Sustainable weed control in managed agricultural systems requires the judicious use of multiple weed control tactics and prevents over-reliance on any one tactic. In this context, sustainable weed management plays a critical role in the mitigation of one of agriculture's most pressing problems- herbicide resistance. Research conducted in Virginia sought to explore the effects of integrating multiple weed management tactics in corn and cool-season turfgrass. Additionally, research was conducted to confirm nicosulfuron and glyphosate herbicide resistance in Virginia johnsongrass and elucidate the molecular mechanisms conferring those resistances. Rye and hairy vetch cover crop residues, combined with reduced rates of preemergence herbicide and postemergence glyphosate applications, were shown to provide sufficient weed control and corn yield. Cover crop type or residue level did not augment weed control in corn production systems, but the use of glyphosate was essential for late-season weed control. Rye and vetch biculture as a cover crop increased corn yield compared to rye cover crop alone. In cool-season turfgrass, the addition of reduced preemergence herbicide rates to corn gluten meal, an organic herbicide product, reduced crabgrass 25%. Moreover, control was dependent on herbicide choice. Herbicides applied at half of recommended labeled rates or less did not control crabgrass at a commercially-acceptable level, regardless of corn gluten meal addition. In field experiments, Virginia johnsongrass expressed resistance to nicosulfuron and glyphosate. Glyphosate at 0.88 kg ae ha-1 controlled johnsongrass 65%. Nicosulfuron at 0.14 kg ai ha-1 controlled the same population 10%. Greenhouse experiments confirmed differential sensitivity of putative herbicide-resistant johnsongrass seedlings to nicosulfuron and glyphosate when compared to a susceptible population. Herbicide resistance was not conferred via target-site mutation. Five ALS-gene site mutations were confirmed absent in Virginia johnsongrass, while three others were located in coding regions that could not be elucidated in johnsongrass. Further investigations showed glyphosate resistance was not conferred via reduction in herbicide absorption or translocation. The susceptible johnsongrass caused an increase in a polar metabolite at Rf = 0.17 with concomitant reduction in glyphosate over time. Although the mechanism is not clear, these data suggests that glyphosate resistance in johnsongrass may be associated with differential metabolism. / Ph. D.

Zoysiagrass (Zoysia spp.) is utilized as a warm-season turfgrass because of its density, visual quality, stress tolerance, and reduced input requirements. Turf managers often exploit winter dormancy in warm-season turfgrass to apply nonselective herbicides such as glyphosate and glufosinate to control winter annual weeds. Although this weed control strategy is common in bermudagrass (Cynodon spp.), it has been less adopted in zoysiagrass due to unexplainable turf injury. Many university extension publications recommend against applying nonselective herbicides to dormant zoysiagrass despite promotional language found in a few peer-reviewed publications and product labels. Previous researchers have used vague terminology such as "applied to dormant zoysiagrass" or "applied prior to zoysiagrass green-up" to describe herbicide application timings. These ambiguous terms have led to confusion since zoysiagrass typically has subcanopy green leaves and stems throughout the winter dormancy period. No research has sought to explain why some turfgrass managers are observing zoysiagrass injury when the literature only offers evidence that these herbicides do not injure dormant zoysiagrass. We sought to explore various herbicides, prevailing temperatures surrounding application, heat unit based application timings, and spray penetration into zoysiagrass canopies as possible contributors to zoysiagrass injury. The results indicated that a wide range of herbicides may be safely used in dormant zoysiagrass. However, as zoysiagrass begins to produce more green leaves, herbicides such as metsulfuron, glyphosate, glufosinate, flumioxazin, and diquat become too injurious. Glufosinate was consistently more injurious regardless of application timing than glyphosate and other herbicides. When temperatures were 10 °C for 7 d following treatment, a delayed effect of glyphosate and glufosinate effect on digitally-assessed green cover loss was noted on zoysiagrass sprigs. In subsequent studies on turf plugs, a 14-d incubation period at 10 °C reduced glyphosate but not glufosinate effects on turf green color reduction. Glyphosate applied at 125, and 200 GDD5C can safely be applied to zoysiagrass while glufosinate applied at the same timings caused inconsistent and often unacceptable zoysiagrass injury in field studies conducted at Blacksburg, VA, Starkville, MS, and Virginia Beach, VA. Zoysiagrass green leaf density was described as a function of accumulated heat units consistently across years and locations but variably by turf mowing height. Turf normalized difference vegetative index was primarily governed by green turf cover but reduced by herbicide treatments, especially when applied at greater than 200 GDD5C. Substantial spray deposition occurred to subcanopy tissue regardless of nozzle type, pressure and height above the zoysiagrass canopy based on spectrophotometric assessment of a colorant admixture. However, increasing nozzle height above the turf canopy and avoiding air induction type nozzles significantly reduced the percentage of green tissue exposed at lower canopy levels. Absorption of radio-labeled glyphosate and glufosinate was up to four times greater when exposed to zoysiagrass stems compared to leaves. Glyphosate translocated more than glufosinate and both herbicides moved more readily from stem to leaf than from leaf to stem / Doctor of Philosophy / Zoysiagrass (Zoysia spp.) is utilized as a warm-season turfgrass because of its density, visual quality, stress tolerance, and reduced input requirements. Being that zoysiagrass is a warm-season turfgrass, it enters a dormancy period during the winter months. During this period, zoysiagrasses' active growth is halted, and leaves lose their green color and turn a golden-brown color. The winter dormancy period presents turfgrass managers with a unique opportunity to apply nonselective herbicides such as glyphosate and glufosinate to control a broad spectrum of winter annual weeds. Although this weed control strategy is common in bermudagrass (Cynodon spp.), it has been less adopted in zoysiagrass due to turfgrass managers observing unexplainable turfgrass injury. Many university extension publications recommend against applying nonselective herbicides to dormant zoysiagrass despite language found in peer-reviewed publications and product labels suggesting they could be safely applied. Previous researchers have used vague terminology such as "applied to dormant zoysiagrass" or "applied prior to zoysiagrass green-up" to describe herbicide application timings. These terms have led to confusion about when to make these applications since zoysiagrass typically has subcanopy green leaves and stems throughout the winter dormancy period. No research has sought to explain why some turfgrass managers observe zoysiagrass injury when the literature only offers evidence that these herbicides do not injure dormant zoysiagrass. Research projects were designed to explore various herbicides, temperatures surrounding herbicide applications, application timings, and spray penetration into zoysiagrass canopies as possible contributors to zoysiagrass injury. The results indicated that a wide range of herbicides may be safely used in dormant and semidormant zoysiagrass. However, as zoysiagrass begins to produce more green leaves and stems, herbicides such as metsulfuron, glyphosate, glufosinate, flumioxazin, and diquat become too injurious and should be avoided. Across multiple research studies, glufosinate was consistently more injurious regardless of application timing than glyphosate and other herbicides. When temperatures were 10 °C for 7-d following treatment, it delayed zoysaigrass response to glyphosate and glufosinate. In a subsequent study, when temperatures were at 10 °C for a 14-d period, glyphosate and the nontreated reached 50% green cover at the same time, which suggests cold temperatures could mitigate glyphosate injury on zoysiagrass over a 14-d period. The 10 ° temperature only delayed glufosinate injury on zoysiagrass, and no safening was observed. The results also indicated that as temperatures increased, glyphosate and glufosinate rate in which injury was observed increased on the zoysiagrass. Glyphosate applied at 125, and 200 GDD5C can safely be applied to zoysiagrass while glufosinate applied at the same timings caused inconsistent and often unacceptable zoysiagrass injury in field studies conducted at Blacksburg, VA, Starkville, MS, and Virginia Beach, VA. Zoysiagrass injury increased when glyphosate and glufosinate were applied later into the spring when more green leaves were present regardless of location. Accumulated heat units and zoysiagrass green leaf density were closely related, indicating that accumulated heat units could be a useful tool for turfgrass managers to track zoysiagrass spring green-up. Substantial spray deposition was found on subcanopy zoysiagrass leaves and stems regardless of nozzle type, pressure, and height above the zoysiagrass canopy based on recovered colorant at the upper, middle and lower levels of the zoysiagrass canopy. However, avoiding air induction-type nozzles and raising spray height may slightly decrease penetration of spray droplets into a zoysiagrass subcanopy, but a large percentage of droplets still reached the middle and lower canopy layers in this research. Absorption of radio-labeled glyphosate and glufosinate was up to four times greater when applied directly to zoysiagrass stolen compared to leaves. Glyphosate translocated more than glufosinate, and both herbicides moved more readily from stem to leaf than from leaf to stem. These data suggest limiting the number of green zoysiagrass leaves at application would be an effective method to avoid injury zoysiagrass when applying nonselective herbicides