Post-harvest Weed Control08/04/2016
Assessment of weed control programs.
With corn harvest underway and soybean and cotton fields approaching maturity, this is a great time to evaluate this year’s weed control programs. Things to consider include: what herbicides were applied, when they were applied in respect to crop and weed growth stages, what were weather conditions like before and after application, and what weed species are present after final weed control efforts. In addition, knowing which fields contain glyphosate-resistant weeds and other difficult to control species that escaped control can help us better plan and budget for more effective herbicide programs. These factors will help critically evaluate weed control programs and may offer insights into becoming more effective at herbicide selection, improving application timing, and how environmental conditions may dictate the need for more aggressive weed control tactics in certain fields.
Post-harvest weed control.
The time period from corn harvest and the first killing frost can range from 1 to 4 months. The average first frost date in North and Central Louisiana is November 15 and 25, respectively. A lot of money and effort is spent in controlling weeds during the growing season to negate yield loss. With the extended window from harvest to first frost, weeds will continue to emerge and produce seed. Timely weed control practices following harvest (post-harvest weed control) can reduce weed seed return to the soil, thus ensuring fewer weeds to fight in future cropping seasons. Post-harvest weed control is especially important in fields containing herbicide resistant weeds. A good example to illustrate the importance of post-harvest weed management is the ability of glyphosate-resistant Palmer amaranth to produce mature seed in as little as 30 days after emergence during late summer and early fall. Many other grass and broadleaf weeds are capable of setting viable seed in a similar time frame.
For weeds that are present in the field at harvest time, mowing and/or tillage should be conducted as soon as possible upon harvest to ensure viable seed set is eliminated or reduced. Rainfall will influence subsequent germination of weed seed and therefore the need for additional weed control. Furthermore, rainfall following cultivation could increase weed seed germination, however, if the weeds are controlled the soil seedbank would be reduced.
Other methods of weed control include the use of herbicides. Herbicide applications should be targeted from late-September through October when the time period from application to first killing frost is shortened. Multiple herbicide applications for post-harvest control of summer annual weeds should be avoided. Residual herbicides such as S-metolachlor, pyroxasulfone, linuron, and diuron, among others, can be applied in the fall following harvest. However, rotation interval restrictions must be followed and length of residual control will be influenced by soil temperature and saturation. Glyphosate plus 2,4-D and/or dicamba or paraquat plus diuron and/or linuron are some choices for late-fall post-harvest applications. Diuron and linuron will offer soil residual; however, if soil temperatures are warm and rainfall frequent, do not expect long residual from these products. Likewise the lack of rainfall to properly activate residual herbicides to minimize weed germination can negatively impact treatment effectiveness. Maximize water volume to ensure good weed coverage as this is critical for good weed control, especially for paraquat plus diuron and/or linuron.
To reiterate, weeds are capable of setting viable seed within 30 days after emergence during late summer and early fall. Post-harvest weed control is especially important when combatting glyphosate-resistant weeds such as Palmer amaranth, waterhemp, or johnsongrass. Problem fields should be identified and receive top priority for preventing seed return. Once harvested these problem fields should be mowed or tilled shortly after harvest to prevent and/or reduce seed set. Fields should then be regularly scouted for emerging weeds and additional control tactics applied prior to seed set. This will require close inspection of weed species to determine when they are flowering. Once a weed species is observed flowering a weed control operation should be implemented. Depending on weather conditions following harvest, weed control tactics may need to be implemented approximately every 3 to 4 weeks until a killing frost has occurred. If glyphosate-resistant Palmer amaranth or waterhemp is an issue, a management tactic (i.e. mowing, tillage, herbicide application) should be done every 3 to 4 weeks.
Source: LSU Ag Center