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Frequent information updates for agricultural audiences

The Use of Preplant Herbicides on Roundup-Ready Crops - from Mike Roegge

Posted by John Fulton -

Over the past few years, we've been discussing the need to use a pre plant or pre emerge herbicide ahead of Roundup Ready soybeans or corn. The reason being to reduce the incidence of resistance developing from continued use of glyphosate herbicide. And most producers have followed through on this management practice, finding it saves them time, money and yield.

It's even more critical to follow this management tactic as we have waterhemp resistant to glyphosate herbicide in the area. We also have waterhemp resistant to the diphenyl ether herbicides (cobra, phoenix, blazer, flexstar, etc.) as well. And the waterhemp populations resistant to diphenyl ethers are much more common that those waterhemp populations resistant to glyphosate. So the need to use a pre-emerge herbicide is even more important, because in soybeans, those are the only two herbicide families that can be used post-emerge to control waterhemp.

A number of herbicides are available in soybean to provide pre control of waterhemp, including: valor, authority, intrro, dual, treflan, prowl (and premixes including these products). And the cost of these products will range from approximately $5-$10 per acre. It's the cheapest form of resistance management you can use if glyphosate is the planned second trip.

The use of a pre herbicide ahead of Roundup Ready crops allows you delay the post herbicide trip (glyphosate) because of the weed control offered. However, keep in mind that weeds do compete with crop yield, and follow the guidelines of 4-6 inch size weeds for corn and 6-8 inch size weeds for soybean when making that glyphosate post application. Otherwise, allowing weeds to grow to larger sizes can cost you big bucks, much more than the cost of the herbicide and the application.

Aaron Hager, U of I Extension Weed Science, recently determined the exact cost of delayed post emergent application, based upon soybean price and yield lost due to delayed application. With a 50 bushel yield, and bean prices of $10, just a one day delay in post emerge application will cost you $5 per acre in lost yield. If you delay that application by 5 days, then the weed competition will reduce your soybean yield to the tune of $25 per acre.

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