Bryan Dillehay1, William Curran2, David Mortensen3, and Marvin H. Hall1. (1) Pennsylvania State Univ., 116 ASI Building, University Park, PA 16802, (2) 116 AG Science and Industry Building, Pennsylvania State Univ., Pennsylvania State University, Dept of Crop and Soil Sciences, University Park, PA 16802-3504, (3) Penn State University, University Park, PA 16802
As a poor competitor with weeds, alfalfa seedling success appears to be strongly related to the critical period of weed control. The critical period of weed control consists of two complementary events that lead to a necessary weed-free period during the crop growth cycle. The first component of the critical period examines the effect of weeds that emerge with the crop that are then removed after a set duration, while the second component examines the effect of weeds that emerge some time after crop emergence and remain until crop harvest. Once these events are combined, a specific critical weed-free period can be identified during the crop growth cycle. The objective of this study was to determine the critical period of weed control in alfalfa by demonstrating how weed competition impacts alfalfa yield as well as the importance of proper herbicide application timing for optimum weed control. Alfalfa was seeded conventionally at two locations in Pennsylvania in the spring of 2004 and 2005, glyphosate and glyphosate-resistant alfalfa were used for weed management. Three weed densities were examined using Japanese millet (Echinochloa frumentacea) as a surrogate weed to simulate varied weed severity. Treatments included a control, four individual glyphosate applications based on time elapsed after planting, a weed-free treatment, and three delayed seedings of Japanese millet after alfalfa planting. The delayed Japanese millet plantings simulated new weed flushes that occur after crop emergence or failure with the initial weed management practice. Plot yields were converted to relative yield of the weed-free control. The relative yield was then regressed against the number of days for which the plots were kept weedy or weed free using nonlinear regression following a sigmoid model. Preliminary analysis suggests that the critical period of weed control for alfalfa in Pennsylvania is between 4 and 6 weeks after planting.
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