Late-planted soybean yield response to reproductive source/sink stress

Research approaches for increasing soybean [Glycine max (L.) Merr.] yield at late planting dates would be better focused if knowledge of source/sink restrictions for yield during the growing season were better understood. Although soybean yield is known to have greater source restriction during the reproductive vs. vegetative period, little is known about differences in source restriction during the early (R1 to shortly after R5) compared with late (effective filling period) reproductive periods, or about potential sink restriction during the late reproductive period. Using crop growth rate as an indicator of source strength and seed number as an indicator of sink strength, our objectives were to: (i) Determine if yield of late-planted determinate soybean had the same degree of source restriction during the early and late reproductive periods; and (ii) Determine the potential for sink restriction of yield during the late reproductive period under field conditions. Field studies in a randomized complete block with four replications were conducted during 1993 and 1994 with 'Centennial' soybean (Maturity Group VI) at Baton Rouge, LA, on a Commerce silt loam soil. Crop growth rate and source/sink ratio (crop growth rate/seed number) were altered by partial defoliation and pod removal treatments. Reduced crop growth rate during the early reproductive period had 147% (in 1993) and 63% (in 1994) greater effect on yield compared with depressed crop growth rate during the late reproductive period, indicating source restriction was greater during the early vs, late reproductive period. In conclusion, soybean yield is most affected by stresses which reduce crop growth rate, when those stresses occur during the early reproductive period.