SOYBEAN (GLYCINE-MAX), COMMON COCKLEBUR (XANTHIUM-STRUMARIUM), AND SICKLEPOD (SENNA-OBTUSIFOLIA) SAP FLOW IN INTERSPECIFIC COMPETITION

Two field experiments were conducted to examine the competitive effect of common cocklebur and sicklepod in soybean. Factors examined includ ed sap flow of both the weed and the crop, as well as the traditional parameters of weed and crop growth and crop yield. The intent was to d etermine to what extent competitive effects could be attributed to dif ferential water utilization. In the first study, soybean planted at 20 plants m(-1) row was infested with either sicklepod at 10 plants m(-1 ) row or common cocklebur at two plants m(-1) row. Sicklepod reduced s oybean sap flow and yield 53 and 49%, respectively. Common cocklebur r educed soybean sap flow and yield 31 and 38%, respectively. Within a w eed species, weed-induced yield and sap flow reductions were comparabl e, indicating that water deprivation was the primary detriment that th ese weeds inflicted on soybean. Individual common cocklebur plants wer e 3.9 times more competitive than sicklepod with respect to reducing s oybean yield. However, sap flow of individual common cocklebur plants was only 1.4 times more than that of sicklepod, indicating that water deprivation was not the only causal factor in weed-crop competition. I n the second experiment, sicklepod was planted in soybean (fixed densi ty) and clipped to either half the height, same height as soybean, or left unclipped, in an attempt to simulate herbicide-induced stunting. Clipping sicklepod had a positive effect on soybean sap flow and yield . Thus, the benefits of reduced weed competition were achieved without killing sicklepod.