Competitive Interactions of Common Bean with Amaranthus palmeri (Palmer amaranth) and Ipomoea purpurea (Tall Morningglory) Evaluated Using a Replacement-Series Design

This study examines the competitive interactions between common bean (Phaseolus vulgaris L.) and two problematic weed species, Amaranthus palmeri S. Watson (Palmer amaranth) and Ipomoea purpurea L. (tall morningglory), using a replacement series design. The analysis evaluates important competition indices, namely relative yield (RY), total relative yield (TRY), and key growth parameters, height and biomass produced by the crop and weeds, along with common bean yield components (i.e., pods per plant, seeds per pod, and pod weight) across the varying proportions of crop-to-weed ratios (100:0, 75:25, 50:50, 25:75, and 0:100). Results demonstrate that I. purpurea exerts greater competitive pressure than A. palmeri. For example, I. purpurea significantly suppressed bean biomass compared to A. palmeri (28.5 vs. 65.5% and 66.7 vs. 87.5% relative to the control for the 75:25 and 25:75 replacement series proportions, respectively). Bean yield and yield components were significantly reduced as weed proportion increased in the replacement series. In particular, the number of pods declined to 2.1 in the 25:75 crop: weed proportion, compared with 7.1 pods plant−1 in bean monoculture. A similar trend was observed for seeds per pod as bean proportion increased in the replacement series. At the 25:75 ratio, for example, bean plants produced 3.9 seeds per pod−1, which increased to 4.3 seeds at 50:50 and 5.2 seeds at 75:25, reaching 5.8 seeds per pod−1 in the pure bean stand. Total relative yield analysis reveals overyielding at the 75:25 proportion, suggesting niche differentiation or competitive equivalence between species. The results indicate that weed competition primarily reduces pod number per plant, with secondary effects on seed set and pod biomass. These findings have provided insights into common bean–weed competition while exerting important implications for integrated weed management strategies in bean production systems.