Two ways to handle weed resistance
Does anybody really care about glyphosate-resistant weeds? It seems there are a few farmers who are avoiding glyphosate-resistant weeds by rotating LibertyLink or conventional varieties in their corn-soybean rotations, and there are the majority who are planning to use mainly glyphosate until it quits working, and then add pres and tankmix partners for the extra $10 to $20 per acre.
• Two main approaches to weed resistance are emerging in South Dakota.
• Both are defendable, says SDSU Extension weed specialist.
• “Somewhat bright” future is seen for controlling resistant weeds.
Either approach is defendable. Those who occasionally avoid using glyphosate can do so at no added extra cost and will be setting themselves up to save on expensive varieties and herbicides in the future. The primary caution for these people is to be sure to research your varieties well as it seems that many of the elite genetics are Roundup Ready.
For those who are going to continue to use glyphosate, the primary caution is to avoid using the same preemergence herbicides or tankmix partners two years in a row, or you may be searching the market for a rotary hoe and row cultivator soon.
Manageable in most species
We all know that glyphosate resistance is not a good thing, but it is manageable for most weed species. Unlike many other Midwestern states, South Dakota is fortunate in that it does not have many problems with resistance to alternative herbicides. Both waterhemp and kochia, which are glyphosate-resistant in some fields, are likely also resistant to ALS herbicides such as Pursuit (imazethapyr). Waterhemp may be effectively controlled in soybeans with Flexstar/Reflex (fomesafen), but there are few effective tankmix partners for kochia control in soybeans.
Therefore, glyphosate-resistant kochia may encourage people to take another look at LibertyLink soybeans. LibertyLink programs are more challenging than the Roundup Ready programs, but they have worked just as well in many South Dakota State University field trials.
Another approach to manage these weeds may be to aggressively manage their primary weakness, the seed bank. Kochia seed has almost no dormancy, and there are reports that waterhemp seed may only survive for about three years. Therefore, it may be possible to aggressively manage these weeds in rotational crops such as corn or wheat (in kochia country), deplete the seed bank, and then manage these weeds with preemergence herbicides in Roundup Ready soybeans and a glyphosate tankmix partner if necessary.
In addition to waterhemp and kochia, other confirmed glyphosate-resistant weeds in South Dakota include common ragweed and horseweed (marestail). Lambsquarters is also likely resistant in some areas.
The future is somewhat bright for managing glyphosate-resistant weeds. In a couple of years, Monsanto may be introducing dicamba-resistant soybeans, Dow may introduce 2,4-D-resistant soybeans, and Syngenta/Bayer may introduce HPPD (bleacher)-resistant soybeans. Both Dow and BASF will likely also introduce low-volatility formulations of 2,4-D and dicamba. These programs will likely be very effective, but may cost a little extra.
Thanks to a robust weed control industry, producers can still be in control regardless of herbicide resistance. If you don’t have glyphosate-resistant weeds yet, consider diversifying your herbicide program. At least use preemergence herbicides in corn, particularly if you have moderate to high weed densities, as that can be a sound investment even without resistant weeds.
Otherwise, watch for resistant weeds and adapt quickly. Selection for glyphosate resistance often occurs slowly, so you may not be convinced you have a resistance problem until the weeds and weed seed banks are dense.
More information may be found at www.sdstate.edu/ps/extension/weed-mgmt.
Moechnig is the SDSU Extension weed specialist.
This article published in the May, 2012 edition of DAKOTA FARMER.