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104 SEEDWORLD.COM DECEMBER 2015 Hula Hoop Method When determining the plant stand with this method randomly toss any round hoop with a known diameter on the ground and count the number of plants within the hoop. Do this at least five times in different areas of the field and calculate the average then multiply that number by the appropriate multiplier Table 3 to get the number of plants per acre plant stand. Replant Threshold Our study showed that the highest yields were achieved with initial plant stands 100000 plants per acre Figure 1. This is consistent with Lee et al. 2008 who stated soybeans in Kentucky require plant stands above 100000 plants per acre to achieve 95 percent of maximum yield. This is further dem- onstrated by the initial seeding rates of 40000 60000 and 80000 seeds per acre with no replanting which produced final plants stands well below 100000 plants per acre and yielded 10 5 and 4 bushels per acre less than the maximum yield respectively Figure 1. However when these same plant stands were filled in and the final plant stands were subsequently increased above 100000 plants per acre significant yield increases of 7 2 and 2.5 bushels per acre were attained respec- tively Figure 1. Replanting initial soybean stands 100000 plants per acre where replant is not beneficial. Therefore the threshold for soybean replanting is 100000 plants per acre. Replanting Options When below threshold soybean stands arise 59000 plants per acre produced similar or higher yields compared to using tillage and replanting with 220000 seeds per acre Figure 1. Therefore filling in soybean stands below the replant threshold 100000 plants per acre is the best method of replanting and replant seeding rates should be high enough to increase the final plant stand over 100000 plants per acre. Figure 2 depicts a stand with 37000 plants per acre being filled in with 100000 seeds per acre. Seed Treatment and Planting Date Effects on the Replant Decision We observed no effect of seed treatment use on replant deci- sions and therefore should not be a factor considered. However seed treatment use especially insecticidefungicide treatments may help avoid replanting because it is an effective manage- ment practice for increasing initial plant stands stands by 20 percent on average Gaspar et al. 2014. Our study indicated a large yield decline as planting was delayed past the first week in May Figure 3. This yield decline is most likely due to decreased light interception of later planted or replanted soybeans. The earliest planting date yielded 73 bushels per acre Figure 3. We observed a 0.25 bushels per acre per day yield decline between the early May and late May planting dates which then doubled to 0.5 bushels per acre per day between the late May and mid-June planting dates. The average yield decline through the whole planting season was 0.32 bushels per acre per day. However the replant decision was not affected by planting date and therefore the replant threshold 100000 plants per acre method fill-in and seeding rates 100000 plants per acre are appropri- ate until June 20 in southern Wisconsin. Replanting past this date greatly increases the risk of fall frost damage Conley and Gaska 2013. Conclusion and Recommendations The first step in deciding if replanting is required is to determine the initial plant stand. Our study demonstrated that replanting soybean stands below the threshold 100000 plants per acre by filling in the existing stand increased yields regardless of the date May-June 20 and seed treatment use. Below threshold Table 2. Length of row to count for 11000th of an acre at different row spacings. Row Width inches Length of Row 30 No 20 Yes 15 Yes 10 Yes 7.5 No Length of Row 43560 row widthft 1000 Table 3. Multiplier to use based upon the diameter of a round hoop. Hoop Diameter inches Multiplier 18 24662 21 18119 24 13872 27 10961 30 8878 33 7337 36 6165 Multiplier 43560 hoop radius2 x 3.14 144