Can Soybean Seeding Costs Be Reduced Through Plant Breeding?.

Soybean seed costs have risen sharply because of adoption of GMO cultivars.   Little research is available identifying cultivars having low minimal optimal plant populations (lowest plant population at which yield is maximized).  The objective of this study was to assess indirect selection criteria for identifying soybean cultivars having minimal optimal plant populations.  Studies were planted on 20 Apr, 2007 and 8 May, 2008 near Baton Rouge, LA.  Experimental design was a randomized complete block in a split plot arrangement with four replications and two years as random factors.  Main plots were eight public soybean cultivars having putative differences for minimal optimal plant population.  Split plots were two plant populations: a normal plant population of 200,000 plant ha^-1 and a low plant population of 10,000 plant ha^-1.  Data were obtained on relative yield [(yield_{low pop.}/yield_{normal pop.}) X 100], and the following growth dynamic parameters for the low plant population treatments:  total dry matter per plant (TDM, g per plant), branch dry matter per plant (BDM, g per plant), partition of TDM into BDM (BDM part, %).  Because of maturity differences between cultivars, these three parameters were normalized by dividing each by the number of days from emergence to R5 (period for formation of vegetative TDM).  A wide range in relative yield was shown (13 to 55%), indicating genetic potential for minimal optimal plant population.  When adjusted for days to R5, all three growth dynamic parameters successfully distinguished cultivars with high vs. low relative yield.  Relative yield was significantly linearly correlated with the three adjusted growth paramerters BDM (R²=0.75-0.84).  Since the three parameters demonstrated similar accuracy at identifying promising cultivars, and TDM is the easiest to obtain, it is recommended that adjusted TDM be used as the indirect selection criteria for cultivars having genetic potential for reducing minimal optimal plant population.