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Addressing early weed management issues

By Mark Loux, Ohio State University Extension herbicide specialist

The warm and relatively dry weather has many growers chomping at the bit to apply herbicides now, whereas this might have been more likely to occur in several weeks in a more typical spring weather pattern. It probably makes sense to apply burndown herbicides now in many no-till fields based on the size of the weeds that are present.

Vegetation in no-till fields is larger than usual for this time of the year due to the warm weather, and lack of fall herbicide application in all but a few fields. Waiting several weeks to apply herbicides will only result in an even more challenging burndown situation. It can make sense to apply burndown herbicides now even where a field will be tilled later, in fields where there is enough vegetation to interfere with spring seedbed preparation.

This is probably the ideal situation for the use of the higher rates of 2,4-D ester in burndown treatments. We have been encouraging use of the higher rates to improve control of marestail, which is fairly advanced in rosette size in some fields already. A reminder that any 2,4-D ester product can be applied 7 days before planting at rates up to 0.5 pound active ingredient per acre. There are several products that can be applied 15 days before planting at rates up to 1 pound of active ingredient per acre (e.g. Weedone 650, E99, Salvo), but otherwise the higher rates require 30 days.

Treating fields now with the 1-pound rate of 2,4-D ester (with glyphosate or other burndown herbicides), with the intention of planting in two weeks or so, can improve control of larger, older weeds. As we move closer to planting, or in especially weedy fields, the addition of Sharpen to glyphosate/2,4-D mixtures can improve control. The addition of products containing chlorimuron (Canopy/Cloak, Valor XLT, Envive, Authority XL) can also improve burndown of some no-till weeds.

One caution about burndown herbicides, especially in a year when weeds are more numerous and larger than usual. We have previously mentioned the alternatives to glyphosate/2,4-D for burndown of marestail, which include Liberty/metribuzin and glyphosate/Sharpen. While the latter two mixtures are effective for burndown of marestail and a number of other weeds, they can struggle some in a situation where the full complement of large no-till weeds is present (they typically work well in fields that have received a fall herbicide application). Glyphosate/2,4-D still has the lowest cost and most utility across a range of no-till burndown situations, and switching to a mixture that does not include 2,4-D should generally occur only where necessary and where the weed spectrum fits the alternative mixtures. Optimum activity from mixtures that contain Sharpen, Liberty or Gramoxone will occur with spray volumes of at least 15 gallons per acre and the appropriate nozzles.

A primary concern of many recent callers about our current situation is whether it is too early to apply residual herbicides. They are undoubtedly aware that applying residual herbicides too early can reduce their effectiveness in late May and early June when the later flushes of marestail and waterhemp are emerging. Applying residual herbicides this early does introduce an element of variability in their activity later in spring, but this depends somewhat on the progress of soybean planting and crop development. The ideal scenario would be that soybeans are planted within the next couple of weeks and conditions are favorable for rapid soybean development and formation of a crop canopy. As a result, the crop is capable of providing control when the residual herbicide activity runs out. The risk of applying this early is that subsequent wet and/or cold weather will prevent timely soybean planting, which ultimately stretches out the time until crop canopy development. In this situation, we are likely to see breaks in residual herbicide activity prior to crop canopy, but the control that does occur is certainly much better than not using residual herbicides at all. It’s a little difficult to predict late-season control of marestail, and we sometimes observe effective residual marestail control regardless of the timing of the residual herbicide application. Here are a couple of suggestions relative to the early application of residual herbicides:

1. Increase residual herbicide rates. We have been suggesting increasing residual herbicide rates above the fairly low “Roundup Ready” rates to improve residual marestail control, and this is especially relevant for early application. It’s probably feasible to increase rates by 20% to 30% above the low rates for many residual products, or use the higher rates within the rate range for a given soil type (either way, do not exceed the maximum rate for the soil type).

2. Split the residual herbicide. Apply some residual herbicide now with the burndown treatment and the rest when the crop is finally planted. The advantage of this approach is that it places some of the residual closer to the time of late-spring weed emergence, and also compensates for late soybean planting. Where soybean planting is delayed, application of the 2nd half of the residual herbicide can also be delayed, and applied when it is most needed.

There are various ways to split the residual herbicide, but something as basic as half the total amount applied early followed by the rest at planting would work. Or mix up the residual herbicides, applying one type of residual early and switching to another later. For products that are relatively short-lived in soil, such as Valor, Authority and metribuzin, the majority of the residual is best applied close to the time of planting.

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