Reliable estimates of weed fecundity require determination under ranges of management practices such as differing crops and tillage systems. We measured components of reproductive output per plant (numbers of primary tillers, panicles, and seeds; and sizes of panicles) in three emergence cohorts of green foxtail and yellow foxtail growing among corn and soybean in moldboard plow (MP), chisel plow (CP), ridge till (RT), spring disk (SD), and no till (NT). Differences in emergence between crops and foxtail Cohorts 1, 2, and 3 were 5, 0, and −7 d, respectively. In MP, Cohort 1 of green foxtail produced 2.3 primary tillers and 5.6 panicles per plant, and Cohort 1 of yellow foxtail produced 4.6 primary tillers and 9.0 panicles per plant. Panicle size was variable for both species across tillage systems, crops, and cohorts both years. Green foxtail plants produced the most seeds per plant (3,811) in NT corn, and cohorts did not vary greatly, whereas fecundity was highly variable across tillage systems and cohorts in soybean, where it averaged 3,240 (± 388) seeds per plant. Green foxtail seed number per plant were closely related to panicle numbers per plant for each year in corn (r 2 = 0.90) and soybean (r 2 = 0.78), and the relationship did not vary among tillage systems. Yellow foxtail seed number per plant was closely related to panicle number per plant, and it was specific for each tillage system in corn (r 2 = 0.60 to 0.85) and soybean (r 2 = 0.65 to 0.92). Estimates for vegetative and reproductive growth were more reliable for green foxtail than for yellow foxtail across tillage systems, crops, cohorts, and years.

Field studies were conducted in 1995 and 1996 to determine the rate response of velvetleaf seedling survival, seed production, and shoot biomass to postemergence herbicides in corn and soybean. Dicamba and imazethapyr were applied to corn and soybean, respectively, at 1, ½, ¼, ⅛, 1/16, 1/32, and 0× labeled rates. Velvetleaf mature plant density was linearly related to seedling density, thus indicating that seedling survival was not density dependent, even after seedling densities exceeded 150 plants m−2. Seedling survival as influenced by herbicide was described by a dose–response curve in corn and soybean. In corn, seedling survival ranged from 0 to 48% across herbicide treatments and years. Seedling survival was greater at the ½× or lower herbicide rates than at the 1× rate. In soybean, maximum seedling survival was 61 and 14% in 1995 and 1996, respectively, and minimum seedling survival was less than 2% in each year. Seedling survival was less in 1996 than in 1995 because velvetleaf was infected with wilt in 1996. In soybean, seedling survival was 20 times greater when treated with herbicides at the ½× rate than when treated at the 1× rate in 1995, but seedling survival was similar when herbicides were applied at 1, ½, ¼, and ⅛× rates in 1996. Velvetleaf fecundity (seeds per plant) was dependent on mature plant density in 1995 but was density independent in 1996. Fecundity as influenced by herbicide was described by dose–response curves in corn each year and in soybean in 1995. In 1995, velvetleaf treated with herbicides at ½× and ¼× rates produced 20 to 30 times more seed per square meter than when treated with herbicides at the 1× rate. Differences in seed per square meter were exaggerated by high densities of velvetleaf. Seed per square meter did not differ between velvetleaf treated with herbicides at 1× or ½× rates in corn or soybean in 1996. Wilt infection of velvetleaf in 1996 was the likely cause of differences in herbicide performance between years. Herbicides at reduced rates were not effective at limiting seedling survival and seed production compared to 1× rates in the absence of wilt. As a result, long-term management of velvetleaf with herbicides at reduced rates likely will be difficult, especially in areas with high densities, unless integrated with other management practices.