Tolerance of two metribuzin-tolerant and two metribuzin-sensitive early maturing soybean cultivars to two rates of acifluorfen, imazethapyr, metribuzin, and thifensulfuron was evaluated under weed-free conditions. Acifluorfen at 1120 g ai ha-1 reduced plant height by 3 cm and thifensulfuron at 18 and 35 g ai ha-1 delayed physiological maturity by 3 d. No herbicide treatment reduced soybean yield when compared with a weed-free check. Soybean yield of the two metribuzin-tolerant and the two metribuzin-sensitive cultivars did not change as metribuzin rate was increased from zero to 280 to 560 g ai ha-1.

EPTC applied at the time of ‘Saranac AR’ alfalfa establishment resulted in higher levels of crop injury from the octanoic acid ester of bromoxynil applied postemergence compared to alfalfa in areas not treated with EPTC. EPTC applied at 1.7 or 3.4 kg ai ha-1 did not injure alfalfa. At Rock Springs, PA, 0.54 kg ai ha-1 bromoxynil caused 28, 43, and 55% foliar injury in areas treated with 0, 1.7, and 3.4 kg ai ha-1 EPTC, respectively. A similar interaction was observed at Landisville, PA, although the level of injury due to bromoxynil was less severe. EPTC also predisposed ‘WL 316’ alfalfa to bromoxynil injury in greenhouse experiments.

At the 1990 meeting of the Weed Science Society of America (WSSA) in Montreal a Symposium titled “Pesticide Transport to Groundwater: Perspectives and Field Methodology” was Chaired by Thanh H. Dao. That symposium made it clear that simulation modeling is being used more and more to integrate the field test data generated by pesticide companies as part of the environmental impact assessment required for pesticide registration with the Environmental Protection Agency (EPA). At the Soil Aspects Section Business Meeting it was decided that a Symposium on the state of the art of this technology would be timely and the authors agreed to organize it.

A field study was established to evaluate symptoms, growth, yield, and nut quality of walnut trees subjected to multiple exposures of simulated bispyribac-sodium drift. Nut yield the year following simulated drift treatment was also evaluated because tissue differentiation for future fruiting position occurs in the prior season. Bispyribac-sodium was applied four times, at weekly intervals, at 0.5% and 3% of the use rate in rice (45 g ai ha-1). Injury from the 0.5% rate exceeded 5% after three applications. In general, the severity of the symptoms peaked 14 d after last application (23% and 40% injury for 0.5% and 3% rate, respectively) and subsequently remained nearly constant over the duration of the study. Growth of shoots treated with the 0.5% rate was initially delayed during the treatment regime but recovered after treatments ended; however, walnut shoots exposed to the higher rate had fewer internodes than nontreated trees at the end of the season. No measurable reduction in walnut yield or average nut weight either in the year of exposure or in the subsequent year was observed. However, both rates negatively affected walnut kernel color in the year of exposure.

Commercialization of 2,4-D–tolerant crops is a major concern for sweetpotato producers because of potential 2,4-D drift that can cause severe crop injury and yield reduction. A field study was initiated in 2014 and repeated in 2015 to assess impacts of reduced rates of 2,4-D, glyphosate, or a combination of 2,4-D with glyphosate on sweetpotato. In one study, 2,4-D and glyphosate were applied alone and in combination at 1/10, 1/100, 1/250, 1/500, 1/750, and 1/1,000 of anticipated field use rates (1.05 kg ha−1 for 2,4-D and 1.12 kg ha−1 for glyphosate) to ‘Beauregard’ sweetpotato at storage root formation (10 days after transplanting [DAP]). In a separate study, all these treatments were applied to ‘Beauregard’ sweetpotato at storage root development (30 DAP). Injury with 2,4-D alone or in combination with glyphosate was generally equal or greater than with glyphosate applied alone at equivalent herbicide rates, indicating that injury is attributable mostly to 2,4-D in the combination. There was a quadratic increase in crop injury and quadratic decrease in crop yield (with respect to most yield grades) with increased rate of 2,4-D applied alone or in combination with glyphosate applied at storage root development. However, neither the results of this relationship nor of the significance of herbicide rate were observed on crop injury or sweetpotato yield when herbicide application occurred at storage root formation, with a few exceptions. In general, crop injury and yield reduction were greatest at the highest rate (1/10×) of 2,4-D applied alone or in combination with glyphosate, although injury observed at lower rates was also a concern after initial observation by sweetpotato producers. However, in some cases, yield reduction of U.S. no.1 and marketable grades was also observed after application of 1/250×, 1/100×, or 1/10× rates of 2,4-D alone or with glyphosate when applied at storage root development.

Weed management in carrot is challenging, given slow and inconsistent crop emergence and early-season growth and the lack of practical season-long management tools such as herbicides. We investigated holistic carrot production systems with a focus on minimizing inputs while optimizing resource use. In an overall sense, results of this work were consistent between years, and stark. The choice of carrot variety had a moderate influence on carrot foliar canopy development and, subsequently, weed density. For example, ‘Cupar’ carrot formed a complete crop canopy sooner than the other dicer-type ‘Canada’ variety. Likely as a result, density of weed species such as spotted ladysthumb and common lambsquarters was less where ‘Cupar’ was grown compared with where ‘Canada’ was grown. Gibberellic acid as a foliar application was not successful in these studies and, in a few cases, may have even increased weed-seed germination and establishment. Adding two carrot rows to the current regional industry-standard three-row bed system not only enhanced competitiveness with weeds but also improved carrot yield without additional fertilizer, water, or pest management inputs. By far, though, the most successful strategy to reduce weed density while maintaining or improving carrot yield was to delay seeding by 17 to 19 d. We anticipate more holistic production system research that integrates low-input alternatives in other crops as herbicide-resistant weeds proliferate while few new herbicides are developed. As was demonstrated in this research, such novel approaches can be successful without adding significant economic burden to the farmer or increasing risk of crop failure.

Red beet growers have expressed interest in adopting the microrate herbicide approach originally implemented in sugarbeet to achieve season-long weed management. Several red beet herbicides were first labeled for use in sugarbeet and lack substantial residual weed control. In response, red beet herbicide programs were evaluated that included a PRE application followed by up to three POST applications of various herbicide combinations. This research, however, indicated that herbicide programs that included PRE herbicides followed by as few as one or two POST applications that involve multiple active ingredients can provide season-long weed control. This observation was consistent across a broad spectrum of weeds, between two study locations that varied in soil type, and during two growing seasons. Herbicide programs that included only a PRE and six-leaf red beet growth stage application were successful at two locations in maintaining weed control and crop yield relative to hand-weeded red beet. Furthermore, these herbicide programs reduced the number of applications by 50% compared with the full programs, reducing crop injury risk and grower cost.

Direct-seeding of cover crops is often promoted to reduce potential soil loss during the winter, enhance soil fertility, and reduce energy use and equipment traffic on fields. The impact of direct-seeding of cover crops on wild-proso millet seedling emergence in subsequent crops is unknown. In this study, a pulse–chase experiment determined the effect of five fall soil management strategies, two spring primary tillage levels, and two herbicide programs on wild-proso millet emergence in a subsequent crop of snap beans. Wild-proso millet seeds were sown following sweet corn harvest in two commercial production fields to simulate seed rain at a density of 500 and 1,000 seeds m−2 in the fall of 2004 and 2005, respectively. The experimental design was a split–split plot. Five fall treatments were applied to main plots and included direct drilling of cover crops, conventionally tilled and drilled cover crops, a winter fallow treatment, and two additional treatments that were direct-drilled and conventionally tilled in the fall but that were not seeded with cover crops to separate the effects of tillage and cover crops on wild-proso millet emergence. Main plots were split in the spring and snap beans planted without primary tillage or conventionally planted after the soil was tilled. Tillage and cover crop treatments applied in the fall influenced emergence in the spring but with slightly different outcomes at the two sites. Direct drilling without a cover crop produced more wild-proso millet seedlings in snap beans than the winter fallow plot; tillage before cover crop planting in the fall produced recruitment levels less than or equal to densities in the winter fallow plots, with one exception. At one site, cover crops increased emergence compared to plots without cover crops. Spring tillage did not alter the affect of the five fall management treatments on wild-proso millet emergence. Direct-seeding of cover crops should be done with equipment or methods that minimize soil disturbance to prevent movement of wild-proso millet seeds into protected and favorable zones of emergence in the soil.

Currently, there are seven herbicides labeled for U.S. tobacco production; however, additional modes of action are greatly needed in order to reduce the risk of herbicide resistance. Field experiments were conducted at five locations during the 2017 and 2018 growing seasons to evaluate flue-cured tobacco tolerance to S-metolachlor applied pretransplanting incorporated (PTI) and pretransplanting (PRETR) at 1.07 (1×) and 2.14 (2×) kg ai ha−1. Severe injury was observed 6 wk after transplanting at the Whiteville environment in 2017 when S-metolachlor was applied PTI. End-of-season plant heights from PTI treatments at Whiteville were likewise reduced by 9% to 29% compared with nontreated controls, although cured leaf yield and value were reduced only when S-metolachlor was applied PTI at the 2× rate. Severe growth reduction was also observed at the Kinston location in 2018 where S-metolachlor was applied at the 2× rate. End-of-season plant heights were reduced 11% (PTI, 2×) and 20% (PRETR, 2×) compared with nontreated control plants. Cured leaf yield was reduced in Kinston when S-metolachlor was applied PRETR at the 2× rate; however, treatments did not impact cured leaf quality or value. Visual injury and reductions in stalk height, yield, quality, and value were not observed at the other three locations. Ultimately, it appears that injury potential from S-metolachlor is promoted by coarse soil texture and high early-season precipitation close to transplanting, both of which were documented at the Whiteville and Kinston locations. To reduce plant injury and the negative impacts to leaf yield and value, application rates lower than 1.07 kg ha−1 may be required in these scenarios.

In recent years, increasing implementation of biological, cultural, and mechanical weed-control methods is desired; however, many of these techniques are not viable in established turfgrass systems. The use of freezing or frost for weed control has previously been researched; however, is not well elucidated. Field and greenhouse experiments were conducted to evaluate liquid carbon dioxide (LCD) for weed control in established turfgrass systems. LCD was applied with handheld prototypes that were modified to reduce the amount of LCD required for weed control. Common annual and perennial turfgrass weeds included common chickweed, corn speedwell, goosegrass, large crabgrass, smooth crabgrass, Virginia buttonweed, and white clover. Turfgrass tolerance was evaluated on the following species: hybrid bermudagrass, Kentucky bluegrass, tall fescue, and zoysiagrass. The final modification allowed for lower output (0.5 kg LCD min−1) when compared with the initial prototype (3 kg LCD min−1). In general, weed control increased as LCD increased. When comparing weed species life cycles, annuals were controlled more than perennials (P < 0.0001) at 14 and 28 d after treatment (DAT). Further, exposure time affected control as white clover, Virginia buttonweed, and large crabgrass control was greater (18, 14, 15%, respectively) from the longer exposure time (30 vs. 15 s), although equivalent amounts of LCD (30 kg m−2) were applied. These data also suggest that plant maturity affects control, as large crabgrass control in one- to two- and three- to four-leaf stages (> 90%) was greater than in the one- to two-tiller stage (< 70%). Turfgrass injury at 7 DAT was unacceptable (> 30%) on all species, but declined to 0% by 28 DAT. These data suggest that LCD has the potential to provide an alternative for weed control of select species where synthetic herbicides are not allowed or desired.

The occurrence of herbicide tank contamination with dicamba or 2,4-D will likely increase with the recent commercialization of dicamba- and 2,4-D-resistant soybean. High-value sensitive crops, including dry bean, will be at higher risks for exposure. In 2017 and 2018, two separate field experiments were conducted in Michigan to understand how multiple factors may influence dry bean response to dicamba and 2,4-D herbicides, including 1) the interaction between herbicides applied POST to dry bean and dicamba or 2,4-D, and 2) the impact of low rates of glyphosate with dicamba or 2,4-D. Dry bean injury was 20% and 2% from POST applications of dicamba (5.6 h ae ha−1) and 2,4-D (11.2 g ae ha−1), respectively, 14 days after treatment (DAT). The addition of glyphosate (8.4 g ae ha−1) did not increase dry bean injury from dicamba or 2,4-D. Over 2 site-years the addition of dry bean herbicides to dicamba or dicamba + glyphosate (8.4 g ae ha−1) increased dry bean injury and reduced yield by 6% to 10% more than when dicamba or dicamba + glyphosate was applied alone. The interaction between 2,4-D (11.2 g ae ha−1) and dry bean herbicides was determined to be synergistic. However, 2,4-D (11.2 g ae ha−1) had little effect on dry bean with or without the addition of a dry bean herbicide program. These studies document that synergy also occurs between dicamba and dicamba + glyphosate and both common dry bean herbicide programs tested: 1) imazamox (35 g ha−1) + bentazon (560 g ha−1), and 2) fomesafen (280 g ha−1). The synergy between dry bean herbicide and dicamba and dicamba + glyphosate can increase plant injury, delay maturity, and reduce yield to a greater extent than dicamba or dicamba + glyphosate alone. This work emphasizes the need to properly clean out sprayers after applications of dicamba to reduce the risk of exposure to other crops.

Field trials were conducted in 2013 and 2014 to investigate the tolerance of limpograss to increasing rates of hexazinone. Dose-response curves were generated using linear and quadratic regression models to determine the hexazinone estimated dose (ED) required to provide 10% (ED10) and 20% (ED20) of visual injury and herbage mass reduction. The ED10 and ED20 for visual estimates of injury were estimated to be 0.05 and 0.14 kg ai ha-1 at 60 d after treatment (DAT). Regarding forage herbage mass reduction, the ED10 and ED20 were estimated to be 0.07 and 0.19 kg ai ha-1 in 2013, whereas in 2014, the ED10 and ED20 were estimated to be 0.03 and 0.06 kg ai ha-1, respectively. The significant difference in herbage mass reduction between 2013 and 2014 was likely due to rainfall patterns, which possibly promoted hexazinone leaching in 2013 and consequently, less activity. Overall, hexazinone resulted in high degrees of limpograss injury across all response variables in both years; therefore, smutgrass control in limpograss pastures with hexazinone may not be a viable option. The presence or absence of smutgrass should be considered before limpograss establishment as there is no viable herbicide to selectively remove smutgrass from limpograss swards.

Glyphosate-resistant (GR) alfalfa offers growers new options for weed control in alfalfa. One potential benefit of using GR alfalfa is increased longevity of an alfalfa stand under frequent harvests. It was hypothesized that GR alfalfa would have a greater longevity because of removal of weed interference with minimal crop injury. To study GR alfalfa yield, weed invasion, alfalfa stand persistence, and relative forage quality (RFQ), a field experiment with three weed control methods (no herbicide, glyphosate, and hexazinone) under two harvest frequencies (high and moderate) was established in August 2003 at the Michigan State University Agronomy Farm in East Lansing, MI. Forage yield of established alfalfa was not adversely affected by herbicide treatments. There were no differences in weed biomass between alfalfa treated with glyphosate and that treated with hexazinone, except in 2007. Average GR alfalfa stand density decreased approximately 90% (from 236 to 27 plant m−2), and yield decreased approximately 30% (from 11.04 to 7.87 Mg ha−1) during the 7-yr period (2004 to 2010) of the experiment. Stand density of GR alfalfa showed natural thinning during the 7-yr period regardless of harvest intensity or herbicide treatment. In most production years (4 out of 5 yr), relative forage quality of GR alfalfa was higher under a high-intensity harvesting system (4 to 5 harvests yr−1) than it was with a moderate intensity harvesting system (3 to 4 harvests yr−1). Relative forage quality was not affected by weed removal with herbicides in most years. Weed removal and harvest intensity in established GR alfalfa had no effect on stand persistence.

Teff is a warm-season C4 annual grass crop grown for forage and food grain that has recently increased in production in parts of the United States. Hay from teff is well suited for livestock, especially horses. The objective of this study was to evaluate teff and weed response to selected herbicides in field studies conducted at the Malheur Experiment Station, Ontario, OR in 2009 and 2010. Herbicides were applied POST when teff was at the four-leaf stage. Broadleaf weed control at 21 d after treatment was greater than 91% across herbicide treatments. Only the premix of 2.5 g ai ha−1 florasulam + 99 g ae ha−1 fluroxypyr + 15 g ai ha−1 pyroxsulam provided acceptable control of barnyardgrass. Due primarily to barnyardgrass competition, teff treated with a premix of 2.5 g ha−1 florasulam + 99 g ha−1 fluroxypyr + 15 g ha−1 pyroxsulam produced 7,200 kg ha−1 of teff hay compared with 4,800 kg ha−1 of teff hay for 2,4-D and dicamba and 4,200 kg ha−1 teff hay when no herbicides were used. Teff grain production was greater with 2.5 g ha−1 florasulam + 99 g ha−1 fluroxypyr + 15 g ha−1 pyroxsulam compared with any of the other treatments. The use of a premix of florasulam + fluroxypyr + pyroxsulam would improve broadleaf and grass weed control in ‘Tiffany' and ‘Dessie' teff varieties, improve hay and grain yield, and reduce production costs.

Niger is a potential crop for the Northern Great Plains, but not until volunteer canola can be controlled. A study at Indian Head, SK, was conducted from 2007 to 2009 to determine the tolerance of niger to flucarbazone, a herbicide that controls volunteer canola. The tolerance was determined by applying three rates of flucarbazone (19, 28.5, and 38 g ai ha−1) at four application stages (two, four, six, and eight-leaf stage). Mean injury did not exceed 22% for any year by treatment combination. Injury was most prominent in 2007, and dissipated as the growing season progressed. Increasing the rate of flucarbazone increased crop injury depending on the year, application timing, and evaluation timing. Injury under 20% in 2007 and under 10% in 2008 and 2009 was observed at the start of flowering when flucarbazone was applied at the two, four, and six-leaf stage. Injury from applications at the two, four, and six-leaf stage decreased as the growing season progressed. When the labelled rate of flucarbazone for wheat (19 g ha−1) was applied at the two, four, or six leaf stage, injury was below 10%. Injury when flucarbazone was applied at the eight-leaf stage was highest during seed filling. Volunteer canola was controlled by flucarbazone. The application of flucarbazone relative to a weedy control increased yield by about 50% (138 to 213 kg ha−1) in 2008 and 2009. Flucarbazone rate did not affect niger yield except in 2007 where yield was about 100 kg ha−1 less with the two highest rates. Delaying flucarbazone application decreased niger yield, especially in the year (2007) with most niger injury. Flucarbazone application at the two- or four-leaf niger stage at a rate of 19 g ha−1 provided a good balance of weed control and crop tolerance.

Greenhouse and field experiments were conducted to evaluate the use of a carbon band to provide a “safe zone” for seedling emergence and growth of three native grass species. ‘KIKA677’ streambed bristlegrass germplasm, ‘Alamo’ switchgrass, and ‘Waelder’ shortspike windmillgrass germplasm were used in combination with several PRE- and POST-applied herbicides including cloransulam, flumioxazin, imazapic, imazethapyr, and 2,4-D. In a greenhouse experiment, switchgrass emergence was improved when a carbon band was used with imazapic or imazethapyr at 0.04 and 0.07 kg ai ha−1 or 2,4-D at 2.12 kg ae ha−1. Windmillgrass emergence was improved when carbon was used in combination with flumioxazin at 0.05 and 0.1 kg ai ha−1, imazapic at 0.04 and 0.07 kg ha−1, imazethapyr at 0.07 kg ha−1, and 2,4-D at 1.06 kg ha−1, whereas bristlegrass emergence was improved when carbon was used in combination with flumioxazin at 0.1 kg ai ha−1, imazapic at both rates, and imazethapyr at 0.04 kg ha−1. Field studies indicated that flumioxazin at 0.05 and 0.1 kg ha−1, imazapic at 0.04 kg ha−1, and imazethapyr at 0.04 and 0.07 kg ha−1, were safened for bristlegrass and switchgrass emergence when used with carbon. Windmillgrass emergence and growth were improved when carbon was used in combination with flumioxazin at 0.1 kg ha−1.

To evaluate the weed control provided by various combinations of PRE applications of napropamide (Nap) and POST applications of mesotrione (Mes) in newly planted and 1-yr-old cranberry vines, six 2-yr experiments were conducted during 2009-2012. Three sites were treated in the year of planting plus the subsequent year (called “new plantings”) and three sites were treated in their second year of growth plus the subsequent year (called “second-year plantings”). Ten treatments, delivered in 3,735 L ha−1 water, were administered each year: Nap at 3.36 kg ai ha−1 applied once, twice, thrice or once followed by (fb) one application of Mes at 210 g ai ha−1; Nap at 5.04 kg ha−1applied once, twice or once fb one application of Mes at 210 g ha−1; Mes at 210 g ha−1 applied once or twice; and nontreated. At new planting sites, plots receiving treatments other than a single Nap application had less total weed biomass compared to nontreated plots. At second-year plantings, all herbicide-treated plots had less total weed biomass than nontreated plots; in addition, plots receiving Mes-only treatments had less total weed biomass than Nap-only treatments. Correlation analysis indicated that monocot biomass production was the primary positive predictor for total weed biomass. Herbicides did not adversely affect cranberry biomass production but overall vine colonization was poor for four of the six sites indicating that other factors, including cultivar choice, planting method, and water management, may play a larger role in rapid cranberry vine colonization than the suppression of initial weed biomass. Given the complementary range of efficacy for Nap and Mes, a combination PRE-POST herbicide program of a low rate of Nap fb Mes may be the most cost-beneficial program in many instances. If cost is a limiting factor, applying a single application of Mes to a new planting should be included as a component of cranberry bed establishment since this practice consistently resulted in significantly less initial weed biomass compared to areas left nontreated.

Poison ivy is a virulent weed that is frequently treated with herbicides. Dicamba, 2,4-D, and two fixed-ratio tank mixtures of dicamba plus 2,4-D were evaluated across a series of rates for poison ivy control. Objective was to test whether tank mixtures were more effective than either herbicide applied alone. Dicamba alone, 2,4-D alone, a 1 : 3, and a 3 : 1 ratio (by weight) mixture of dicamba plus 2,4-D, respectively, were applied at eight rates to 2-yr-old, container-grown poison ivy plants. The eight rates ranged from 0.036 to 1.79 kg ae ha−1, which, in terms of phytotoxicity, generally ranged from none to death. Percentage of control was determined from plant fresh-weight reduction relative to a nontreated control and was determined at 1 and 4 mo after treatment (MAT). Rates required for 95% control at 1 MAT and for control of regrowth at 4 MAT and the cost of those treatments were determined for the dicamba and 2,4-D applied alone and in the two mixtures. At the 1-MAT evaluation, 2,4-D alone was more cost effective than either dicamba alone or the two mixtures. By the 4-MAT evaluation, however, which followed clipping at 3 MAT, dicamba alone was more cost effective than either mixture. The 2,4-D alone failed to provide 95% control at the 4 MAT evaluation, even at the highest rate evaluated (1.79 kg ha−1). Response curves for the two mixtures were equivalent to the response curves of the components applied alone at the 1 MAT evaluation and fell between the response curves of the components at the 4 MAT evaluation. Hence, 2,4-D plus dicamba mixtures were neither antagonistic nor synergistic. Results indicate that dicamba applied alone is far more effective than 2,4-D is for control of established and perennial poison ivy, assuming the intent is to obtain control with a single, one-time application.

Feral populations of cultivated crops have the potential to function as bridges and reservoirs that contribute to the unwanted movement of novel genetically engineered (GE) traits. Recognizing that feral alfalfa has the potential to lower genetic purity in alfalfa seed production fields when it is growing in the vicinity of foraging pollinators in alfalfa seed fields, industry has established production standards to control feral plants. However, with the commercialization of GE glyphosate-resistant (GR) alfalfa and the need to support the coexistence of both GE and conventional production, effective methods to control transgenic feral alfalfa need to be developed. Therefore, a study was conducted in 2012, 2013, and 2014 to determine the effect of several synthetic auxin herbicides on seed development in GR alfalfa. GR alfalfa, var. Genuity (R44BD16), was treated with dicamba, 2,4-D, triclopyr, and aminopyralid when alfalfa plants contained green seed pods. Two weeks after herbicide application, plants were harvested, air dried, and seed yield, seed germination, and seedling emergence from the soil were determined. In 2013, dicamba, triclopyr, and 2,4-D decreased alfalfa seed yield per plant compared wih nontreated plants, whereas in 2014, all four herbicides decreased alfalfa seed yield per plant 24 to 49% (by weight) compared with nontreated plants. The same trend was evident in 2012, but seed yield was variable and was not significantly different among treatments. Seed germination averaged 43, 50, and 72% in 2012, 2013, and 2014, respectively, and was not affected by the four herbicides applied at early pod-fill stage. However, seeds harvested from plants treated with dicamba, 2,4-D, and triclopyr often produced deformed and abnormal seedlings, and when planted in soil, frequently failed to emerge. The combined effects of dicamba, 2,4-D, and triclopyr in reducing seed yield, seedling emergence, and seedling growth could contribute to managing feral alfalfa populations.

Turfgrass managers apply nonselective herbicides to control winter annual weeds during dormancy of warm-season turfgrass. Zoysiagrass subcanopies, however, retain green leaves and stems during winter dormancy, especially in warmer climates. The partially green zoysiagrass often deters the use of nonselective herbicides due to variable injury concerns in transition and southern climatic zones. This study evaluated zoysiagrass response to glyphosate and glufosinate applied at four different growing degree day (GDD)-based application timings during postdormancy transition in different locations, including Blacksburg, VA; Starkville, MS; and Virginia Beach, VA, in 2018 and 2019. GDD was calculated using a 5 C base temperature with accumulation beginning January 1 each year, and targeted application timings were 125, 200, 275, and 350 GDD5C. Zoysiagrass injury response to glyphosate and glufosinate was consistent across a broad growing region from northern Mississippi to coastal Virginia, but it varied by application timing. Glyphosate application at 125 and 200 GDD5C can be used safely for weed control during the postdormancy period of zoysiagrass, while glufosinate caused unacceptable turf injury regardless of application timing. Glyphosate and glufosinate exhibited a stepwise increase to maximum injury with increasing targeted GDD5C application timings. Glyphosate applied at 125 or 200 GDD5C did not injure zoysiagrass above a threshold of 30%, whereas glufosinate caused greater than 30% injury for 28 and 29 d when applied at 125 and 200 GDD5C, respectively. Likewise, glyphosate application at 125 or 200 GDD5C did not affect the zoysiagrass green cover area under the progress curve per day, whereas later applications reduced it. Glyphosate and glufosinate caused greater injury to zoysiagrass when applied at greater cumulative heat units and this was attributed to increasing turfgrass green leaf density, because heat unit accumulation is positively correlated with green leaf density. Accumulated heat unit-based application timing will allow practitioners to apply nonselective herbicides with reduced injury concerns.