Weed management options in agronomic crop production have been severely limited by widespread populations of weeds resistant to herbicides, including waterhemp [Amaranthus tuberculatus (Moq.) Sauer (syn. rudis)] resistant to foliar applications of herbicides that inhibit protoporphyrinogen oxidase (PPO; EC 1.3.3.4) activity (PPO-R). Herbicides within this site of action (WSSA site of action #14) remain efficacious when soil-applied to PPO-R waterhemp populations. Therefore, the continued use of these herbicides for soil-residual control of PPO-R waterhemp, especially in soybean production, is paramount with limited postemergence herbicides that remain effective. An improved understanding of the selection for PPO-R waterhemp would provide information to help minimize future loss of residual PPO-inhibiting herbicide activity. Five studies, consisting of 14 experiments, were conducted to improve our understanding of the selection for herbicide-resistant individuals. Soil-residual herbicides have been suspected to select for herbicide-resistant individuals; however, this phenomenon has never been observed experimentally in field conditions. This dissertation provides direct evidence from greenhouse and field experiments that significant selection pressure can occur from soil-residual herbicides; however, this selection for resistance could be delayed when using full commercial herbicide rates and effective herbicides from multiple sites of action. Also, the frequency of heterozygous individuals (RS) and PPO-inhibiting herbicide efficacy on RS individuals is a factor in the selection for herbicide resistance; however, current information on the these topics is limited. To provide additional information on RS individuals, a large-scale genotypic and phenotypic screen of multiple PPO-R waterhemp populations was conducted. It was determined that RS individuals were less frequent than expected and PPO-inhibiting herbicide efficacy on RS individuals was population-dependent. Finally, the hormetic effects of soil-residual herbicides have been paradoxically implicated as a means of both mitigating and exacerbating the selection for herbicide resistant biotypes; however, limited information was available on the hormetic effects of soil-residual PPO inhibitors. Greenhouse and growth chamber experiments were conducted to improve understanding of hormesis and soil-residual PPO-inhibiting herbicides. Experiments indicated that PPO-inhibiting herbicides may exert a limited hormetic effect on waterhemp germination below doses that cause a phytotoxic effect of the emerging seedling, indicating this effect may exacerbate the issue of selection for PPO-R waterhemp. Overall, data presented in this dissertation provides important information on the under-studied interaction between PPO-inhibiting herbicides and PPO-R waterhemp to safeguard the sustained efficacy of herbicides within this site of action.

Research was conducted in 2012 and 2013 to evaluate the use of saflufenacil in rice (Oryza sativa L.). Studies included a preemergence evaluation of different rates of saflufenacil in comparison to one rate of carfentrazone, a postemergence evaluation of saflufenacil at different rates and carfentrazone at one rate at different postemergence timings, an adjuvant evaluation to assess rice injury and weed control from different adjuvants when mixed with saflufenacil, a Clearfield program evaluation where saflufenacil was compared to other broadleaf herbicides in a Clearfield weed control program, and a cultivar tolerance evaluation where postemergence applications of saflufenacil were compared to carfentrazone on five different commercial rice cultivars.

Cogongrass management generally requires multiple herbicide applications, however, success is limited if not integrated with other methods. Experiments were conducted to evaluate the use of cover cropping systems with herbicides on cogongrass control. Field studies determined that sequential glyphosate applications in the summer were necessary to achieve 80% or greater control, but a single application could be effective if weather conditions allowed early planting and good cover crop establishment of Roundup Ready soybeans. Studies also indicated that the use of ALS-resistant Italian ryegrass and white clover crop combinations showed no effect, but imazapyr applications made in May or June provided 80% or higher control by October. Greenhouse experiments showed that delayed planting at least 1 month after imazapyr preemergence applications from 70 to 280 g ae ha-1, significantly reduced emergence failure, height and biomass reductions of legumes used for revegetation.

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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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