Crop Update: So grateful for some rain for much of the area last week! Updated soil moisture status at http://jenreesources.com. The crops are rapidly growing now as are the weeds. Some were seeing Palmer shooting heads at soil level already…last year we
didn’t see that till late July. Many have been in the process of postemergence herbicide applications. We revisited a CropWatch article regarding best management practice considerations for postemergence dicamba-based applications to corn based on the research that is available. Please see the full article with more explanation at http://cropwatch.unl.edu. Briefly, those practices include:
- Consider a quick irrigation (rainfast/irrigation timing based on label requirements for the product you’re applying) of only 1000 gal/ac to help reduce any potential volatility.
- Don’t use dicamba products in both corn and soybean to reduce selection pressure and resistance.
- Check for temperature inversions and wind speed. Temperature inversions can be tested by using Innoquest SpotOn® inversion tester and testing the temperature at 1 meter and 3 meters. If the temp is cooler at 1 meter than 3 meters, a temperature inversion is occurring and spraying is not recommended.
- Consider using the more restrictive RUP dicamba requirements regarding wind speed, boom height, etc. Also consider not using AMS with any dicamba product even though it is labeled for use in some of the corn dicamba products. This may result in you needing to increase the glyphosate rate to the highest labeled rate to increase efficacy. Amit Jhala will showcase research on efficacy of dicamba products with and without AMS at the South Central Ag Lab Weed Science Field Day on June 27.
Volunteer corn is also a major issue in many corn and soybean fields in the area and there’s two articles in CropWatch at http://cropwatch.unl.edu addressing this topic. A number of grass herbicides are available for control in soybean. The challenge is in the continuous corn fields. If you had glyphosate resistant corn last year and used a different technology such as Liberty or Enlist, you have some other options this year. For Liberty Link corn this year, two applications, each of 32 to 43 fl oz/acre, could be made. Remember that Liberty will NOT be effective if Roundup Ready + Liberty Link hybrid corn was planted last year. Regarding Enlist corn, Assure® II is the only grass herbicide labeled to control volunteer corn with this technology. It can be applied at 5 to 12 fl oz/acre in Enlist Corn for selective control of volunteer corn. Please be sure to read and follow all label requirements. A few farmers have also discussed their past experiences with cultivation, using either one or two passes and their concern about the soil moisture situation this year.
So how much yield loss can be anticipated from volunteer corn? Perhaps more than one would think with more loss occurring in soybean! Studies were conducted in several mid-western states at various densities including 3500, 5000, 7000 and greater volunteer corn plants per acre. To envision this, imagine 3.5, 5, and 7 volunteer corn plants respectively in 1/1000 of an acre (17’5” in 30” rows). Some fields this year have much higher densities than this! Clumps of corn impact yield more than individual plants.
UNL research found a volunteer corn density of 3500 plants/acre led to 10% yield reduction in soybean. Doubling the density to 7000 plants/acre led to a 27% yield reduction. South Dakota State University data revealed similar trends. A volunteer corn density of 5000 plants/acre resulted in a 20% yield reduction (12 bu/acre yield loss in 60 bu/ac soybean).
Clumps of volunteer corn in soybean led to greater yield loss as they were more competitive than individual plants. In the UNL study a density of 3500 clumps of corn/acre resulted in a 40% yield reduction. Researchers in Minnesota and Illinois also found increased competition with clumps of volunteer corn versus individual plants. Clumps of corn (7-10 plants/clump) were established at different densities. Depending on the location and year, soybean yield was reduced 1% for every 75-115 clumps/acre.
A recent UNL research study found highest yield reduction occurred when volunteer corn was left uncontrolled or when it was controlled too late at the R2 (full flower) soybean growth stage. The combined density at this greatest yield reduction was at 24,710 volunteer corn plants per acre plus 1,235 volunteer corn clumps per acre.
In corn, UNL research found a volunteer corn population of 3500 plants/acre resulted in a 2% yield reduction in corn. Doubling the density to 7000 plants/acre caused a 5% yield reduction. Clumps of volunteer corn led to greater yield loss as they were more competitive than individual plants. A density of 7000 clumps of corn/acre resulted in a 14% yield loss compared to a 5% yield loss with individual plants. So volunteer corn in general can be fairly competitive especially to our legume crops. It can also be a problem regarding harboring corn rootworm in soybean fields, reducing the advantage of the rotation from that perspective.
Also, an update on my soybean planting date demo at the fairgrounds: I wasn’t counting on rabbits! All the soybeans were reduced to stems below the point of recovery. So there is no demo but the groundskeeper preferred they took out my soybeans instead of his petunias 🙂 Will try again next year.
Last week was the first week I hosted dicamba trainings. Socially I haven’t seen the divisiveness within conventional ag that I’ve observed around this topic.
As I showed the dicamba videos, I stopped on a couple of slides to make a few points that were true for our area of the State last year; some farmers thanked me for doing so at the end of my trainings. This column is going to focus on these points because they’re not being discussed and by not talking about them, we’re not sharing the entire story of what happened in Nebraska last year. Caveat: I’m speaking only from my observations for the area of the State I serve and my news column is not peer reviewed.
The majority of my calls still come from Clay, Nuckolls, Thayer, and to a lesser extent Fillmore Counties because we’ve built relationships and I continue to serve you till we have a new educator in Clay County. I received my first dicamba call in June. All but three soybean fields along the way to the field I was asked to look at were cupped for the entire field. There was drought stress in the area and at first I wondered if there was something environmental occurring. Then I started making phone calls as to what herbicides were applied to crops in the area. No one met me at the fields that day-I just spent the entire afternoon/early evening walking fields and taking pictures for a 20 mile radius. In my inquiries I learned numerous corn fields all had corn dicamba formulations applied to them; the soybean fields sprayed at that time only had burndown apps but not post-apps. I continued to look at damaged soybean fields for six weeks in 10 counties.
When we think about last May, it was wet, cold and windy delaying corn post
applications. The first few weeks of June were very hot and humid and in my notes I mentioned “seemed like the whole countryside was spraying corn at one time”. Humidity may have decreased during the night and some felt light winds may have shifted directions some evenings. There was potential for increased volatility and temperature inversions and in discussions a handful of agronomists agreed that “herbicide seemed to hang in the air”. Palmer had gotten too tall very quickly and corn dicamba formulations went on hundreds of thousands of acres in the area-more than what we’ve before experienced-and, they did a nice job against palmer in most situations.
Much of what we see and hear is about the three now RUP dicamba products that were used last year. I’m not saying that damage didn’t potentially occur from these formulations. However, at least 90% of off-target movement I looked at primarily in Thayer, Fillmore, Nuckolls, and Clay started with corn dicamba formulations. I don’t know if that is the case in other parts of the State. I was able to determine the same thing in specific situations I was called out to in York, Seward, and a few central Nebraska counties. As the summer progressed, the soybean formulations came more into play and we were also finding 2,4-D damage based on samples sent to the South Dakota lab, which is also interesting. There’s fields I looked at that were affected by off-target movement twice and a couple even perhaps three times as the summer progressed.
Why do I say so much damage started with corn apps? When I couldn’t figure out what
was occurring, I let the plants tell the story. UNL Research from Dr. Jim Specht says a soybean will produce a new node every 3.75 days. Research on dicamba shows that it takes 7-14 days for leaf damage to occur on susceptible plants. So I started counting nodes. I counted how many total nodes were on the plant and multiplied by 3.75 to figure out how many days old the plant was. I then figured out about the date of the trifoliate with the leaf cupping damage and counted back on the calendar 7-14 days. This correlated over 90% of the time to a corn dicamba product applied…and often several farmers or Coops in the area applied products so it was hard to tell where it came from. Most I talked to agreed to just wait till harvest because with whole-field damage, there wasn’t a good way to compare yields. There may be a better method and I wasn’t aware of others doing this till I started sharing it last summer in my news column-but it’s the only thing I could figure out at the time. I chose not to report the number of calls I received nor acres damaged as I was unsure how the information would eventually be used; thus, to answer the questions why on power-point presentations, the areas I serve remain blank regarding reporting when you all know we had large numbers of acres damaged.
Another point. There’s a number of ways that pesticides can move off-target including particle drift, through tank contamination, temperature inversions, and volatility. New research is also looking into movement on dust particles. If we just look at the potential for volatility, we know the three RUP products do not have ammonium sulfate (AMS) in them and it is off-label to add AMS. Research has also shown these three RUP dicamba formulations to be 50-70% less volatile than other dicamba formulations. There’s over 30 corn dicamba formulations registered for use in Nebraska; some have AMS in them or most allow AMS to be added to them.
Why is this important? AMS can increase the potential for volatility. In research from the University of Arkansas and from Purdue, soil was treated and placed in low tunnels between two soybean rows. The low tunnels were removed after 48 hours then percent soybean injury was measured. For example, adding AMS to Xtendimax (which is off-label but allowed for research purposes) resulted in a 20-30% increase in soybean injury due to volatility. The injury observed was similar to that of Banvel. The non-RUP corn dicamba labels allow for use of AMS while the RUP dicamba labels don’t. The label is the law and a legal document.
So what do we recommend for best management practices for corn dicamba apps? We’ve had numerous conversations within Nebraska Extension. I’m truly hoping we can come to some consensus based on the research that is known to provide BMPs for you in the next few weeks.
One consideration as we think about resistance management: UNL research found it only took three generations of spraying palmer amaranth in greenhouse settings before resistance occurred. One best management practice would be to not use dicamba in both corn and soybeans each year as it’s a tool too critical for us in managing palmer.
Ultimately, while I’m speaking of dicamba here, the overarching issue is pesticide applications in general and what off-target movement does to all sensitive plants and even what we’re breathing. I realize we can’t control weather. However, we all need to do what we can to always read and follow label requirements to avoid off-target movement of pesticides in all situations. To have another year like last year with great off-target pesticide movement may potentially impact pesticide applications in large ways in the future.