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An update from the soybean and pulse agronomy lab
Kristen P. MacMillan, MSc, PAg, Research Agronomist, Department of Plant Science, University of Manitoba
WHAT SEEDING DATE and depth maximizes soybean yield? Seeding within the first three weeks of May and within the range of 0.5 to 1.5 inches has provided the most consistent yield advantage among the environments tested so far in these experiments. But, that doesn’t come without variability.
SOYBEAN SEEDING DEPTH
Soybean seeding depths between 0.25 and 2.25 inches were tested at Arborg (clay soil) and Carman (loam soil) in 2017 and 2018. Trials were seeded between May 14 to 24 into dry soil conditions, much like many crops these past two years.
At the time of seeding, moist soil was down around 1.25 inches in 2018 and an accumulated 25 mm of rain took about 14 and 21 days in 2017 and 2018, respectively. In the combined analysis (excluding Arborg in 2017), both environment, seed depth and their interaction were significant. In other words, the effect of seed depth on soybean yield varied by environment (Figure 1).
In both years at Arborg, seed depth did not affect soybean yield. This made sense in 2017 when the depth range tested was only 0.25 to 1.75 inches and the trial was seeded into fallow land. In 2018, a trend was evident but variability restricted statistical differences. Now let’s turn to Carman – at Carman in 2017, soybean yield was reduced by 25% when seeded at 2.25 inches compared to 0.5 and 0.75 inches. The other seed depths produced yields similar to all other treatments. This was the only environment where yield was significantly reduced with deep seeding.
At Carman in 2018, soybean yield was reduced by 20% with shallow seeding (0.25 inch) compared to seeding at 1.25 to 1.5 inches. The other depths were statistically similar to all others. It’s important to discuss interactions when they occur, like we just did, but it also makes recommendations across a range of environments difficult. That’s why we say “it depends” a lot. When looking at the combined analysis and overall effect of seed depth on yield, the same trend exists at each environment – although to different degrees, which leads to the interaction.
Even though yield loss with deep or shallow seeding is not large every time, when it does occur (one in four environments thus far), it is substantial (20–25%). This leads me to the preliminary conclusion that seeding within the range of 0.5 to 1.5 inches will likely optimize yield in the long run. Factors such as soil moisture, soil type, residue management, land rolling, moisture holding capacity, compaction risk and rain forecast should guide your actual depth within that range.
What factors are driving the yield effect? Plant establishment is one factor that may be contributing to the effect of seed depth on yield as shown in Figure 2. Across environments, it shows a similar trend to yield where emergence is reduced with shallow and deep seeding. However, it is not the only factor. At Carman in 2017, soybeans seeded at 2.25 inches had a good plant population and still had a 25% yield reduction.
There is a visible impact of deep seeding on soybean seedlings – they emerge slower and symptoms of hypocotyl swelling and chlorosis are evident, leading to reduced seedling vigour (Figure 3). Seeding soybeans too shallow (0.25 inches) are also more prone to moisture fluctuations, resulting in wetting and drying of the seed which leads to poor germination and establishment. To identify other mechanisms contributing to yield differences, we measured the effect of seed depth on pod height in 2018 and we plan to measure nodulation and root rot in 2019, which will be the last year of the study.
SOYBEAN SEEDING WINDOW
We seeded soybeans within four seeding windows (very early, early, normal and late) from April 28 to June 4 at Arborg, Carman, Dauphin (2018 only) and Melita over the past two years to determine if there is an optimum time to plant soybeans and if that varies by environment.
Overall, soybean yields ranged from 21–40 bu/ac in this study, except for Dauphin in 2018, which yielded an incredible 64 bu/ac. Preliminary results show that the seeding window that optimized yield varied by environment. However, I am going to present the overall main effect of seeding date because the trend was present in each environment. Overall, seeding within the first three weeks of May optimized yield (Figure 4). There is no clear recommendation to seed soybeans as early as possible, which we hear from southern growing regions.
Whether you should seed the first, second or third week of May to maximize yield varies by site-year/environment (data not shown). For example, yield maximization occurred in the first/very early seeding window at three out of seven environments (Arborg 2017, Carman 2018 and Melita 2018) – out-yielding the second and third dates by 2–12%. In the remaining four environments, the second/early seeding window maximized yield by 1–14% compared to the first and third. These differences between the first three seeding windows were rarely statistically different and, on average, were within 4% of the maximum yield (range 1–14%). To me, these differences are acceptable and difficult if not near impossible to predict.
Seeding itself is also not that precise – it’s dictated by weather, field conditions, crop priority and equipment. This data tells us that the soybean seeding window in Manitoba is quite flexible up to about May 24, with yield maximization generally occurring in the first or second week. Exactly when you seed within that window will depend on the conditions listed previously and your last spring frost date.