Field experiments were conducted on the Eastern Shore of Virginia from 1999 to 2001 to evaluate the effects of tank mixture applications of isopropylamine or trimethylsulfonium salts of glyphosate with two liquid formulations of manganese (Mn lignin or Mn chelate) on spray solution pH and weed control in glyphosate-resistant soybean. Additions of manganese to herbicide solutions resulted in a reduction in the acidifying effects of the herbicides as well as in the control of common lambsquarters, large crabgrass, morningglory spp., and smooth pigweed. Reduced control caused by manganese could be overcome with higher rates of the herbicides on some species, but reduced control of common lambsquarters was seen when manganese was included with any herbicide application rate. For most species, Mn chelate caused a greater reduction in control than did Mn lignin. Although manganese caused significant decreases in weed control, soybean yield was not influenced by glyphosate salt, application rate, or manganese. Reduced weed control caused by the addition of manganese to herbicide solutions may be due to the complexing of the herbicide formulations, which could result in the formation of insoluble salt complexes that are not readily absorbed through the plant cuticle, resulting in decreased glyphosate phytotoxicity.

Competition from annual grasses and yellow starthistle can severely reduce perennial grass forage production and quality in pasture and rangeland. The purpose of this study was to determine yellow starthistle and weedy annual grass control by imazapic applied alone or in combination with picloram, and smooth brome tolerance to these treatments. Sixty days after treatment (DAT) imazapic applied at 70 and 140 g ae/ha reduced annual grass (downy brome, medusahead, and annual bluegrass) plant density and biomass by 66 to 76% compared with the untreated control. Picloram applied at 280 and 420 g ae/ha reduced yellow starthistle plant density and biomass by over 93%. Imazapic applied at 70 and 140 g/ha reduced smooth brome biomass by 79 to 95% at 60 DAT. Picloram did not affect the density or the biomass of annual grasses or smooth brome, whereas imazapic did not markedly affect the density or the biomass of yellow starthistle. Downy brome control increased to a maximum of 97% with increasing imazapic dose (maximum of 280 g/ha) at 30, 60, and 90 DAT.

Laboratory and greenhouse studies were conducted to determine if the reported red morningglory control failures with atrazine in sugarcane are caused by triazine-resistant mutants. Plants were grown from seeds collected at 24 locations in Louisiana and Arkansas where atrazine had never been used, and where it had been used in sugarcane with poor results. Terminal fluorescence of leaf material from all locations increased after treatment with 10−3 M atrazine, indicating electron transport inhibition and, hence, triazine susceptibility. However, small differences in the magnitude of fluorescence increase were observed among populations, possibly indicating the existence of biotypes with slightly different inherent tolerances to atrazine. Some phenotypic differences were observed among the red morningglory populations. In a separate study, postemergence application of atrazine at 1.1 kg ai/ha plus nonionic surfactant controlled greenhouse-grown plants from all populations at least 99%, which supports the findings of the fluorescence assay. This research was unable to verify that reduced red morningglory control with atrazine was the result of an altered binding site mutation, even in populations exposed to atrazine annually for more than 10 yr. Other factors should be evaluated to determine their impact on atrazine performance.

Field research was conducted for a period of 2 yr to evaluate the response of soybean and cotton to simulated drift rates representing 12.5, 6.3, 3.2, 1.6, and 0.8% of the usage rates of 1,120 g ai/ha glyphosate (140, 70, 35, 18, and 9 g/ha, respectively) and 420 g ai/ha glufosinate (53, 26, 13, 7, and 4 g/ha, respectively). Early-postemergence applications were made to 2- to 3-trifoliate soybean and 2- to 3-leaf cotton, and late applications to soybean at first flower and cotton at early bloom. A mid-postemergence application was also made to cotton at pinhead square (first flower bud development). Soybean and cotton injury and height reductions occurred in most cases for only the two highest rates of the herbicides with variation noted between years. Soybean height was reduced by no more than 11%, regardless of herbicide rate or timing. On the basis of visual injury, soybean was more sensitive to glyphosate than to glufosinate when applied early in 1998, but sensitivity was equal for both the herbicides in 1999. When herbicides were applied late, soybean was more sensitive to glufosinate in the first year. Cotton was more sensitive to glufosinate 7 d after application in both years, regardless of timing, but by 28 d differences between herbicides were less apparent. Cotton maturity was not delayed by either herbicide, on the basis of days to first square or flower and nodes above white flower. Both crops were able to recover rapidly from herbicide injury, and yields were not affected negatively.

Field studies were conducted in 1999 and 2000 at Belleville, IL, to compare herbicide efficacy and economic return on investment (EROI) for various herbicide systems in conventional, imidazolinone-resistant, glufosinate-resistant, and glyphosate-resistant corn hybrids. Corn injury 14 d after RPA 201772 preemergence (PRE) treatments ranged from 0 to 10%. Treatments with nicosulfuron + rimsulfuron + atrazine injured corn 5 to 24% at 7 d after treatment (DAT); however, no injury was visible at 14 DAT. Corn injury by all herbicide treatments dissipated by 28 DAT. All treatments controlled giant foxtail at least 88%. Common waterhemp was controlled at least 94% with all treatments, except for imazethapyr + imazapyr postemergence (POST), which did not control common waterhemp. All treatments controlled at least 92% of common cocklebur, giant ragweed, and ivyleaf morningglory, with the exception of the total PRE herbicide system of S-metolachlor + atrazine in 2000. Average grain yield ranged from 7,290 to 9,180 kg/ha in 1999, with the yield of the glyphosate-resistant hybrid lower than that of all other hybrids. In 2000, average grain yield ranged from 8,440 to 11,520 kg/ha, with the grain yield of the glyphosate-resistant hybrid greater than that of all other hybrids. There were no differences in economic return in 1999 because of the treatment, the herbicide system, or the hybrid. In 2000 EROI was greater with the glyphosate-resistant hybrid than with the other hybrids. EROI was influenced more by the grain yield of the corn hybrid than by the associated weed control costs. It has been demonstrated by this research that effective and economical weed control can be achieved in all hybrids, with minimal injury.

Field experiments were conducted at four locations in Kansas and Wyoming during the 1998 to 1999, 1999 to 2000, or 2000 to 2001 winter wheat–growing seasons to compare BAY MKH 6561 and MON 37500 at four postemergence timings for weed control and crop response. BAY MKH 6561 at 45 g ai/ha and MON 37500 at 35 g ai/ha controlled cheat 95% or more when applied early-fall postemergence (EFP), late-fall postemergence (LFP), early-spring postemergence (ESP), or late-spring postemergence (LSP) at Hays, KS, in 1999 and 2001. At Manhattan, KS, in these years, cheat control was 98 to 100%, except for MON 37500 (83 to 90%) applied LSP. Downy brome control with BAY MKH 6561 or MON 37500 declined as application timing was delayed at Hays, Archer, WY, and Torrington, WY. Averaged over herbicides, downy brome was controlled 38 to 95% when herbicides were applied EFP and 10 to 79% when applied LSP. Late-season wheat stunting was 11% or less, regardless of herbicide treatment, at Manhattan each year; no late-season injury was visible at Hays, Archer, or Torrington. Yields did not differ between treated and untreated wheat at Manhattan in 1999 or 2001, at Archer in 2000, or at Hays in 1999. Wheat receiving BAY MKH 6561 EFP yielded 8% more grain than did wheat treated with MON 37500 LSP at Hays in 2001 and at Torrington. At these two locations, all herbicide-treated wheat yielded 10 to 20% more grain than did the untreated wheat.

Jointed goatgrass is a problem weed in winter wheat production areas of the Great Plains. Winter wheat seeding rates are easily adjusted by the growers and influence competition by some weeds. Field experiments were initiated in Kansas, Nebraska, and Wyoming using winter wheat cultivars selected from leading adapted cultivars from each region to determine the effect of wheat plant density in the fall on jointed goatgrass competitiveness. Three winter wheat seeding rates (50, 67, and 84 kg seeds/ha) were used at Hays, KS, and Sidney, NE, and four seeding rates (33, 50, 67, and 101 kg seeds/ha) were used at Torrington and Archer, WY. An analysis of covariance model was fit with winter wheat fall plant density as the covariate. In 1996, winter wheat grain contamination (dockage) was reduced at the rate of about 6% for every 10 additional wheat plants/m2 above the threshold density of 70 plants/m2 at Archer, WY, and at the rate of about 0.5% for every 10 additional wheat plants/m2 above the threshold density of 110 plants/m2 at Hays, KS. At Hays the reduction occurred only with the semidwarf cultivar ‘Vista’. Increased wheat density reduced jointed goatgrass reproductive tillers in four out of six location–year combinations and biomass in two out of four location–year combinations. Despite the lack of a consistent reduction in jointed goatgrass competitiveness as the result of increased wheat density, increased seeding rates may be a good, low-cost, long-term investment as part of an integrated jointed goatgrass control program in winter wheat.

In traditional simulated herbicide drift research, dose response is evaluated using a constant carrier volume. The influence of carrier volume was evaluated in field experiments with drift rates representing 12.5 and 6.3% of the use rates of 1,120 g ai/ha glyphosate (140 and 70 g/ha, respectively) and 420 g ai/ha glufosinate (53 and 26 g/ha, respectively). Corn and soybean were exposed to herbicide rates applied in constant carrier volume of 234 L/ha and in proportional carrier volumes of 30 L/ha for the 12.5% rate and 15 L/ha for the 6.3% rate. Averaged across herbicides, corn height reduction 14 d after treatment (DAT) was greater for the 12.5% rate when applied in proportional 30 L/ha carrier volume (45%) compared with constant 234 L/ha carrier volume (28%). The 6.3% rate reduced corn height 38% when applied in proportional 15 L/ha carrier volume but not when applied in 234 L/ha carrier volume. When carrier volume was changed from constant to proportional, corn injury 14 DAT increased from 33 to 51% for the 12.5% rate and 18 to 38% for the 6.3% rate. Compared with constant spray volume, corn yield reduction was 1.5 times greater for the 12.5% rate but 4 times greater for the 6.3% rate when spray volume was varied proportionally to the herbicide rates. Differential response due to carrier volume was not observed when herbicides were applied to soybean. Soybean was injured more by glyphosate than by glufosinate, but recovery was rapid and yield was not negatively affected. Results suggest that drift research using constant spray volume may underestimate the yield reduction expected for sensitive crops exposed to glyphosate or glufosinate.

Potassium azide (PA) (112 kg/ha), oxadiazon + 1,3-dichloropropene (1,3-D) (168 kg/ha + 140 L/ha), dazomet (392 kg/ha), dazomet + chloropicrin (392 + 168 kg/ha), dazomet + 1,3-D (392 kg/ha + 140 L/ha), iodomethane (IM) (336 kg/ha), metam-sodium (MS) (748 L/ha), MS + chloropicrin (748 L/ha + 168 kg/ha), and MS + 1,3-D (748 + 140 L/ha) were evaluated at Jay and Arcadia, FL, in 1998 and 1999 as alternatives to methyl bromide (MeBr) fumigation for the management of common turfgrass weeds. Potassium azide was as effective as MeBr in controlling ‘Coastal’ bermudagrass, yellow and purple nutsedges, alexandergrass, broadleaf signalgrass, tall and sharppod morningglories, and various winter annual broadleaf weeds, but it failed to provide acceptable control of redroot pigweed. 1,3-Dichloropropene + oxadiazon did not control yellow nutsedge, purple nutsedge, or Coastal bermudagrass. Similarly, this combination treatment failed to control carpetweed but did provide 83% control of the winter annual weed species, 71% control of alexandergrass and broadleaf signalgrass, and ≥ 80% control of tall morningglory, sharppod morningglory, and redroot pigweed. Dazomet + combination treatments provided control of Coastal bermudagrass at Jay; however, control of common bermudagrass, alexandergrass, and broadleaf signalgrass was not acceptable at Arcadia. Sedge species control with dazomet + combinations was poor (< 63%) at both sites. Iodomethane, a treatment not yet registered by the U.S. Environmental Protection Agency (EPA), controlled weedy grass species, sedge species, and broadleaf weeds present at the two locations under different environmental conditions. Metam-sodium alone and MS + chloropicrin, tarped and untarped, and MS + 1,3-D provided acceptable weed control; however, MS + chloropicrin covered with a plastic tarp for 48 h was the best MS treatment. Metam-sodium + chloropicrin, with plastic tarp, controlled weedy grass and broadleaf species equal to MeBr; however, unacceptable sedge species control at Jay and Arcadia was 56 and 79%, respectively. Metam-sodium applied alone failed to control redroot pigweed; however, MS + combinations provided control. These studies confirm that no EPA-registered fumigant alternative to MeBr, applied alone or in combination for preplant turf soil fumigation, exists. Consequently, until such time that an effective alternative is identified, turf managers will be forced to forego fumigation, or they will have to choose a less-effective alternative and accept the consequences of contamination.

Field trials were conducted in 1998 and 1999 to determine the influence of harvest-aids on dry bean desiccation, seed yield, and quality. Light Red Kidney and Great Northern dry beans were pulled from the soil and placed on the soil surface to dry or treated with glufosinate at 440 g ai/ha, with glyphosate at 840 g ae/ha, or with paraquat at 560 g ai/ha at three stages during the maturation process. Dry bean seed weight and seed yield were reduced by all harvest-aid treatments applied when only 5 to 7% of the seedpods were yellow. Herbicides did not affect dry bean seed yield, weight, or germination if treatments were delayed until 77 to 85% of the seedpods had turned yellow. Application of harvest-aids at early stages of the maturation process enhanced desiccation of dry bean plants, pods, and seeds, but reduced yield. Herbicides applied at later stages of the maturation process avoided seed yield reductions, but were not as effective in desiccating plants. Therefore, applying glufosinate, glyphosate, or paraquat to reduce the time from dry bean maturation to harvest may not be beneficial when the crop is grown on the high plains of the United States.

Field studies were conducted at Lewiston and Rocky Mount, NC, in 1996 and 1997 to evaluate weed control and peanut response to preplant incorporated (PPI) treatments of diclosulam alone and in systems with postemergence (POST) commercial standard herbicides. All plots received ethalfluralin PPI at 840 g ai/ha. In both years, ethalfluralin plus diclosulam PPI at 17 or 26 g ai/ha followed by (fb) acifluorfen plus bentazon POST, paraquat plus bentazon POST, or imazapic POST controlled common lambsquarters, ivyleaf morningglory, pitted morningglory, prickly sida, smooth pigweed, spurred anoda, and yellow nutsedge in a manner similar to or better than did the commercial standards of ethalfluralin PPI fb metolachlor preemergence (PRE) fb acifluorfen plus bentazon or ethalfluralin PPI fb acifluorfen plus bentazon POST. Yield from peanut treated with diclosulam systems that included POST herbicides was equivalent to or higher than that from peanut treated with ethalfluralin PPI fb metolachlor PRE fb acifluorfen plus bentazon POST or ethalfluralin PPI fb acifluorfen plus bentazon POST. Peanut exhibited excellent tolerance to diclosulam PPI at all rates.

Field studies were conducted in 1999 and 2000 to evaluate the response of three runner market-type peanut cultivars to diclosulam applied preplant incorporated at 0,18, 27, or 54 g ai/ha in a weed-free environment. Peanut cultivars evaluated included ‘Georgia Green’, ‘C-99R’, and ‘MDR-98’. Peanut injury was not observed with diclosulam at any rate or with any cultivar. Diclosulam did not affect peanut canopy development, percentage extra-large kernels, sound mature kernels, sound splits, total sound mature kernels, other kernels, or yield for any cultivar.

Preemergence (PRE) herbicides may affect the ability to reestablish warm-season turfgrasses in winter-injured areas. Experiments were conducted in 1996 and 1997 to evaluate the effects of fall or spring applications of six PRE herbicides on the vegetative establishment of Tifway bermudagrass, and Meyer and El Toro zoysiagrass. PRE herbicides were applied at the recommended rates during the fall of 1995 and 1996 and at recommended or reduced rates during the spring of 1996 and 1997. Oxadiazon, benefin plus trifluralin, or oryzalin did not inhibit Tifway bermudagrass or zoysiagrass sprig establishment. Fall applications of prodiamine and dithiopyr at full rates suppressed Tifway bermudagrass establishment as much as 25%, but recovery was evident by the end of the growing season. Reduced spring rates of prodiamine diminished its suppressive effects on Tifway establishment. Zoysiagrass establishment was suppressed as much as 20% by full or reduced rates of prodiamine but was less affected by dithiopyr. Pendimethalin had lesser and briefer suppressive effects than prodiamine and dithiopyr had on either species. Results suggested that avoidance of or reduced rates of prodiamine or dithiopyr may be warranted in areas prone to winter injury.

Field studies were conducted in 1999 and 2000 at Belleville, IL, to evaluate herbicide efficacy, grain yield, and economic return on investment (EROI) from various herbicide systems in conventional, sulfonylurea-tolerant, glufosinate-resistant, and glyphosate-resistant soybean varieties. Several grower-oriented herbicide systems were developed within each soybean variety using current weed control options. These herbicide systems included herbicides with soil-residual, herbicides with different modes of action, and combinations of preemergence (PRE) followed by (fb) postemergence (POST) herbicides. Soybean injury ranged from 1 to 16% 28 d after (DA) PRE applications of sulfentrazone plus chlorimuron plus pendimethalin. Injury 7 DA POST treatments ranged from 2 to 39%, with the greatest injury occurring from the conventional herbicide combination of fomesafen plus fenoxaprop plus fluazifop-P. No injury symptoms persisted through soybean harvest. Control of giant foxtail was the most consistent (97% or greater) in the PRE broadleaf fb POST herbicide system 56 DA planting. Common waterhemp control was at least 85% for all treatments, with the exception of the single POST herbicide systems of chlorimuron plus thifensulfuron plus quizalofop-P, and glufosinate and the standard sequential herbicide system of pendimethalin fb chlorimuron plus thifensulfuron. All treatments provided at least 87% control of common ragweed and ivyleaf morningglory. Soybean grain yield was similar between varieties in the weed-free control plots in 1999. In 2000, grain yield of the glyphosate- and glufosinate-resistant soybean varieties was greater than for the conventional and sulfonylurea-tolerant varieties. Overall, soybean grain yield and EROI were less for some treatments because of poor control of common waterhemp. Weed control had a greater influence than herbicide-related costs on EROI.

Three studies were conducted in 1999 and 2000 to determine whether acetolactate synthase (ALS)–resistant common ragweed and giant ragweed biotypes were present in Ohio. Results of field studies indicated that biotypes of both species had cross-resistance to three chemical families of ALS-inhibiting herbicides. Cloransulam-methyl applied postemergence at 9, 18, and 36 g/ha controlled more than 85% of two susceptible populations of common and giant ragweed 28 d after treatment, whereas less than 35% control of resistant populations was achieved at the same rates. Fomesafen, lactofen, and glyphosate applied alone at the recommended rates provided the most effective control of ALS-resistant common and giant ragweed. Mixtures of cloransulam-methyl with either fomesafen or lactofen did not significantly increase ALS-resistant common and giant ragweed control compared with each diphenylether herbicide used alone. Dose–response bioassays conducted in the greenhouse indicated that susceptible common and giant ragweed tended to be more sensitive to cloransulam-methyl and chlorimuron than to imazamox. ALS-resistant common ragweed demonstrated a high level of resistance to all the herbicides tested because GR50 values were not reached with rates 1,000 times higher than the recommended rate. ALS-resistant giant ragweed treated with 13,000 g/ha of chlorimuron and 18,000 g/ha of cloransulam-methyl was not inhibited enough to obtain a GR50 value, thus also demonstrating a high level of resistance. The GR50 for ALS-resistant giant ragweed treated with imazamox was 1,161 g/ha. Results of these studies confirmed the presence of ALS–cross-resistant populations of common and giant ragweed in Ohio and suggest that herbicides with different mechanisms of action will be required to manage these weeds effectively.

Littleseed canarygrass, a troublesome wheat weed, has developed resistance against isoproturon in India. Laboratory, pot, and field experiments were conducted to quantify the level of isoproturon resistance in various littleseed canarygrass biotypes and to evaluate the response of isoproturon-resistant littleseed canarygrass to alternate herbicides. For quantification of resistance, 10 littleseed canarygrass biotypes were collected from fields with a varied history of cropping and herbicide use. Pot studies indicated that 6 out of the 10 biotypes tested were resistant to isoproturon. The resistant (R) biotypes had 50% growth reduction values, 5 to 10 times greater than that of the most susceptible biotype. The R biotypes were from areas where isoproturon had been used continuously for more than 10 yr in wheat under a continuous rice–wheat cropping system. The isoproturon resistance was not detected where crops and herbicides were rotated. Under field conditions, postemergence isoproturon at 1,000 g/ha 32 d after sowing failed to control littleseed canarygrass and as a result wheat grain yield decreased by 65% compared with the weed-free control. The herbicides observed to be effective in wheat against isoproturon-resistant littleseed canarygrass were CGA 184927 (50 to 60 g ai/ha), fenoxaprop-p-ethyl (100 to 120 g/ha), ICIA 0604 (350 g ai/ha), and MON 37503 (25 g ai/ha). The use of these herbicides increased the grain yield by 200% or more when compared with the weedy check. Among the herbicides tested, MON 37503 at 25 g ai/ha was the best because of complex weed flora control and produced wheat yield similar to that of the weed-free control. Based on this study, the absence of resistance when herbicides and crops were rotated indicates that growers should follow herbicide and crop rotation in order to avoid development of herbicide resistance in the near future. The integration of this system (crop and herbicide rotation) with cultural and mechanical means of weed control could be further targeted as a long-term strategy for resistance management.

Outside its native range, smooth cordgrass is a noxious weed in estuaries throughout the world. Trials were conducted to evaluate smooth cordgrass control with imazapyr at various timings, rates, and spray volumes, and with several surfactants; comparisons were made with glyphosate, the sole herbicide registered in the United States for estuarine use. Across years, all sites, application dates, and experiments, it was found that smooth cordgrass control with imazapyr at 1.68 kg/ha was better and more consistent than with glyphosate at 8.4 kg/ha. Control with imazapyr at 0.84 kg/ha was comparable to control with glyphosate at 8.4 kg/ha. The time between application and tidal inundation over the canopy affected the efficacy of imazapyr less than that of glyphosate. There was a lack of consistent effects across dates of application. For imazapyr, a higher application volume (467 L/ha) usually provided better control than did the low volume (93 L/ha), but excellent control was achieved using ultra-low application volumes (23 and 47 L/ha) if the dry time was greater than 12 h. Several surfactants provided comparable efficacy for imazapyr.

This article presents an approach to managing and evaluating soil survey data that can aid in soil-applied herbicide decision making. Field experiments were used to quantify the dose responses of corn, shattercane, and velvetleaf to isoxaflutole over a range of soil conditions within an agricultural field. Isoxaflutole doses eliciting crop injury (20% greenness reduction) and weed control (80% biomasss reduction) were projected for a county located in Nebraska based on associations between the plant response and the soil properties. The biologically effective dose of isoxaflutole increased with increasing organic matter and mineral surface area. Mean biologically effective doses for velvetleaf (1 to 27 g/ha) were considerably lower than that for shattercane (42 to 206 g/ha). Over 60% of the surface texture is silty clay loam for Saunders County, suggesting that 17 and 158 g/ha are the minimal doses required to suppress velvetleaf and shattercane, respectively, for a majority of the county. Conceivably, this approach could be used as an initial step to assess the relative value of field-specific applications and variable dose application technologies.

Greenhouse studies verified that smooth crabgrass from a golf course tee in southern New Jersey was resistant to fenoxaprop-P, although smooth crabgrass from a nearby untreated rough was susceptible to this herbicide. Fenoxaprop-P-resistant plants were injured, however, when fenoxaprop-P was applied at levels above the maximum use rate for turf. Fenoxaprop-P applied postemergence at 0.76 kg ai/ha, four times the maximum rate, reduced shoot weight of the resistant biotype by 35%, whereas application of 1.52 kg/ha, eight times the maximum rate, reduced shoot weight by 64%. Large crabgrass and the susceptible biotype of smooth crabgrass were controlled by fenoxaprop-P applied at 0.1 kg/ha. Fenoxaprop-susceptible and -resistant biotypes of smooth crabgrass had a similar response to MSMA, dithiopyr, and quinclorac applied postemergence. The fenoxaprop-P–resistant smooth crabgrass biotype was controlled by the cyclohexanedione herbicides sethoxydim and clethodim, but the aryloxyphenoxypropionate herbicides fluazifop and quizalofop reduced shoot weight by only 15 to 66% depending on herbicide and rate.

Glyphosate-resistant canola has been widely adopted in western Canada. This has prompted producer interest in the timing of glyphosate application, particularly under zero tillage, where glyphosate is often applied preseeding. Field experiments were conducted at Lacombe, Edmonton, and Beaverlodge in Alberta in 1997, 1998, and 1999 to determine the importance of preseeding glyphosate and the most effective growth stage to apply glyphosate to canola to optimize yield and weed management. Treatments consisted of zero-tillage systems, with and without preseeding glyphosate, and a conventional-tillage system involving preseeding tillage operations. Glyphosate was applied at the one- to two-, three- to four-, or five- to six-leaf stages of canola in each tillage system. Canola yield and weed dry weight were largely unaffected by the tillage system. In most instances, the highest canola yields occurred when glyphosate was applied early to the crop. The opposite occurred at Lacombe and Edmonton in 1999, however, where canola yield increased as glyphosate was applied at later crop growth stages. This yield benefit likely resulted from the control of late-emerging weeds that exerted competitive pressure on canola. Early glyphosate timing in glyphosate-resistant canola may eliminate the need for preseeding glyphosate in zero-tillage systems, and optimize yield and weed control.