Application Timing and Plant Size
We have heard often about the strategy of waiting until soybeans are about to canopy to apply post herbicides. The principle here appears to be that the herbicide will control emerged weeds and then the shade from the crop canopy will prevent later-emerging weeds from competing with the soybeans. This can work where the residual herbicides applied prior to planting actually control most of the weeds up to this point, so that the post treatment is primarily addressing a few small weeds. However, in fields where the residual herbicides are not this effective, or where certain tougher weeds are present (giant ragweed, waterhemp, etc.), applying post herbicides just prior to canopy closure is really too late. No-till soybeans may require eight weeks to canopy, and delaying POST applications until this late inevitably means that at least some of the weeds are large. It’s almost always a more effective strategy in the end to make an earlier POST application to smaller weeds, and then a second application three weeks later if needed. Keep in mind also that as soybean planting is delayed later into the season, the first post application occurs later in the overall weed emergence cycle, so the likelihood of needing a second post application is reduced.
When soybeans are planted later in the season, the progression through growth stages occurs during a shorter period of time. Flowering can occur when plants are relatively small, and delaying post applications until plants are a certain size may result in treatment of plants at a more advanced growth stage compared with early planting. Glyphosate labels specify that the herbicide can be applied through the R2 stage. The R3 stage starts when a pod of at least 3/16 in. long appears at one of the four uppermost nodes with a fully developed leaf on the main stem. Applying after the R2 stage increases the risk of injury to soybeans. Injury is not necessarily likely for late applications, but can range from transient “yellow flash” to abortion of pods on the upper part of the plant. Labels for other herbicides may also specify application prior to flowering. Late application of some post herbicides with substantial residual in soil, such as Flexstar, can also increase the risk of carryover, especially if dry weather occurs afterward.
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