Degree Type


Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Matt Liebman


Most agricultural systems are designed without regard to their intrinsic effects upon weed populations. Yet cropping system characteristics may affect weed population dynamics by altering key demographic rates of weeds. This dissertation comprises two field experiments and two modeling studies that examine how tillage timing, legume green manure and compost affect the demography of giant foxtail (Setaria faberi Herrm.), an important summer annual grass weed of corn-soybean production systems in Iowa. Demographic data were collected for giant foxtail grown in a wheat-corn-soybean crop sequence in 2000 and 2001, with either a wheat sole-crop ('W') or wheat/red clover crop mixture ('R') in the wheat phase. In experiment 1, residues from the wheat phase were incorporated either in fall ('FT') or spring ('ST'). In experiment 2, residues from the wheat phase were incorporated in spring, with ('C') or without ('NC') a supplemental application of composted swine manure made the previous fall. Demographic rates estimated from the field data included seed survival from October to March (sigmas( w)) and March to October (sigmas (s)), seedling recruitment (gamma), plant survival (sigmap), fecundity (&phis;) and proportion of newly dispersed seeds not consumed by seed predators (sigmas( pred)). These demographic rates were used to parameterize matrix models of cropping system effects on giant foxtail demography either without (modeling study 1), or with (modeling study 2), some form of external weed control. The models were subjected to perturbation analysis to determine which giant foxtail life stages were responsible for treatment effects upon giant foxtail population dynamics. Compost was not found to have any effect upon giant foxtail demographic rates. Red clover residues were associated with very low values of sigmas( pred), compared to residues of wheat, corn or soybean. Tillage timing and red clover residues interacted in their effects upon sigma s(w), gamma, and &phis;. Simulation results indicated that high rates of giant foxtail seed predation, combined with low giant foxtail fecundity, led to consistently low population growth of giant foxtail in the FT/R cropping system treatment compared to all other treatments studied. These results suggest that cropping system effects on weed demography may constitute an important design consideration for agricultural producers.



Digital Repository @ Iowa State University,

Copyright Owner

Adam Samuel Davis



Proquest ID


File Format


File Size

171 pages