By Clarke McGrath
We are now past the Summer Solstice, which means daylight hours will be getting shorter and we will starting seeing more flowers on soybean plants in fields, which has triggered questions about when we have to stop using dicamba on Xtend beans.
There isn’t really a true “cutoff date” for applying dicamba post-emergence on soybeans. The label tells us to stop applications based on growth stages of the soybean plants.
Reading the labels, you see that in-crop applications of the approved dicamba herbicide products can be made up to and including beginning bloom (R1 growth stage of soybeans). The labels specifically say “do not make in-crop applications after beginning bloom (R1 growth stage).”
That is a little complicated, and as applicators we are hearing different ideas about when to stop spraying. As a retail agronomist and commercial and private applicator, trying to decipher convoluted label statements is part of the game, so I’ll give this a shot. Interpretation from your chemical dealer and the product reps may help with clarity as well. Now would be a good time to share the reminder to “read and follow label directions” just to cover our bases; the label trumps any other advice.
Since the labels say we can make applications through R1, if we can meet all the other parameters laid out in the directions on the label, we may not have to stop spraying dicamba at the very first flower we see, as some farmers and applicators are hearing. But we also have to know when to stop cold, which is when we hit R2. That means we need to know exactly what defines R1 growth stage and when it ends, and we have to scout our fields prior to loading up to spray.
The R1 stage, or beginning bloom, means a plant has one flower open at any node on the main stem. Typically, soybeans are around V6 to V10 at that time, but this depends on many factors. In my experience, flowering starts on the third to sixth node on the main stem of the soybean plant.
As a point of further clarification, a growth stage begins when half or more of the plants in a field are at or beyond that stage.
So when does R1 end and R2 start? It’s when there’s an open flower at one of the two top nodes of the main stem and at least one of these two upper nodes is showing a fully developed leaf.
How many days does the R1 timeframe last? As usual with any agronomy answer, “It depends.” Scientific literature says from 0-7 days. In my experience, usually 3 to 5 days is what I see as R1; it goes pretty fast. Scouting and good recordkeeping is the only way to know for each of your fields.
The bottom line: the timeframe “including beginning bloom” that the dicamba label lists as the end of the application window…it is short and unpredictable.
When does soybean flowering usually start? Again, that depends, and it is triggered by not only longer nights, but is also subject to planting date, varietal differences, environmental conditions and a slew of other factors. Around the Summer Solstice is often when we start noticing flowers on soybean plants in the fields, but in many years we have soybeans starting R1 a week or more ahead of the solstice. Scouting is the only way to know for sure. Saturday morning I looked at a lot of soybean fields in SC IA and found about a third had flowers, but not all of the fields were at 50% flowered yet. A few were truly R1, and the ones with some flowers but not quite to 50% were close and will probably hit R1 in the next day or two. The other 2/3rds were close but didn’t have any flowers I could find, and I didn’t find any at R2 but that doesn’t mean they aren’t out there somewhere.
It’s important that we understand the label and application requirements for the new dicamba products to be successful. So, we need to be wrapping these dicamba herbicide applications up pretty darn soon! I don’t want any of our clients to be made an example of—for spraying past the end of R1. Be sure you understand your soybean growth stages, scout well prior to spraying, follow the label and err on the side of caution if there is any doubt as to whether the beans are past R1 growth stage when you are considering making a post-emergence application of dicamba.
Source: istate.edu