Updated December 2, 2016:
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Project Title: Optimizing soybean plant population for early, ideal and late planting dates

Dr. Yvonne Lawley, Department of Plant Science, University of Manitoba, Project Lead
Cassandra Tkachuk, Department of Plant Science, University of Manitoba, M.Sc. Student

Soybean producers make important decisions at planting to maximize seed yield and economic return by optimizing planting date and plant population. With the expansion of soybean acres in northern growing regions of Manitoba and North Dakota, current information on soybean yield response to different planting windows is limited. Planting time is critical, as growers in these short growing season regions face the risk of both late spring and early fall frost. If a poor soybean stand occurs, it can be difficult to determine if replanting is necessary due to the ability of soybean plants to compensate for stand loss by adding branches, pods, and seed. Although it is currently recommended to increase seeding rates with late planting to compensate for loss in yield potential, it can still be risky to replant or delay planting until June. The objectives of this study were to: 1) identify the impact of reduced soybean plant stand on soybean yield, 2) monitor the compensation level of soybean plant growth based on planting date and plant population density, and 3) compare economic optimum soybean plant populations for early, ideal, and late seeding windows.
Six target plant populations: 80,000, 110,000, 140,000, 170,000, 200,000 and 230,000 plants ac-1 were seeded on early (May 4), ideal (May 22), and late (June 10) calendar dates in 2015 at the Ian N. Morrison Research Farm in Carman, Manitoba. The lowest plant population was intended to mimic a 50% stand loss, and the “late” planting date was intended to mimic a replanted soybean crop. The results of this study can be compared to the results reported from Carman in 2014 and to a partnered study at the NDSU Carrington Research Extension Center in Carrington, North Dakota.
Yields increased with increasing plant population until reaching maximum yields. These maximums were similar for the early and ideal dates but different for the late planting date at Carman in 2015 (Figure 1). Beyond the maximum level, yields declined with further increases in plant population for all planting dates.
Marginal cost analysis was used to calculate the economic optimum seeding rate for soybeans to incorporate yield maximization and economic return. Assumptions for these calculations included a soybean seed cost of $44 per 1000 seeds excluding seed treatment, a soybean price of $10 bu-1, and the average mortality from the experiment of 22%. The economic optimum seeding rates calculated for the treatments in the Carman 2015 experiment were 154,000, 144,000, and 198,000 seeds ac-1 for early, ideal, and late planting, respectively. Economic optimum seeding rates were also calculated for the results previously reported from Carman 2014. They were 157,000 and 147,000 seeds ac 1 for the ideal and late planting (no early treatment in 2014). The econmic optimum seeding rates calcualted for all planting dates in both years were similar but always lower than the plant population that achieved maximum yield.
Comparing trends in economic optimum seeding rates between 2015 and 2014 reveal that likelihood of late season moisture should be a seeding rate consideration for late seeded soybeans. At Carman in 2014, the economic optimum seeding rate was lower for late compared to ideal planting dates, the opposite of what was expected. This may be explained by the early fall frost and lack of precipitation during the seed filling period (R5 to R7 stages) that occurred in 2014. In contrast, 2015 results coincide with current recommendations to increase seeding rate for late planting, where adequate precipitation was received during seed-fill. Thus the results from this study suggest that soybean growers should consider both growing season length and the potential for late season moisture when deciding rates for late planting.