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Title: Mid-South Soybean Yield and Net Returns as Affected by Plant Population and Row Spacing

Author
item THOMPSON, NATHANAEL - University Of Tennessee
item LARSEN, JAMES - University Of Tennessee
item ROBERTS, ROLAND - University Of Tennessee
item LAMBERT, DAYTON - University Of Tennessee
item Mengistu, Alemu
item Bellaloui, Nacer
item WALKER, ERIC - University Of Tennessee

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/23/2014
Publication Date: 5/5/2015
Citation: Thompson, N., Larsen, J., Roberts, R., Lambert, D., Mengistu, A., Bellaloui, N., Walker, E. 2015. Mid-South Soybean Yield and Net Returns as Affected by Plant Population and Row Spacing. Agronomy Journal. 107, 3: 979-989.

Interpretive Summary: Frequent late-season droughts and rising seed costs have caused farmers in the rolling uplands region of the Midsouthern United States to reevaluate their production practices. This study evaluated economically optimal plant populations considering maturity group, row spacing, input-output prices, and weather. Estimated economically optimal plant populations were generally higher for earlier maturing cultivars and for soybeans planted in narrow row spacing (15 inches or 38 cm) as was expected. However, soybeans planted in narrow row spacing consistently generated higher yields and net returns than soybeans planted in wider row spacing (30 inches or 76 cm), with the exception of years of extreme water stress, such as 2007 in this analysis. Maturity group IV cultivars maximized net returns based on expected weather conditions. However, results suggest that MG V cultivars may maximize returns if extreme surplus or deficit precipitation is experienced, such as 2005 and 2007 in this analysis. Although there has been growing interest into the use of earlier maturing cultivars in the rolling uplands region of the Midsouth, MG III soybean cultivars did not maximize net returns under any conditions evaluated in this analysis. Not only did these cultivars require higher seed costs, but they also failed to generate higher yields than MG IV soybean cultivars. Hence, the key finding of this analysis indicates that farmers in the rolling uplands region of the Midsouth may plant MG IV cultivars in narrow row spacing at seeding rates necessary to achieve final plant population density of approximately 46,559 plants per acre (115,000 plants per hectare).

Technical Abstract: Traditionally grown maturity group (MG) V and VI, and more recently adapted MG IV soybean [Glycine max (L.) Merr.] cultivars, are subject to late-season drought conditions in the Midsouthern United States when planted in mid-May resulting in yield limitations. Thus, the use of earlier maturing cultivars has been generating interest among soybean farmers in the region. The objective of this research was to determine economically optimal plant population density considering seeding rate, MG, row spacing, input-output prices, and weather in the rolling uplands region of the Midsouth for dryland soybean production. Field experiments conducted during 2005, 2006, and 2007, using MG III, IV, and V cultivars, were planted in mid-May in wide (30 inches or 76 cm) and narrow (15 inches or 38 cm) row spacing at a range of seeding rates from 24,292 to 240,081 seeds per acre (60,000 to 593,000 seeds hectare) to determine the production system that may maximize net returns. Given that farmers must make planting decisions based on expected weather, originally estimated yield response functions were weighted by year based on the Angstrom weather index. Results of the evaluation of weighted average response functions suggest that MG IV cultivars planted in narrow row spacing at seeding rates necessary to achieve a final plant population of 46,559 plants per acre (115,000 plants per hectare) may maximize net returns. Earlier maturing MG III cultivars did not maximize net returns under drought or normal weather conditions.