Seedling growth of soybean [Glycine max (L.) Merr. ‘Forrest’], common cocklebur (Xanthium strumarium L. # XANST), and tall morningglory [Ipomoea purpurea (L.) Roth. # PHBPU] was compared in field studies. Root growth was determined by measuring roots restricted to the inside of a permeable membrane that contained no soil. Leaf area duration and net assimilation rate between 4 and 9 weeks were greatest for common cocklebur compared to the other species. At 9 weeks, shoot and root dry weight and leaf area were greatest for common cocklebur compared to soybean and tall morningglory. Relative growth rate of common cocklebur roots was greater than the other species between 4 and 9 weeks. Tall morningglory had greater shoot length or height at 4 and 9 weeks and greater shoot or height elongation rate between 2 and 4 weeks, and 4 and 9 weeks than the other species.

The influence of temperature and relative humidity on the activity of acifluorfen, fomesafen, lactofen, and acifluorfen plus bentazon on prickly sida, pitted and entireleaf morningglory, and common cocklebur was evaluated in a growth chamber. Reduced control of all species was observed at 50% relative humidity as compared to 85% relative humidity when temperatures were higher (32/55 C day/night). Similar response to relative humidity was observed at the lower temperature (25/15 C) when treatments were applied 14 days after emergence (DAE). Changes in temperature at the same relative humidity did not alter herbicidal activity. Delaying application timing from 7 to 14 DAE decreased control by all herbicides except lactofen applied at high relative humidity, which controlled prickly sida at both 7 and 14 DAE.

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.

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.

Field experiments were conducted using a stale seedbed production system to determine the effect of herbicide application time on preplant, preplant incorporated (PPI), and at-planting treatments on weed control and soybean yield. Herbicides were applied on the surface preplant (PPL) or PPI at 6 to 7, 4 to 5, and 2 to 3 wk before planting and just prior to planting. The differences in weed control and soybean yield among years were due to rainfall patterns 2 wk after herbicide application and during the growing season. Preplant treatments applied 2 to 5 wk before planting generally controlled common cocklebur and pitted morningglory better than preplant treatments applied 6 to 7 wk before planting due to persistence of herbicide activity or treatments at planting due to a greater chance of obtaining adequate rainfall for herbicide activation, more uniform seedbed at planting, and larger weeds at application. Metribuzin plus chlorimuron was less suited than imazaquin as a preplant treatment when applied more than 2 weeks before planting.