Soybean Seeding Date Effects on Productivity Jane Froese 1, Bruce Brolley 2 and Derek Lewis 1

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1 Soybean Seeding Date Effects on Productivity Jane Froese 1, Bruce Brolley 2 and Derek Lewis 1 1 Dept. of Plant Science, University of Manitoba, Winnipeg, MB R3T 2N2 jane_froese@umanitoba.ca, derek_lewis@umanitoba.ca 2 Manitoba griculture, Food and Rural Initiatives, arman, MB RG J bbrolley@gov.mb.ca Introduction Soybeans are a good news story in Manitoba, particularly if one considers that it is a recently introduced crop to our province. creage of soybeans has increased from 8 in 1996 to a high of 363, in 26. Over the course of a decade, soybeans have established a foothold in Manitoba; one that adds to species diversity, as well as has the capability to fix atmospheric nitrogen and provide high potential cash value. One of the primary challenges, however, to growing soybeans in Manitoba is our relatively short growing season. Indeed, previous Manitoba gronomist onference presentations have touched on this difficulty with Bruce Brolley discussing Soybean Production in Short Season reas in 21 and Jason Voogt presenting the question in 24 Pushing the Geographic Envelope: How Far do we go with Warm Season rops?. The Seed Manitoba: Variety Selection and Growers Guide for Manitoba farmers lists soybean varieties according to their ompany Heat Unit rating. These heat unit values are provided by the company that produced the variety and are a relative measurement of the heat required for that variety to mature in trials across anada. While these heat units provide farmers some direction for selecting varieties, these heat units do not always reflect actual maturity in Manitoba (Seed Manitoba 27). In general terms, the Red River Valley area of Manitoba where soybeans are primarily grown may be divided into three soybean zones according to heat unit recommendations: < 24 heat units - north of Hwy #1 and east of Winnipeg 24-2 heat units - between Hwy #23 and Hwy 31 >2 heat units - south of Hwy #23 Objective The objective of this study was to determine the effect of four seeding dates (mid-may to mid-june) on soybean height (reflecting pod clearance at harvest), yield, seed weight, as well as seed protein and oil content. Materials and Methods This study was conducted in four southern Manitoba locations in 26: arman, Morden, rborg and Rosebank. Two glyphosate-resistant varieties were examined at each site: 9M1 (a 27 heat unit, large sized soybean) and 2-2R (a 2 heat unit, medium/large sized soybean). Due to the shorter growing season in rborg, pollo (a 24 heat unit, large sized soybean) was substituted for 9M1. Each of the two soybean varieties were sown at four approximately evenly spaced seeding dates ranging from the second week of May to the third week of June. Data were collected on soybean height, seed yield and weight, days to maturity, as well as seed protein and oil content. (Data from Rosebank was not analyzed in time to be included in this report.) Results and Discussion Seeding Date and ccumulated Growing Degree Days (GDD) Due to weather variability and equipment availability, it was impossible to set precise seeding dates (Table 1). lthough the intervals between seeding dates were not equally spaced, the seeding dates at all locations were always within to 6 GDD of each other. 1

2 Table 1. Soybean seeding dates and accumulated growing degree days (GDD) at arman, Morden and rborg in 26. Seeding date 1 Seeding date 2 Seeding date 3 Seeding date 4 alendar ccum'd alendar ccum'd alendar ccum'd alendar ccum'd Site day GDD z day GDD day GDD day GDD arman May May May June Morden May May June 2 38 June rborg May May June June z T base = 6 Plant Height Data pertaining to plant heights were collected at arman and rborg, however, treatment responses were only found at arman. That is, 2-2R was taller than 9M1 (P <.1) and significant differences (P =.2) due to seeding date were exhibited by the 2-2R variety (Fig. 1). The height of plants sown on the first two dates were similar to each other, however, height was observed to decline over the final two dates. The loss of approximately 16 cm between the second and fourth seeding dates may be substantial enough to affect pod clearance at harvest and the ability to capture maximum yield. 1 8 B B Height (cm) R 9M1 Figure 1. Plant height of two soybean varieties seeded at four seeding dates, arman 26. Within each variety, columns marked with different letters are significantly different at P<.. Seed Yield No significant yield differences due to date or variety were detected at Morden (data not shown) and it is hypothesized that water stress masked treatment effects. Evidence from the arman and rborg sites indicate that late seeding dates have a significantly negative effect on yield. Yield of soybeans planted on the fourth date are between one-third and one-half that of soybeans planted on the first date (Figs. 2 and 3). t neither site is there is any meaningful reduction in seed yield between the first and second seeding dates, however, yield declines substantially after both the second and third seeding dates. The notable exception is 2-2R at rborg where there is a precipitous decline in yield between the second and third seeding dates, but no significant decline between the final two seeding dates. It is hypothesized that the 2-2R variety (a 2 heat unit soybean) sown on the third seeding date experienced photoperiodtriggering of flowering prior to full physiological readiness, thereby reducing its yield potential. That the 2

3 same variety (2-2R) sown on the fourth seeding date yielded as well as that sown on the third seeding date may reflect the effect of late rains. It is also hypothesized that the yield of pollo (a 24 heat unit soybean) sown on the third seeding date was greater than that of 2-2R because this shorter season soybean was not as affected (i.e., more mature at the time of the photoperiod flowering trigger) by the late seeding date. Yield (kg/ha) Yield (kg/ha) R 9M1 2-2R pollo Figure 2. Soybean seed yield of two soybean varieties seeded at four different seeding dates, arman 26. Within each variety, columns marked with different letters are significantly different at P<.. Figure 3. Soybean seed yield of two soybean varieties seeded at four different seeding dates, rborg 26. Within each variety, columns marked with different letters are significantly different at P<.. Seed Weight Seed weight data analyzed from arman indicated significant effects of both variety (P <.1) and date (P =.2). s expected, the larger sized soybean (9M1) had fewer seeds lb -1 than 2-2R (Fig. 4). The data also indicate that later sown soybeans produce smaller sized seeds. Seed Weight (seeds/lb) B 2-2R 9M1 Figure 4. Soybean seed weight of two soybean varieties sown at four seeding dates, arman 26. Within each variety, columns marked with different letters are significantly different at P<.. Days to Maturity The number of days for soybeans to reach maturity was measured at rborg (Fig. ). ccording to expectations, pollo required fewer days than 2-2R to mature, and soybeans sown later in spring took less time to mature than those sown later. (Soybeans sown on the fourth date, June 22, required two 3

4 weeks less to mature than soybeans sown on the first date, May 12.) However, as the yield and seed size data indicate, there is a yield penalty to pay for photoperiod-triggered faster maturity. Days R pollo Figure. Days to maturity of two soybean varieties sown at four seeding dates, rborg 26. Within each variety, columns marked with different letters are significantly different at P<.. Seed Quality: Protein and Oil ontent Protein and oil contents were measured for the Morden site only (Figs. 6 and 7). There was no evidence of either variety or seeding date effects on either protein or oil contents. Protein ontent (%) R 9M1 Oil ontent (%) R 9M1 Figure 6. Soybean seed protein content of two soybean varieties seeded at four different seeding dates, Morden 26. Figure 7. Soybean seed oil content of two soybean varieties seeded at four different seeding dates, Morden 26. onclusions While the data in this report reflect the response of two soybean varieties at three locations over only one field season, several responses are notable. In terms of plant height, although only one variety (2-2R) at one location (arman) displayed a reduction of height with delayed seeding date, the observed difference of 16 cm would likely have a meaningful effect on pod clearance at harvest. Soybeans sown in the last week of May or later will require fewer days to mature and have a reduced seed yield and weight. The amount of this reduction is a function of the growing season, particularly weather conditions between flowering and onset of frost. In fact, research from other studies suggests that the actual amount of yield loss, if any, is related to the amount of stress in the growing season in late July to early September. Neither protein nor oil content was affected by seeding date. 4

5 Based on the results of this study (which will be repeated for a second year in 27), farmers are encouraged to focus on the first half of the seeding window, that is, the second and third week of May. cknowledgements The authors appreciate the contributions of Eric Klassen (Johnson Seeds), l Sloan (F Morden) and Daryl Rex (gricore United) in managing the rborg, Morden and Rosebank sites, as well as summer students ndrew McPherson, Stacey Dyck and Meagan Simpson. Funding for this project was provided by the Manitoba Pulse Growers ssociation and RDI. Reference Seed Manitoba: Variety Selection and Growers Guide. 27. Farmers Independent Weekly, Winnipeg, Manitoba.