Don’t Neglect Fall Weed Management
Josh Copes, Daniel Stephenson, Donnie Miller, and Lauren Lazaro
Trends in earlier crop harvest has resulted in adequate time for weeds to set seed between harvest and a killing frost. This time period can range from one to four months. The average first frost date in North and Central Louisiana is November 15 and 19, respectively. Since a lot of money and effort is spent in controlling weeds during the growing season to negate yield loss, timely weed control practices following harvest is important. The objective of post-harvest weed management is to reduce viable 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. Some common weeds infesting fields after harvest include barnyardgrass, morningglory species, prickly sida/teaweed, browntop millet, Palmer amaranth, and waterhemp. Special attention should be made to ditch banks and other non-cropland areas infested with Palmer amaranth and/or waterhemp, since their seed is easily spread in water.
For weeds 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. Very little time will be required for these weeds to set a substantial amount of seed. 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.
In a stale-seedbed production system, herbicide applications should be targeted from late-September through October when the time period from application to first killing frost is shortened. In minimum tillage systems, or where weeds emerge after field prep operations, herbicides should be applied before or shortly after flowering. This implies that weeds will be large and more difficult to control, and therefore water volume should be maximized to ensure good weed coverage, as this is critical for good weed control. Multiple post-harvest herbicide applications for control of summer annual weeds should be avoided, so as to minimize herbicide selection pressure that can lead to herbicide-resistance. Utilizing multiple effective modes of action will help minimize selection pressure, e.g. 2,4-D plus glyphosate or glufosinate plus 2,4-D etc. Herbicide choice should depend on weed species present in the field. Some soil residual herbicides 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. Do not expect winter long weed control from soil residual herbicides applied from August to early October. Likewise, the lack of rainfall to properly activate residual herbicides can negatively impact treatment effectiveness.
Fall herbicide applications can be effective for control of perennial weed species such as johnsongrass, bermudagrass, alligatorweed, and redvine. Johnsongrass escapes are becoming more apparent across the state. Studies conducted by LSU AgCenter weed scientists have determined that fall applications should be made from September 15 to October 15 when environmental conditions favor weed growth (http://www.lsuagcenter.com/portals/communications/publications/agmag/archive/2006/summer/longterm-management-of-perennial-weeds-starts-in-the-fall). 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.
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. Budgets are typically tight in the fall and spending additional money on weed control when no crops are in the field is difficult, but by identifying fields in need of post-harvest weed management and by implementing field prep in a timely, well-spaced manner can go a long way in reducing future weed numbers in your fields. Below are a list of herbicides labeled for use following main crop harvest and for non-cropland use (ditch banks etc.). Always read and follow label guidelines and restrictions.
If you have any questions please contact us or your local county agent.
Herbicides Labeled for Post-harvest Weed Control
Glufosinate – Liberty 280 SL
Glyphosate – Roundup PowerMax
Gramoxone 2 SL
2,4-D LV4 and 2,4-D Amine
Dual II Magnum – Italian Ryegrass (Sept.1 to Dec. 1)
Herbicides Labeled for Non-Cropland Areas/Farmstead Use