By: Nick Monnig
If you have picked up a farm magazine or listened to a presentation on increasing soybean yields over the last 5-10 years, you have seen plenty of data demonstrating the advantages of planting soybeans early. As many soybean experts have put it, there are few management factors that can increase soybean yields as much as early planting.
It is no longer a question of whether to plant soybeans early, but now the questions move to the specifics around the process of planting early. Questions like: what is the right maturity to plant early, the right seeding rate and the right row spacing?
- If I am planting early, what maturity do I plant?
This is one of the most common questions I get when it comes to planting soybeans early, do I plant a maturity on the early side of my geography, or one on the later side? First off, the yield potential and defense traits (i.e., Sudden Death Syndrome tolerance, Phytophthora tolerance, etc.) needed trump maturity when it comes to varietal selection. What I mean by this, is if I have a 3.8 maturity in Central Missouri that comes with high yield potential and excellent defense traits, versus a 4.5 maturity with moderate yield potential and weaker defense traits, then I would select the 3.8. Now, let’s assume the two varieties have similar yield potential and defense traits, then I would select the 4.5 maturity. The University of Illinois and the University of Wisconsin found when planting early, the longer maturity offers more yield potential. When planting early, utilize a variety on the longer side of maturity for your geography with high yield potential and good agronomic, defense traits. This will extend the reproductive period and offer more yield potential.
The effect of full- and short-season cultivars on soybean grain yield at an early and normal planting date at 58 on-farm locations distributed throughout Illinois and Indiana in the growing seasons of 2011 and 2012. Vertical bars with the same letters are not significantly different at an α level of P ≤ 0.10. Error bars represent the 90% confidence intervals. Vossenkemper et al., 2016. Early Planting, Full-Season Cultivars, and Seed Treatments Maximize Soybean Yield Potential.
- What population do I utilize when planting early?
Soybeans can compensate a lot for missing plants. The initial population you plant should be driven by the yield potential of the field you are planting into (in general, less population in highly productive fields, and more population in lower producing fields).
While it is true that early planted soybeans might be more prone to adverse cool and wet conditions that challenge stands, they also have a greater ability to compensate for missing plants. So, planting rates for early-planted soybeans are probably not much different than rates at normal planting dates. There has been a trend to lower planting rates in general, over the last several years. Research from the University of Wisconsin (Table 1 below) may help explain part of this movement.
They showed in 2019 and 2020 research that precision placement of seed enabled them to plant at a lower population and maintain statistically similar yield to the highest treatment, than random seed placement (i.e., the comparison would be like a planter maintaining high placement accuracy vs. a drill without meters). The precision placement was able to maintain yield at lower populations because it provided faster canopy closure than random placement. In essence, advances in planter equipment technology are one factor playing into this reduction in soybean seeding rates.
Does Precision Planting Really Matter in Soybean? Spyros Mourtzinis, Adam C. Roth, John M. Gaska, and Shawn P. Conley. https://coolbean.info/wp-content/uploads/sites/3/2021/08/2021_SoybeanPlantSpacing_final.pdf
Bottom line, to use a conservative number, some would say 100,000 emerged soybean plants will maximize yield potential. Some folks on high-yielding ground may already be planting below 100,000. Recent Purdue research states there is no advantage to filling in a stand of soybeans above 66,000. So, there is a wide range in what many are gauging as optimal seeding rates. I am not going to tell you the exact population to plant, utilize previous experience, row spacing, weather forecast and field productivity. Try doing some testing at various rates, pushing your comfort on the high and low end.
- What row spacing is best for early-planted soybeans?
Early planting is more important than row spacing. This means there is more potential yield to be gained by planting early, regardless of the row spacing you use. Secondly, the productivity of the field will influence the yield difference between row spacings (i.e., there may not be much yield difference between 30” and 15” rows on a highly productive field, but there could be a huge advantage to 15” rows on a tough drought-prone field).
Data across the Midwest over several years has consistently shown an advantage to narrow rows (less than 30”) versus wide rows (30” or greater). A stressful year where soybeans are slow to canopy can make this advantage greater. Even with an early planting date, the data says narrow rows are advantageous over wide rows.
So, which row spacing do I use when planting early? My answer is to get the beans planted early regardless of row spacing. If you have a choice, the data supports narrow rows (less than 30”) over wide rows (30” or greater) regardless of planting date. Keep in mind the caveat that field productivity and environment can influence the difference.
- Should my fertility program change with early-planted soybeans?
For things like pH, phosphorus, potassium and others, we will want to follow soil test recommendations. If we are pushing yield potential with early planted soybeans, then our fertility must compensate to keep up with our goals. Outside of that, I want to look at early-season fertility. If you start thinking about early-planted soybeans, you start thinking about potentially cool and wet conditions. Conditions that are not good for the mineralization of nutrients from organic matter. These early-planted soybeans are not receiving a lot of nutrients like nitrogen and sulfur. Plus, atmospheric deposition of sulfur has decreased dramatically as we have cleaned up air quality in the U.S.
The bottom line is that we have seen big responses to sulfur fertility on early planted soybeans. Early spring applied sulfate forms of sulfur like ammonium sulfate, or ammonium thiosulfate. Data from Purdue shows that combining some nitrogen with sulfur in these cool conditions can provide a lot of synergy in terms of yield. This helps the early planted soybeans through adverse conditions before they start fixing their own nitrogen. Keep the nitrogen rate lower (30-40 lbs./a) to keep it from impacting nitrogen fixation later.