Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2010
Publication Date: 3/1/2010
Citation: Walker, E.R., Mengistu, A., Bellaloui, N., Koger III, C.H., Roberts, R.K., Larson, J.A. 2010. Plant population and row spacing effects on maturity group III soybean. Agronomy Journal. 102(3):821-826. Interpretive Summary: Soybean is an essential crop to the United States, with a gross value of production of approximately $27 billion in 2008, and $1.1 billion in the upper mid-southern states of Tennessee, Kentucky, eastern Mississippi, and northern Alabama. Due to its rolling uplands, highly erodible soils, and small fields, there are very few acres of irrigated soybean in this region. Recently, earlier maturing soybean cultivars than historically produced in the region were introduced to utilize rains in the spring and early summer and avoid seasonal dry periods in the mid- to late summer to increase and stabilize yields. Given the relative newness of utilizing early maturing soybean cultivars in a region previously planted in later maturing cultivars, production practices have not been optimized, and full yield potential has not been realized. Research was conducted to determine seeding rates and final plant populations that produce optimum seed yields of early maturing soybean produced on narrow (15 inches apart) and wide rows (30 inches apart) in the upper midsouth. Although yields were inconsistent, there were occurrences of seed yield increases due to narrow rows, and narrow rows always produced seed yields that were at least equal to those obtained from wide rows. The results suggest that in nonirrigated sites of the upper mid-southern United States, early maturing soybean should be planted on narrow rows at a seeding rate of approximately 180,000 seed per acre, giving an expected population of 120,000 plants per acre at harvest to provide optimum seed yields in most years.
Technical Abstract: Maturity group IV and V soybean produced in the upper mid-southern states of Kentucky, Tennessee, eastern Mississippi, and northern Alabama are primarily nonirrigated due to rolling uplands, highly 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 results in inconsistent yields. Recently, earlier maturing soybean cultivars (MG III) were introduced to this region to utilize rains in the spring and early summer and avoid seasonal dry periods to increase and stabilize yields. During 2005, 2006, and 2007, MG III soybean cultivars were planted in mid-May on narrow (38 cm apart) and wide (76 cm apart) rows to determine seeding rates and final plant populations that produce optimum seed yields in this region. Soybean seeding rate and final plant population were fitted to linear, curvilinear, and quadratic models to determine the best indicator for optimum soybean yields of MG III cultivars, and regression equations usually called for higher seeding rates and early plant populations than necessary to provide optimum yields. Sporadic, heavy rainfall among extended dry periods occurred in 2005, and seeding rates of 284,100 to 464,900 seed per hectare were required to produce 95% of the maximum yield, while 125,000 to 232,000 plants per hectare at harvest produced optimum yields. Optimal rainfall in 2006 necessitated seeding rates of 358,400 to 378,600 seed per hectare and final plant populations of 288,900 to 326,500 plants per hectare for 95% of the maximum yield. Extreme 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 increased self-thinning produced low final plant populations. Soybean planted on 38-cm row spacing produced optimum yields from 192,800 plants per hectare at harvest, while soybean planted on 76 cm row spacing required a final plant population of only 92,100 plants per hectare to produce 95% of the maximum yield. Overall, yield response to row spacing was inconsistent, but there were occurrences of seed yield increases due to narrow rows, and narrow rows always produced seed yields that were at least equal to those obtained from wide rows. The results suggest that MG III cultivars should be planted on narrow row spacing at approximately 450,000 seed per hectare, giving an expected population of 300,000 plants per hectare at harvest, to provide optimum seed yields in most years when planted on nonirrigated sites in the upper mid-southern United States.