Intraspecific interference of sicklepod and interspecific interference between sicklepod and soybeans were investigated in field experiments in 1982 and 1983. Sicklepod growth and reproductive parameters were reduced as intraspecific spacing between plants decreased from 90 to 2.5 cm. Sicklepod plants growing intraspecifically were competing at spacings below 90 cm. Soybean plant heights were affected only slightly by sicklepod interference. Soybeans grew little in height from 35 to 84 days after emergence, while sicklepod plants continued to grow until they were 30 to 45 cm taller than the soybeans. Sicklepod plants spaced 10 and 30 cm apart in soybeans reduced soybean yield (25 to 35%) more than sicklepod spaced 90 cm apart. Sicklepod produced 900 to 2000 seeds per plant even when growth was restricted by interference from soybeans.
