Prolonged rains, coupled with high August temperatures, were optimal for weed growth in late summer and into this fall. People have asked about control options for weeds post-harvest.
Earlier harvest trends have resulted in adequate time for weeds to set seed between harvest and a killing frost. This time period can range from 1 to 4 months. The average first frost date in North and Central Louisiana is November 15 and 25, respectively.
Since a lot of money and effort goes into controlling weeds during the growing season to negate yield loss, timely weed control practices following harvest is important. These practices can reduce weed seed return to the soil seedbank, 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 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. Producers in no-till systems will have to rely on mowing and herbicides to prevent weed seed production.
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, some 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 to the soil seedbank. 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 employed every 3 to 4 weeks.
Fall herbicide applications can be made for control of perennial weed species such as johnsongrass, bermudagrass, alligatorweed, and redvine. Studies conducted by LSU AgCenter weed scientist have determined that fall applications should be made from September 15 to October 15 when environmental conditions favor weed growth (here).
For johnsongrass, bermudagrass and alligatorweed control, 1.0 lb ai/acre of glyphosate should be applied. Two lb ai/acre of glyphosate or dicamba are effective control options for redvine. Glyphosate (2.0 lb ai/acre) plus dicamba (1.0 lb ai/acre) can also be an effective control option. Fields should be scouted the fall following herbicide application to determine whether an additional application is needed. Do not mow or till fields for several weeks following herbicide application.