Authors:  James E. Board and Dinesh Maricherla

Explanation for the inverse relationship between harvest index (HI) and yield in soybean may help identify yield-limiting processes that lead to research strategies allowing for greater yield potential.  Our objective was to assess the efficacy of two hypotheses postulated to play a role in this phenomenon:  1)  The inverse relationship between HI and yield is caused by an inverse relationship between node production efficiency [node no. per area per unit total dry matter (TDM)] with TDM; 2)  The inverse relationship between HI and yield is caused by yield component compensation between complementary yield components at specific levels of yield formation.  Data were obtained from six studies conducted at Baton Rouge, LA (30^o N Lat) across a 10-year period (1987-1996).  Complementary yield components and their respective yield formation level were:  1)  Primary yield components affecting yield :   seed number per area and seed size;  2)  Secondary yield components affecting seed number per area:  seed per pod and pod number per area;  3)  Tertiary yield components affecting pod number per area:  pod per reproductive node  and reproductive node number per area; and 4)  Quaternary yield components affecting reproductive node number per area:  percentage reproductive nodes and node number per area.  Data also were obtained on production efficiencies for all yield components (yield component/TDM).  All TDM levels were determined at R5, because it is at this stage that vegetative dry matter is determined.  Results indicated that the inverse relationship between yield and HI clearly originated from the inverse relationship between node production efficiency with TDM(R5).  This basic relationship then sets the pattern for how production efficiencies of other yield components important in affecting yield (reproductive node number, pod number,  and seed number) respond to increasing TDM(R5).  Limited yield component compensation occurred between complementary yield components at any yield formation level.