Isoxaflutole and mesotrione have been used to control creeping bentgrass in cool season turf, these experiments evaluate these two products for selective nimblewill control. Three experiments were conducted in Virginia and Tennessee to evaluate selective control options for nimblewill in cool-season turfgrass. Single applications of isoxaflutole control nimblewill more effectively than single applications of mesotrione. Nimblewill control 8 wk after initial treatment (WAIT) increased as mesotrione rates increased from 28 to 280 g ai/ha, and isoxaflutole rates increased from 28 to 168 g/ha. Isoxaflutole at 84 and 168 g/ha were the only single application treatments that controlled nimblewill greater than 80% 8 WAIT. However, sequential applications of isoxaflutole and mesotrione at 28 g/ha, 10 d apart controlled nimblewill 94 and 80%, respectively 8 WAIT. Triclopyr and fenoxaprop-p did not effectively control nimblewill and caused unacceptable turfgrass injury. Long-term control was not evaluated in these studies. However, it was concluded that isoxaflutole and mesotrione at appropriate rates and applied in sequence selectively control nimblewill without harming desirable turf.

Field studies were conducted in 1998 and 1999 at Simcoe and Exeter, ON, to evaluate the tolerance of nine sweet corn cultivars, representing a range of genotypes, to RPA 201772 applied preemergence at 105 and 210 g ai/ha. The location × year interaction was significant for visible plant injury, plant height, and yield. All cultivars showed significant injury at Simcoe in 1999, especially at 210 g/ha. Calico Belle was the most sensitive cultivar, with RPA 201772 resulting in severe injury at Simcoe in 1999. Increased injury in 1999 was due to significant rainfall (48.5 mm) within 10 d of RPA 201772 application at Simcoe. Calico Belle also showed injury at Exeter in 1999 and at Simcoe in 1998. Plant height of all cultivars was reduced at the Simcoe location in 1999. Yield of five cultivars was also reduced at Simcoe in 1999. Sensitivity to RPA 201772 increased as application rate increased. Plant height and yield was not reduced at Exeter.

The majority of dry pea (pea) cultivars in current production have semileafless architecture. Although these cultivars tend to yield well and resist lodging, they may not be the best cultivars in terms of competition with weeds. Experiments were conducted at Lacombe and Lethbridge, Alberta, Canada, from 2003 to 2005 to compare semileafless and leafy pea cultivars under different seeding and herbicide regimes. During the study, in terms of weed emergence and biomass, oat (seeded) was usually more dominant than broadleaf weeds. Higher-disturbance sweep seeding led to slightly greater oat populations than lower disturbance seeding with knives. Leafy pea had lower yield potential in less weedy conditions than the semileafless pea, but the former were less susceptible to yield reduction as weed competition increased. Leafy pea often led to lower weed biomass and dockage than the semileafless pea. However, because semileafless pea yield at least as well as leafy pea in all of the environments encountered in this study, integrated weed management principles are currently best served by growing semileafless pea. Breeding to improve leafy cultivar yield potential may lead to greater opportunities for integrated weed management and herbicide input reductions in the future.

New hybrid bluegrass cultivars have been bred for increased heat and drought tolerance compared with traditional Kentucky bluegrass cultivars. Research was conducted to evaluate hybrid bluegrass tolerance and smooth crabgrass and goosegrass control with mesotrione and quinclorac during seeded establishment. Mesotrione applied as a single application 14, 28, or 42 d after emergence (DAE) injured hybrid bluegrass acceptably from 1 to 11% 7 d after treatment (DAT). A sequential mesotrione application at 0.14 kg/ha 28 DAE did not injure hybrid bluegrass greater than the initial 14 DAE application; however, a sequential application at 0.28 kg/ha 28 DAE injured hybrid bluegrass greater than the initial 14 DAE application. Three sequential mesotrione applications did not reduce hybrid bluegrass ground cover 63 DAE. Quinclorac applied singly at 1.68 kg/ha 14 DAE and sequentially at 0.84 kg/ha 14 and 28 DAE injured hybrid bluegrass that continued throughout the rating period resulting in decreased ground cover at 63 DAE. Quinclorac at 0.84 kg/ha plus mesotrione at 0.28 kg/ha injured hybrid bluegrass < 20% throughout the rating period but reduced ground cover 63 DAE. All quinclorac-containing treatments controlled smooth crabgrass ≥ 92%. All mesotrione sequential-application treatments controlled smooth crabgrass and goosegrass ≥ 87%. These data indicate that sequential mesotrione applications can be used for smooth crabgrass and goosegrass control during seeded hybrid bluegrass establishment. Quinclorac should be applied according to label recommendations at 0.84 kg/ha no earlier than 28 DAE.

An integrated approach to weed management in pinto bean is needed since available herbicides seldom adequately control all weed species present in a field. A two-year study was conducted to assess weed control efficacy and pinto bean tolerance to mechanical weeding from a rotary hoe or flex-tine harrow at crook, unifoliolate, and trifoliolate stages of bean development. Weed control was similar for both implements and all timings in 1993. In 1994, mechanical weeding at trifoliolate and both crook and trifoliolate stages controlled more weeds than at other growth stages, regardless of type of implement. Using the flex-tine harrow reduced pinto bean stand, but results based on growth stage were not consistent each year. Damage to pinto bean hypocotyls and stems was observed with the flex-tine harrow used at both crook and trifoliolate stages in 1994. Rotary hoeing did not reduce pinto bean stand or cause injury. Yield and seed weight did not differ among treatments in either year.

A 4-yr study (1985, 1987, 1988, 1989) was conducted on the same field plots at Shafter, CA to evaluate the efficacy of prometryn in controlling black nightshade on planting beds of cotton. Two rates (1.7 and 2.2 kg ai ha–1) were applied at two times (mid March before the preplant irrigation and early April at cotton planting) each year. Incorporation of prometryn into moist planting beds with a powered rotary tiller operated at 10 cm deeper resulted in excellent control of black nightshade under low to moderate weed pressure in 1985 and 1987. Control of nightshade with early and late applications of 1.7 kg ha–1 of prometryn under high weed pressure in 1988 was only 70% at harvest, and yield losses of cotton averaged 25%. Yields of cotton treated with 2.2 kg ha–1 of prometryn in 1988 were not significantly different from weed-free plots. Only the late application of 2.2 kg ha–1 of prometryn prevented significant cotton losses under extreme weed pressure in 1989. Cotton yield losses with the other prometryn treatments ranged from 78 to 100%. Losses of cotton in weedy-check plots that received only cultivation ranged from 22% in the absence of rain or irrigation at cotton planting in 1987 to as much as 100% when rain fell in 1988 or plots were irrigated at planting in 1989. Plots hoed one time 4 wk after cotton planting yielded an average of 84% as much seed cotton as weed-free plots. In an attempt to determine why the efficacy of prometryn declined between 1985 and 1989, several experiments were conducted in 1988 to 1990 to discover reasons for this poor control of nightshade. Because efforts failed to provide evidence for the movement of the herbicide with water, the development of weed resistance to prometryn, or accelerated degradation of this herbicide in soil, increasing weed seed populations in soil were believed to have contributed greatly to the declining nightshade control from prometryn. The fact that prometryn applied and incorporated into flat soil provided excellent control of nightshade in 1990 under sprinkler irrigation indicated that both soil moisture and incorporation techniques limited activity of prometryn in planting beds in 1988 and 1989. Incomplete control of nightshade plus good soil moisture at planting contributed to the high weed populations in 1988 and 1989.

Volunteer potato is highly competitive with onion and few control tactics are effective for removing this weed from an onion crop. Both volunteer potato density and duration of interference reduce onion yield, but the interaction of these factors is unknown. Field trials were conducted in 2003 in Idaho, Oregon, and Washington to determine the influence of volunteer potato density on the critical time of weed removal (CTWR) in onion. Yield losses of 2.5, 5.0, and 10% were estimated to occur at 534, 654, and 830 growing degree days (GDD) after onion emergence, respectively, with a volunteer potato density of 0.5 plants/m2. At 2.0 volunteer potato plants/m2, yield losses of 2.5, 5.0, and 10% were estimated to occur at 388, 481, and 598 GDD after onion emergence, respectively. Volunteer potato at 2.0 plants/m2 had to be removed at least one onion leaf stage sooner, compared to a weed density of 0.5 plants/m2, to avoid yield loss. Yield loss due to volunteer potato density or duration of interference was greatest among jumbo, colossal, and supercolossal market grades (P ≤ 0.1). Lowering potato tuber density in crops preceding onion will extend the critical time for weed removal and reduce the risk of crop loss.

Spring-applied herbicides were evaluated in field studies conducted in 1985 through 1987 in western Nebraska for selective weed control in irrigated established alfalfa. Imazethapyr, norflurazon, pendimethalin, and prodiamine did not injure alfalfa but varied in their weed control spectrums. Overall weed densities were lowest in plots treated with norflurazon at 2.8 kg ai/ha and diuron at 1.8 kg/ha.

Herbicide-resistant canola dominates the canola market in Canada. A multiyear field experiment was conducted at three locations to investigate the effect of time of weed removal (two-, four-, or six-leaf canola) and herbicide rate (50 or 100% recommended) in three herbicide-resistant canola systems. Weeds were controlled in glufosinate-resistant canola (GLU) with glufosinate, in glyphosate-resistant canola (GLY) with glyphosate, and in imidazolinone-resistant canola (IMI) with a 50:50 mixture of imazamox and imazethapyr. Canola yields were similar among the three canola cultivar–herbicide systems. Yields were not influenced by 50 vs. 100% herbicide rates. Timing of weed removal had the greatest effect on canola yield, with weed removal at the four-leaf stage giving the highest yields in most cases. Percent dockage was often greater for GLU and IMI than for GLY. In comparison with the other treatments, dockage levels doubled for GLU after application at 50% herbicide rates. The consistency of monocot weed control was usually greater for GLY than for GLU or IMI systems. However, weed biomass data revealed no differences in dicot weed control consistency between IMI and GLY systems. Greater dockage and weed biomass variability after weed removal at the six-leaf stage or after low herbicide rates suggests higher weed seed production, which could constrain the adoption of integrated weed management practices in subsequent years.

Multiflora rose was defoliated completely 320 days after spring foliar application of metsulfuron. Metsulfuron and 2,4-D plus dicamba spring foliar applied controlled multiflora rose equally. Metsulfuron applied to soil using a spotgun at 20 mg per m diam of multiflora rose in the spring resulted in 95% control 320 days later in one study but only 50% control in another. Control was less when lower rates of metsulfuron were soil-applied with a spotgun. Tebuthiuron spring soil-applied completely controlled multiflora rose. All spring-applied foliar and soil applications seemed to control multiflora rose better than fall treatments.

Isoxaflutole, a preemergence herbicide for use in corn, causes bleaching of plant tissue and plant death at low rates. A concern regarding widespread use of isoxaflutole is the unintentional exposure of high-value, minor hectareage crops that may be sensitive. Unintentional exposure could occur because of carryover from a previous application, spray drift, or contamination of irrigation water. The objective of this study was to determine the potential for injury to nine minor hectareage Michigan crops. Crops evaluated were: adzuki bean, alfalfa, carrot, cucumber, dry bean (navy and black beans), onion, sugar beet, and tomato. Experiments were conducted in the greenhouse to evaluate injury from low rates of isoxaflutole applied to soil to simulate carryover as well as low concentrations of isoxaflutole in 2.54 cm of irrigation water applied over the course of 1 h to 15-cm-tall plants. Isoxaflutole rates and concentrations that cause 20% injury (I20) were calculated using Seefeldt's log-logistic dose–response model. Regardless of application type, onion was always the least sensitive plant to isoxaflutole (I20 = 37 g/ha applied to soil and 194 μg/L in irrigation water), whereas navy bean and black bean were the most sensitive (I20 = 9 g/ha applied to soil and 5 μg/ L in irrigation water). The remaining plants exhibited intermediate sensitivity. All of the rates that resulted in injury were substantially less than the rates used for weed control in corn. Carryover from isoxaflutole applications in corn production may require plant back restrictions for certain sensitive crops.

Field and greenhouse experiments were conducted in 2005 and 2006 in Guam to evaluate the effects of glyphosate on field dodder control and to describe glyphosate dose–response curves on selected ornamental plants grown with and without dodder infestation. Visual quality of dodder-free plants decreased with increasing dose of glyphosate. The most sensitive species was king's mantle, whereas the most tolerant was hibiscus. The values for the effective dose for a 10% reduction in visual quality (ED10) of glyphosate were 800, 280, 1,250, 370, 590, 830, 660, and 170 g ai/ha for dodder-free croton, allamanda, hibiscus, paper gardenia, ixora, duranta, schefflera, and king's mantle, respectively. However, dodder-infested plants were less tolerant to glyphosate because of the confounded stress from both the parasite and herbicide. Field dodder parasitizing ornamental plants could be adequately controlled on all ornamental species at a dose of about 140 g/ha of glyphosate.

Spreading dogbane is a troublesome weed of wild blueberry fields. Field studies were conducted in 2008 and 2009 to evaluate efficacy of different herbicides and application techniques on spreading dogbane as well as blueberry tolerance. Results indicated that summer-broadcast nicosulfuron at 25 g ai ha−1 with 0.5% v/v blend of surfactant with petroleum hydrocarbons suppressed (> 60%) spreading dogbane at three of four sites. Spot sprays with dicamba at 1 kg ae ha−1 effectively controlled (> 80%) spreading dogbane with minimal (19 to 23%) blueberry damage at three of four sites. Glyphosate spot sprays at 5 g ae L−1 water provided more effective and longer control than hand pulling. Wiping with glyphosate at 154 g ae L−1 water or wiping triclopyr at 29 g ae L−1 water onto the shoots is also an effective control method for localized patches of spreading dogbane. Although low to moderate crop damage may accompany these techniques, it may still be tolerable for growers to apply these options to limit long-term yield loss caused by spreading dogbane.

Dodder seeds are physically dormant because of hard seed coats and do not readily germinate without scarification. Reliable methods of scarification for small lots of dodder seed are needed to facilitate laboratory, greenhouse, and field research projects. Dodder seed was scarified for varying times using a handheld rotary tool at the 10,000 rpm setting with a conical grinding-stone bit attached. Percentage of germination and weight change of seeds were assessed using scarification times between 0 and 4 min at 0.5-min increments. Mean seed weight loss and mean number of germinated seeds increased quadratically as scarification time increased. Scarifying for 2.5 min was judged the shortest time with maximal germination. Another study evaluated the effect of seed number (100 to 400 seeds sample−1) on the efficacy of rotary tool scarification when scarification time was held constant at 2.5 min. Percentage of germination decreased linearly as seed batch size increased. The handheld rotary tool provides consistent and repeatable scarification of dodder seed with germination rates greater than 80%.

Greenhouse experiments were conducted in 2011 to evaluate the effect of mowing frequency and mowing height on four summer annual weed species (large crabgrass, barnyardgrass, giant ragweed, and common lambsquarters). Plants were clipped at three heights (5, 10, or 20 cm) and at two frequencies (single clipping or repeated clippings at the same height) to simulate mowing. A nonclipped control was also grown for each species. When clipped once, large crabgrass, barnyardgrass, and giant ragweed produced at least 90% of the total dry weight (DW) of the nonclipped plants, and common lambsquarters produced at least 75%. A single cut was generally not sufficient to prevent weed seed production or kill any of the weeds in this study. Repeated clipping reduced large crabgrass, giant ragweed, and common lambsquarters reproductive DW to 46, 27, and 10% respectively, of the nonclipped control. Barnyardgrass plants that were repeatedly clipped produced between 0 and 8% of the seed DW of nonclipped plants, depending on clipping height. Repeated clipping reduced weed total DW to below 40% for all species compared to nonclipped plants. Our results suggest that, unless combined with other weed management practices, repeated mowing may be necessary to limit the growth and seed production of these weed species.

Nutsedge species are problematic in plastic-mulched vegetable production because of the weed’s rapid reproduction and ability to penetrate the mulch. Vegetable growers rely heavily on halosulfuron to manage nutsedge species; however, the herbicide cannot be applied over mulch before vegetable transplanting due to potential crop injury. This can be problematic when multiple crops are produced on a single mulch installation. Field experiments were conducted to determine the response of broccoli, cabbage, squash, and watermelon to halosulfuron applied on top of mulch prior to transplanting. Halosulfuron at 80 g ai ha−1 was applied 21, 14, 7, and 1 d before planting (DBP), and 160 g ai ha−1 was applied 21 DBP. In all experiments, extending the interval between halosulfuron application and planting reduced crop injury. For squash and watermelon, visual injury, plant diameters/vine runner lengths, marketable fruit weights, and postharvest plant biomass resulted in similar values when applying 80 g ha−1 21 DBP and with the nontreated weed-free control. Reducing this interval increased injury for both crops. Visual crop injury and yield reductions up to 40% occurred, with halosulfuron applied 14, 7, or 1 DBP in squash and 1 DBP in watermelon. Broccoli and cabbage showed greater sensitivity, with injury and plant diameter reductions greater than 15%, even with halosulfuron applied at 80 g ha−1 21 DBP. Experimental results confirm that halosulfuron binds to plastic mulch, remains active, and is slowly released from the mulch over a substantial period, during rainfall or overhead irrigation events. Extending the plant-back interval to at least 21 d before transplanting did overcome squash and watermelon injury concerns with halosulfuron at 80 g ha−1, but not broccoli and cabbage. Applying halosulfuron over mulch to control emerged nutsedge before planting squash and watermelon would be beneficial if adequate rainfall or irrigation and appropriate intervals between application and planting are implemented.

Rattail fescue is a problematic weed for small grain producers in the Pacific Northwest when no-till production practices are used. Pyroxsulam and pyroxasulfone are two herbicides not previously evaluated for control of rattail fescue. Pyroxasulfone provided levels of control (> 74%) similar to flufenacet. Pyroxsulam did not consistently control (21 to 71%) rattail fescue. Rattail fescue biomass was reduced by pyroxasulfone and flufenacet compared to the nontreated control. Effective consistent rattail fescue control was only achieved where PRE herbicides were used. When managing rattail fescue, PRE herbicides pyroxasulfone and flufenacet plus metribuzin are essential components of an integrated management strategy.

As the amount of methyl bromide approved for use in Florida strawberry diminishes, growers are faced with a forced transition to alternative fumigants. Many of these methyl bromide alternatives have been associated with reductions in weed control, requiring additional but complementary measures. POST herbicide options for annual strawberry are limited, resulting in significant portions of the strawberry acreage in Florida being hand-weeded when troublesome weeds escape conventional control methods. Strawberry has shown acceptable tolerance to clopyralid in other areas and production systems; however, its integration into the Florida production system and ramifications of applications during fruiting warrants further research. Eight trials were conducted, with three common strawberry cultivars grown in West Central Florida subjected to POST spray and drip-tape-injected applications of clopyralid. Formation of new strawberry leaves was not affected by clopyralid application, except for a reduction in new leaves of the cultivar ‘Strawberry Festival' at the highest rate of application of 261 g ae ha−1 in comparison with the nontreated control. Strawberry leaf malformation was best explained by an exponential growth equation, whereas marketable yield followed the trend of a Weibull peak. At the maximum labeled rate (66 g ha−1), leaf malformation was less than 5% for all cultivars tested, and marketable yield was estimated at 104% of the nontreated control.

Field experiments were conducted in 2014 and 2015 in Fayetteville, Arkansas, to evaluate the residual activity of acetyl-CoA carboxylase (ACCase)–inhibiting herbicides for monocot crop injury and weed control. Conventional rice, quizalofop-resistant rice, grain sorghum, and corn crops were evaluated for tolerance to soil applications of six herbicides (quizalofop at 80 and 160 g ai ha–1, clethodim at 68 and 136 g ai ha–1, fenoxaprop at 122 g ai ha–1, cyhalofop at 313 g ai ha–1, fluazifop at 210 and 420 g ai ha–1, and sethoxydim at 140 and 280 g ai ha–1). Overhead sprinkler irrigation of 1.3 cm was applied immediately after treatment to half of the plots, and the crops planted into the treated plots at 0, 7, and 14 d after herbicide treatment. In 2014, injury from herbicide treatments increased with activation for all crops evaluated, except for quizalofop-resistant rice. At 14 d after treatment (DAT) in 2014, corn and grain sorghum were injured 19% and 20%, respectively, from the higher rate of sethoxydim with irrigation activation averaged over plant-back dates. Conventional rice was injured 13% by the higher rate of fluazifop in 2014. Quizalofop-resistant rice was injured no more than 4% by any of the graminicides evaluated in either year. In 2015, a rainfall event occurred within 24 h of initiating the experiment; thus, there were no differences between activation via irrigation or by rainfall. However, as in 2014, grain sorghum and corn were injured 16% and 13%, respectively, by the higher rate of sethoxydim, averaged over plant-back dates. All herbicides provided little residual control of grass weeds, mainly broadleaf signalgrass and barnyardgrass. These findings indicate the need to continue allowing a plant-back interval to rice following a graminicide application, unless quizalofop-resistant rice is to be planted. The plant-back interval will vary by graminicide and the amount of moisture received following the application.

Glyphosate-resistant Amaranthus species are a recognized risk to U.S. agriculture. With affected cropland exceeding 1.2 million ha, this epidemic is particularly pertinent to agricultural regions that utilize an intensive glyphosate-based management program to control weedy pests. Before 2006, Texas had no identified glyphosate-resistant populations. Two independent common waterhemp populations exhibiting poor control by glyphosate were identified in Wharton County and Fort Bend County, TX in 2006 and 2008, respectively. The objective of the present research was to characterize the level of glyphosate resistance (50% lethal dose [LD50] and 50% reduction in growth rate [GR50]) in each population. Resistance levels in four putatively glyphosate-resistant common waterhemp biotypes selected from these two populations were compared with confirmed glyphosate-resistant and -susceptible common waterhemp populations under greenhouse conditions. The LD50 value for the susceptible population (736 g ae ha−1) was equivalent to the 0.9× labeled rate of glyphosate, whereas the putatively resistant lines exhibited a broad range of resistance with LD50 values ranging from 3.5 to 59.7× the labeled rate of glyphosate. The GR50 value for the most resistant line was 2.5-fold greater than the susceptible biotype (317 g ae ha−1 of glyphosate). These results confirm the first documented case of a glyphosate-resistant weed species in Texas.