Explanations for decreased harvest index with increased yield in soybean

Yield in soybean [Glycine max (L.) Merr] is controlled by total dry matter (TDM) accumulation and partition of that TDM into yield as measured by harvest index (HI). Previous research indicated that yield responded to TDM at R5 (seed initiation) [TDM(R5)] in an asymptotic relationship that plateaued at about 600 g m(-2). As yield rose with TDM(R5), HI declined, thus acting as a yield limiting factor. Explanation for the inverse relationship between yield and HI would help develop research strategies for yield improvement. Our objective was to determine if this negative relationship resulted from an inverse relationship between node production efficiency [node no. per gram TDM(R5)] with TDM(R5). Data for this study were collected from previous studies containing a variety of cultural treatments (planting dates, row spacings, plant populations, and waterlogging) conducted near Baton Rouge, LA (30 degrees N lat), between 1987 and 1996 and combined into a single data set. Regression analyses between yield, TDM(R5), yield components, and production efficiencies of yield components [yield component level per gram TDM(R5)] were conducted. Results indicated that the inverse relationship between HI and yield (R-2 = 0.54; b(1) = P < 0.0001, b(2) = P < 0.05) resulted mainly from a negative relationship between node production efficiency and TDM(R5) (R-2 = 0.81; b(1) = P < 0.0001, b(2) = P < 0.05), which resulted in less efficient production of pods and seeds (the main yield components controlling yield) as TDM(R5) increased. In conclusion, reduced node production efficiency with increased TDM(R5) created the inverse relationship between HI and yield.