeds. Consider applications of 2,4-D, dicamba, Gramoxone, or Liberty if these weed species continue growing or start to germinate after harvest.
If harvesting a field infested with Palmer or waterhemp, make sure to thoroughly clean the combine or chopper before moving to the next field or farm. Consider moving to a field that will be planted to corn next season since better herbicide options exist in that crop to control these weeds. If possible, it is best to harvest contaminated fields last to spend time this winter removing components and doing a comprehensive cleaning of the machine. Since the seeds are very small, it is extremely difficult to remove them, but this is necessary to prevent spread.
Regarding other annual weeds, not all species set seed during the same timeframe. For giant foxtail, mature foxtail seeds are usually produced in late August and peak seed rain occurs from late September through the month of October. Yellow foxtail seed rain can start in early August and continue into late October. Pigweed species can begin to produce mature seed by mid-August, while lambsquarters and ragweed generally do not mature until the month of September. (These timeframes can slightly vary depending on location and environmental conditions.)
So, depending on the weed species in question, after silage harvest may be too late. In a fallow setting say after small grain harvest or vegetable production, to prevent seed production, fields can be sprayed with an effective herbicide or mowed once or twice. Glyphosate is particularly effective at stopping grass growth and reproduction. The plant growth regulators (2, 4-D and dicamba) would probably be a better choice for broadleaf weeds. With giant foxtail, even treating the field by mid-September could greatly reduce seed production. If seed heads are present, check suspect fields to determine how advanced flowering and seed rain are and time control practices accordingly. If seeds are already starting to mature, then applying an herbicide would be of no value. Taking the time to prevent seed production this year can make a big difference next year.