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Residual herbicide injury: Another challenge to this year’s soybean crop
Author: Bob Scott, Professor & Extension Weed Scientist

Almost as soon as soybeans started coming up I starting getting calls on herbicide injury.  At first, there were some standard calls on injury from Valor or Valor-containing products such as Envive and Fierce.   Then a few calls came in on the new BASF products Sharpen and Verdict.  It is important to mention before I go any further that these fields are the exception and not the rule; for every one injury field I saw or was contacted about, there are probably 100 fields that are not hurt or even more.  Most of the Valor and Sharpen injury has occurred when a rain is received just as soybeans are emerging, which is good for activation, but can result in injury if the product is “splashed up” in the emerging soybean terminals.  Injury can range from moderate to severe, and the symptoms are short plants with yellow necrotic terminals.  In addition to the rain, another common factor for herbicide injury has been application date relative to planting date.  When we do these trials in the research plots, generally we plant and then immediately apply the PREs.  What I am hearing on most of these fields is that the PRE treatment is often delayed until just before soybean emergence, which may explain why I rarely see injury in my research plots.

The result of unnecessarily mixing Sharpen and Valor-stunted soybean seedling

Fig. 1 – The result of unnecessarily mixing Sharpen and Valor

Figure 1 is of a field that received an application of both Sharpen and Valor—I would consider this “overkill”.  It would be like mixing Prowl and Treflan, which both do pretty much the same thing in this scenario, so why use both?  In addition to the herbicide or perhaps preceding the herbicide, these plants appear to have wind scaring and the soil is pretty light.  These wind scars are a good way for the herbicide to get in and cause problems.

The Sharpen label says to only apply 1 oz Sharpen PRE or 5 oz of Verdict (1 oz sharpen + 4 oz Outlook) PRE per acre on soybean.  On lighter soils and at higher rates there is a plant back interval to soybean.

LeadOff herbicide is a combination of rimsulfuron (Resolve) and thifensulfuron (Harmony), and it is currently labeled for use as a burndown herbicide in the spring for soybean.  A few fields have turned up this year with LeadOff carryover following these burndown applications.  Again, this is just a handful of fields in comparision to the hundreds of thousands of acres this product was used on last year and this year.

The current LeadOff label states that soybeans or cotton can be planted 30 days following a 1.5 oz/A application.  If the rate goes over 1.5 oz/A, the rotation interval jumps to 60 days.  There was quite a bit of LeadOff used this year in burndown programs.  It provides excellent burndown and residual control of many spring and winter weeds including non-ALS resistant ryegrass and is very economical.

Several soybean fields have turned up with ALS carry-over symptoms following LeadOff applications this year.  These symptoms include: stunting, purple veins on the backs of leaves, bottle brush roots, and yellow terminals.   Some injury symptoms are shown in the picture below (Figure 2).

Fig. 2 - Symptoms of LeadOff injury

Fig. 2 – Symptoms of LeadOff injury

Some of these fields have been associated with planting back too soon and higher application rates than labeled; however, a few have been within the label and may be the result of high pH, wet or waterlogged soils and/or salt.  In addition, one field was bedded after application which may have concentrated the herbicide on one side of the bed.  So far, STS beans have not been affected; however, the current label states that the rotation to STS beans is also 30 days.  Dupont is investigating these issues and will possibly change the label in 2014.  I know we have a lot of soybeans left to plant in the state so if you are in doubt about your LeadOff application I suggest planting an STS soybean.

In addition to these herbicides we have seen the normal amount of injury from other products such as the contact burning type of injury associated with Prefix, Flexstar and Blazer applied early POST.  One way to mitigate this injury is to make applications when beans are small, do not load up the tank with too many “hot” adjuvants, and avoid adding adjuvants other than 1% v/v of a standard COC or 0.25% v/v NIS unless the label calls for it.  Remember Dual is a pretty good adjuvant and most Glyphosates are fully loaded to begin with.  This holds true for Liberty in LL soybean.  We usually add only a COC to Liberty.  If using Dual or Prefix you don’t really need any.  An exception to this would be if you are spraying in early morning or late at night or a cloudy day.  Optimum Liberty activity occurs from 9 am to around 5 pm in bright sunshine.

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