PARENT SESSION
Organized Oral Session 14: Ecology of arable plants: Linking invertebrate and weed population dynamics
Organizer(s): J Cussans, A Mauchline, and K Gross
Tuesday, August 9, 8:00 AM - 11:30 AM, Meeting Room 511a, Level 5, Palais des congrès de Montréal

Modeling the impact of seed predators on weed population dynamics.

Westerman, Paula^*,1, Liebman, Matt¹, Heggenstaller, Andrew¹, Dixon, Philip¹, Menalled, Fabian², Davis, Adam³, ¹ Iowa State University, Ames, Iowa, prwester@iastate.edu² Montana State University, Bozeman, Montana³ USDA-ARS Invasive Weed Management Unit, Urbana, Illinois

ABSTRACT- The introduction of herbicides in arable fields has greatly simplified crop protection, increased the productivity of farm labor, and fostered the development of conservation tillage systems. Despite these advantages, intensive herbicide use can threaten environmental quality and human health, and promote resistance in weed populations. Consequently, increasing numbers of farmers and scientists now seek less chemically dependent approaches for weed management. We used data from a field experiment and matrix models to examine how crop rotations with different herbicide inputs affected two weed species, velvetleaf (Abutilon theophrasti) and giant foxtail (Setaria faberi). Compared with velvetleaf, giant foxtail has a less persistent seedbank, greater seed production, and a more plastic growth habit. Herbicides were applied at conventional rates in a 2–year (corn–soybean) rotation, whereas applications in a 3–year (corn–soybean–triticale + red clover) and a 4–year (corn–soybean–triticale + alfalfa–alfalfa hay) rotation were 72% and 79% lower. Rates of weed seedling emergence, survival, fecundity, and seed loss to predators were quantified in each crop in each rotation system. Data collected from four replicate blocks in two years were used to parameterize depth–structured periodic matrix models and generate eight stochastic environments for all crop × rotation system combinations. Analyses of stochastic population growth rates, log _s, using the bootstrapped distributions, indicated that velvetleaf densities in all three rotation systems and foxtail densities in the 2–year rotation should drop below 10 viable seeds m⁻² in 15 years. In contrast, giant foxtail densities in the 3– and 4–year rotations should increase (_s 1.1). Variability in stochastic growth rates for foxtail in the 3– and 4–year rotations was high compared with the 2–year rotation, and seed losses of at least 40–50% would be required to ensure decreasing densities in periods < 20 years. Seed predation data indicated that achieving those levels of seed loss was feasible. Results of this study lead to the conclusion that diverse rotations that exploit multiple stress and mortality factors, including weed seed predation, can suppress weeds with less reliance on herbicides.

Key words: weed seed predation, weed population dynamics, diverse crop rotations, stochastic growth rates