Amount of crop residues in no-till farming affects weed-crop ecosystems

Successful weed management in no-till systems is dependent upon herbicides. However, issues such as herbicide resistant weeds, environmental contamination, and the impact of herbicides on non-target organisms have prompted weed scientists to search for other strategies to incorporate into weed management in no-till farming. Experiments were conducted during 1992 to 1995 to test the hypothesis that increasing wheat residue (CR) levels reduces giant foxtail infestations in no-till systems, as well as to study the mechanisms involved in the reduction of foxtail infestation, and to assess the impact of CR levels on weed-crop interference and crop yield. Increasing CR to 12 Mg/ha decreased foxtail density, delayed foxtail emergence, and changed foxtail morphology. A minimum of 6 Mg/ha of CR was necessary to reduce foxtail density by 50% relative to bare soil. Increasing CR levels to 12 Mg/ha positioned foxtail crown node 2 cm above the soil and in the middle of the CR. Growth chamber experiments indicated the allelopathic potential of some phenolics present on wheat straw was (in decreasing order): ferulic acid $\u3e$ catechol $\u3e$ salicilic acid. Up to 27 mM of caffeic acid did not affect giant foxtail germination. Adsorption of these phenolics to soil ranged from 10 to 40%. None of the phenolics tested would be present in no-till soil at concentrations high enough to inhibit foxtail germination. We speculate that allelopathy is limited in space and time because of adsorption, degradation and reduced concentration, and cannot account for the reduction of giant foxtail in the field. Computer simulation and growth chamber experiments indicated that reduced temperature is, at least in part, responsible for reduction of foxtail density by CR in no-till systems. Growth chamber experiments showed that light interacts with temperature in affecting foxtail emergence. In weedy plots in 1993 and 1994, increasing CR to 9 and 3 Mg/ha, respectively, reduced soybean yield losses to less than 10%, compared with bare soil no-till plots. Increase in CR levels decreased critical weed-free time, and increased critical time for weed removal