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.

A brief review is presented for the use of soil lysimeters in studying transpiration, evapotranspiration, moisture, and nutrient movement in earlier times and pesticide dissipation and movement, and mass-balance of pesticide dissipation in more recent times. The important factors needed to understand research findings and to model pesticide dissipation such as key soil and site characteristics, climatic conditions, and the methods involved are discussed. Several case studies carried out by Ciba and North Carolina State University are discussed and current developments in soil column field lysimeters are presented.

Soybean response to simulated drift of the corn herbicides nicosulfuron and primisulfuron applied POST at 10 to 50% (3.5 to 17.4 and 4.0 to 20.2 g ai ha−1, respectively) of the total rates at the V3 and R1 growth stages was evaluated in field studies in 1991 and 1992. Primisulfuron reduced soybean height and increased leaf chlorosis, cupping, and necrosis more than nicosulfuron with both applications at all five rates. The symptoms of injury caused by both herbicides often increased linearly with increasing rate. At 50% of label rate, primisulfuron reduced height 75% and decreased yield 58%. Nicosulfuron reduced soybean height as much as 27%, but did not reduce seed yield either year. Height reduction, leaf chlorosis, cupping, and necrosis were correlated with yield loss caused by primisulfuron.

The effect of ammonium sulfate on absorption of 14C-sethoxydim applied alone and with bentazon was studied in large crabgrass. Adding ammonium sulfate to 14C-sethoxydim increased 14C absorption 6-fold at 0.5 h after application but did not affect the amount of 14C absorbed 1 or 4 h after application. Applying bentazon with 14C-sethoxydim reduced 14C absorption 26%. When 14C-sethoxydim was applied with bentazon and ammonium sulfate, 14C absorption and translocation were similar to those with 14C-sethoxydim alone.

Field studies were conducted at two locations in North Carolina in 1998 and in 1999 to evaluate weed control and peanut response following diclosulam at 27 g ai/ha preemergence (PRE) or flumioxazin at 87 g ai/ha preemergence (PRE) alone and in systems with postemergence (POST) commercial standards. All plots received a preplant incorporated (PPI) treatment of metolachlor at 1,400 g ai/ha. Metolachlor PPI plus diclosulam or flumioxazin PRE controlled common lambsquarters, common ragweed, entireleaf morningglory, large crabgrass, and yellow nutsedge as well as or better than metolachlor PPI followed by (fb) acifluorfen plus bentazon POST or paraquat plus bentazon early postemergence fb acifluorfen plus bentazon POST. Metolachlor PPI plus diclosulam PRE or flumioxazin PRE controlled ivyleaf morningglory as well as metolachlor PPI fb acifluorfen plus bentazon POST. Metolachlor PPI plus flumioxazin PRE controlled common lambsquarters better than metolachlor PPI plus diclosulam PRE while diclosulam PRE controlled common ragweed better. There was no difference in common lambsquarters control between flumioxazin and diclosulam PRE when POST herbicides were used. There was only one difference in peanut yield and net returns between metolachlor PPI fb either diclosulam or flumioxazin PRE when POST herbicides were used.

Field experiments were carried out during 1997 and 1998 in northern Greece to investigate the effects of tank mixing rimsulfuron and primisulfuron with atrazine or dicamba against johnsongrass in corn. Sequential applications, where the johnsongrass herbicides were applied 5 d after the broadleaf herbicides, were also evaluated. Rimsulfuron applied alone at 10 g ai/ha gave very good control (91%) of johnsongrass, which was significantly higher than that provided by 30 g ai/ha of primisulfuron (43%). Rimsulfuron applied in tank mixture with atrazine (1.0 kg ai/ha) or dicamba (0.28 kg ai/ha) gave 12 and 17% lower johnsongrass control, respectively, than of rimsulfuron applied alone, whereas the corresponding reduction for primisulfuron was 18 and 43%. Efficacy of rimsulfuron applied 5 d after the application of atrazine or dicamba was similar to that applied alone; however, this was not the case for primisulfuron, where reduced antagonism was observed compared to that produced by its tank mixture treatments. Again, primisulfuron was affected more by dicamba than by atrazine. Corn yield with rimsulfuron and primisulfuron applied alone was more than double that of the untreated control and similar to that of the weed-free control. Also, rimsulfuron applied with atrazine or dicamba (in tank mixture or sequentially) produced similar corn yield to that applied alone. However, primisulfuron applied in tank mixture or sequentially with dicamba gave 22 and 14% lower yield, respectively, than when applied alone, and slightly lower when applied with atrazine.

The synergistic interaction between mesotrione, a hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide, and atrazine, a photosystem II (PS II)-inhibiting herbicide, has been identified in the control of several weed species. A series of dose–response studies examined the synergistic effect of these herbicides on a susceptible (S) wild radish population. The potential for this interaction to overcome target-site psbA gene-based atrazine resistance in a resistant (R) wild radish population was also investigated. Control of S wild radish with atrazine was enhanced by up to 40% when low rates (1.0 to 1.5 g ha−1) of mesotrione were applied in combination. This synergistic response was demonstrated across a range of atrazine–mesotrione rate combinations on this S wild radish population. Further, the efficacy of 1.5 g ha−1 mesotrione increased control of the R population by a further 60% when applied in combination with 400 g ha−1 of atrazine. This result clearly demonstrated the synergistic interaction of these herbicides in overcoming the target-site resistance mechanism. The mechanism responsible for the observed synergistic interaction between mesotrione and atrazine remains unknown. However, it is speculated that an alternate atrazine binding site may be responsible. Regardless of the biochemical nature of this interaction, evidence from whole-plant bioassays clearly demonstrated that synergistic herbicide combinations improve herbicide efficiency, with lower application rates required to control weed populations. This, combined with the potential to overcome psbA gene-based triazine resistance, and, thereby, regain the use of these herbicides, will result in more sustainable herbicide use.

The introduction of transgenic crops with resistance to 2,4-D will provide growers with new weed management options in soybean. Field and greenhouse experiments were conducted in 2010 and 2011 to determine the influence of weed height on compatibility of glufosinate and 2,4-D in resistant soybean. Results from these experiments indicate a significant increase in the rate of glufosinate or 2,4-D is needed when applied alone in order to achieve similar levels of weed control as lower rates of glufosinate and 2,4-D combinations. Mixtures of glufosinate and 2,4-D were more effective in controlling common waterhemp, Asiatic dayflower, and barnyardgrass than either herbicide alone, and with the magnitude of difference greater as the height of common waterhemp increased. Large crabgrass control decreased when glufosinate was applied with 2,4-D compared with glufosinate alone, with the magnitude of the reduction greater as weed height increased. Sequential application of glufosinate and 2,4-D eliminated the antagonism noted with tank mixtures. Soybean yields were also reduced by approximately 3% in response to 30 to 35–cm compared to 10 to 15–cm herbicide applications because of the combination of weed interference and reduced control. Overall, results from these experiments indicate that glufosinate plus 2,4-D combinations are likely to provide increased control of problematic weeds like Asiatic dayflower and common waterhemp while providing similar grass control as herbicide programs that contain glufosinate alone.

Glyphosate-resistant (GR) weeds, especially GR Palmer amaranth, are very problematic in cotton-producing areas of the midsouthern region of the United States. Growers rely heavily on PRE residual herbicides to control Palmer amaranth since few effective POST options exist. Interest in integrating high-residue cover crops with existing herbicide programs to combat GR weeds has increased. Research was conducted in 2013 and 2014 in Tennessee to evaluate GR Palmer amaranth control when integrating cover crops and PRE residual herbicides. Cereal rye, crimson clover, hairy vetch, winter wheat, and combinations of one grass plus one legume were compared with winter weeds without a cover crop followed by fluometuron or acetochlor applied PRE. Biomass of cover crops was determined prior to termination 3 wk before planting. Combinations of grass and legume cover crops accumulated the most biomass (> 3,500 kg ha−1) but by 28 d after application (DAA) the cereal rye and wheat provided the best Palmer amaranth control. Crimson clover and hairy vetch treatments had the greatest number of Palmer amaranth. These cereal and legume blends reduced Palmer amaranth emergence by half compared to non–cover-treated areas. Fluometuron and acetochlor controlled Palmer amaranth 95 and 89%, respectively, at 14 DAA and 54 and 62%, respectively, at 28 DAA. Cover crops in combination with a PRE herbicide did not adequately control Palmer amaranth.

Adjuvants are used to improve spray delivery, to increase spray retention on weed foliage, and to enhance foliar penetration by postemergence herbicides for increased herbicide efficacy. Numerous factors govern adjuvant efficacy, including the specific herbicide, plant species, and environmental conditions. Adding superior surfactants as adjuvants to a sethoxydim tank mix with Na-bentazon may overcome the Na-bentazon antagonsim of sethoxydim activity by facilitating increased sethoxydim absorption and by masking the antagonism similar to increasing the sethoxydim rate. Herbicides such as Na-bentazon contribute alkaline or alkaline earth cations that can form salts of a weakly acidic herbicide such as sethoxydim that are not absorbed readily by the plant. Adding abundant ammonium from ammonium sulfate or other sources may prevent and/or may overcome this antagonism.

Field trials were conducted from 2005 to 2007 at two locations in southwestern Ontario to investigate how weed control in corn was affected by the time of day that herbicides were applied. Weed control following the application of six POST herbicides (atrazine, bromoxynil, dicamba/diflufenzopyr, glyphosate, glufosinate, and nicosulfuron) at 06:00, 09:00, 12:00, 15:00, 18:00, 21:00, and 24:00 h was assessed. For many weed species herbicide efficacy was reduced when applications were made at 06:00, 21:00, and 24:00 h. Velvetleaf was the most sensitive to the time of day effect, followed by common ragweed, common lambsquarters, and redroot pigweed. Annual grasses were not as sensitive to application timing; however, control of barnyardgrass and green foxtail was reduced in some environments at 06:00 h and after 21:00 h. Only in the most severe cases was the grain yield of corn reduced due to decreased weed control. Daily changes in air temperature, relative humidity, and light intensity that cause species-specific physiological changes may account for the variation in weed control throughout the day. The results of this research suggest that there is a strong species-specific influence of ambient air temperature, light intensity, and leaf orientation on the efficacy of POST herbicides. These results should aid growers in applying herbicides when they are most efficacious, thus reducing costs associated with reduced efficacy.

Field studies were conducted during 1999 and 2000 to compare weed control after fall and early-preplant (EPP) herbicide applications in no-till soybean. Three residual treatments (chlorimuron plus metribuzin, chlorimuron plus sulfentrazone, and metribuzin) were applied at two rates and timings (fall and 30 d EPP) either alone or in combination with glyphosate and 2,4-D. The addition of glyphosate and 2,4-D to fall-applied residual herbicides significantly increased control of common chickweed, annual bluegrass, cressleaf groundsel, and shepherd's-purse. The effect of application rate on weed control was species dependent. Fall-applied residual herbicides were comparable with EPP treatments with respect to winter annual weed control; however, at planting control of summer annual weed species with fall treatments was less consistent compared with EPP residual herbicides.

Two kinds of dinitroaniline-resistant biotypes of goosegrass have been found in counties in South Carolina where trifluralin is used for weed control. One biotype, termed the resistant (R) biotype, resists trifluralin about 1000- to 10 000-fold. The other biotype is intermediate (I) in resistance between the R and susceptible (S) biotypes, resisting trifluralin about 50-fold. Growth studies and root tip squashes were used to determine the level of resistance to other dinitroaniline herbicides. The R biotype is cross resistant to all dinitroaniline herbicides, and the I biotype is highly resistant to some and is resistant to others only at a low level. The cross resistance of the I biotype to other dinitroaniline herbicides does not correlate with structural or solubility characteristics of the herbicides. Electron microscopic investigations indicate that cell plate formation (controlled by phragmoplast microtubule arrays) is affected at lower herbicide concentrations than spindle microtubule formations. The level of herbicide resistance, the pattern of cross resistance, and the ultrastructural effects induced by the herbicide indicate that the I biotype has a unique mechanism of resistance from the R biotype.

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.

A practical and objective system is needed to determine the lowest rates of the most efficacious herbicides to meet each producer's specific weed control problems. Determining which method of weed control to utilize is difficult today with increasing product choices, the growing use and complexity of herbicide mixtures, regulatory pressures to reduce rates, and the closer integration of weed control with other crop decisions. Expert computer systems could improve current practices and use herbicide mixtures as a tool to increase herbicide efficiency. Such systems would account for herbicide dose and mixture responses; select most economical herbicides; optimize adjuvants; recommend control at economic thresholds; and vary rates according to weed spectrum, density, and local environmental conditions. An example using chlorimuron and thifensulfuron illustrates how these systems could use quantitative dose response and mixture information.

Plant growth regulators paclobutrazol and flurprimidol were evaluated for suppression of Poa annua spp. reptans, a perennial biotype of annual bluegrass, in a creeping bentgrass green. Paclobutrazol at 0.6 kg ai/ha applied on Mar. 15, followed by 0.3 kg/ha on Apr. 29, 0.6 kg/ha on Oct. 1, and 0.3 kg/ha on Nov. 12 for each of 2 yr suppressed P. annua spp. reptans 28% approximately 4 mo after the final treatment. When paclobutrazol was applied at 0.3 kg/ha in each of four applications during the same period, suppression was similar to that with the higher rate 1 and 2 mo after final treatment but declined to 7% by 4 mo. Suppression of P. annua spp. reptans from flurprimidol applied four times during each of 2 yr was 22 to 27% 1 mo after final treatment but no suppression was noted 2 mo after the final treatment. Maximum creeping bentgrass injury was less than 20% when either paclobutrazol or flurprimidol was applied in the spring. However, creeping bentgrass injury was approximately 30% in the fall when either chemical was applied in the spring and repeated in October at 0.6 kg/ha.

Field and greenhouse studies were conducted to evaluate sulfentrazone and flumioxazin as preemergence (PRE) herbicides for broadleaf weed control in potato. Sulfentrazone and flumioxazin were applied alone and in combination with s-metolachlor to determine the crop response and the weed spectrum controlled. These treatments were compared with metribuzin or rimsulfuron plus s-metolachlor treatments. Potato variety response to sulfentrazone and flumioxazin was evaluated in a separate field study. Sangre, Chipeta, Russet Norkotah, and Russet Nugget were treated with sulfentrazone from 0.14 to 0.28 kg/ha or flumioxazin from 0.035 to 0.07 kg/ha. Sulfentrazone and flumioxazin provided excellent broadleaf weed control at all the rates tested, whereas grass control increased as rate increased. Grass control improved when combined with s-metolachlor. Sulfentrazone and flumioxazin treatments were comparable with metribuzin and rimsulfuron treatments in weed control and total yield. Flumioxazin was safe when applied PRE to four selected varieties, whereas sulfentrazone produced initial phytotoxicity to Sangre and Chipeta at high rates but did not affect yields. Sulfentrazone increased the yield of U.S. No.1 potatoes compared with other treatments in the variety response study. Dose–response curves were used to generate the sulfentrazone, flumioxazin, and metribuzin herbicide rates required to reduce biomass by 50% (I50) for eight common weed species. Herbicides were applied PRE at several rates, and plant response was recorded. Log-logistic analysis was performed on bioassay data generated to estimate species sensitivity to each herbicide. Sulfentrazone reduced the biomass of hairy nightshade, black nightshade, redroot pigweed, kochia, common lambsquarters, and redstem filaree by more than 90% at 0.0175 kg/ha (the lowest rate evaluated), whereas flumioxazin had a similar effect on all broadleaf species except on kochia at 0.004 kg/ha (the lowest rate evaluated). Therefore, it was not possible to calculate I50 or even I80 values for most broadleaf species. Metribuzin I50 values could be calculated for most of the species tested. The metribuzin I50 value for hairy nightshade was 0.28 kg/ha, which was 16 and 70 times higher than the sulfentrazone and flumioxazin rates, respectively, that reduced hairy nightshade biomass by more than 90%. Sulfentrazone and flumioxazin appeared to be sufficiently safe when applied on potato and controlled several weed species common to potato production in the western United States.

The transfer of herbicide resistance genes from crops to related species is one of the greatest risks of growing herbicide-resistant crops. The recent introductions of imidazolinone-resistant wheat in the Great Plains and Pacific Northwest regions of the United States and research on transgenic glyphosate-resistant wheat have raised concerns about the transfer of herbicide resistance from wheat to jointed goatgrass via introgressive hybridization. Field experiments were conducted from 2000 to 2003 at three locations in Washington and Idaho to determine the frequency and distance that imidazolinone-resistant wheat can pollinate jointed goatgrass and produce resistant F1 hybrids. Each experiment was designed as a Nelder wheel with 16 equally spaced rays extending away from a central pollen source of ‘Fidel-FS4’ imidazolinone-resistant wheat. Each ray was 46 m long and contained three rows of jointed goatgrass. Spikelets were collected at maturity at 1.8-m intervals along each ray and subjected to an imazamox screening test. The majority of all jointed goatgrass seeds tested were not resistant to imazamox; however, 5 and 15 resistant hybrids were found at the Pullman, WA, and Lewiston, ID, locations, respectively. The resistant plants were identified at a maximum distance of 40.2 m from the pollen source. The overall frequency of imazamox-resistant hybrids was similar to the predicted frequency of naturally occurring acetolactate synthase resistance in weeds; however, traits with a lower frequency of spontaneous mutations may have a relatively greater risk for gene escape via introgressive hybridization.

Organic growers need additional tools for weed control. A new technique using abrasive grit propelled by compressed air was tested in field plots. Grit derived from corncobs was directed at seedlings of summer annual weeds growing at the bases of corn plants when the corn was at differing early stages of leaf development. Season-long, in-row weed control exceeded 90% when two or three abrasion events were coupled with between-row cultivation. Timing of weed abrasion was critical, with highest levels of control corresponding to the one- and five-leaf stages or the one-, three-, and five-leaf stages of corn development. Corn yields associated with these treatments were equivalent to those of hand-weeded controls in which no abrasive grit was applied. Thus, air-propelled abrasive grit applications at the one-, three-, and five-leaf stages of corn controlled weeds sufficiently to prevent weed-induced reductions in corn grain. Additionally, these applications were not harmful to corn plants. This new concept for weed control may be of interest to organic crop managers.