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Title: Potential synergistic effects of cereal rye biomass and soybean planting density on weed suppression

item RYAN, MATT - Pennsylvania State University
item Mirsky, Steven
item MORTENSEN, DAVID - Pennsylvania State University
item Teasdale, John
item CURRAN, WILLIAM - Pennsylvania State University

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2010
Publication Date: 3/1/2011
Citation: Ryan, M.R., Mirsky, S.B., Mortensen, D.A., Teasdale, J.R., Curran, W.S. 2011. Potential synergistic effects of cereal rye biomass and soybean planting density on weed suppression. Weed Science. 59:238-246.

Interpretive Summary: Simulatenously reducing tillage and external input costs are important features of a sustainable grain production system. Cover crops play an important role in achieving these goals but require a multi-tactical approach in order to achieve adequate weed managmenet. From past research, we know that synergistic interactions can occur between selected tactics (i.e., cover crops and herbicides). Cover crop residue can serve as an initial agent of weed suppression while a closed soybean leaf canopy can effectively suppress weed establishment after leaf canopy closure. By combining the early weed suppression of the mulch with the late weed suppression of the soybean canopy, the synergistic potential of this combination of tactics was demonstrated. We developed functional models that described weed responses to cereal rye mulch rate and soybean population density, individually, and then to the combination of these tactics using synergistic variations on a multiplicative model. The two factor system including cover crop residue plus a high soybean population can synergistically suppress weeds compared to expected suppression based on either factor alone. This research shows one approach to achieving optimum weed management efficacy in high-residue, reduced-tillage cropping systems by employing multiple ecologically based tactics that may be marginally effective individually but can provide effective control when used in combination.

Technical Abstract: Increasing crop density is a cultural weed management practice that can complement the use of cover crops for weed suppression. In this research, we created a range of cover crop biomass and soybean densities in order to assess their weed suppressive ability alone and in combination. The experiment was conducted in 2008 and 2009 in Maryland and Pennsylvania using five levels of cereal rye (Secale cereale L.) residue representing 0, 0.5, 1.0, 1.5, and 2.0 times the ambient level and five soybean densities ranging from 0 to 74 seeds m-2 . Weed biomass decreased with increasing rye residue and weeds were completely suppressed at levels above 1,500 g m-2 Weed biomass also decreased with increasing soybean density in two 24 of four site-years. We evaluated weed suppression by fitting an exponential decay model of weed biomass as a function of rye biomass and a hyperbolic model of weed biomass as a function of soybean density at each of the five tactic levels. We multiplied these individual tactic models and included an interaction term to test for tactic interactions. In two of the four site-years, the combination of these tactics produced a synergistic interaction that resulted in greater weed suppression than would be predicted by the efficacy of each tactic alone. Our results indicate that increasing soybean planting rate can compensate for lower cereal rye biomass levels when these tactics are combined.