Field experiments were conducted in 1986 and 1987 to determine the effects of shoot and root interference of common cocklebur and soybean. Plants were grown in porous membrane envelopes. Common cocklebur plants had a longer vegetative growth period than soybean and were twice as tall as soybean at maturity. Soybean root and shoot dry weight and seed yield were reduced by shoot and whole plant (root and shoot) interference of common cocklebur, with whole plant interference giving the greatest reduction. Common cocklebur growth was affected little by soybean interference. Common cocklebur shoot interference alone reduced total soybean seed weight by 48%, the amount caused by common cocklebur whole plant interference.

Experiments were conducted in the greenhouse and growth chamber to evaluate competitive mechanisms of soybean and common cocklebur during seedling growth. Soybean was approximately 1.5 to 2 times taller and was more competitive aboveground than common cocklebur during the first few weeks of growth. Common cocklebur had an approximate 20 to 50% finer root system and was more competitive belowground than soybean during early growth.