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Competition of Soybean with Blackjack ( Bidens pilosa L.) and Pigweed ( Amaranthus hybridus L.) Chivinge, O.A. & Schweppenhauser, M.A. Abstract Field studies were conducted to evaluate competitiveness between blackjack or pigweed and soybean using replacement series. Plant height of the three species was not affected by interspecific competition. Branching was reduced in soybean at 75 days after plant emergence (DPE), but increased in the weed species. Soybean, blackjack and pigweed shoot dry weight was reduced in most mixtures. The weed species had higher relative growth rate (RGR) and leaf area index (LAI) than soybean. The highest LAI occurred in monoculture pigweed. Competitive ration (CR) values showed that pigweed was the most competitive of the three species. Soybean pod number per plant, 1000 seed weight and grain yield per plant were significantly less in mixtures with the two weed species. Pigweed caused more reduction in soybean yield components than blackjacks. The number of blackjack seeds per head, seed heads per branch and seeds per plant were significantly reduced in mixtures while seed production by pigweed in mixture increased by 30 to 253%. It is concluded that both weed species reduce soybean vegetative growth except for height and components of yield. Keywords Interspecific competition, intraspecific competition, mixtures, replacement series, yield components.

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