Source: Dallas Peterson, Weed Management Specialist, Kansas State University
Broadleaf weeds can sometimes grow rapidly in wheat fields toward the end of the growing season. When this happens, the weeds can grow above the wheat canopy. This not only interferes with harvest, it can also result in dockage problems. This year, preharvest weed problems are starting to appear in some wheat fields around Kansas, especially in thin fields and areas with abundant rainfall. Any further delays in harvest will likely exacerbate the problem.
What can producers do about this, and are the potential control measures economically feasible? Several herbicides are labeled as preharvest treatments in wheat, but it is critical to only use herbicides labeled for this application and follow all of the application and preharvest intervals specified on each respective label. Application of herbicides not labeled for preharvest application in wheat could result in crop destruction and severe penalties. The options for preharvest weed control in wheat include:
- Aim. Aim is a very quick-acting herbicide and provides a rapid desiccation of green foliage, which is often a primary goal of a preharvest treatment. However, Aim has limited translocation and thus plants may not die, and often will begin to regrow eventually. Apply after wheat is mature, but at least 3 days before harvest.
- Rage D-Tech. Rage D-Tech is a combination of Aim plus 2,4-D. It will have the same attributes as Aim, but should provide more long-term suppression of weeds because of the 2,4-D. Apply after wheat reaches the hard dough stage to control large, actively growing broadleaf weeds. Do not harvest wheat until at least 7 days after application. Do not graze dairy or meat animals for 14 days after application and do not feed treated straw to livestock.
- Dicamba + 2,4-D. Apply when wheat is in the hard dough stage and green color is gone from nodes of the stems. A waiting period of 7-14 days is required before harvest depending on dicamba and 2,4-D product. Do not allow grazing or use feed from treated area.
- Glyphosate + 2,4-D. This treatment will not dry weeds down quickly, but probably will provide the most complete weed control eventually, which may be especially beneficial for control of summer annual grasses and perennial weeds, or if planning to doublecrop after wheat harvest. Apply when wheat is in the hard dough stage and at least 7 days before harvest. Not recommended for wheat being harvested for use as seed. Do not feed treated straw or permit dairy animals or meat animals being finished for slaughter to graze treated grain fields within 2 weeks after treatment.
- Ally + 2,4-D. Apply when wheat is in the dough stage, and at least 10 days before harvest. Weeds under drought stress may not be controlled.
- 2,4-D LVE. Apply when wheat is in the hard dough stage. Weeds under drought stress may not be controlled. Consult individual 2,4-D products for use guidelines and preharvest intervals. Do not use treated straw for livestock feed.
The effectiveness of preharvest treatments will depend on thorough spray coverage and canopy penetration. Follow label guidelines regarding application guidelines and minimum spray volumes. Producers should be especially careful about avoiding spray drift of these products at this time of year. In particular, 2,4-D, dicamba and glyphosate can cause significant off-site injury to many crops and area gardens.
It is very difficult to estimate the value of preharvest weed treatments as it will depend on the differences a treatment would have on harvest efficiency and dockage. It may not pay to use these treatments at lower weed densities unless harvest continues to be delayed. If the weeds are about to set seed, a preharvest treatment can go a long way toward reducing weed problems in future years by preventing seed production.