Progress in bioherbicide development has been hindered by the strict moisture and temperature requirements of the living active ingredient. Application of a jute fabric to areas treated with a Sclerotinia minor granular bioherbicide improved broadleaf weed control and broadened the effective application period to include the warm summer season. When turfgrass plots treated with the bioherbicide were covered with burlap fabric for 3 d, broadleaf weed (dandelion, white clover, broadleaf plantain, buckhorn plantain, ground ivy, and prostrate knotweed) control was greatly enhanced. The cover was made of natural jute fibers that retained water but had sufficient transparency to allow 33% light penetration for continued growth of the grass. Virulence of the bioherbicide was maintained under elevated temperatures that would otherwise reduce efficacy. The bioherbicide was ineffective in the summer unless covered, but dandelion density, broadleaf weed ground cover, and dandelion survival were all reduced by the bioherbicide when plots were covered, even if applications were made in July. The efficacy of the bioherbicide was also enhanced under favorable conditions, and covering permitted reduced application rates without loss of efficacy. When applied at a rate of 20 g/m2 and covered, S. minor granules exerted significantly greater biocontrol of dandelion than 40 g/m2 without covering. Covering for up to 5 d did not cause any adverse effects on the turfgrass. This approach may overcome one obstacle to the commercialization of the Sclerotinia minor bioherbicide, permitting its deployment under challenging environmental conditions.

The selection of herbicide-resistant weeds in grain sorghum production has prompted researchers to explore alternative herbicides to prevent, delay, and manage herbicide-resistant weed biotypes. Greenhouse and field experiments were conducted to evaluate the differential response of sorghum hybrids to POST application of mesotrione. In a greenhouse experiment, 85 sorghum hybrids were treated with 0, 52, 105, 210, and 315 g ai/ha mesotrione when plants were at the three- to four-leaf collar stage. Sorghum response ranged from susceptible to tolerant sorghum hybrids. ‘Pioneer 84G62’, ‘Pioneer 85G01’, and ‘Triumph TR 438’ were the three most susceptible, whereas ‘Dekalb DKS35-70’, ‘Frontier F222E’, and ‘Asgrow Seneca’ were the three most tolerant hybrids. One week after treatment (WAT), the mesotrione rate causing 50% visible injury ranged from 121 to 184 and 64 to 91 g/ha in the most tolerant and susceptible hybrids, respectively. Mesotrione dose–response studies were conducted under field conditions on four sorghum hybrids. One WAT, injury symptoms were greater (up to 23%) in Pioneer 85G01 than in Asgrow Seneca (< 14%). However, all plants appeared normal by the end of the growing season. In addition, sorghum yields were not reduced by mesotrione treatments as verified by correlation coefficient analysis.

Field experiments were conducted from 2004 through 2006 to evaluate cressleaf groundsel control following fall or early-spring preplant herbicide applications. Glyphosate, glyphosate + imazethapyr, glyphosate + 2,4-D ester, paraquat, paraquat + simazine, chlorimuron + tribenuron, and pendimethalin + glyphosate + 2,4-D ester applied in the fall controlled at least 93% of the cressleaf groundsel. Glyphosate, glyphosate + imazethapyr, glyphosate + 2,4-D ester, chlorimuron + tribenuron, and pendimethalin + glyphosate + 2,4-D ester applied in the spring provided at least 94% control of the cressleaf groundsel. Chlorimuron + tribenuron provided at least 98% control, regardless of application timing. These results indicate that herbicides applied in the fall or early spring can control cressleaf groundsel. However, certain herbicides provide greater control when applied in the fall compared with spring.

Herbicide rotations and mixtures are widely recommended to manage herbicide resistance. However, little research has quantified how these practices actually affect the selection of herbicide resistance in weeds. A 4-yr experiment was conducted in western Canada from 2004 to 2007 to examine the impact of herbicide rotation and mixture in selecting for acetolactate synthase (ALS) inhibitor resistance in the annual broadleaf weed, field pennycress, co-occurring in wheat. Treatments consisted of the ALS-inhibitor herbicide, ethametsulfuron, applied in a mixture with bromoxynil/MCPA formulated herbicide (photosystem-II inhibitor/synthetic auxin), or in rotation with the non-ALS inhibitor at an ALS-inhibitor application frequency of 0, 25, 50, 75, and 100% (i.e., zero to four applications, respectively) over the 4-yr period. The field pennycress seed bank at the start of the experiment contained 5% ethametsulfuron-resistant seed. Although weed control was only marginally reduced, resistance frequency of progeny of survivors increased markedly after one ALS-inhibitor application. At the end of the experiment, the level of resistance in the seed bank was buffered by susceptible seed, increasing from 29% of recruited seedlings after one application to 85% after four applications of the ALS inhibitor. The level of resistance in the seed bank for the mixture treatment after 4 yr remained similar to that of the nontreated (weedy) control or 0% ALS-inhibitor rotation frequency treatment. The results of this study demonstrate how rapidly ALS-inhibitor resistance can evolve as a consequence of repeated application of herbicides with this site of action, and supports epidemiological information from farmer questionnaire surveys and modeling simulations that mixtures are more effective than rotations in mitigating resistance evolution through herbicide selection.

Field experiments were conducted in Nebraska with the experimental herbicide KIH-485 on soils with three different levels of organic matter (OM) to ascertain a dose response for weed control and corn tolerance. Dose–response curves based on the log-logistic model were used to determine the effective dose that provides 90% weed control (ED90 values) for three grasses (green foxtail, field sandbur, large crabgrass) and two broadleaf weeds (velvetleaf, tall waterhemp). The ED90 values for green foxtail control were 143, 165, and 202 g ai/ha for soils with 1, 2, and 3% OM, respectively at 28 d after treatment (DAT). The highest dose of 371 g ai/ha was needed for field sandbur control at 28 DAT, compared with 141 g ai/ha for large crabgrass, 152 g ai/ha for tall waterhemp, and 199 g ai/ha for velvetleaf. There was no significant corn injury observed. Grain yield increased with increasing doses of KIH-485; optimum yield was achieved at about 195 g ai/ha. From the dose–response curves it is clear that the proposed label rate of KIH-485 of 200 to 300 g ai/ha will provide excellent control of most grasses and certain broadleaf weeds in corn for at least the first 4 wk of the growing season on soils up to 3% OM in the state of Nebraska.

Blackberry is a troublesome species across much of the southeastern United States. Control of blackberry with the pyridine herbicides is often variable among different locations. Experiments were conducted to determine whether application timing, either spring or fall, affected efficacy of the pyridine herbicides triclopyr, fluroxypyr and picloram, and metsulfuron. The pyridine herbicides provided greater control when applied in the fall. At 12 mo after treatment, fluroxypyr plus picloram and fluroxypyr plus triclopyr provided 83% control when applied in the fall and 65% when applied in the spring. Conversely, metsulfuron provided 85% control, and application timing was not significant. Although metsulfuron effectively controls blackberry, it is also highly injurious to bahiagrass. Therefore, chlorosulfuron was tested to determine whether it would provide blackberry control while not injuring bahiagrass. Blackberry control with chlorosulfuron was similar to metsulfuron. These data indicated blackberry control in bahiagrass pastures can be effectively accomplished with chlorosulfuron.

Two field studies were conducted in 2007 and 2008 to evaluate at-planting burndown and POST herbicide applications targeting volunteer glyphosate-resistant (GR) soybean in rice. In the burndown study, paraquat, glufosinate, and a thifensulfuron plus tribenuron mixture were applied immediately after rice seeding. Paraquat controlled volunteer GR soybean at least 95% at all evaluations both years. Control with glufosinate was greater in 2007 than 2008 due to rainfall that occurred following application the second year. The thifensulfuron plus tribenuron mixture provided similar control in both years, but control never exceeded 71%. Additionally, a study was conducted evaluating POST-applied rice herbicides including propanil (4,480 and 2,240 g ai/ha), triclopyr (420 and 210 g ai/ha), bispyribac-sodium (38 and 19 g ai/ha), penoxsulam (40 and 20 g ai/ha), and halosulfuron (70 and 35 g ai/ha). Control across all POST herbicides and application rates was equivalent (> 95%) 28 and 56 d after application except for propanil, which controlled volunteer GR soybean less than other treatments. Volunteer GR soybean can be effectively managed in a rice production system with at-planting burndown or POST herbicide applications in rice.

This research explored the use of downy brome (BROTE) as a cover crop in irrigated corn. Although BROTE is a difficult weed to control, it could not be maintained as a cover crop in no-till irrigated corn for more than one season. A 10-fold reduction in BROTE occurred in the second year of corn. By the fourth year, only one BROTE plant could be found at the two locations. Because BROTE did not persist across years, soil coverage decreased 5 to 18% in the later location-years. At one location, normal herbicide rates decreased Johnsongrass biomass more than 22-fold both years it was applied. Increasing herbicide input decreased Palmer amaranth density more than 3-fold, but only in a single location-year. In three of six location-years, level of herbicide input had no significant effect on evapotranspiration (ET). Increased BROTE biomass decreased ET 0.033 to 0.083 cm/d during the first season at both locations. Increased irrigation increased corn yield by 240 to 1,900 kg/ha in five of six location-year combinations. Half rates of in-season herbicides reduced yield only in one of six location-years. High BROTE density reduced ET but did not translate into increased crop yield. In three of six location-year combinations, high BROTE density decreased yield by 300 to 1,000 kg/ha. In a single location-year, increased surface residues provided by BROTE increased yield by 560 kg/ha. Increased irrigation inputs decreased water use efficiency (WUE) by 6.3 kg/ha-cm in a single location-year and increased WUE by 10.8 to 121.6 kg/ha-cm in four of six location-years. Increased herbicide inputs increased WUE by 10.3 kg/ha-cm in one location-year. BROTE density had no significant effect on WUE at location 1. At location 2 in the first 2 yr, WUE was increased 9.4 to 22.2 kg/ha-cm.

Research was conducted in North Carolina to determine peanut response to flumioxazin as influenced by rate and timing of application and cultivar. Delaying application of flumioxazin from 1 d after planting until peanut emergence increased injury regardless of rate. The Virginia market-type cultivar ‘NC-V 11’ was injured more by flumioxazin than the cultivars ‘Gregory’ or ‘Perry’. However, pod yield was not affected by flumioxazin even though significant injury was observed early in the season regardless of flumioxazin rate, application timing, or cultivar. Diclosulam was more effective than flumioxazin in controlling eclipta when these herbicides were applied PRE with metolachlor or following pendimethalin PPI. However, control by flumioxazin prevented yield loss when compared with metolachlor alone.

Western Australian growers are adopting no-tillage farming systems, which have a greater reliance on integrated weed management systems that include competitive cultivars, high seeding rates, strategic fertilizer placement, and narrow row spacing. At the same time, they are sowing more of their barley area to cultivars with a semidwarf habit. This study compared six two-row, spring malting barley cultivars with different morphological traits at two dates of planting and three crop densities, with nil-added and added rigid ryegrass at five sites. Four cultivars, ‘Buloke’, ‘Flagship’, ‘Hamelin’, and ‘Vlamingh’, had an erect early growth habit; the other two, ‘Baudin’ and ‘Gairdner’, had a prostrate, semidwarf early growth habit. Increasing the density of rigid ryegrass plants from 16 to 125 plants/m2 decreased barley grain yield by reducing crop biomass production, tiller number, grain number, and harvest index. Average grain weight was also reduced at most sites. The impact of competition on grain yield varied across sites and cultivars, but did not interact with date of planting. Baudin, Flagship, and Hamelin were more competitive with rigid ryegrass than Buloke, Gairdner, and Vlamingh. Differences in competitiveness between cultivars did not appear to be strongly related to morphological traits that affect light interception such as canopy closure, plant height, and tiller number. Differences in cultivar competitiveness were cultivar-specific and not generic. Rigid ryegrass productivity (dry matter and tiller number) tended to be lower under the more competitive cultivars. Increasing barley plant density increased grain yield, and reduced both rigid ryegrass dry matter and tiller number. Barley density had a larger impact on rigid ryegrass productivity than crop cultivar. The impact of barley density on reducing rigid ryegrass tiller number was larger with the first planting date than the second, and higher in the added rigid ryegrass plots than the nil-added plots. This study highlights the importance of high barley seeding rate for the integrated management of rigid ryegrass.

Field studies were conducted to determine if mesotrione alone or in combinations with other corn herbicides would control horseweed and other winter annual weeds associated with no-till corn. Mesotrione alone controlled horseweed 52 to 80% by 3 wk after treatment (WAT); however, by 7 WAT control diminished to between 37 to 68%, depending on mesotrione rate. Mesotrione at 0.16 kg ai/ha plus atrazine at 0.28 kg ai/ha controlled 99% of horseweed and annual bluegrass and 88% of yellow woodsorrel. Combinations of mesotrione at 0.16 kg/ha plus acetochlor at 1.79 kg ai/ha plus 1.12 kg ai/ha glyphosate (trimethylsulfonium salt of glyphosate) or 0.7 kg ai/ha paraquat provided 93% or greater control of all three weed species. Glyphosate alone also controlled all weed species 97 to 99%, while paraquat alone provided 99% control of annual bluegrass, 72% control of horseweed, and 36% control of yellow woodsorrel. Mixtures of paraquat plus acetochlor improved control of horseweed (93%) and yellow woodsorrel (73%) over control with either herbicide applied alone.

Five field trials were conducted over a 2-yr period (2007, 2008) at various locations in Ontario to evaluate the tolerance of black, cranberry, kidney, otebo, pink, pinto, small red Mexican (SRM), and white bean to halosulfuron applied PPI, PRE, and POST at 35 and 70 g ai/ha. There was minimal injury (3% or less) with halosulfuron applied PPI or PRE in dry bean. At Exeter and Ridgetown, halosulfuron applied POST at 35 and 70 g/ha caused 3 to 5% and 4 to 8% injury in dry bean, respectively at 1 wk after herbicide application (WAA). The injury was transient with no significant injury at 2 and 4 WAA. At Harrow, halosulfuron POST at 35 and 70 g/ha caused as much as 4% injury at 35 g/ha and 14% injury at 70 g/ha in dry bean. Halosulfuron applied PPI, PRE, and POST at 35 and 70 g/ha caused no decrease in plant height of dry bean except for kidney bean, which was reduced 6% at 70 g/ha, and white bean, which was reduced 3% at both 35 and 70 g/ha. Halosulfuron applied PPI, PRE, and POST at 35 and 70 g/ha caused no decrease in dry bean yield except for kidney bean, which was reduced 9% at 35 g/ha and 10% at 70 g/ha; otebo bean, which was reduced 3% at 70 g/ha; and white bean, which was reduced 7% at both 35 and 70 g/ha. On the basis of these results, there is an adequate margin of crop safety in dry bean to halosulfuron applied PPI or PRE at 35 and 70 g/ha. In addition, there is an adequate margin of crop safety in black, cranberry, pink, pinto, and SRM bean to halosulfuron applied POST at 35 and 70 g/ha. However, further research is required to ascertain the tolerance of kidney, otebo, and white bean to halosulfuron applied POST.

Military bases in the United States were mandated to reduce the amount of pesticide used to 50% of 1993 levels by 2000. Historically, 2,4-D was applied to control common sunflower, which establishes itself in disturbed soils and obstructs gunners' views of targets. A 25-ha lowland field in Camp Forsyth was selected to compare efficacy of alternative herbicides with that of 2,4-D low-volatile ester (LVE), with the goal of reducing the amount of herbicide applied by at least half. Site vegetation was mostly native tallgrass prairie dominated by warm-season C4 grasses (e.g., big bluestem, Indiangrass, little bluestem, and switchgrass) and including less abundant C3 species in the Asteraceae, Fabaceae, Brassicaceae, and other families. Initially, the site had a high population of common sunflower. All herbicide treatments from 3 yr of field trials were highly and equally effective at reducing common sunflower, decreasing stem density by 83 to100%. Treatments that offer substantial reductions in the amount of herbicide applied are chlorimuron (0.01 kg ae/ha), dicamba + 2,4-D amine (0.07 kg ae/ha + 0.20 kg ae/ha), clopyralid + 2,4-D amine (0.06 kg ae/ha + 0.28 kg ae/ha), 2,4-D LVE (0.56 kg ae/ha), and metsulfuron + 2,4-D amine (0.002 kg ai/ha + 0.28 kg ae/ha). Use of these herbicides at Ft. Riley would reduce total active ingredient applied by 73 to 99% and lower chemical costs for this particular use by as much as 88%.

Experiments were conducted during 2000 and 2001 at a total of 13 locations throughout Alabama, Georgia, Florida, North Carolina, and Texas to evaluate efficacy of herbicides at or below the manufacturer's suggested use rate. Herbicide applications included diclosulam and flumioxazin applied PRE alone or followed by imazapic applied early postemergence (EPOST). All possible combinations of diclosulam at 0, 13.5, or 27 g ai/ha and flumioxazin at 0, 53, or 105 g ai/ha applied PRE were included. Imazapic was applied at 35 g ai/ha. Ivyleaf morningglory was controlled more than 87% when imazapic was applied EPOST regardless of PRE herbicide. Pitted morningglory control > 67% was observed with applications of diclosulam (27 g/ha) followed by imazapic, diclosulam (13.5 g/ha) plus flumioxazin (53 g/ha), diclosulam (13.5 g/ha) plus flumioxazin (105 g/ha), and diclosulam (27 g/ha) plus flumioxazin (105 g/ha). Sicklepod was controlled more than 74% with flumioxazin (53 g/ha) followed by imazapic and diclosulam (27 g/ha) plus flumioxazin (105 g/ha) followed by imazapic. Florida beggarweed was controlled more than 84% by all PRE herbicide combinations except flumioxazin (53 g/ha) alone or diclosulam (27 g/ha) alone or with imazapic. Yellow nutsedge was controlled at least 90% with diclosulam at either rate followed by imazapic and by diclosulam plus flumioxazin followed by imazapic regardless of rate. Pod yield was generally higher when herbicides were applied regardless of herbicide combination or rate. Peanut yield was maximized with the lowest rates of flumioxazin or diclosulam PRE followed by imazapic EPOST.

Infestations of glyphosate-resistant (GR) horseweed have become widespread in the eastern United States. This biotype is problematic in no-tillage production that relies extensively on glyphosate for weed control. Because horseweed is treated at various stages of growth, a greenhouse study explored rate response of glyphosate-resistant and -susceptible horseweed at three growth stages. GR horseweed was more responsive to glyphosate at the seedling stage than at the large rosette or bolting stages. A field study evaluated GR horseweed response when treated with glyphosate at soybean planting time, POST in-crop (about 45 d after planting), or both at planting and POST in-crop. There was a cumulative effect of the at-planting followed by POST in-crop glyphosate applications. When evaluating single glyphosate applications, the at-planting application was more effective at suppressing GR horseweed than a POST in-crop application. Because glyphosate cannot control GR horseweed, this biotype should be controlled with an herbicide with an alternate mode of action and applied at the most effective timing.

A 3-yr field study was conducted from 2005 to 2007 at Stoneville, MS, to determine efficacy of in-crop and autumn-applied glyphosate on purple nutsedge density and yield of no-till glyphosate-resistant (GR) corn and GR soybean. Separate experiments were conducted in GR corn and GR soybean in areas maintained under a no-tillage system after the autumn of 2004. Each experiment was conducted in a split-plot arrangement of treatments in a randomized complete-block design with and without autumn application of glyphosate at 1.68 kg ae/ha as main plots and in-crop herbicide application (glyphosate- and nonglyphosate-based programs) as subplots with three replications. In GR corn, glyphosate applied in the autumn reduced purple nutsedge density by 40 to 67% compared with no glyphosate during 3 yr. In GR corn, glyphosate applied in-crop reduced purple nutsedge shoot density by 48% in 2005, 92% in 2006, and 100% in 2007 compared with no herbicide. However, GR corn yields were unaffected by either in-crop or autumn-applied glyphosate. In GR soybean, glyphosate applied in the autumn reduced purple nutsedge shoot density by 64 to 83% compared with no glyphosate during 3 yr. Glyphosate applied in-crop in GR soybean reduced purple nutsedge density by 81% in 2005 and by 100% in 2006 and 2007 compared with no herbicide. GR soybean yields were similar in 2005, but yields were 34 and 18% higher in 2006 and 2007, respectively, with autumn-applied glyphosate compared with no glyphosate. GR soybean yields were higher with glyphosate applied in-crop compared with no herbicide in 2 of 3 yr. These results indicate that purple nutsedge density could be reduced with glyphosate applied in-crop in no-till GR corn and GR soybean. In addition, autumn-applied glyphosate was effective in reducing purple nutsedge populations following harvest of crops and could be an effective purple nutsedge management strategy regardless of GR trait.

Field studies were conducted near Clayton, Lewiston, and Rocky Mount, NC in 2005 to evaluate weed control and cotton response to preemergence treatments of pendimethalin alone or in a tank mixture with fomesafen, postemergence treatments of glufosinate applied alone or in a tank mixture with S-metolachlor, and POST-directed treatments of glufosinate in a tank mixture with flumioxazin or prometryn. Excellent weed control (> 91%) was observed where at least two applications were made in addition to glufosinate early postemergence (EPOST). A reduction in control of common lambsquarters (8%), goosegrass (20%), large crabgrass (18%), Palmer amaranth (13%), and pitted morningglory (9%) was observed when residual herbicides were not included in PRE or mid-POST programs. No differences in weed control or cotton lint yield were observed between POST-directed applications of glufosinate with flumioxazin compared to prometryn. Weed control programs containing three or more herbicide applications resulted in similar cotton lint yields at Clayton and Lewiston, and Rocky Mount showed the greatest variability with up to 590 kg/ha greater lint yield where fomesafen was included PRE compared to pendimethalin applied alone. Similarly, an increase in cotton lint yields of up to 200 kg/ha was observed where S-metolachlor was included mid-POST when compared to glufosinate applied alone, showing the importance of residual herbicides to help maintain optimal yields. Including additional modes of action with residual activity preemergence and postemergence provides a longer period of weed control, which helps maintain cotton lint yields.

In the spring of 2007, a widespread freeze occurred that led to the replanting of about 81,000 hectares of corn in Tennessee. Limited research was available on effective herbicide options to control failed stands of glyphosate-tolerant corn where replanting to corn was desired. Therefore, in 2007 on three failed freeze-damaged corn stands (27,000 plants/ha) and in 2008 on two non–freeze-damaged corn stands (81,000 plants/ha), studies were initiated to determine how to control the failed stand while not harming subsequent replant corn. The results from this research clearly show that producers have several effective options to manage undesirable glyphosate-tolerant corn before replanting to corn. Clethodim at 0.14 kg/ha, paraquat at 0.84 kg/ha, or paraquat at 0.70 kg/ha plus a photosystem II (PSII) inhibitor provided very good control of the original corn stand without affecting the yield of the replanted corn. Moreover, the addition of a PSII inhibitor could provide residual weed control in the replanted corn. Paraquat should be applied at rates of 0.70 kg/ha or greater to obtain consistent control of an undesirable corn stand.

Field experiments were conducted in Platte County, Missouri, during 2006 and 2007 to evaluate PRE, POST, and PRE followed by (fb) POST herbicide programs for the control of glyphosate-resistant waterhemp in soybean. All PRE fb POST treatments resulted in at least 66 and 70% control of glyphosate-resistant waterhemp in 2006 and 2007, respectively. Control of glyphosate-resistant waterhemp was less than 23% with lactofen and acifluorfen in 2006, but at least 64% in 2007. Variability in control likely resulted from differences in trial locations and a population of protoporphyrinogen oxidase (PPO)–resistant waterhemp at the Platte County site in 2006 compared with 2007. In both years, glyphosate resulted in less than 23% control of glyphosate-resistant waterhemp and provided the least control of all herbicide programs. Programs containing PRE herbicides resulted in waterhemp densities of less than 5 plants/m2, whereas the POST glyphosate treatment resulted in 38 to 70 plants/m2. Waterhemp seed production was reduced at least 78% in all PRE fb POST programs, from 55 to 71% in POST programs containing lactofen and acifluorfen and by only 21% in the POST glyphosate treatment. Soybean yields corresponded to the level of waterhemp control achieved in both years, with the lowest yields resulting from programs that provided poorest waterhemp control. PRE applications of S-metolachlor plus metribuzin provided one of the highest net incomes in both years and resulted in $271 to $340/ha greater net income than the glyphosate-only treatment. Collectively, the results from these experiments illustrate the effectiveness of PRE herbicides for the control of glyphosate-resistant waterhemp in glyphosate-resistant soybean and the inconsistency of PPO-inhibiting herbicides or PPO-inhibiting herbicide combinations for the control of waterhemp populations with multiple resistance to glyphosate and PPO-inhibiting herbicides.

Glyphosate-based, ready-to-use weed control products often contain pelargonic acid (PA) at a concentration equivalent to that of the glyphosate. It remains unclear what benefit, if any, this combination provides. Greenhouse experiments using large crabgrass, yellow nutsedge, longstalked phyllanthus, and prostrate spurge were conducted to determine whether the addition of PA improved weed control efficacy compared to glyphosate alone. Glyphosate was applied at a series of rates, ranging from 0.11 to 1.12 kg ae/ha, either alone or with an equal rate of PA. Addition of PA to glyphosate was synergistic only in longstalked phyllanthus and yellow nutsedge, and this synergism was manifested only as an increase in the amount of early (i.e., 5 to 7 d after treatment) visual injury. Conversely, longer-term control and control of regrowth was either not affected or reduced by the addition of PA. We conclude that the addition of PA to glyphosate in ready-to-use weed control products is neither warranted nor justified. However, we also note that the increase in early injury that was observed in only two of the four species evaluated could be an important attribute for the consumers for which these products are targeted.