One Shot May Not Get Them All
As the presence of glyphosate-resistant Palmer exploded in North Carolina fields this year, some growers who thought they were doing the right thing by incorporating a pre-emergence residual herbicide still managed to find themselves in a predicament when Palmer plants started peaking through the top of their crop. One North Carolina consultant contributes this to a canopy effect that takes place when a weed flush occurs. Because all weeds don't germinate at the same time, within a flush, some weeds are taller than others when it comes time to make a pre-emergence application. At first glance, a producer may assume the herbicide killed all the weeds because he/she sees the herbicide working on the taller ones. However, what some don't realize and don't scout for are those smaller weeds that survive because they miss out on the herbicide thanks to the protection of their taller siblings. Those protected siblings need a second pre-emergence herbicide application to be controlled. This may sound like an expensive strategy, but growers and consultants in North Carolina are finding it's worth the extra expense up-front to keep menacing weeds out of their fields because it saves them money and yield loss in the long run.
As pictured below, taller weeds can act as a canopy and protective barrier against herbicide applications for smaller weeds that have just emerged. Have you seen this effect in your operation?
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