Wallaces Farmer

Soybean Source: Consider planting dates and field history when deciding whether to add fungicide and insecticide treatments.

Clarke McGrath

February 20, 2020

6 Min Read
soybeans treated with a fungicide
EARLY PLANTING: If you’re planting soybeans early, fungicide seed treatments can offer good insurance as cool, wet soils are very conducive to poor stands. Farm Progress

What might seem like a pretty simple question can be a pretty loaded one, too. This time of year, some version of “Should we treat our soybean seed, and if so, with what?” comes up fairly often. Sometimes the answers are short and sweet, but more often, the question turns into a discussion. The nearly infinite combinations of field conditions, management decisions, weather, seed genetics and many other factors make the seed treatment decision too dynamic for a simple yes or no. 

Just deciding exactly what seed category of treatments you might benefit from is challenging. This list seems to grow yearly, and includes fungicides, bio-fungicides, insecticides, nematicides, inoculants, growth promoters, micronutrient mixtures, biologicals and other amendments. The good news is many of our choices can be made near planting time since most seed dealers can customize your treatments on-site. Combining knowledge of each of your soybean field’s agronomic challenges, your soil conditions at planting time and a decent short-term weather forecast can help dial in what treatments may be a good idea. 

Let’s take a quick look at some of the various types of seed treatments and factors to consider in making decisions about how they fit in your situations:  

Fungicides. ISU research and regional academic publications find the odds of the right product benefiting you go up when you:  

  • have a field with a history of soybean death syndrome or seedling disease 

  • plant into wet, cool soils below 60 degrees F, or into compacted soils 

  • seed at rates under 140,000 per acre 

  • practice no-till or reduced tillage 

  • have a lot of surface residue 

  • have a field with a history of floods 

  • have high levels of seedborne fungal infection 

  • did not rotate crops — second year (or more) of soybeans 

  • plant bean varieties susceptible to soilborne diseases, such as phytophthora root and stem rot and SDS 

The length of protection and spectrum of diseases suppressed varies with products and application rates. To maximize performance of fungicide seed treatments, work with your seed supplier to match the right combination of fungicides to the disease problems you expect to encounter. 

Nematicides. Fighting soybean cyst nematode with seed treatments is a relatively new frontier, so good data is at a premium. Lucky for us, researchers like Greg Tylka have been at the forefront working with many of the products to provide information.  

For example, while nematicide seed treatments offer some early-season seedling protection, they’re not a silver bullet for season-long nematode control. Consider a seed treatment as just one additional nematode management tool. The two best tools for managing nematodes continue to be rotating crops and planting nematode-resistant soybean varieties. 

To fine-tune the best fit for nematicide seed treatments, they are more likely to help if you have fields with a history of nematodes; combine the seed treatment with nematode-resistant bean varieties; have limited crop rotation options; and have a manageable population of nematodes (nematicide seed treatments offer little protection with severe populations). 

You’ve probably heard “Take the test. Beat the pest.” So before using a seed treatment, have recent SCN soil tests taken of your fields and use the results to determine if populations are in a range where treatment has a shot at being beneficial. 

Inoculants. This one’s pretty straightforward, as inoculants are a good bet in fields that haven’t had beans for several years. Some experts will include fields that had significant areas of beans flooded out in previous years. It may not be quite as simple as yes or no, as many of today’s commercial inoculants are promoted as having novel or new strains of rhizobium that can increase yields even under normal circumstances.

Sometimes inoculants are co-packaged with growth promoters, micronutrients, bio-fungicides or other ingredients and are along for the ride. With relatively low cost for the most part and no known agronomic downside, there isn’t much more to say on these. 

Insecticides. This one isn’t quite as cut and dried. Sure, you can use insecticide seed treatments to manage early-season insect pests, especially insects that injure soybeans before growth stage V2. However, most soybean insect pests occur after that growth stage, so why invest in them? 

Recent research and analysis of insecticide on soybean seed led to interesting conclusions. A total of 23 researchers analyzed soybean seed yield data from 194 randomized and replicated field studies, conducted specifically to evaluate the effect of seed treatments on soybean seed yield. The trials were run at sites across 14 states from 2006 through 2017. The 14 states included account for about 85% of U.S. soybean acreage. You can read a summary of the study here at coolbean.info

To paraphrase a few key points, across the entire region, while there was a roughly 2-bushel-per-acre yield benefit, a partial economic analysis showed inconsistent evidence of reaching a breakeven cost. Their analysis was that blanket applications of insecticide seed treatments don’t pay off consistently. 

Several ag chemical companies issued statements in response to the study. One company stated, "The entomologists and agronomists who published this paper might not think a 2-bushel-per-acre yield increase is a big deal, but clearly growers do, and we do, too." 

Another ag chem firm pointed out that neonicotinoid seed treatments target soil-dwelling pests that are difficult to scout for ahead of planting. "Neonic seed treatments provide farmers with an economical level of risk management and means of protecting seeds and seedlings against early-season insect pests. This is especially important as farmers reduce the number of seeds planted per acre to manage their expenses." 

Depending on your individual situation, both statements may merit consideration along with the conclusions of the researchers. But for the most part, my experience lines up with what the data says: Unless you have a compelling reason, skip the insecticide seed treatment. 

Circumstances certainly do exist where insecticide seed treatments are more likely to pay off, so don’t totally dismiss them. That list is included along with other good information at cropprotectionnetwork.com. It’s a rock-solid look at a wide range of seed treatments in addition to insecticides, so I highly recommend checking it out. 

Along with recommending a search of industry and Extension resources, look at the Iowa Soybean Association’s research programs. They’ve taken a good “on-farm” look at a broad selection of seed treatments, much of it replicated data gleaned from member producers. 

Each year it seems more of our traditional seed treatments have generic equivalents, which can save a significant amount of money if the generics happen to fit your farm’s pest or disease profile. Another way to save money is a do-it-yourself seed treatment. Rather than buy seed already treated, you can apply the treatment yourself if you have the skill set and equipment to give it a shot.  Have a safe and successful spring. 

McGrath is the on-farm research and Extension coordinator for the Iowa Soybean Research Center at ISU.  


About the Author(s)

Clarke McGrath

Clarke McGrath is the On-Farm Research and Extension coordinator for the Iowa Soybean Research Center at ISU.

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