Date of Award
Master of Science
Plant and Soil Science
With herbicide efficacy declining as weeds continue to evolve and resist key modes of action, long-term, multi-faceted control practices need to be investigated. Two studies, involving cover crops and tillage, were implemented to understand how management practices influence the weed seed bank. The objectives of both studies were to examine long-term changes in the weed seed bank in response to tillage methods, fertility, and cover crops, and to analyze differences in distribution and community composition between individual species in the weed seed bank. To assess the weed community present in both seed banks, soil samples were collected, and a soil grow out was conducted in the greenhouse. To examine the field-emerged seed banks, percent coverage was collected for each weed species, crop residue, bare ground, and cover crop if present. The first study was established in 1970, evaluating four tillage systems: moldboard plow (CT), chisel plow (RT), alternative (AT), and no-tillage (NT). From 1970 to 1990, this study was continuous corn (Zea mays L.), but in 1991, soybeans (Glycine max L.) were added into the rotation, marking the beginning of the current corn-soybean rotation. Fertilizer treatments (no-fertilizer, nitrogen only, and NPK) were also evaluated. Each tillage and fertility treatment were replicated four times in the field in 6 m by 8 m plots. Weed seeds were found to be distributed within the soil profile differently by tillage treatment. No-till treatments maintained most of the seed bank near the surface of the soil. Based on the response of individual species to fertility treatments, community shifts in seed bank composition were found. LAMAM, STEME, and SIBVI had the greatest richness in NPK treatments compared to no fertilizer and nitrogen only. CERVU tended to favor treatments without any fertilizer. Tillage and fertility were also found to interact and influence species presence and community composition. The second study was established in 2013, to examine changes and differences in distribution and composition between individual species in the weed community in response to cover crop rotations and tillage. A split-plot design with three crop rotation systems was implemented: 1) corn (Zea mays L.) – cereal rye (Secale cereale L.) – soybean (Glycine max (L.) Merr.) – hairy vetch (Vicia villosa Roth) [CcrShv], 2) corn-cereal rye-soybean-oats + radish (Avena sativa L. + Raphanus sativus L.) [CcrSor], and 3) corn-no cover crop-soybean-no cover crop [NOCC], and two tillage treatments: conventional tillage and no-till. This field study also supported previous findings of higher weed diversity in no-till systems. ANOVA performed in R suggested species richness was significantly higher in no-tillage treatments in comparison to tillage treatments. For the field-emerged weed community, a pairwise comparisons test suggested cover crop treatments have significantly lower weed richness compared to plots with no cover crop present, but there was no interactive effect of tillage. 3-Way ANOVAs suggested time, tillage, and crop rotation influenced each weed species differently. Due to individual weed species having different requirements for germination and seed longevity, these data suggest the importance of developing and implementing a quality, integrated weed management program to maintain low levels of weed emergence and seed credits to the seed bank.
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