1. Wet weather has delayed POST herbicide applications in both corn and soybeans. This can result in weeds and crops that are larger and more advanced in growth stage than anticipated. The larger crop is primarily a problem in corn, where a more advanced growth stage can start to limit herbicide options. Be sure to check labels and the OH/IN/IL Weed Control Guide for information on maximum crop size and stage for herbicides (Table 8 on page 68 of 2015 edition). Larger weeds may require higher rates or more complex POST herbicide mixtures. Glyphosate and Liberty rates can be increased in Roundup Ready and LibertyLink crops, respectively. Glyphosate usually does not need much help to control large grasses, but the addition of a clethodim product or Fusion to Liberty in LibertyLink soybeans will be required for grasses more than a few inches tall (and always for control of barnyardgrass, yellow foxtail, and crabgrass). Maximum rates of glyphosate can be required for control of large giant ragweed, especially if they have developed some resistance. The addition of fomesafen or Cobra can improve control of glyphosate-resistant giant ragweed populations, and crop oil should be included where these herbicides are used. Various herbicides can be mixed with glyphosate in corn to improve giant ragweed control, including Status, Impact/Armezon, Callisto, Laudis, etc. Increasing spray volume can improve penetration of herbicide into a taller, denser weed and crop canopy, especially for contact herbicides.
2. Consider altering the weed management strategy in fields with late-planted soybeans. While we advocate strongly for the use of residual herbicides in soybeans, the need for full rates of residual premix products applied in late June is debatable. Two issues here: a) some residual herbicide labels specify 10 month or greater interval between application and corn planting next year; and b) we are through the period of peak weed emergence, so that the residual herbicide activity does not have to last as long (assuming that POST herbicides will be applied). In addition, soybeans grow more rapidly when planted in late June compared with early May, so there is less time until a crop canopy develops to help with weed control. Where residual herbicides will be applied, check labels and the Weed Control Guide for information on re-crop intervals to corn or other crops, and consider switching to a less persistent herbicide with shorter re-crop restrictions where appropriate. It’s probably also possible to forgo the residual herbicides in some fields with low weed pressure, and just use POST herbicides. Be aware however that Marestail can emerge into July, and relying on POST herbicides for control of this weed is not a good idea unless Liberty can be used.
3. Can I plant soybeans in fields where corn has failed when I have used pre-emergence herbicide X, Y. and/or Z? There’s a very straightforward answer to this question, and then a more complicated answer with less certainty requiring some knowledge not found on herbicide labels. Labels for corn herbicides provide re-crop intervals that should occur between application of corn herbicides and soybean planting next year, and these typically range from 6 to 10 months for any residual corn herbicides that are not also used in soybeans. Based on this, it’s not possible to plant soybeans anytime the same season where an atrazine premix has been used, and also Lexar/Lumax, Acuron, Halex GT, Balance, Corvus, or SureStart/Tripleflex among others. The grass herbicide components (metolachlor, acetochlor, dimethenamid, pyroxasulfone) of these products are not the problem since they are used in soybeans anyway. It’s the atrazine, mesotrione, isoxaflutole, and clopyralid components that result in re-crop intervals of 6 months or more. One of the options here always is to plant a preliminary test strip of soybeans in a field, and see how it looks after a few weeks, before planting the entire field (i.e. field bioassay). This approach can be impractical this late in the season, when planting as soon as possible can maximize soybean yield potential. Aside from this, we usually advise contacting a manufacturer representative directly or through a dealer to get their assessment of the situation. There may not be a concrete answer provided, but this can result in some information along the lines of “we think there is a good shot the soybeans might make it” or “absolutely do not do it”. The only one of these herbicides that we can provide any advice on really with regard to soybean planting the same season is atrazine, since it’s been around so long and subject to almost every situation possible. It is possible to sample soil and have it tested for atrazine levels, and the resulting numbers can provide some guidance on replanting (page 12 of the current Weed Control Guide). Beyond this, our experience based on feedback from growers and advisors over the past couple decades is that when 6 to 8 weeks and a lot of rainfall has occurred since atrazine application, there is a reasonably good chance that soybeans can survive and grow. However, this doesn’t mean there won’t be injury or reduced yield potential, and there isn’t any way to know really without a field bioassay and/or the lab analysis of soil.