If there's one thing we have seen in our agricultural production fields over the past several decades, it's been a change in our predominant weed species.
These weed "shifts" usually occur in response to some specific pressure that has been placed on them. Since 1996, this pressure has come in the form of the rapid adoption of Roundup Ready cropping systems and the continuous use of glyphosate. As a result, we are starting to see a shift in our fields to those weeds that glyphosate does not control. One of these weeds is Asiatic dayflower (Commelina communis).
In the past two weeks I have received numerous calls about the control of this species in no-till corn and soybean fields. We have given presentations and written several newsletter articles on this species in the past, so I will only briefly summarize here.
First, I believe there is an increasing awareness about this species among our clientele, but I also believe that we have more work to do and that there are many who are completely unaware as to the identity of this plant. In some surveying we have done over the past several years, we have encountered this species on a fairly regular basis throughout the state, especially in no-till soybean fields. It is a species that can form thick canopies and cause significant yield losses, not to mention the fact that it is difficult to control.
Asiatic dayflower seedlings more closely resemble a "wide-leaved" grass plant when they first emerge in the spring (Figures 1 and 2). Asiatic dayflower can have an erect growth habit but more commonly creeps along the ground and is capable of rooting at the nodes. The leaves occur alternately along the creeping stem and are ovate to lanceolate in outline, as much as 5 inches long and 2 inches wide. All leaves and stems are hairless, and each leaf has a membranous sheath which encircles the base of the leaf and stem (Figure 3). The flowers of Asiatic dayflower consist of two, very distinctive large blue petals with one white petal below (Figure 4). Asiatic dayflower generally blooms from mid- to late-summer in Missouri, with each flower blooming for a single day (thus the name). Several authors have found that the seed of Asiatic dayflower are capable of germinating throughout the growing season and that the seed can also remain viable in the soil for more than 4 ½ years.
Few herbicides provide acceptable control of Asiatic dayflower in soybeans. Firstrate, Sencor, and the Authority products are some of the only herbicides that will provide acceptable Asiatic dayflower control when applied as a pre-emergence treatment. Similarly, Firstrate is one of the only conventional herbicides that will provide acceptable control of this species when applied as a post-emergence treatment in soybeans, but applications must be made before this species reaches six inches in height. Even high rates of glyphosate in Roundup Ready soybeans will usually only provide a moderate degree of suppression (<50% control).
In corn, there is much less information available about the control of this weed in the weed science literature. I usually get far fewer questions on the control of this weed in corn, and I suspect that the use of atrazine in our corn production systems is the reason. S-metolachlor, or products that contain this herbicide (Dual, Bicep II Magnum, Lexar, Lumax, etc.) provide some degree of residual control of this species. Based on some research on the control of a similar dayflower species that occurs in the south, it appears that Aim has good post-emergence activity on dayflowers, and that a post-emergence program that includes Aim + S-metolachlor + glyphosate provides good control of populations that weren't initially controlled with a pre-emergence herbicide program. As mentioned previously, I'm sure there are other post-emergence programs that provide equivalent or higher levels of Asiatic dayflower as well; there just isn't much information on this species in the literature.
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REVISED: October 1, 2015