It's that time of year when I always get a few calls from producers asking what to do about the weeds they are finding in their winter wheat. The situation that makes these questions maybe not as straight-forward as some might think is that many of these winter annual weeds are flowering and/or dying back naturally on their own right now and they just aren't very competitive with wheat. I've often thought that winter wheat is one of the most competitive crops we plant, and winter annual weeds are some of the least competitive weeds that we have. And so the question often becomes whether a herbicide application is worth it or not at this stage of the game. While there's not a lot of information out there on this subject, I present here a few highlights of what we have found in our own research, or what others have reported in the literature, for some of the most common weeds that infest wheat in Missouri.
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 years ago, we found that wheat yields were reduced by 28% with common chickweed densities of 169 plants per square meter.
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% 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% have been reported in fields with 200 downy brome plants per square meter (Blackshaw 1993).
Much like the situation with cheat and downy brome, herbicide applications are almost always warranted for Italian ryegrass. This weed has a tendency to evolve resistance to herbicides, is capable of taking up nutrients like nitrogen and potassium at a greater rate than wheat, and is a very competitive species. There have been numerous studies done on the effects of Italian ryegrass on wheat yields, and they all show yield reductions. In one of the more recent studies conducted on this topic, Liebl and Worsham (2017) showed that wheat grain yields were reduced an average of 4.2% for every 10 Italian ryegrass plants per square meter.
Henbit is one of those weeds that does not compete as effectively with wheat as some of these other winter annuals weeds, but still could cause yield reductions when present at high enough densities. I would put purple deadnettle in this same category, but can't find any data to support that statement. Research we conducted in several locations in Missouri years ago showed that season-long competition from henbit at densities of 18 plants per square meter did not cause any wheat yield loss at all but densities of 82 plants per square meter reduced yields by 13%. However, the stage of growth that these species are in at the time you wish to treat is critically important. As I mentioned previously, almost all of the henbit and purple deadnettle I've seen in Missouri now is flowering or past flowering, and already starting to senesce and die back on its own. While a herbicide application might be justified from the standpoint of preventing seed production, chances are that these weeds are not going to be competitive enough with wheat to make it an economically justifiable treatment.
Although wild garlic is not considered much of a competitor with wheat either, control of wild garlic in wheat is usually necessary because of the dockage that will occur at the grain elevator if the bulblets of this species contaminate the harvested grain.