Are Those Weeds Worthy Of Treatment?

DR. KEVIN BRADLEY

COLUMBIA, MO.
   I’ve gotten a few calls lately from producers asking what density of winter annual weeds justifies treatment in winter wheat. While there’s not a lot of information out there in the weed science literature on this subject, I present here a few highlights of what I could find for some of our most common weeds that infest wheat in Missouri.
   Common Chickweed. Most of the available research on common chickweed indicates that this winter annual will cause wheat yield reductions when this weed is present at densities of at least 30 plants per square meter and higher.  In research conducted throughout Missouri, wheat yields were reduced by as much as 28 percent with common chickweed densities of 169 plants per square meter.  
   Cheat/Downy Brome. In fields with cheat and downy brome infestations, herbicide applications are almost always warranted, especially when these grasses emerge at or within the first few weeks after wheat planting. Researchers in Oklahoma have observed a 49 percent reduction in wheat yield due to cheat infestations of 86 plants per square meter (Koscelny and Peeper, 1997). Similarly, wheat yield reductions greater than 60 percent have been reported in fields with 200 downy brome plants per square meter (Blackshaw 1993).
   Henbit/Purple Deadnettle. Henbit is one of those weeds that may not compete as effectively with wheat as some of the other winter annuals like chickweed, cheat, and downy brome, but still may cause yield reductions when present at high densities. I would put purple deadnettle in this same category, but can’t find any data to support that statement. Research conducted in several locations in Missouri has revealed that henbit densities of 18 plants per square meter will not cause wheat yield reductions but henbit densities of 82 plants per square meter can reduce yields by as much as 13%. Another thing to consider before making a decision to treat henbit and purple deadnettle especially is their stage of growth. If these species are blooming at the time of application, they have already entered into their natural state of senescence and their sole goal at this point is to complete seed production. So while you may reduce seed production in these species, chances are these weeds are not going to be competitive enough with wheat to make it an economically justifiable treatment.
   Wild Garlic and Wild Onion. Although wild garlic is not considered much of a competitor with wheat, control of wild garlic in wheat is an absolute necessity because of the dockage that will occur at the grain elevator. So there really can’t be any allowance for this species in wheat.∆
   DR. KEVIN BRADLEY: Associate Professor, Division of Plant Sciences, University of Missouri