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Field Notes talks a tale of two springs and weed management

Phyllis Bongard, Extension content development and communications specialist, Debalin Sarangi, Extension weed specialist, Joe Ikley, Extension weed specialist, North Dakota State University, and Ryan Miller, Extension educator-crops

Photo: Liz Stahl, UMN Extension
After a dry winter and early start to the growing season, rain has turned this season into a tale of two springs with a range of planting dates and herbicide applications. Extension weed specialists, Dr. Debalin Sarangi from UMN and Joe Ikley from NDSU, teamed up with moderator Ryan Miller, Extension educator-crops, to discuss weed management concerns in the June 5 session of Field Notes.

A tale of two springs

In the west central and northwestern part of the state, early-seeded small grains and most of the corn was planted before the frequent, heavy rains started in mid-May. Small grains are approaching jointing and appear to be doing well, despite having wet spots in the field. However, it’s been a challenge to plant soybeans and other later-planted crops like sunflowers and dry beans in the region. Some growers may be looking at Plans B and C for these later planted crops.

Although corn and soybean planting is roughly on track with the 5-year average statewide, cool weather, rain, and the lack of sunshine have limited crop and weed growth in the entire region. Where we might normally expect to see soybeans in the V3 or V4 leaf stages in southern Minnesota, the crop is mainly at the V1-V2 stages. Weed growth is also slow. Kochia in the northern part of the state and waterhemp in southern Minnesota had not yet reached a 4-inch height by June 5 in most of the fields. However, once sunshine returns and temperatures warm into the 70s and 80s, weeds are going to take off!

Watch the weed growth!

When the sun and warm temperatures do return, weeds like waterhemp are capable of growing an inch every day. It won’t take long for them to get beyond the recommended maximum 4-inch treatment height for most of the herbicides, so fields need to be monitored closely. Because weeds will continue to emerge due to so much soil moisture, adding a Group 15 residual herbicide like Warrant (acetochlor), Dual (metolachlor), or Outlook (dimethenamid) to the tank mix will help with control. This is particularly important for waterhemp control, since its emergence window extends into August.

Herbicide activity expectations

Where growers took a break in planting to apply preemergence (PRE) herbicides, the herbicides have been working quite well. On the flip side, where growers didn’t get a chance to apply PREs and have applied early postemergence herbicides, their activity has been very slow due to the cool temperatures and slow weed growth.

With a contact herbicide, you should have a general sense of how well it’s working within a 3- to 4-day window. While weeds may not be dead, you should still be able to gauge the herbicide’s activity. It’s a different story with systemic herbicides. Weeds need to be actively growing to move the herbicide to growing points, so conditions that delay growth also delay translocation. 

What about over-the-top dicamba?

Registration of the few dicamba products that were available for dicamba-tolerant soybean (Engenia, Tavium, and Xtendimax) was vacated in February of this year. However, the EPA issued an “existing stocks order” which allowed for limited sale and use of these products that were already in the pipeline. The selling deadline date was May 13 south of I-94 and May 31 north of I-94. The application cutoff restrictions are the same with a present or forecast temperature of 85F and dates of June 12 south of I-94 and June 30 north of I-94. While the window is closing in southern Minnesota, there are more opportunities to use this technology, particularly for kochia, on the north side of I-94. Note that these three herbicides are restricted-use products that require a certificate or license and mandatory training.

It's been windy!

If you’ve felt it’s been windier than normal, Dennis Todey with the Midwest Climate Hub would confirm your impressions. With wind speeds trending upward, herbicide drift concerns also increase. This is particularly true in this tale of two springs, where crops are at varying stages of development. For example, with soybean planting behind in the northwest, drift from burndown or glyphosate applications in soybeans into neighboring tillering or jointing wheat will be readily apparent. However, the situation could also be reversed where products applied to small grains drift and injure later-seeded, emerging crops. While challenging, check your weather app to find the best time for your postemergence applications.

Other herbicide application restrictions

With temperatures forecast to be warmer, corn will start to grow quickly. Many products recommend application at V2 or have applications cutoffs at 11 or 12-inch corn. In southern MN, some of the earlier planted crops have already exceeded that threshold.

In northwestern MN, opportunities for PRE applications were limited, so growers will be relying on that early POST timing from V1 to V3 in corn. If an atrazine application is planned, it must be applied before the corn reaches 12-inches. Other products, like acetochlor, have different restrictions depending on the premix and the cutoffs range from 11- to 24-inches. In corn, there are many herbicide products that can be applied either as a PRE or POST application. Because of this, growers can likely stick with their weed control programs until the crop gets taller. Read your label for growth stage cutoffs.

In soybeans, there are a few products that are PRE-only, including sulfentrazone (Spartan, Authority products), flumioxazin (Valor), and metribuzin. If the PRE didn’t get applied and the field got rained out, it might mean going to Plan B and switching chemistries.

Problem weeds

Waterhemp and kochia are our favorite weeds to discuss because they cause the most trouble. Two years ago, Dr. Sarangi’s team surveyed 90 populations of waterhemp in 47 counties to get a picture of herbicide resistance in the state. What they found was alarming. Waterhemp populations with multiple herbicide resistance are spreading in Minnesota, with some populations surviving 6 different herbicide sites of action. Liberty (glufosinate) is the only postemergence herbicide that waterhemp has not shown resistance to. That puts a lot of pressure on this herbicide, so it’s important to preserve its effectiveness. Spraying smaller weeds is important, as is using multiple herbicide sites of action, rotating crops and herbicide traits, and considering use of cover crops and tillage. Waterhemp management practices need to focus on the long-term to keep it from becoming as problematic as it is in the southern U.S.

For kochia, most populations have glyphosate resistance and there are increasing pockets of dicamba resistance. Dr. Ikley has also focused on Group 14 (ex. Sharpen) resistance, since kochia is resistant to the Group 14 herbicides that are used in burndown applications. However, other Group 14 herbicides used for kochia control in dry beans (Reflex) and sugarbeets (Ultra Blazer) are still effective. although control decreases rapidly once kochia exceeds two inches. Glufosinate is also still largely effective, but there’s concern over the amount of selection pressure put on this herbicide.

To control kochia, Dr. Ikley recommends at least three effective active ingredients. So far, the Groups 5 (ex. atrazine) and 6 (ex. bromoxynil) herbicides are still fairly effective in small grains and corn, but their heavy use puts long-term pressure on these products. In corn, combining a Group 27 herbicide with a Group 5 or 6 herbicide provides effective kochia control, as long as the weed size is right. Several common premixes are available for small grains.

Rescue – or revenge – applications

Neither weed scientist is a fan of rescue – or revenge – treatments. The herbicide application will kill off the most susceptible of the tall weeds, but given their size, there will likely be survivors. Are they resistant? Maybe or maybe not, but they would at least be more tolerant. Long term, that can become problematic for shifting the weed population toward either tolerance or resistance.

Dr. Sarangi’s group conducted a trial on rescue treatments a couple of years ago and looked at Enlist One and Roundup versus Liberty. What they found was that if there had been one postemergence application before the rescue treatment, both treatments had good yields and weed control. However, control was more variable with Liberty as a rescue treatment. If the weeds are smaller, Liberty provides good control, but if they’re larger, then Enlist One and Roundup would be a better option.

Thanks to the Soybean Research and Promotion Council and the Corn Research and Promotion Council for their generous support of this program.

Reference to product names is for educational purposes only and does not imply endorsement.

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