INFLUENCE OF COHORTS ON CHENOPODIUM-ALBUM DEMOGRAPHY

Peak germination and emergence of common lambsquarters usually occur i n early to mid-spring, but both processes can occur during summer and fall. Seeds produced by different common lambsquarters cohorts (seedli ngs that emerge at nearly the same time) may vary in dormancy status, response to environmental conditions, and response to management facto rs. Therefore, experiments were conducted to determine the influence o f different cohorts on common lambsquarters demography. Field experime nts determined plant density, biomass, and seed production of differen t common lambsquarters cohorts within a crop-free community of annual weed species that included redroot pigweed, giant foxtail, and velvetl eaf. Common lambsquarters plant density and aboveground biomass were g reater for a mid-May cohort than for early June, late June, mid-July, or early August cohorts, but seed production of the mid-May and early June cohorts did not differ (about 192,000 seeds m(-2)) and was greate r than chat of other cohorts (111,500 seeds m(-2) or less). In the lab oratory percent germination prior to stratification (exposure of seeds to low temperatures) was less for seeds harvested from early May and late May cohorts (greater than or equal to 9%) than those of mid-June or early July cohorts (less than or equal to 75%). After stratificatio n in the field, percent emergence (seedlings per number of planted see ds) and mean emergence time were similar among early May, late May, mi d-June, and early July cohort seed sources, and were not influenced by shallow burial in soil. These results suggest that recruitment from s eeds produced by different common lambsquarters cohorts is similar, bu t proportional to the number of seeds produced by each cohort.