At this point there should be little concern about a lack of rain to “activate” preemergence herbicides where it was possible to get them applied. We have had some challenges with wind and water and it’s another one of those years when a lot of the corn planted within the past several weeks could emerge before there is opportunity to apply preemergence herbicides and 28%. Most preemergence corn herbicides can be applied to emerged corn, and some of them have enough foliar activity to control small, emerged weeds without the need to add postemergence herbicides. However, the majority of the corn hybrids are resistant to glyphosate and/or glufosinate (Liberty), and these can be combined with preemergence herbicides for additional control. It’s also possible to mix in other POST herbicides such as Impact, dicamba, 2,4-D, Capreno, etc to control emerged weeds, instead of glyphosate or Liberty. Some issues to be aware of with regard to POST application of preemergence corn herbicides follow.
- Only a few herbicides can be applied using 28% as the spray carrier. These are Degree, Degree Xtra, and Bullet. Degree Xtra and Degree can be applied in 28% on corn up to 6 inches tall, when air temperatures are less than 85 F. Bullet can be applied up to 5-inch corn with the same temperature restriction. Expect some leaf burn from these mixtures. All other herbicides must be applied using water as the spray carrier.
- Usually a maximum corn size is specified, which can be based on growth stage or corn height. This can be as small as the V2 stage for some herbicides, such as Corvus and Balance Flex.
- Most premixes or tank mixtures that contain atrazine can adequately control small broadleaf weeds, and especially those that contain another broadleaf herbicide (e.g. Lexar, Lumax, Instigate, SureStart). Grasses more than about an inch tall will require the addition of glyphosate, Liberty, or other herbicide with effective grass activity.
- Follow adjuvant recommendations closely to minimize the risk of injury and do not assume that it is always possible to use an adjuvant once corn has emerged.
- Based on our research with this type of approach to herbicide management, it can be more effective to wait to apply until the first flush of weeds has emerged before applying, compared to applying just before the weeds start to emerge. The latter timing can result in weeds emerging before the field receives enough rain to move herbicides down to the shoots that are just beneath the surface. Herbicides should be applied when weeds are less than about two inches tall to ensure that risk of yield loss from early weed interference is minimized.
- Further information can be found on herbicide labels and also in the herbicide descriptions in the “Weed Control Guide for Ohio and Indiana”. However, we find that labels can lack enough information on adjuvants and tank-mix partners for this situation. Be sure to check with dealers and manufacturer/distributor representatives to get specific information when necessary.
There is just not as much flexibility when it comes to POST application of the preeemergence soybean herbicides that are most prevalent in Ohio. Anything that contains metribuzin, flumioxazin (Valor), and sulfentrazone (Authority) cannot be applied to emerged soybeans. Herbicides with substantial residual activity that can be applied early POST: FirstRate, metolachor products, Outlook, Warrant, Prefix/Vise (fomesafen + metolachlor), Pummel (metolachlor + Pursuit), Pursuit, Scepter, Torment (fomesafen + Pursuit), and Zidua. There are also some premixes of glyphosate with residual herbicides that can be applied POST, including Sequence (metolachlor + glyphosate) and Extreme/Tackle/ThunderMaster (Pursuit + glyphosate). All of these herbicides would typically have to be applied with glyphosate in an early POST weed control situation. Some of these are commonly being applied POST in the south already as part of the “overlapping residual” strategy for management of Palmer amaranth, but they can lack residual activity on giant ragweed and marestail, which are still our main problems here. Check the ratings of residual herbicides in the current version of the “Weed Control Guide for Ohio and Indiana” when trying to make a choice for this situation.
Source : osu.edu