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Thinking Spring Inputs: Setting soybeans up for success

Megan Bourns, On-Farm Network Agronomist, MPSG

Spring input decision-making can be stressful. Decisions about soybean seeding rate, inoculant and seed treatment can all affect yield, and certainly your bottom line. At what rate should soybeans be seeded? Do I need double inoculant or single inoculant? What about seed treatment? What factors should be considered in making decisions on each of these inputs? MPSG production knowledge and On-Farm Network (OFN) research offer information to help guide these decisions.

Seeding Rate

Seeding rate, while an essential aspect for all crop production, is of particular importance in soybean production given the high cost of seed. It’s important to note that the seeding rate, target population and plant stand are not the same. Seeding rate should be based on your target population, or ‘ideal plant stand,’ factoring in seed survivability. That means your seeding rate will be greater in seeds/ac than what you are targeting for plants/ac since not all of the seed will survive, germinate and grow into a yield-producing soybean plant. Plant stand is the actual number of plants/ac that you determine by doing plant counts in the field.

Farmers may want to try out lower seeding rates to reduce seed costs. When making seeding rate decisions, it’s important to keep in mind the competitive ability of the crop, yield potential and economic return. The MPSG Bean App has a seeding rate calculator that can help guide this decision, determining the economic optimum plant population and seeding rate. Typically, the Bean App’s recommended populations range from 140,000 to 160,000 plants/ac and will depend on seed cost, expected yield and grain price.

Soybean seeding rate research through MPSG’s OFN has been ongoing. Typical seeding rates compared in these trials include 130,000 seeds/ac, 160,000 seeds/ac and 190,000 seeds/ac. Results from these trials are in the process of being analyzed and information on the results will be available soon. You can view single-site reports for these trials at manitobapulse.ca/on-farm-network/on-farm-research-reports/.

Inoculant: Double? Single?

One of the great attributes of a soybean crop is its ability to produce its own nitrogen — if provided with the correct tools to do so. Bradyrhizobium japonicum, the bacteria that create a symbiotic relationship with soybean to form the N-fixing nodules, is not native to Manitoba soils and therefore must be introduced. Dr. Ivan Oresnik, at the University of Manitoba, found that after introduction to the soil, the rhizobia population decreased over time, but the population did not reduce to zero. That indicates there is some persistence in the population. The question for farmers is what are the implications for their inoculant strategies. Does a double inoculant practice become redundant after a certain number of soybean years in a field? How much of a yield benefit does single inoculant provide, once soybean history has built up?

In 2013, questions started to surface about the benefits of doubling up on inoculant after soybeans had been grown on the same piece of land for a couple of years. That is when the OFN began investigating soybean inoculant practices. Double vs. single inoculant trials were set up in fields where there were at least two years of soybean history in the last ten years. There have been a total of 36 double vs. single inoculant trials since. Only two trials had a significant yield increase with double inoculant compared to single inoculant. For the other 34 trials, there was no significant yield difference between double and single inoculant strategies.

As soybean production history in the province continued to expand, the OFN began investigating a second inoculant question­ — comparing single vs. no inoculant in soybean fields where there were at least three years of soybean history in the last 10 years. Since starting these trials in 2016, there have been a total of 29 single inoculant trials through the OFN. To date, there has been no significant yield response to single inoculant compared to no inoculant in these trials.

Although it doesn’t seem as though there is a significant benefit to double inoculating soybean when there has been an established soybean history, or even single inoculating once that history increases, it is very important to keep in mind that successful nodulation depends on several factors. Crop nutrition, environment and soil conditions, in addition to the B. japonicum population, play into successful nodulation.

MPSG has developed a checklist to help guide the decision on double vs. single inoculant (see above). We don’t have a similar checklist designed for making a single vs. no inoculant decision, however, the trial data is being explored to determine if a reliable list should be developed. Before making a widespread inoculant management change across the farm, it would be wise to test out the new practice in one or two fields — perhaps even setting up an on-farm trial to compare strategies, to check nodule formation and yield.

Seed Treatment: Is it effective? Is it necessary?

The OFN began comparing soybean yield with and without seed treatment in 2015. We now have five growing seasons of data, with a total of 41 sites. Out of all 41 locations, there have been eight statistically significant yield increases, and one significant yield decrease, for soybean with seed treatment compared to soybean without seed treatment (Figure 3). Most of the responsive sites were from 2015 and 2016, likely due to the seeding conditions in 2015 and 2016 vs. 2017, 2018 and 2019.

Making seed treatment decisions can be tricky. Right now, there are no rapid methods available for farmers or agronomists to quantify the fungal pathogen and insect pressure pre-seeding that could help guide a seed treatment decision. The OFN results across the 41 sites indicate that some of the time, seed treatment does result in a yield increase, but not all the time. Considering seeding conditions and knowledge of early-season fungal and insect pressure in past years can be helpful. As with most agronomic decisions, the best verification method is to get your boots on the ground and scout. Digging up seedlings shortly after emergence to look at the roots for symptoms of fungal disease, sifting through the soil to search for wireworms, these are things that can give you clues about the necessity, or lack thereof, for seed treatment on your soybeans. While there isn’t a reliable, rapid method to quantify the extent of seedling pest pressure in your fields yet, the qualitative observations are still very informative.

When in Doubt, Get Out to Scout

Spring input decisions can make or break your crop. Drastic, farm-wide changes in spring input decision making are probably not the smartest choice. Instead, look to test changes you’re interested in trying in one or two fields, either independently or through the OFN. Part of testing those changes and evaluating their efficacy means getting boots on the ground — you can never go wrong with getting out to scout. It may be too late to make a change for that growing season, but your observations can inform you for the next one. Take plant stand counts to determine how your seeding rate is stacking up to your target population. Dig up seedlings and look for disease on the roots and insect pests in the soil. Look at roots mid-season to check how your inoculant strategy is working for nodulation. If you don’t look, you don’t know. Combining data and visual observations give you powerful tools to guide decision making on your own farm.