Final Plant Populations for Optimum Soybean Yields in Non-Irrigated Fields of the Upper Mid-Southern United States.

Soybean produced in the upper mid-southern United States are primarily nonirrigated due to rolling uplands, erodible soils, and small fields common to the region.  Sole reliance on rainfall and the coinciding of the soybean reproductive period with the dry late summer months often result in inconsistent yields in maturity group (MG) IV and V soybean.  Recently, earlier maturing soybean cultivars (MG III) were introduced to this region to utilize early season rains and avoid later dry periods to increase and stabilize yields.  From 2005 to 2007, MG III soybean cultivars were planted in mid-May on rows spaced 38 and 76 cm apart to determine seeding rates and final plant populations that produce optimum seed yield (95% of the maximum yield) in this region.  Sporadic, heavy rainfall among extended dry periods occurred in 2005, and 284,100 to 464,900 seed ha^-1 and 125,000 to 232,000 plants ha^-1 at harvest were required to produce optimum yield.  Regular rainfall in 2006 necessitated 358,400 to 378,600 seed ha^-1 and 288,900 to 326,500 plants ha^-1 at harvest for optimum yield.  Drought in 2007 resulted in the lowest yields of the study.  No relationship between seeding rate and seed yield could be determined, and reduced emergence and survival produced low final plant populations. Soybean planted on 38-cm and 76-cm row spacings produced optimum yields from 192,800 and 92,100 plants ha^-1 at harvest, respectively.  Yield response to row spacing was inconsistent, but narrow rows always produced seed yields that were equal to or greater than those obtained from wide rows.  The results suggest that MG III cultivars should be planted on narrow row spacing at approximately 450,000 seed ha^-1, giving an expected population of 300,000 plants ha^-1 at harvest, to provide optimum seed yields in most years when planted on nonirrigated sites in the upper mid-southern United States.