residual herbicide in growing cover crop

Asked February 23, 2017, 1:08 PM EST

I have been using Prowl H2O and LV4 to kill early broadleaf weeds in my overwintering cover crop blend ahead of soybeans. These fields are sprayed in April, and I do not want to kill the rye/triticale until near planting in May. My question is whether the residual Prowl (to control lambsquarter, etc.) is effective or if it is metabolized by the covers which often reach 3 feet tall or more before burndown?

Buchanan County Iowa

1 Response

That question comes up often, but I’m not aware of any work looking at herbicide wash off from freshly killed small grains. Back in the 70’s and 80’s there was considerable work looking at the fate of herbicides applied onto crop residue present on the soil surface at planting. The herbicides weren’t bound to the crop residue, but would wash off the residue into the soil with the first rainfall. With heavy levels of crop residue it might take a more rain to activate the herbicide than without (usually ~0.5" as a rule of thumb), but it isn’t tied up by the crop residue.

I think a rye or triticale cover crop would have a similar effect to heavy levels of crop residue. One difference is that since the rye is often living when the herbicide is applied, some of the herbicide might actually be absorbed into the tissue. That might initially reduce the availability of the herbicide to be washed into the soil where it is needed.

Field experience suggests this issue likely isn't a huge problem, but in cases where the rye is allowed to grow that large prior to termination, it may make sense to delay the application of the preemergence herbicide where it is feasible or possible. If the rye is a dense, uniform stand that produces sufficient biomass to provide some early season weed control, delaying the preemergence herbicide application may make sense. That dense mat of rye would be effective at suppressing weeds early and as the rye degrades, weeds would begin to emerge through the cover. In these cases, the herbicide would contribute more when the application could be delayed to coincide with that rye degradation. This would limit herbicide choices as the application of the preemergence herbicide would likely occur after crop planting.

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