OFN Updates, Pulse Beat Individual Articles

Findings from the On-Farm Network

Do higher soybean seeding rates pay?

Megan Bourns, MSc, On-Farm Network Agronomist, MPSG

Soybeans are an attractive addition to diversify crop rotations in Manitoba as a nitrogen-fixing, low input crop. Seems ideal for your bottom line, right? However, there is one large limiting factor to the low input cost, and that is the cost of seed. Relative to most other crops grown in Manitoba, soybean seed cost is quite high. With that in mind, efficient use of seed inputs is critical for protecting positive economic outcomes at the end of the season.

The On-Farm Network (OFN), MPSG’s in-house research program, has been investigating soybean seeding rates since 2012. Over seven years, a total of 75 soybean seeding rate trials were conducted, comparing seeding rates ranging from 125,000 to 239,000 seeds/ ac (Figure 1). Each trial compared two or three seeding rates. Although various combinations of seeding rates have been used across the 75 trials, the main question remains consistent: “Can I lower my seeding rate while maintaining my yield?” Generally speaking, the answer seems to be yes.

Out of the total 75 trials, only 16 (or 21%) had a significant yield difference between seeding rates (Figure 2). Of these 16 trials, the higher seeding rates significantly out-yielded the lower seeding rates at 15 site-years, as expected. One site in 2012 had the opposite result, but that site had less than 45% emergence in the spring due to poor seed quality. The positive trend with greater seeding rates might make it seem as though reducing rates is not such a great idea after all. However, we need to break down the results further to look at the economics.

The paired bars in Figure 3 show the yield results for the 16 trials with significant yield differences between seeding rate treatments. Below the bars, the bu/ac value is the yield difference between the high and low seeding rate for each trial. Along with that yield difference is the soybean price required to offset the additional seed cost of the higher seeding rate at planting, based on the yield increase at harvest. These break-even prices range from $3.85/ bu, an obvious economically favourable yield response, to $23.75/bu, an out-of-the-question soybean price in today’s market. Examining the sites in Figure 2, only a handful of higher seeding rates resulted in yield increases great enough to cover the additional seed cost per acre, assuming realistic soybean prices. If we assume prices are below $10.00/ bu, about 8% of the trials had enough of a yield increase with a higher seeding rate to pay for the increased seed cost in the spring (Figure 2). If we are optimistic that prices will continue to be closer to $11/ bu in the future, then about 14% of the sites show a positive economic return from increased seeding rate. Whether we are optimistic, pessimistic or somewhere in between regarding soybean prices, 8–14% isn’t a very high success rate of favourable economic outcomes. Does this mean we can all reduce our seeding rates next spring? If so, how low can we go? Unfortunately, there isn’t likely to be a one-size-fits-all answer.

Seeding rate trials are ongoing in the OFN, and as the dataset continues to expand, so too will our analysis into the factors affecting yield outcomes for different seeding rates. A very important piece of the seeding rate puzzle is seed survivability­ — how much of what we put in the ground actually grows into a yield-producing plant? How can we optimize seed survivability? Is this the key to lowering seeding rates even further? How low is too low?

These are the types of questions we seek to answer in the OFN and we need your participation to make it happen! If you are interested in investigating different seeding rates on your farm, contact the OFN (204-751-0439 or megan@manitobapulse.ca). Stay tuned for the spring edition of Pulse Beat, where we will dive further into the seeding rate trial results, including those from the 2020 growing season, and discuss seed survivability, the factors affecting it and its influence on yield response.