There is obviously a lot going on with the dicamba issue, resulting in uncertainty as to where exactly we are headed with regard to future labels, restrictions, and stewardship. However, we are fairly confident that the label for early-season use of dicamba in preplant/premeergence burndown programs will be preserved into next year at least (“dicamba” in this article refers to the three dicamba products approved for use in Xtend soybeans – Engenia, XtendiMax and FeXapan). Dicamba is more effective than 2,4-D on marestail in the spring, and has a good fit in burndown programs to help with this weed. And using dicamba early-season is one way to minimize the risk of off-target movement and injury to surrounding vegetation, compared with later-season POST applications when temperatures are higher, inversions are more prevalent, and non-target plants are more developed and sensitive.
As we move into the fall herbicide application season, one question coming up is whether the spring dicamba burndown treatment is good enough on overwintered marestail plants to eliminate the need for fall herbicides. The answer is yes, maybe, and no, depending. In OSU research, inclusion of dicamba in burndown treatments prior to early May has resulted in effective control of small emerged marestail. Most of these treatments have also included residual herbicides and glyphosate. When application is delayed past early May and marestail get taller and older, we have occasionally observed reductions in control. It appears that among other reasons, the residual herbicides can antagonize and reduce the activity of the dicamba on larger marestail sometimes. Large marestail can be tough to control – period – and cold weather increases the difficulty. We have also observed this with 2,4-D so it’s not unexpected.
Our research results indicate that where the spring burndown treatment is applied early enough and includes comprehensive residual herbicides that control later-emerging marestail, there should be little need for a POST application of dicamba (for marestail at least). In a year when burndown applications and planting get delayed and overwintered marestail plants are large, expect there to more variability in the effectiveness of dicamba on existing marestail, and an increased need for follow up POST dicamba treatments. Hence the “yes” and “maybe”. Certainly one way to minimize the potential for problems with spring marestail control in Xtend soybeans is to keep using fall-applied herbicides. The fall treatment will accomplish the same thing in Xtend soybeans that is does for all other soybeans – control the marestail that emerges in late summer and fall so that fields are devoid of weeds in spring. The spring burndown then just has to take care of small spring-emerged marestail. This is still the most consistently effective way to manage marestail for not a lot of money.
With regard to the “no”, we need to consider all of the reasons why fall herbicide treatments became commonplace. While they became an essential component of marestail management programs over the past decade, fall treatments first gained traction in the decade before that for management of dense infestations of winter annual weeds and dandelion. Comments from growers at that time were that spring infestations of chickweed and deadnettle and dandelion were interfering with tillage and planting, and spring-applied burndowns just did not cause the weeds to die and dessicate fast enough. Dandelion and other cool-season perennials are more susceptible to herbicides in the late fall compared with spring. Dandelion in particular became extremely problematic for a few years. Some winter annual weeds also serve as a host for soybean cyst nematode and other insects in late-fall and/or spring. The adoption of fall herbicide treatments resolved many of these issues. So moving forward, the omission of fall-applied herbicides is likely to mean the return of some of these problems, even if spring-applied dicamba can handle the marestail adequately.
The bottom line here is that applying herbicides in fall still results in a weedfree seedbed well into spring that allows for maximum soil warming and drying, greatest ease of tillage and planting, and the most consistently effective marestail control. And fall treatments help take the pressure off of spring herbicide programs in a year when spring weather is less than ideal. Risk management 101.