Cropping System Effects on Giant Foxtail (Setaria faberi) Population Dynamics: Modeling Analyses.

Crop rotation sequences create complex agroecological environments that affect weed population dynamics by altering key processes of the weed life cycle. We investigated the effects of a conventionally managed 2-year rotation (corn-soybean) and a diversified 4-year rotation (corn-soybean-small grain+alfalfa-alfalfa) on giant foxtail seed bank dynamics using matrix population models and demographic data collected in 2003-2005 and 2008-2010 from a field study in Boone, IA. Using mean values for seed decay from three years of observations, modeling analyses projected that over a 20-year period, giant foxtail seed banks would decline in the 2-year rotation (λ=0.83) and increase in the 4-year rotation (λ=1.04). Elasticity analysis indicated that changes in the proportion of over-winter and summer seed survival at a depth of 11-20 cm would have the greatest impacts on the population growth rate in the 2-year rotation, whereas several parameters, including over-winter and summer seed survival, and seedling emergence and survival, would have strong effects on the population growth rate in the 4-year rotation. Modeling analyses also showed that giant foxtail summer seed decay values higher than 20% would reduce the seed bank in the 4-year rotation. These findings highlight the importance of preventing or reducing the incorporation of weed seeds into deep soil layers, and focusing attention on ecological processes that occur at the soil surface and below ground, such as seed predation and seed decay.