Submitted to: Western Society of Weed Science Research Reports
Publication Type: Research Notes
Publication Acceptance Date: 12/1/2002
Publication Date: 3/13/2003
Citation: ANDERSON, R.L. ESTIMATING A ROTATION'S SELECTION PRESSURE FOR WEEDS, BASED ON JOINTED GOATGRASS DEMOGRAPHICS. WESTERN SOCIETY OF WEED SCIENCE RESEARCH REPORTS. 2003. p. 220-221. Interpretive Summary: Producers in the semiarid Great Plains are rapidly changing their crop rotations. This change provides an opportunity for producers to develop ecologically-based crop sequencing, where the benefits of natural ecological processes are incorporated into the production system. For example, weed survival in soil follows a characteristic pattern; viable seed numbers decline rapidly within two years. Less weed seed density in soil leads to less weeds in the following crops. Using a simulation model based on jointed goatgrass population dynamics, we evaluated the impact of rotating crops with different life cycles on weed density. The most favorable rotation to minimize jointed goatgrass density consisted of two winter annual crops (fallow serving as one crop) followed by two summer annual crops. Compared to winter wheat-fallow, selection pressure for jointed goatgrass was 1 million less with winter wheat-corn-proso millet-fallow. Producers in the Great Plains are fortunate in that both winter and summer annual crops are economically-viable options; thus, producers can reduce weed community density considerably with appropriate crop sequencing.
Technical Abstract: Rotations are rapidly changing in the Great Plains because of no-till systems. In place of winter wheat-fallow, producers are seeking rotations comprised of a diversity of crops. To help producers plan alternative rotations, we developed an empirical simulation model that estimated the impact of various rotations on weed populations. We used jointed goatgrass demographics for parameters of the model, and calculated selection pressure among rotations based on jointed goatgrass population changes. The rotation least favorable for jointed goatgrass was winter wheat-corn-proso millet-fallow; jointed goatgrass density was 1-million fold less with this rotation compared to winter wheat-fallow. The reason for this dramatic impact is the natural decline of jointed goatgrass seeds in the soil; density drops rapidly after two years. A further impact on weed dynamics is the winter wheat canopy. Improving winter wheat's competitiveness with cultural practices reduced jointed goatgrass density 35- to 50-fold with some rotations, compared with the common practices used by winter wheat producers. Producers can enhance weed management by rotation design, as arranging crops in a cycle-of-four with two winter annual crops followed by two summer annual crops will minimize density of the winter annual weed community.