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Soybean planting dates based on soil temperature in Manitoba Cassandra Tkachuk 1 , Yvonne Lawley 1 , Robert Gulden 1 , Francis Zvomuya 2 1 Department of Plant Science, University of Manitoba, Winnipeg, Manitoba 2 Department of Soil Science, University of Manitoba, Winnipeg, Manitoba Cassandra Tkachuk: [email protected] Yvonne Lawley: [email protected] INTRODUCTION Soybean [Glycine max (L) Merr.] production has increased dramatically in Manitoba over the past decade with 1.63 million acres planted in 2016 (Stat Can, 2016). Due to this expansion, there are many new soybean growers in the province faced with the decision of when to plant soybeans. It is currently recommended in Manitoba to plant soybeans when the soil temperature at seed depth is at least 10°C on the day of planting. However, current information on the effects of soil temperature at planting is limited. OBJECTIVES MATERIALS & METHODS 1) To determine if soil temperature at planting was an influential factor for soybean yield, emergence, and physiological maturity in Manitoba. 2) To identify the soil temperature that produced the highest soybean yield. CONCLUSIONS REFERENCES RESULTS Soybean Yield Soybean Emergence Figure 6. Range of frost damage to soybean seedlings at Morden, MB on May 30, 2015 caused by air temperatures ranging from -0.5 to -1.6°C for a total of three hours. • Two soybean varieties: Dekalb 23-10RY (early variety) Dekalb 25-10RY (late variety) • Six planting dates assigned by the target soil temperatures of: 6, 8, 10, 12, 14, and 16°C • Operational soil temperature definition: The soil temperature at 10:00 AM at a 5 cm depth for two consecutive days Treatments seeded on the second day • Three field sites in MB: Carman (2014-2015) Melita (2014-2015) Morden (2015) • Randomized complete block design with four replicates • Emergence data modelled using the sigmoidal logistic function [E = M / (1 + exp (-kt + b))] Fitted to each soil temperature treatment Model parameters used to calculate maximum plant population at 100% emergence and days to 50% emergence Figure 2. The relationship between soybean yield and soil temperature at planting at (A) Carman, MB in 2014 and (B) Carman, MB in 2015. Note: calendar dates for corresponding soil temperatures at planting are labeled for each planting date treatment. Figure 3. The relationship between maximum soybean plant population at 100% emergence and soil temperature at planting at (A) Carman, MB and (B) Morden, MB in 2015. Figure 4. The relationship between days to 50% soybean emergence and soil temperature at planting for combined Carman, Melita, and Morden, MB site years in 2015. Figure 5. The relationship between percentage soybean seedling mortality and soil temperature at planting at Morden in 2015. • Results from this two-year field study suggest that there is no soybean yield penalty from low soil temperatures at planting. This finding should be tested with additional site years to strengthen planting date recommendations for Manitoba. • Calendar date had a greater effect on soybean yield than soil temperature at planting. • Late spring frost, rather than low soil temperatures at planting, increased soybean seedling mortality and reduced plant stands of early-seeded soybeans at one in two site years. • Overall, it is still recommended that growers should consider calendar date, seedbed conditions (Pedersen, 2006), the weather forecast following seeding, tolerance to loss from spring or fall frost, and timeline to complete seeding and harvest, when determining soybean planting dates (Figure 7). • No differences between cultivars were found for soybean yield. Reported results are averaged over the two cultivars. • A significant quadratic relationship between soybean yield and soil temperature at planting was only found for one in two site years at Carman in 2015 (Figure 2). Maximum soybean yield occurred at 9°C, beyond which yield declined with increasing soil temperature (Figure 2B). As Carman 2015 was the only site year with planting delayed into June, the decline in yield was likely influenced by later planting rather than warming soil. • Yields were overall greater at Carman in 2014 and 2015 when soybeans were planted into cooler soil temperatures, or on earlier calendar dates (Figure 1). • Although soybean yield was represented as a response to soil temperature at planting for both Carman 2014 and 2015, it is more likely that yield differences occurred due to calendar date rather than soil temperature at planting (Figure 1B). • No differences between cultivars were found for soybean emergence. Reported results are averaged over the two cultivars. • A significant positive relationship between soybean plant stand at 100% emergence and soil temperature at planting occurred for one in two site years at Morden in 2015. At this site, established spring plant populations increased with increasing soil temperature at planting (Figure 3). • Days to 50% soybean emergence from combined 2015 site years clustered into “cool” (6 to 12°C) and “warm” (14 to 22°C) soil temperatures. Cool temperatures generally caused delayed soybean emergence, and warm soil temperatures resulted in rapid emergence (Figure 4). A significant linear relationship was found between days to 50% emergence and soil temperature at planting for only cool soil temperatures, where days to emergence decreased with increasing soil temperature at planting (Figure 4). Contrastingly, days to 50% emergence were unresponsive to warm soil temperatures (Figure 4). This result suggests that soil temperatures of at least 14°C at planting are ideal for soybean emergence. • A significant negative linear relationship was also found between soybean seedling mortality and soil temperature at planting at Morden in 2015 (Figure 5). High soybean seedling mortality (Figure 5) coincided with low plant stands at 100% emergence (Figure 3B), suggesting that seedling mortality was responsible for low soybean plant stands at Morden in 2015. Figure 7. Time of seeding compromise diagram for Manitoba soybean growers (adapted from Buss, 2015). Buss, T. 2015. MAFRD CropTalk Eastman May 7, 2015. Available at https://www.youtube.com/watch?v=jbOvhB2ACnc&list=PLD7OiKfhYvB9GOFDK_GeBLXsFoZQK_fML&index=2 (verified Sep 2016). Pedersen, P. 2006. Soybean planting date. Integrated Crop Management Iowa State University. Available at http://www.ipm.iastate.edu/ipm/icm/2006/4-3/soyplant.html (verified May 2016). Statistics Canada. 2016. Estimated areas, yield, production, average farm price and total farm value of principal field crops, in imperial units. Available at http://www5.statcan.gc.ca/cansim/a01?lang=eng (verified Sep 2016) Frost Effect ACKNOWLEDGEMENTS Ian N. Morrison Research Farm Westman Agricultural Diversification Organization (WADO) Agriculture and Agri-Food Canada (AAFC) Morden Research Station -2.0 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 12 AM 6 AM 12 PM 6 PM 12 AM Temperature (°C) Time Soil Temperature Air Temperature Figure 1. Example of diurnal air and soil temperature fluctuation at Carman, MB on May 20, 2015. This example meets the criteria for 6°C at 10:00 AM. • Late spring frost events occurred at both Carman and Morden, MB on May 30, 2015; however, Carman experienced a milder frost event compared to Morden. Air temperatures ranged from -0.4 and -0.7°C over two hours at Carman, whereas temperatures ranged from -0.5 to -1.6°C for three hours at Morden. More extensive seedling damage was observed at Morden due to frost (Figure 6). • At the time of the late spring frost in Morden 2015 the first four treatments had emerged. As a result, these four treatments had greater seedling mortality (Figure 5). Thus, this late spring frost damage was likely responsible for higher seedling mortality (Figure 5) and lower soybean plant stands in these four treatments (Figure 3B). • Overall, this late spring frost rather than soil temperature at planting had an effect on soybean emergence variables. (B) Morden 2015 (A) Carman 2015 (B) Carman 2015 (A) Carman 2014
