PARENT SESSION
Oral Session # 26: Agro-Ecology II.
Presiding: K Gross
Tuesday, August 5. 1:30 PM to 5:00 PM, SITCC Meeting Room 100.

Assessing patch dynamics of common sunflower (Heliathus annuus) in response to weed management in row crop agriculture.

Humston, Robert^{*,1, 2}, Mortensen, David¹, Bjørnstad, Ottar¹, ¹ Penn State University, University Park, PA, USA² Washington and Lee University, Lexington, VA, USA

ABSTRACT- Establishment and spread are important processes in pest weed invasion, yet few studies have quantified these dynamics with respect to weed management approaches in row-crop agriculture. Weed patches with a range of initial starting densities were established under a range of management practices and spread measured over a three year period. Common sunflower, Helianthus annuus, patches were established in year 0 in 1.3 m² quadrats centrally located in 9.91 m by 15.85 m plots at three densities: 1) low (500 seeds / 1.3 m²); 2) medium (1500 seeds / 1.3 m²); and 3) high (2500 seeds / 1.3 m²). Patches were established in corn and in soybean and monitored for three years in a corn-soybean rotation cropping system. High and low management intensities were implemented by applying a full or half strength herbicide application (bentazon). Cultivation, herbicide application and harvest activities were oriented unidirectionally across the experimental area. Spatial variation in density of H. annuus was described by quantifying the number of seedlings in every 76 x 61 cm (0.46 m²) cell of the overlain sampling grid. Patch structure was evaluated with one- and two-dimensional metrics to examine extent of patch development in the first two years of growth. Reaction-diffusion models were then parameterized by assessing patterns of spatial patch density resulting from seed dispersal in year 0 and seedling emergence in year 1. Results indicate that crop type influences the intrinsic rate of increase of the number of plants in a patch, as well as the relative contribution of natural and anthropogenic forces to seed dispersal and patterns of patch expansion. Initial seed density had the most significant impact on patch dynamics, influencing growth rate, coefficient of diffusion, and lateral velocity of patch expansion. Experimental treatments had negligible impact on the asymptotic velocity of spatial expansion of the weed population. Restricting the magnitude of the initial infestation appears to hold the greatest potential as a management strategy for controlling expansion of pest weed populations in agricultural systems.

Key words: reaction-diffusion, Helianthus annuus, dispersal, pest weed management