Field experiments were conducted across the north-central United States to determine the benefits of various weed control strategies in corn. Weed control, corn yield, and economic return increased when a preemergence (PRE) broad-spectrum herbicide was followed by (fb) postemergence (POST) herbicides. Weed control decisions based on field scouting after a PRE broad-spectrum herbicide application increased weed control and economic return. Application of a PRE grass herbicide fb a POST herbicide based on field scouting resulted in less control of velvetleaf and morningglory species, corn yield, and economic return compared with a PRE broad-spectrum herbicide application fb scouting. Cultivation after a PRE broad-spectrum herbicide application increased weed control and corn yield compared with the herbicide applied alone, but economic return was not increased. An early-postemergence herbicide application fb cultivation resulted in the highest level of broadleaf weed control, the highest corn yield, and the greatest economic return compared with all other strategies. Weed control based on scouting proved to be useful in reducing the effect of weed escapes on corn yield and increased economic return compared with PRE herbicide application alone. However, economic return was not greater than the PRE fb planned POST or total POST strategies.

Greenhouse and field experiments were conducted under weed-free conditions in 2000 and 2001 to investigate the responses of 10 soft red winter wheat cultivars to postemergence applications of the experimental herbicide AE F130060 03 at 15 g ai/ha with the crop safener AE F107892 at 30 g ai/ha. In the greenhouse, AE F130060 03 injured wheat 7 to 12% and reduced height 11 to 14% at 3 wk after treatment (WAT) across all cultivars but did not reduce biomass of any cultivar. In the field, AE F130060 03 injured wheat 11 to 32%, reduced tiller number of all cultivars except ‘Roane’, ‘Coker 9663’, and ‘VA98W-593’, and reduced height of all cultivars except ‘USG 3209’ and VA98W-593 at 3 WAT. By 9 WAT, tiller number and height of treated wheat was similar to that of nontreated wheat. AE F130060 03 did not influence moisture content or kernel weight of any cultivar. However, AE F130060 03 reduced grain yield in ‘FFR 518’, Coker 9663, and VA98W-593 in both years as well as in ‘AgriPro Patton’ in 2001. These yield reductions suggest that further investigation into soft red winter wheat cultivar tolerance to AE F130060 03 is needed.

Widespread groundwater contamination has prompted studies on the fate and transport of solute through soil. Large quantities of atrazine and metribuzin are applied annually in the Humid Pampas of Argentina, creating the need to study the fate of these herbicides in soils of the region. The objective of this work was to study the vertical transport of atrazine and metribuzin in packed soil columns for three loam soils representative of the Humid Pampas of Argentina. Bromide was used as a nonreactive tracer. The convection dispersion equation was fitted to chemical breakthrough data to obtain a parameter characterizing chemical transport. Bromide breakthrough curves (BTCs) were similar among soils. BTCs for atrazine and metribuzin revealed significant interaction among soils and herbicides. The average values for the organic carbon (OC) partition coefficients (Koc) derived from column flow experiments were 119 and 48 ml/g for atrazine and metribuzin, respectively. Metribuzin in leachate was 97.3% of the total recovered, whereas atrazine was 3.5%. This behavior can be explained by their different affinities to OC. The OC contents of the Balcarce, Necochea, and Nueve de Julio soils were 4.1, 3.4, and 1.9%, respectively. The lowest leaching values of herbicides were found in the Balcarce soil, suggesting that OC content was the main factor in controlling herbicide transport in these soils.

Imazamox ammonium salt at 53 g ae/ha applied postemergence in the fall to imidazolinone-resistant wheat controlled Italian ryegrass 98 to 100% 10 wk after treatment (WAT). Control 22 WAT was 88 to 98% at two locations and 55% at a third location. Imazamox was more effective applied in fall to three- to four-leaf Italian ryegrass than when applied in spring to one- to three-tiller Italian ryegrass. Split applications, with 27 g/ha applied in fall and spring, were no more effective than 53 g/ha applied in fall. Pendimethalin preemergence in combination with fall-applied imazamox increased control 22 WAT 10 to 33 percentage points at two of three locations. Imazethapyr ammonium salt plus imazapyr isopropylamine salt applied at 47 plus 16 g ae/ha and imazamox at 44 or 53 g/ha were similarly effective, whereas imazethapyr at 70 g/ha was ineffective. Control by imazapic ammonium salt at 70 g ae/ha was equal to or greater than control by imazamox at 44 or 53 g/ha. Imazamox, imazethapyr plus imazapyr, and imazapic controlled diclofop-resistant and -susceptible Italian ryegrass. Thifensulfuron plus tribenuron mixed with imazamox increased Italian ryegrass control in field and greenhouse experiments, whereas dicamba reduced control compared with imazamox applied alone. Control by imazamox plus 2,4-D was similar to or greater than control by imazamox alone.

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.

Studies were conducted to compare foliar- and root-based absorption and translocation of SC-0051 in corn and soybean using radio-tracer techniques. Visual injury symptoms expressed in soybean by SC-0051 were bleaching in newly formed (meristematic) tissue followed by necrosis. Corn expressed little to no visual injury. SC-0051 was absorbed more rapidly by foliage of corn and soybean than by their roots. Although soybean absorbed more SC-0051 through roots than did corn, there was no difference between species in foliar absorption of SC-0051. SC-0051 was rapidly translocated throughout corn plants but was concentrated mainly in the meristematic tissue of soybean. Even though there were slight differences in absorption and translocation of SC-0051 in corn and soybean, it is likely that differential metabolism of the herbicide explains selectivity.

Studies were conducted in 1999, 2000, and 2001 to evaluate cotton response to CGA 362622 applied postemergence with various adjuvants. In field studies, CGA 362622 was applied at 3.8 or 7.5 g ai/ha with nonionic surfactant, crop-oil concentrate (COC), or a urea-based adjuvant. A nontreated control was maintained weed free for comparison. Crop injury over all years at 1 wk after treatment (WAT) was 27 and 34% from 3.8 and 7.5 g/ha CGA 362622, respectively, when rates were pooled over adjuvants. At 4 WAT, injury was 6 to 14% with 3.8 g/ha and 10 to 21% with 7.5 g/ha CGA 362622 during the 3-yr study. Cotton heights at 2 WAT were reduced by 16 to 31% of nontreated cotton heights by CGA 362622. Heights of treated cotton did not differ and were generally equivalent to the nontreated control at 8 WAT. Cotton injury and height reduction were greatest when CGA 362622 was applied with COC. Cotton lint yields and fiber quality were not affected by CGA 362622 rate or adjuvant treatment. Cotton injury from CGA 362622 in the greenhouse was similar to that in the field. Initial cotton injury and subsequent reduction in leaf area or shoot dry weight were generally lowest when CGA 362622 was applied with no adjuvant or UBA in the greenhouse.

Broomrapes (Orobanche spp.) are root holoparasitic plants that cause severe damage to economically important crops, especially in Mediterranean countries. Egyptian broomrape is the most troublesome weed on tomatoes grown for processing in Israel. In the present study, we tested the efficacy and selectivity of four sulfonylurea herbicides in controlling Egyptian broomrape on tomatoes grown in pots under greenhouse conditions. MON 37500, rimsulfuron, HOE 404 and SL-160 were applied postemergence (POST) and preplant incorporated (PPI) followed by POST applications. MON 37500 and rimsulfuron were more selective to tomato and controlled the parasite more effectively than HOE 404 and SL-160. MON 37500 and rimsulfuron at 50 and 100 g ai/ha and at 100, 150, and 200 g ai/ha, respectively, applied on tomato foliage 14, 28, and 42 d after planting (DAP) and followed by sprinkler irrigation to field capacity, resulted in complete control of the parasite. However, a significant reduction in control efficacy was observed when the experiment was repeated with charcoal-topped pots, suggesting that the herbicides act mainly through the soil. Except for rimsulfuron, the PPI followed by two POST treatments was more phytotoxic to tomato plants than the POST treatments. The PPI plus POST applications controlled Egyptian broomrape effectively, but tomato plants were injured by HOE 404 at all PPI application rates and by MON 37500 at the high rate at 150 g/ha. The present study determined that three POST applications or a PPI application followed by two POST applications of MON 37500 at 50 or 100 g/ha, or rimsulfuron at 100, 150, or 200 g/ha were effective and selective in controlling Egyptian broomrape on tomato, under greenhouse conditions.

Field studies were conducted in 1999, 2000, and 2001 to evaluate broadleaf weed control in glyphosate-resistant cotton by glyphosate plus CGA 362622 applied postemergence. Treatments included 560 and 1,120 g ai/ha glyphosate-isopropylamine alone or in mixtures with CGA 362622 at 3.8 and 7.5 g ai/ha, and CGA 362622 at 7.5 g/ha alone. Cotton injury 7 d after treatment (DAT) was 3 to 11% from glyphosate alone and 16 to 24% from glyphosate plus CGA 362622. Injury 28 DAT with CGA 362622 or herbicide mixtures did not exceed 6%. Broadleaf weed control by herbicide mixtures was generally more consistent than control from either herbicide applied alone. Glyphosate plus CGA 362622 controlled common cocklebur and smooth pigweed better than glyphosate alone. In most instances, the mixtures also controlled common ragweed, common lambsquarters, ivyleaf morningglory, pitted morningglory, and tall morningglory better than glyphosate applied alone. Common cocklebur and smooth pigweed were controlled at least 85% by all treatments. CGA 362622 did not control spurred anoda or jimsonweed. Cotton yields generally reflected weed control. According to these results, glyphosate plus CGA 362622 mixtures can consistently control many broadleaf weeds in cotton.

The presence of row crops, such as field corn, improves herbicidal control of weeds, but the impact of crop row position on herbicide dose–response relationships for weeds is unknown. At midseason at three site-years in Missouri, total weed cover (WC) was reduced by increasing soil residual herbicide rate in a dose-dependent response and was as much as 20% lower in-row (IR) than between-row (BR). Preemergence atrazine + S-metolachlor + clopyralid + flumetsulam at different rates (0×, 0.25×, 0.5×, 0.75×, and 1×, where 1× rate was 2,240 + 1,750 + 210 + 67 g ai/ha, respectively) were applied at planting in field corn to control giant foxtail, the chief weed present, and annual broadleaf weeds, largely common waterhemp. Lower herbicide rates were required to reduce IR WC to the same extent as BR WC, but these rates varied between site-years. At all three site-years, a least squares regression equation adequately described data variability relating corn yield to IR or BR WC (or both) (i.e., Y = a + bBR2, where Y is corn yield in kg/ha, BR is BR WC [%], and a and b are coefficients).

Weed control in white beans is currently limited by the small number of registered herbicides. The tolerance of two white bean cultivars, ‘AC Compass’ and ‘OAC Thunder’, to various postemergence (POST) herbicides at the maximum use rate and twice the maximum use rate for soybean or corn was evaluated at two Ontario locations in 2001 and 2002. Generally, the two cultivars did not differ in their response to the POST herbicides. POST applications of imazamox plus fomesafen, imazamox plus bentazon, and cloransulam-methyl decreased plant height, shoot dry weight, and yield by as much as 29, 41, and 55%, respectively, and increased seed moisture content up to 3.9%. POST applications of thifensulfuron, chlorimuron, and bromoxynil decreased plant height as much as 57%, shoot dry weight by up to 71%, yield as much as 93% and increased seed moisture content up to 15.5%. Based on these results, AC Compass and OAC Thunder white beans do not possess sufficient tolerance to support the registration of imazamox plus bentazon, imazamox plus fomesafen, cloransulam-methyl, thifensulfuron, chlorimuron, and bromoxynil.

This study compared the effect of weed control and orchard floor management (OFMA) options including organic mulch on summer annual weed interference in a newly established peach orchard. Weed interference where no preemergence (PRE) herbicides were applied, including vole damage, caused 29% peach tree mortality, reduced tree trunk cross-sectional area (TCSA) 62% by the fourth year of orchard establishment, and reduced fruit yield and fruit number in 1999 by 73 and 75%, respectively, but had no effect on fruit size. Compared with a no-till or conventionally tilled orchard floor, the population of grassy weeds within the tree row was greater in killed perennial ryegrass sod (PRS) plus hard fescue residue mulch treatments but was less in killed PRS plus tall fescue residue mulch treatments. Among the no-PRE treatments, the tree row broadleaf weed populations were suppressed in killed PRS with or without the addition of fescue residue mulch to the tree row when compared with the no-till or conventionally tilled orchard floor treatments. PRE herbicide treatments strongly affected peach fruit yield and TCSA but not average fruit size. There was no effect among the killed PRS, with or without hard or tall fescue residue mulch treatments, on peach fruit yield, TCSA, or average fruit size when compared with the no-till or conventionally tilled orchard floor treatment options. All treatments with herbicide had higher yields in 1999 than those without herbicides.

Imazethapyr applied to the soil at 0, 35, 53, 70, 87, 105, and 140 g ai/ha followed by (fb) early-postemergence (EPOST) or late-postemergence (LPOST) applications at 140, 105, 87, 70, 53, 35, and 0 g ai/ha, respectively, was evaluated for weed control, crop tolerance, and grain yield in imidazolinone-tolerant rice. In drill- and water-seeded rice, imazethapyr effectively controlled barnyardgrass 91 to 97% with a soil fb EPOST or LPOST application except when applied at 105 fb 35 g/ha EPOST at 49 d after late-postemergence treatment. Rice yields were 5,920 and 3,920 kg/ ha in the drill-seeded study with a single application of imazethapyr at 140 g/ha EPOST in 2000 and 2001, respectively. These yields were greater than or equal to those from treatments with two imazethapyr applications.

Field experiments were conducted near Chickasha, OK, in 1999 and 2000 and near Perkins, OK, in 2000 to evaluate the effects of Palmer amaranth on harvest efficiency of grain sorghum and full-season competition with that crop. Weed densities were 0 (the weed-free check), 1, 2, 4, 6, 9, 12, and 18 plants/15 m of row. In the harvest efficiency experiments, each additional weed per 15 m of row increased grain moisture before cleaning by 0.7 and 0.2% at Chickasha in 1999 and at Perkins, respectively. After cleaning, it increased moisture by 0.2% in both the experiments for each weed. Foreign material increased 67, 2, and 3 kg/ha at Chickasha in 1999, Chickasha in 2000, and Perkins, respectively. At Chickasha in 2000, sorghum seed loss through the combine increased 11 kg/ha for each additional weed per 15 m of row. Grain grades improved at Perkins at higher weed densities. In the competition experiments, grain yield decreased by 1.8 to 3.5% for each increase of 1 weed/15 m of row. More weeds resulted in higher weed dry weight. Each kilogram of Palmer amaranth dry weight per plot reduced grain yield by 5.3 to 9.1%. In 2000, sorghum seeds per panicle were reduced by 27 to 50 for each weed. Grain grades generally decreased as weed density increased at Chickasha in 2000 but not at Perkins.

Field experiments were conducted in 1994 and 1995 under sprinkler irrigation at the University of Wyoming Research and Extension Center at Torrington to evaluate the effects of season-long interference and the effects of duration of interference of several common sunflower and green foxtail densities, alone or in combination, on pinto bean yield. Green foxtail densities did not significantly affect pinto bean yield in 1994 and reduced yield only at the highest density in 1995. In contrast, sunflower densities reduced pinto bean yield, except at the lowest density in 1994. Pinto bean yield was reduced as the combined density of green foxtail and sunflower increased. Compared with yield losses from each weed species alone, yield reductions from mixed species were additive in 1994 and at low weed densities in 1995 and less than additive at higher weed densities in 1995. The minimum number of weeds per m of row that will economically reduce pinto bean yield was estimated to be 1.6 to 2.9 for green foxtail and 0.12 to 0.2 for sunflower. Pinto bean yield reduction increased as the duration of green foxtail and sunflower interference increased, whether grown alone or in combination. The maximum duration that green foxtail, sunflower, and green foxtail plus sunflower can interfere with pinto bean before causing economical losses was estimated to be 4.5, 3.2, and 2.5 wk, respectively.

Current plant bioassays included in the guidelines for testing pesticides do not include the measurement of reproduction endpoints. A bioassay, based on reduction of flowering of cress was developed to detect soil residues of hexazinone and simazine at levels of 0.02 and 0.10 ppm, respectively. The endpoint used in the described bioassay is the percentage of plant viability that implies that the tested plants have reached the flowering stage. It was found that sensitivity of cress is lower in soils containing higher organic matter.

Plant responses to various doses of herbicides usually follow a sigmoid model where the potency is given by the 50% inhibition (I 50) value. To assess the potency of a herbicide under a range of environmental conditions, a series of independent bioassays are necessary to account for assay-to-assay variation. Analysis has conventionally been done by separate analysis of the individual bioassays or by simply pooling data. Analyzing the individual bioassays separately throws up relevant information on interassay variation. Such a model becomes too complex because a full set of model parameters is needed for each data set. Pooling data instead, and analyzing the bioassay jointly, inflates parameter uncertainty because of oversimplification. Such a simple model would have too few variables, and the fixed-effect estimates would be more uncertain because they would be explaining the interassay random effects. This means that the underlying statistical model is not realistic. Therefore, we propose a new technique of intermediate complexity that outperforms either technique and provides biologically realistic estimates that allow us to compare herbicide potencies. With this technique, we simultaneously analyze independent experiments by using a combination of nonlinear regression and mixed models. The case study uses a group of independently run bioassays with two photosystem II–inhibiting herbicides, diuron and bentazon, by measuring the oxygen evolution of thylakoid membranes. The introduction of random elements in the nonlinear regression parameters reduces the uncertainty in the parameters of interest. We demonstrate that it is possible to pool data from independent experiments to assess which parameters can be assigned a random element, to conduct hypothesis testing, and to calculate stable confidence limits and thus obtain a more precise interpretation of the biologically relevant parameters, such as I 50, compared with the conventional nonlinear regression models of the individual bioassays.

Integrated weed management practices, such as crop rotation and increased seeding rates, potentially improve weed management. Yet, few studies compare competitive interactions of weeds with different crops. This research quantified the impact of Persian darnel on spring wheat, canola, and sunflower yield across different seeding rates. Increasing crop density increased yield when Persian darnel affected crop yield early in physiological development. Crop yield loss was estimated to reach 83, 70, and 57% for spring wheat, canola, and sunflower, respectively, at high Persian darnel densities. Persian darnel reduced spring wheat yield by limiting the number of tillers per plant and seed per tiller; reduced canola yield by limiting the number of branches per plant, pods per branch, and seed per pod; and reduced sunflower yield by limiting the number of seed per plant. Persian darnel affected crop growth early in physiological development, indicating that interspecific interference occurred early in the growing season. Cultural and resource management aimed at reducing Persian darnel impact on resource availability and crop yield components will reduce Persian darnel impact on crop yield.

The earliest possible seeding of wheat crops in the southern Australian dryland cropping zone is prevented by the lack of a weed control practice that adequately controls initial weed seedling emergence at the start of the growing season. The objective of this study was to determine the potential for using residual herbicides applied up to 1 mo before the start of the growing season to control rigid ryegrass seedlings that emerge after the season-opening rains. In a series of glasshouse studies, S-metolachlor and propyzamide were found to effectively persist on the soil surface through prolonged exposure to hot, dry, and intense sunlight conditions, preventing the establishment of rigid ryegrass seedlings. In addition, these herbicides caused little or no effect on subsequently seeded wheat. It also was determined that S-metolachlor had the potential to retain efficacy on rigid ryegrass seedlings after 12 wk of exposure on the soil surface to these conditions. These studies have identified two herbicides with the potential for use at the novel application timing, i.e., before the commencement of the growing season, in Mediterranean climates of southern Australia.

Field studies were conducted over 3 yr at two locations to evaluate the effect of glyphosate rate and time of application on common lambsquarters control, density, dry weight, seed production, and the number of seedlings emerging from soil cores taken the year after herbicide application in glyphosate-resistant corn. Glyphosate was applied at 0, 112, 225, 450, 675, or 900 g ai/ha when common lambsquarters were at the two-, four-, or six-leaf stage of growth. Nicosulfuron was applied to all experimental areas to control annual grasses. Visual estimates of percent control increased, whereas density, dry weight, seed production, and seedlings emerging the year after treatment decreased as the rate of glyphosate was increased from 0 to 450 g/ha. Increasing the glyphosate rate above 450 g/ha had little effect on these parameters. Corn yield declined only at glyphosate rates below 450 g/ha. Time of application had no effect on common lambsquarters control and corn yield because little emergence occurred after the first glyphosate application. There was no interaction between glyphosate rate and time of application for any of the parameters evaluated. In these studies, the application of glyphosate at half the manufacturer's registered rate provided control of common lambsquarters equivalent to the full-registered rate with no measured increase in weed seed production and no increase in weed seedlings emerging from soil cores the year after herbicide application. The results suggest that in some cases the use of reduced herbicide rates can provide excellent weed control and maintain crop yields, while reducing the cost of production and the environmental impact of herbicides. The use of extremely low rates (112 or 225 g/ha), however, resulted in reduced corn yields, increased common lambsquarters seed production and seedlings emerging the year after application, and possibly increased weed management costs in subsequent years.