As we move into the seasonal time frame of applying more post herbicides, this is just a reminder to always consider herbicide resistance management. With many acres of Roundup Ready crops being planted, properly managing herbicides, especially glyphosate, to prevent resistance is always a challenge.
The first consideration is using the proper rate of glyphosate depending on the size of the weeds. In most cases, for 4–6 inch tall weeds use 0.75 lb ae of glyphosate http://extension.psu.edu/agronomy-guide/pm/tables/table-2-2-1a. Be sure to increase glyphosate rates when required by larger weed sizes.
Avoid thinking that glyphosate can control any weed, at anytime. Glyphosate does not provide equal control of all weed species. Glyphosate tends to be weak on annual morningglories, lambsquarters, velvetleaf, nightshade, smartweed, and ragweeds, especially if they are larger in size (8 inches or taller).
Another consideration is to tank mix with other herbicides which have different modes of action yet control a similar complement of weeds. Also, tank mixing broadens the weed control spectrum and provides additional insurance for reducing resistance. In RR soybeans, tank mix with Pursuit, Scepter, Raptor, FirstRate, Classic, Reflex, Warrant, or Harmony SG.
In RR corn, consider adding atrazine, Clarity, Status, Impact/Armezon, Capreno, Yukon, Northstar, Resolve Q, or Steadfast Q. Keep in mind there are certain products that are already pre-mixed with glyphosate including, Extreme, Flexstar GT 3.5, and Halex GT. Some of these herbicide combinations provide some soil residual activity after the post application.
In order to preserve glyphosate’s usefulness now and in the future, it is important to use necessary resistance management tactics. Otherwise we will lose another valuable weed control option.
Other things to watch for: ALS herbicide injury in corn. With all the wet weather, we are setting up for a repeat of last year with corn injury from ALS herbicide products applied preemergence.
Watch out for fields that are stunted and chlorotic. Also, we are expecting to see more weed escapes as soil-applied, residual herbicides are degrading and/or are being washed away with the abundance of rainfall. If these happen, we will provide more details in future newsletters.