Tuesday, January 25, 2011
Pre-emerge, Post-emerge, and Glyphosate: Solving New Issues
Have you seen Monsanto’s promotional advertising for Warrant? That is a new product, designed to be applied to soybeans with glyphosate, and stick around for a while to control weeds that emerge after the residual effect of glyphosate is long gone. Sounds like a good idea. Or does it?
Warrant is a not a foliar herbicide as is glyphosate. It is a post emergent herbicide, but instead it has the active ingredient in Harness, which is acetochlor. And instead of burning your beans, it is encapsulated, which will provide a protection against herbicide injury to your soybeans, but is designed to strike at the weeds which emerge after the glyphosate spray. It is designed to control grasses and small seeded weeds and is applied in a tank mix with glyphosate. That is the theory.
Iowa State weed specialist Bob Hartzler wants you to think about the impact of Warrant and the concept for its use. In his fact sheet on Warrant, he says there is no doubt that it will improve your weed control. But he is concerned about whether it will hinder your attempt to prevent glyphosate resistance. The recommendations of Hartzler and his colleagues around the Cornbelt are to apply a pre-emergent herbicide if you are using glyphosate as a post emergent herbicide. Hartzler says the purpose is to control as many weeds as possible with the pre-emergent herbicide and clean up the rest with glyphosate, which should be very few. By depending on glyphosate to control most of the weeds, then the potential for resistance will increase.
With the use of an herbicide like Warrant, which might control 20% of the weeds that are left after the glyphosate does its work, there is no effort being made to prevent glyphosate resistance. Hartzler says the field might be cleaner with the combination of Warrant and glyphosate, but farmers are still depending on glyphosate to be the workhorse, instead of the fail-safe. He says the tank mix with the Warrant is better than relying only on glyphosate, but there is still the risk of increased resistance.
Hartzler and other weed specialists say the key to managing resistance is to use multiple control tactics, and surprise your weeds with something different each time. That includes both a pre-emergent herbicide in addition to a post emerge such as glyphosate, and use different pre-emergent herbicides from year to year to ensure they do not develop resistance to the same chemistry.
Hartzler says, “To optimize this benefit, products and rates used must be selected to control the major weeds present in the field. A product such as Warrant may enhance the performance of many weed management programs, but in many situations its benefit in reducing the risk of glyphosate resistance will be much less than a residual herbicide applied before or at planting. An excellent alternative would be to use a traditional PRE product followed by a POST application of glyphosate plus Warrant.”
Summary:
Adding another post emergent herbicide in a tank mix with glyphosate may help with cleaner soybean fields, but it may not reduce the potential for development of glyphosate resistance as good as first using a pre-emergent herbicide and then cleaning up the rest of the weeds with glyphosate.
Posted by Stu Ellis on 01/25 at 04:02 PM | Permalink