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 (r² = 0.90) and soybean (r² = 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 (r² = 0.60 to 0.85) and soybean (r² = 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.

Nomenclature: Green foxtail, Setaria viridis (L.) Beauv. SETVI; yellow foxtail, Setaria pumila (Poir.) Roem. & Schult. [= Setaria glauca (L.) Beauv.] SETLU; corn, Zea mays L. ‘Pioneer 3893’; soybean Glycine max (L.) Merr. ‘Pioneer 9091’.