By Liz Stahl, Extension Educator – Crops, Seth Naeve, Extension Soybean Agronomist, and Debalin Sarangi, Extension Weed Specialist
Crop progress, weed control, and what effect variability in precipitation was having on the crop were topics of discussion on the June 10 Strategic Farming: Field Notes episode. Drs. Seth Naeve, Extension Soybean Agronomist, and Debalin Sarangi, Extension Weed Specialist, were joined by moderator Dave Nicolai, Extension Educator - Crops to discuss these issues and more.
We are in the middle of the time period where drastic yield reductions occur if soybean planting/replanting would be needed. Up until May 31, yield potential averages around 87% of maximum. By the time we get to July 1, however, yield potential is about half what we would expect compared to an optimal planting date of May 1. This means we lose about 45% of our yield potential over a month, or a point and a half per day. Soybean populations would have to be considerably reduced to make replanting worth it at this time, just due to the reduction in yield potential from a later planting date.
Another part of the replanting equation is plant stand distribution. Large gaps mean more potential for weed emergence and growth, so this needs to also be considered if one is faced with reductions in stand at this point in the season. Useful resources to help with this decision include the When and how to plant soybean and the When and how to plant soybean on-line resources.
Spiking in seed with an earlier maturity variety (at least 0.5 earlier RM) to help increase stands is a possibility, but one does need to consider potential damage to the existing stand. Drown-out spots should be less overall this year, but they still can occur. While it can be logistically challenging to get to spots in the middle of a field, seeding soybeans yet at this time can help field average yield and assist with weed management.
Iron deficiency chlorosis, or IDC, is showing up in western Minnesota this year, and earlier than Naeve has historically seen it. Soybeans may be able to come through this with the right patterns of rainfall and temperature, but right now it looks like it could be a tougher IDC year. There is not a lot a person can do at this point in the season, but Managing iron deficiency chlorosis in soybean outlines strategies to consider in the future.
Keep in mind that preemergence (PRE) herbicides are losing their efficacy by now, so it is a good strategy to layer a residual herbicide with postemergence (POST) applications. Group 15 products (e.g. Warrant, Outlook, Dual, Zidua) will help with upcoming waterhemp flushes. A layered residual should help with control until the crop canopy closes.
You can instead make a sequential POST application to target waterhemp too, but effectiveness will depend on weed emergence timing, weed density, and how much control you get from the herbicide.
It is critical to target weeds when they are 3 to 4 inches tall. Once weeds exceed 4 inches in height, efficacy can be dramatically impacted. In particular, it can be difficult to get the coverage needed with a contact herbicide like glufosinate, and you may just end up burning off the top of the weed before it gets over 4 inches tall. With the recent confirmation of glufosinate-resistant waterhemp in the state, proper application is more important than ever, as well as using tankmixes with more than one mode of action. See the article Don't break glufosinate (Liberty)! for more strategies and reminders.
Over the last few years, Sarangi and his team have tested seed from plants that survived 2,4-D application. To date, the seed tested was determined to be susceptible and not resistant, since plants did not survive applications in greenhouse tests. One possibility is that suspect plants in the field were sprayed when they were too tall. Even though Enlist is a systemic herbicide (a.i. 2,4-D), it is still critical to make applications when weeds are 4 inches or less. This guideline holds for ALL herbicide modes of action. Weeds can also survive if they are under the crop canopy and they get a sublethal dose of the herbicide used.
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| Soybeans growing in a terminated cereal rye cover crop. Photo: Liz Stahl |
Overview of soybean progress and replant questions
Precipitation has been variable across the state, with some areas having very heavy rainfall over a short period of time, while other parts of the state are still quite dry. Naeve notes that dry springs seem to benefit MN farmers as this helps conditions at planting as long as we get timely rains.We are in the middle of the time period where drastic yield reductions occur if soybean planting/replanting would be needed. Up until May 31, yield potential averages around 87% of maximum. By the time we get to July 1, however, yield potential is about half what we would expect compared to an optimal planting date of May 1. This means we lose about 45% of our yield potential over a month, or a point and a half per day. Soybean populations would have to be considerably reduced to make replanting worth it at this time, just due to the reduction in yield potential from a later planting date.
Another part of the replanting equation is plant stand distribution. Large gaps mean more potential for weed emergence and growth, so this needs to also be considered if one is faced with reductions in stand at this point in the season. Useful resources to help with this decision include the When and how to plant soybean and the When and how to plant soybean on-line resources.
Spiking in seed with an earlier maturity variety (at least 0.5 earlier RM) to help increase stands is a possibility, but one does need to consider potential damage to the existing stand. Drown-out spots should be less overall this year, but they still can occur. While it can be logistically challenging to get to spots in the middle of a field, seeding soybeans yet at this time can help field average yield and assist with weed management.
Iron deficiency chlorosis, or IDC, is showing up in western Minnesota this year, and earlier than Naeve has historically seen it. Soybeans may be able to come through this with the right patterns of rainfall and temperature, but right now it looks like it could be a tougher IDC year. There is not a lot a person can do at this point in the season, but Managing iron deficiency chlorosis in soybean outlines strategies to consider in the future.
Weed management considerations
Where drier conditions have prevailed, waterhemp populations may be looking low at the moment. Sarangi has observed that some plants which emerged in earlier in May did not survive, possibly due to dry conditions. However, waterhemp loves water (there is a reason it is in the name), and he expects waterhemp will still come since rainfall will trigger emergence.Keep in mind that preemergence (PRE) herbicides are losing their efficacy by now, so it is a good strategy to layer a residual herbicide with postemergence (POST) applications. Group 15 products (e.g. Warrant, Outlook, Dual, Zidua) will help with upcoming waterhemp flushes. A layered residual should help with control until the crop canopy closes.
You can instead make a sequential POST application to target waterhemp too, but effectiveness will depend on weed emergence timing, weed density, and how much control you get from the herbicide.
It is critical to target weeds when they are 3 to 4 inches tall. Once weeds exceed 4 inches in height, efficacy can be dramatically impacted. In particular, it can be difficult to get the coverage needed with a contact herbicide like glufosinate, and you may just end up burning off the top of the weed before it gets over 4 inches tall. With the recent confirmation of glufosinate-resistant waterhemp in the state, proper application is more important than ever, as well as using tankmixes with more than one mode of action. See the article Don't break glufosinate (Liberty)! for more strategies and reminders.
Dicamba, 2,4-D, and other reminders
June 12 was the last date dicamba products labelled for use in dicamba-tolerant soybeans could be applied south of I-94 in Minnesota. North of I-94, the cutoff date is June 30. See the MN Department of Ag Over-the-top Dicamba website for more details on federal and state restrictions, as well as the herbicide label for more details.Over the last few years, Sarangi and his team have tested seed from plants that survived 2,4-D application. To date, the seed tested was determined to be susceptible and not resistant, since plants did not survive applications in greenhouse tests. One possibility is that suspect plants in the field were sprayed when they were too tall. Even though Enlist is a systemic herbicide (a.i. 2,4-D), it is still critical to make applications when weeds are 4 inches or less. This guideline holds for ALL herbicide modes of action. Weeds can also survive if they are under the crop canopy and they get a sublethal dose of the herbicide used.
The window is closing fast for timely POST herbicide applications as warmer temperatures and adequate moisture result in rapid weed growth. Be sure to check herbicide labels for application cut-offs.
University of Minnesota’s Strategic Farming: Field Notes series, offered Wednesdays morning from 8 to 8:30 am through August 12, provides useful, timely, and relevant research-based information on cropping issues throughout the growing season. For more information and to register as well as listen to a recording of all previous sessions, visit z.umn.edu/strategic-farming.
Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their generous support of this program!
Join Us Wednesday mornings!
Join us next week, June 24, when we welcome Dr. Bob Koch, Extension Entomologist, and Matt Pfarr, Extension Educator - Crops, for an update on crop conditions and soybean pests as we prepare for the scouting season.University of Minnesota’s Strategic Farming: Field Notes series, offered Wednesdays morning from 8 to 8:30 am through August 12, provides useful, timely, and relevant research-based information on cropping issues throughout the growing season. For more information and to register as well as listen to a recording of all previous sessions, visit z.umn.edu/strategic-farming.
Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their generous support of this program!
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