| Surface mulch of dried leaf and stem residues of field-grown sunflower 'Peredovik' reduced seedling emergence of barnyardgrass, broadleaf signalgrass, fall panicum, goosegrass, green foxtail, johnsongrass, shattercane, entireleaf morningglory, tall morningglory, hemp sesbania, prickly sida, and spurred anoda.;Returning fresh residues to the soil where sunflower plants were grown reduced shoot growth of cotton, Palmer amaranth, pitted morningglory, sicklepod, shattercane, and velvetleaf. Residue maturity and planting time of test species elicited varying responses among test species. Older residues were more phytotoxic than younger residues. Without nitrogen application, Palmer amaranth shoot growth was substantially reduced by sunflower mulched residues. This growth inhibition was completely overcome by 50 and 100 kg N ha;In field experiments, sunflower mulch reduced shoot growth of test species. Inhibition persisted up to 30 days of residue decomposition. However, under extreme weed pressure, residues did not provide significant weed control. The duration of exposure to sunflower residues was important in the expression and persistence of growth inhibition. Cotton 'DPL 51', sunflower, and pitted morningglory required 30 days exposure to decaying residues before significant inhibition occurred. Shoot growth of shattercane, barnyardgrass, and prickly sida decreased at 15 but recovered at 30 days after seeding. In another field study, sunflower mulch had no adverse effect on seedling emergence, seedling plant height, shoot dry weight and leaf area of seven cotton cultivars evaluated.;No interaction was observed between sunflower seeding rates and reduced and full recommended herbicide rates. Averaged over herbicide treatments, sunflower residues reduced total weed density and weight in 1994 but not in 1995. In 1995, insufficient biomass produced in plots where sunflower was previously grown resulted in more intense weed pressure. Sunflower as a fall-planted or spring-planted cover crop did not result in the production of sufficient biomass. Recommended rates of herbicides in either standard or on as-needed-based program provided good to excellent weed control with or without sunflower residues. In unweeded plots, large crabgrass, spotted spurge, and prickly sida densities increased with time.;Results of this study showed that in a no-till cotton production system, exploiting the allelopathic potential of sunflower as a cover crop, was not a practical option. |
