Random Stand Deficiency and Replanting Delay Effects on Soybean Yield, Yield Components, Canopy, and Morphological Responses

Poor soybean [Glycine max (L.) Merr.] emergence presents difficult replanting decisions. Most plant population research has involved deficient stands with even distribution rather than the more commonly encountered random gap distribution. The objective was to provide guidelines for making profitable replanting decisions. Field experiments were conducted in 1984 and 1985 to compare growth parameters and seed yield of an adapted cultivar in northern, central, and southern Illinois at 42,40, and 38° N Lat, respectively. Initial and delayed (21 d) plantings were made in rows 0.76 m apart. Two plant densities (291 550 and 202 350 plants ha⁻¹) and five random stand deficiencies levels (0 to 60%) were established within each planting. The largest effect on seed yield was caused by random stand deficiency followed by environment and planting date. Stands with randomly imposed 0.31 m gaps totaling 40 and 60% of the plot areas resulted in 10.8 and 21.1% yield reductions, relative to uniform stands in the initial planting, respectively. Replanting delays resulted in an average relative yield loss of 9.0%. Photosynthetic photon flux density measurements were made when the initial planting reached R5 and were expressed as percent light interception (PLI). The PLI at R5 was not related to the 9.0% yield loss due to replanting delay. At the 60% stand reduction level the low plant density mean yield was 6.2% less than the high plant density mean over both plantings. Seed number per plant ranged from 63.9 in replanted full stands to 195.1 in the most deficient initial planted stands. Seed weight effects were minimal and inconsistent. The PLI decreased quadratically with increasing stand reductions from 92.5% in full stands to 80.4% in stands reduced by 60% (over all other factors). Relative yield (RY) was regressed on PLI as follows: (i) initial planting: RY = 39.65 + 1.4781 PLI (r² = 0.941, P < 0.01); and (ii) Delayed replanting: RY = 47.75 + 1.4598 PLI (r² = 0.944 P < 0.01). In most environments only stand reductions of over 50% could be justifiably replanted. The results provide guidelines for decisions of whether or not to replant, based on expected yield losses due to irregular initial stand emergence compared with expected losses due to replanting delay. Research was supported in part by grants from Illinois Soybean Program Operating Board.