For effective full-season control of dodder in alfalfa produced for seed, herbicides applied to soil in late winter or early spring should remain active through June. Pendimethalin or prodiamine at 2.2 kg ai ha-1 applied in mid March controlled 97 to 100% of dodder through May and 92 to 100% through June. At 3.4 kg ha-1, both controlled 96 to 100% of dodder through June each year, and control of 95 or 96% persisted through July in 1 of 3 yr. In the greenhouse, trifluralin was only about 1/8 as active against dodder as pendimethalin and prodiamine. In the field, trifluralin at 2.2 kg ai ha-1 controlled dodder satisfactorily through May in 3 of 4 yr and through June in 1 of 4 yr. At 4.5 kg ha-1, trifluralin controlled 99% of dodder through May in each of 3 yr; control was satisfactory through June in 2 of 3 yr. Oryzalin was less effective than the other three dinitroaniline herbicides. A standard herbicide, chlorpropham at 6.7 kg ai ha-1, controlled 97 to 100% of dodder through May, but control usually did not persist through June. Another standard herbicide, DCPA at 11.2 kg ai ha-1, controlled 97 or 98% of dodder through June in 2 of 3 yr. Pendimethalin and prodiamine at 3.4 and 6.7 kg ai ha-1 did not affect the yield or quality of seed produced by established alfalfa. Heavy sprinkler irrigation (15 cm of water) after application did not diminish dodder control from pendimethalin or prodiamine.

Immunochemical techniques offer many advantages over chromatographic methods used for pesticide trace analysis of substrates such as soil, water, plants, urine, and blood. These advantages include speed of processing samples, high specificity for detecting a pesticide, reduced amount of preparation and cleanup of the sample before analysis, and a dramatic increase in the number of samples that can be analyzed. Immunoassays are based on the principle that antibodies to pesticides can be prepared, in animals, that can recognize and attach with exquisite specificity to certain chemical configurations displayed on the surface of a molecule. Small molecules such as herbicides usually are not immunogenic but can be made so by chemically bonding them to a large immunogenic protein such as bovine serum albumin before injection into an animal. Development of herbicide-specific antibodies and their use in direct and indirect enzyme-linked immunosorbent assays (EIA or ELISA) as well as radioimmunoassays (RIA) are discussed. The principles behind monoclonal antibody production are outlined, and immunoassays using polyclonal and monoclonal antibodies are compared. Specific reference is made to the development and use of indirect ELISA and RIA procedures for trace analysis of 2,4-D and picloram.

Avena spp. are world weeds with many cases of evolved herbicide resistance. In Australia, Avena spp. (wild oat and sterile oat) are a major problem, especially in grain crops. Acetyl-CoA carboxylase (ACCase)–inhibiting herbicides have been used extensively since the late 1970s for Avena spp. control. However, continued reliance on these herbicides has resulted in the evolution of resistant Avena spp. populations. Resistance across many ACCase-inhibiting herbicides was characterized in four Avena spp. populations from the Western Australian grain belt. Dose–response experiments were conducted to determine the level of resistance to the aryloxyphenoxypropionates and cyclohexanediones and to the phenylpyrazoline herbicide pinoxaden. On the basis of resistance index values, all four resistant populations exhibited high-level diclofop resistance but varied in the level of resistance to other ACCase-inhibiting herbicides tested. It is evident that Avena spp. populations from the Western Australian grain belt have evolved resistance to a number of ACCase-inhibiting herbicides.

A 2-yr field study was conducted during 2007 and 2008 at Stoneville, MS, to determine the effect of twin-row (two rows 38 cm apart on 102-cm beds) and single-row (on 102-cm beds) patterns and glyphosate POST applications with and without fluometuron + S-metolachlor PRE on cotton canopy closure, weed control, and lint yield in two cultivars (‘DP117B2RF’, early maturity, hairy leaf; ‘DP164B2RF’, mid to full maturity, smooth leaf) under an irrigated environment. The experiment was conducted in a split–split plot arrangement of treatments in a randomized complete block design with row pattern as the main plot, cultivars as the subplot, and herbicide programs as the subsubplot. Cotton canopy closed 2 wk earlier in the twin-row pattern compared to the single-row pattern. Canopy closure was unaffected by cultivars and herbicide programs. Control of nine predominant weeds was sufficient (≥ 95%) to support cotton production. Total weed dry biomass was reduced by 35% in twin rows compared to the single-row pattern, 15% in DP117B2RF compared to DP164B2RF cultivar, and ≥ 97% with glyphosate early POST (EPOST), EPOST followed by (fb) mid-season POST (MPOST), EPOST fb MPOST fb late POST (LPOST) following PRE herbicides or three applications of glyphosate POST only without PRE herbicides compared to no herbicide. Cotton grown in twin-row pattern produced 6% higher lint yield than single-row cotton. Cultivar DP117B2RF produced 23% higher lint yield than cultivar DP164B2RF. Lint yields were higher with glyphosate EPOST fb MPOST, EPOST fb MPOST fb LPOST following PRE herbicides or three applications of glyphosate POST only without PRE herbicides (1,210 to 1,230 kg/ha) compared to glyphosate EPOST following PRE herbicides (1,130 kg/ha). These results demonstrated that cotton grown in twin-rows closed canopy early and produced higher lint yields than cotton grown in single-rows.

Dicamba, 2,4-D, picloram, and commercially available premixes of glyphosate plus 2,4-D or glyphosate plus dicamba were evaluated alone and in combination for field bindweed control in a winter wheat-fallow system in Colorado, Wyoming, Kansas, and Montana. Approximately one year after application, herbicide mixtures containing picloram at 0.14 or 0.28 kg ai ha-1 provided the best control. In five of seven locations, the control provided by picloram in herbicide mixtures was greater than the control provided by glyphosate plus 2,4-D, 2,4-D, or dicamba when these products were mixed with picloram. Glyphosate plus 2,4-D or glyphosate plus dicamba premixes, or 2,4-D added to dicamba were less effective for long-term control of field bindweed than mixtures containing 0.14 kg ai ha-1 or more of picloram. Under drought conditions in Kansas in 1988, picloram did not control field bindweed as well as in Colorado, Wyoming, or Montana where rainfall was normal.

“Pesta” is a granular, extruded product made from a cereal grain flour and any biological control agent. A strain of Pseudomonas fluorescens, BRG100, which is a pathogen of green foxtail, has been formulated into a Pesta product. P. fluorescens BRG100 survived processing best in oat flour, and the addition of 20% (wt/wt) maltose extended the shelf life of the product to more than 32 wk. Field studies of 8-wk duration showed that a Pesta product containing BRG100 suppressed green foxtail emergence by as much as 90%. An optimally formulated and processed Pesta product has potential for the biocontrol of green foxtail.

Because of a previously reported antagonism of clethodim activity by other herbicides, greenhouse experiments were conducted to determine goosegrass control with clethodim and glufosinate postemergence alone, in tank mixtures, and as sequential treatments. Herbicide treatments consisted of glufosinate at 0, 290, or 410 g ai/ha and clethodim at 0, 105, or 140 g ai/ha, each applied alone, in all possible combinations of the above application rates, or sequentially. Glufosinate at either rate alone controlled goosegrass at the two- to four-leaf growth stage <44%, and control was less for goosegrass at the one- to two- and four- to six-tiller growth stages. Clethodim controlled two- to four-leaf and one- to two-tiller goosegrass 91 and 99% at application rates of 105 and 140 g/ha, respectively, and controlled four- to six-tiller goosegrass 68 and 83% at application rates of 105 and 140 g ai/ha, respectively. All tank mixtures of glufosinate with clethodim reduced goosegrass control at least 52 percentage points when compared to the control with clethodim alone. Glufosinate at 290 or 410 g/ha when applied sequentially 7 or 14 d prior to clethodim reduced goosegrass control at least 50 percentage points compared to the control obtained with clethodim applied alone. Clethodim at rates of 105 or 140 g/ha when applied 7 or 14 d prior to glufosinate controlled goosegrass equivalent to the control obtained with each respective rate of clethodim applied alone at the two- to four-leaf and one- to two-tiller growth stage. Clethodim should be applied to goosegrass no larger than at the one- to two-tiller growth stage at least 7 d prior to glufosinate application or 14 d after a glufosinate application for effective goosegrass control.

The effect of spurred anoda competition in narrow- (35 cm) and wide-row (70 cm) soybean was studied in field experiments for 2 yr. Vigorous early soybean growth in narrow- compared with wide-row soybean resulted in lower radiation transmitted through the canopy, which can partially account for greater competitiveness of narrow-row than wide-row soybean. Soybean plant height was not significantly influenced by the row spacing. Relative yield total (RYT), which is the relationship between yield in mixtures and in monocultures of the crop or the weed and indicates resource complementarity, was equal to 1 with 12 spurred anoda/m2 in the year with less precipitation. Regardless of the row spacing, spurred anoda gave resource use complementarity with the crop (RYT > 1) in all other treatments; therefore, partial avoidance of competition in mixed species was evident. Soybean aggressivity, which takes into account the effect of competition on both the crop and the weed and indicates competitive ability, decreased with weed density in both row spacings. Soybean yield loss at harvest was linearly related to relative dry weight 40 d after planting. Weed-free narrow- and wide-row soybean produced similar yields. In the presence of the spurred anoda, soybean yield was greater in narrow-row compared with wide-row soybean only in the most humid year. A management system that uses quick canopy closure with narrow-row soybean can provide excellent soybean yield and suppression of low spurred anoda densities.

Model simulations predict that lowering herbicide efficacy by reducing the application rate would slow the rate of enrichment of herbicide-resistant individuals in a weed population, but the resulting increase in density of susceptible plants would reduce crop yield and increase the weed seed bank. A study was conducted at three sites in Saskatchewan, Canada, from 1997 to 2000 to examine the implication of reduced rates of acetyl-CoA carboxylase (ACCase) inhibitors in a diverse 4-yr crop rotation, in conjunction with variable crop seeding rates, on the enrichment of resistant wild oat in a mixed (resistant and susceptible) population. Main-plot treatments were crop (barley, canola, field pea, and spring wheat), subplot treatments were crop seeding rate (recommended and high), and sub-subplot treatments were ACCase inhibitor rate (0, 0.33, 0.67, and 1.0 times the recommended rate). Herbicide rate frequently interacted with seeding rate in affecting wild oat seedling density, seed return, the viable fraction of the weed seed bank, and crop seed yield. As simulation models predict, reduced herbicide efficacy decreased the proportion of resistant individuals in the population. The high crop seeding rate compensated for a one-third reduction in herbicide rate by limiting total wild oat seed return and by reducing the number of resistant seedlings recruited from the seed bank. The level of resistance in the seed bank can be reduced without increasing the total (resistant plus susceptible) seed bank population by manipulating agronomic practices to increase crop competitiveness against wild oat when ACCase inhibitor rates are reduced to a maximum of two-thirds of that recommended.

Studies were conducted with one glyphosate-resistant and three conventional soybean cultivars to determine the effects of reducing row spacing from 76 to 38 and 19 cm, in combination with increased soybean population and herbicide applications (single and sequential), on sicklepod control and soybean yield. ‘Hutcheson’ was the only cultivar, and only under adverse conditions (1998), for which the 19-cm rows provided greater sicklepod control than the 38-cm rows. It was only under optimum conditions (1997), and only with conventional cultivars and sequential herbicide applications, that the 19-cm–row yield of 3,350 kg/ha was 21% greater than the 38-cm–row yield in medium population (455,375 plants/ha), and 21 and 64% greater than the 19- and 76-cm–row yield in low populations (241,000 plants/ha), respectively. In 1998, the same treatment showed similar late-season sicklepod control (> 80%) and a yield of 1,890 kg/ha, with no difference between the 19- and the 38-cm rows; but the yields were 15 and 24% greater than the yield of the narrow (19 and 38 cm) and that of the 76-cm row in low populations, respectively. Low populations with sequential applications in narrow rows and in the 76-cm row showed similar late-season sicklepod control (59 to 70%) and yield in 1998; but in 1997, the narrow rows showed 18 and 32% greater late-season control and yield than the 76-cm row, respectively. In 1997 and 1998, for ‘Hartz 5088RR’ (glyphosate resistant) with medium population and narrow rows, the single and sequential applications provided similar late-season sicklepod control (> 80%) and yield, and at least 10 and 24% greater yield than the narrow and the 76-cm rows in low populations, respectively.

Field experiments were conducted at five locations in Kansas, Nebraska, and Wyoming to determine the effects of imazamox rate and application timing on winter annual grass control and crop response in imidazolinone-tolerant winter wheat. Imazamox at 35, 44, or 53 g ai/ha applied early-fall postemergence (EFP), late-fall postemergence, early-spring postemergence (ESP), or late-spring postemergence (LSP) controlled jointed goatgrass at least 95% in all experiments. Feral rye control with imazamox was 95 to 99%, regardless of rate or application timing at Hays, KS, in 2001. Feral rye control at Sidney, NE, and Torrington, WY, was highest (78 to 85%) with imazamox at 44 or 53 g/ha. At Sidney and Torrington, feral rye control was greatest when imazamox was applied EFP. Imazamox stunted wheat <10% in two experiments at Torrington, but EFP or LSP herbicide treatments in the Sidney experiment and ESP or LSP treatments in two Hays experiments caused moderate (12 to 34%) wheat injury. Wheat injury increased as imazamox rate increased. Wheat receiving imazamox LSP yielded less grain than wheat treated at other application timings in each Hays experiment and at Sidney in 2001. No yield differences occurred in one Torrington experiment. However, yields generally decreased as imazamox application timing was delayed in the other Torrington experiment. Generally, imazamox applied in the fall provided the greatest weed control, caused the least wheat injury, and maximized wheat yield.

Field and greenhouse studies were conducted to investigate the compatibility of MSMA in a tank mixture with glyphosate or glufosinate for broadleaf and grass weed control. Glyphosate, glufosinate, and MSMA were evaluated at 0.5×, 1×, and 2× rates, with 1× rates of 0.84 kgae/ha, 0.5 kgai/ha, and 2.2 kgai/ha, respectively. Glyphosate and glufosinate provided similar levels of control for most weed species and were often more efficacious than MSMA alone. Glyphosate controlled Palmer amaranth better than glufosinate. Glufosinate controlled hemp sesbania, pitted morningglory, and ivyleaf morningglory better than glyphosate at one location. Weed control was not improved with the addition of MSMA to glyphosate or glufosinate when compared with either herbicide alone. MSMA antagonized glyphosate efficacy on barnyardgrass, browntop millet, hemp sesbania, Palmer amaranth, and redroot pigweed. MSMA antagonized glufosinate efficacy on browntop millet, hemp sesbania, ivyleaf morningglory, johnsongrass, Palmer amaranth, pitted morningglory, prickly sida, redroot pigweed, and velvetleaf. Antagonism of glyphosate or glufosinate by MSMA was often overcome by applying the 2× rate of either herbicide alone. MSMA is not a compatible tank-mixture partner with glyphosate or glufosinate for weed control in cotton.

Field experiments were conducted in 2002 and 2003 to evaluate total POST weed control in corn with mixtures of mesotrione, atrazine, and the commercial mixture of nicosulfuron plus rimsulfuron plus atrazine at registered and reduced rates. Treatments were compared with nicosulfuron plus rimsulfuron plus atrazine POST, and S-metolachlor plus atrazine PRE alone and followed by (fb) nicosulfuron plus rimsulfuron plus atrazine POST. All treatments controlled common lambsquarters 8 wk after the postemergence treatments (WAPT). Common ragweed control with POST mesotrione plus nicosulfuron plus rimsulfuron plus atrazine combinations was greater than 89%. Mesotrione plus the registered rate of nicosulfuron plus rimsulfuron plus atrazine POST controlled common ragweed more effectively than the PRE treatment alone. Addition of atrazine to mesotrione improved common ragweed control by at least 38 percentage points over mesotrione alone. Nicosulfuron plus rimsulfuron plus atrazine at the registered rate and in mixtures with mesotrione controlled morningglory species (pitted and ivyleaf morningglory) 89 to 91%. Large crabgrass control varied between 2002 and 2003. In 2002, large crabgrass control was 58 to 76% with all POST treatments, but in 2003, nicosulfuron plus rimsulfuron plus atrazine POST alone controlled large crabgrass greater than 86%. Large crabgrass was more effectively controlled by treatments with S-metolachlor plus atrazine PRE than by the total POST treatments in 2002. Giant foxtail was controlled at least 97% with nicosulfuron plus rimsulfuron plus atrazine treatments. S-metolachlor plus atrazine PRE fb nicosulfuron plus rimsulfuron plus atrazine POST controlled all weed species greater than 85%. Corn yields by total POST treatment combinations of mesotrione plus either rate of nicosulfuron plus rimsulfuron plus atrazine were comparable to S-metolachlor plus atrazine PRE alone or fb nicosulfuron plus rimsulfuron plus atrazine POST.

Protoporphyrinogen IX oxidase (PPO)–inhibiting herbicides (WSSA Group 14) have been used in agronomic row crops for over 50 yr. Broadleaf weeds, including glyphosate-resistant Palmer amaranth, have been controlled by this herbicide site of action PRE and POST. Recently, Palmer amaranth populations were reported resistant to PPO inhibitors in 2011 in Arkansas, in 2015 in Tennessee, and in 2016 in Illinois. Historically, the mechanism for this resistance involves the deletion of a glycine at position 210 (ΔG210) in a PPO enzyme encoded by the PPX2 gene; however, the ΔG210 deletion did not explain all PPO inhibitor–resistant Palmer amaranth in Tennessee populations. Recently, two new mutations within PPX2 (R128G, R128M) that confer resistance to PPO inhibitors were identified in Palmer amaranth. Therefore, research is needed to document the presence and distribution of the three known mutations that confer PPO inhibitor resistance in Tennessee. In 2017, a survey was conducted in 18 fields with Palmer amaranth to determine whether resistance existed and the prevalence of each known mutation in each field. Fomesafen was applied at 265 g ai ha–1 to Palmer amaranth infestations within each field to select for resistant weeds for later analysis. Where resistance was described (70% of surviving plants), the ΔG210 mutation was detected in 47% of resistant plants. The R128G mutation accounted for 42% of resistance, similar to the frequency of the ΔG210 mutation. The R128M mutation was less frequent than the other two mutations, accounting for only 10% of the resistance. All mutations detected in this study were heterozygous. Additionally, no more than one of the three PPX2 mutations were detected in an individual surviving plant. Similar to previous research, about 70% of PPO resistance was accounted for by these three known mutations, leaving about 30% of resistance not characterized in Tennessee populations. Survivors not showing the three known PPO mutations suggest that other resistance mechanisms are present.

Pyroxasulfone (KIH-485) is a seedling growth-inhibiting herbicide developed by Kumiai America that has the potential to control weeds in sunflower. However, little is known about how this herbicide will interact with various soil types and environments when combined with sulfentrazone. The objective of this research was to evaluate sunflower injury and weed control with pyroxasulfone applied with and without sulfentrazone across the Great Plains sunflower production area. A multisite study was initiated in spring 2007 to evaluate sunflower response to pyroxasulfone applied PRE at 0, 167, 208, or 333 g ai ha−1. In 2008, pyroxasulfone was applied alone and in tank mixture with sulfentrazone. In 2007, no sunflower injury was observed with any rate of pyroxasulfone at any location except Highmore, SD, where sunflower injury was 17%, 4 wk after treatment (WAT) with 333 g ha−1. In 2008, sunflower injury ranged from 0 to 4% for all treatments. Adding sulfentrazone did not increase injury. Sunflower yield was only reduced in treatments in which weeds were not effectively controlled. These treatments included the untreated control and pyroxasulfone at 167 g ha−1. Sunflower yield did not differ among the other treatments of pyroxasulfone or sulfentrazone applied alone or in combination. The addition of sulfentrazone to pyroxasulfone improved control of foxtail barley, prostrate pigweed, wild buckwheat, Palmer amaranth, and marshelder, but not large crabgrass or green foxtail. The combination of pyroxasulfone and sulfentrazone did not reduce control of any of the weeds evaluated.

Glyphosate-resistant (GR) common waterhemp has become a significant problem weed in Nebraska and several Midwestern states. Several populations of GR common waterhemp are also resistant to acetolactate synthase (ALS)-inhibiting herbicides, making them difficult to control with POST herbicides in GR soybean. Glufosinate-resistant (GFR) soybean is an alternate system for controlling GR common waterhemp, justifying the need for evaluating glufosinate-based herbicide programs. The objectives of this study were to compare POST-only herbicide programs (including one-pass and two-pass POST programs) with PRE followed by (fb) POST herbicide programs for control of GR common waterhemp in GFR soybean and their effect on common waterhemp density, biomass, and soybean yield. Field experiments were conducted in 2013 and 2014 near Fremont, NE in a grower’s field infested with GR common waterhemp. Glufosinate applied early- and late-POST provided 76% control of GR common waterhemp at 14 d after late-POST (DALPOST) compared with 93% control with a PRE fb POST program when averaged across treatments. The PRE application of chlorimuron plus thifensulfuron plus flumioxazin, S-metolachlor plus fomesafen or metribuzin, saflufenacil plus dimethenamid-P fb glufosinate provided ≥95% control of common waterhemp throughout the growing season, reduced common waterhemp density to ≤2.0 plants m─2, caused ≥94% biomass reduction, and led to 1,984 to 2,210 kg ha─1 soybean yield. Averaged across treatments, the PRE fb POST program provided 82% common waterhemp control at soybean harvest, reduced density to 23 plants m─2 at 14 DALPOST, and caused 86% biomass reduction and 1,803 kg ha─1 soybean yield compared with 77% control, 99 plants m─2, 53% biomass reduction, and 1,190 kg ha─1 yield with POST-only program. It is concluded that PRE fb POST programs with multiple effective modes of action are available for control of GR common waterhemp in GFR soybean.

Herbicide-resistant weeds pose a severe threat to sustainable vegetation management in various production systems worldwide. The majority of the herbicide resistance cases reported thus far originate from agronomic production systems where herbicide use is intensive, especially in industrialized countries. Another notable sector with heavy reliance on herbicides for weed control is managed turfgrass systems, particularly golf courses and athletic fields. Intensive use of herbicides, coupled with a lack of tillage and other mechanical tools that are options in agronomic systems, increases the risk of herbicide-resistant weeds evolving in managed turfgrass systems. Among the notable weed species at high risk for evolving resistance under managed turf systems in the United States are annual bluegrass, goosegrass, and crabgrasses. The evolution and spread of multiple herbicide resistance, an emerging threat facing the turfgrass industry, should be addressed with the use of diversified management tools. Target-site resistance has been reported commonly as a mechanism of resistance for many herbicide groups, though non–target site resistance is an emerging concern. Despite the anecdotal evidence of the mounting weed resistance issues in managed turf systems, the lack of systematic and periodic surveys at regional and national scales means that confirmed reports are very limited and sparse. Furthermore, currently available information is widely scattered in the literature. This review provides a concise summary of the current status of herbicide-resistant weeds in managed turfgrass systems in the United States and highlights key emerging threats.

Herbicide-resistant Echinochloa spp. pose a significant threat to U.S. rice production. Two surveys were conducted to characterize Echinochloa resistance to common rice herbicides and provide important demographic information on the populations in Arkansas: one was the Echinochloa Herbicide Resistance Confirmation Survey conducted annually since 2006; the other was the Echinochloa Herbicide Resistance Demographics Survey conducted since 2010. The Resistance Confirmation Survey showed that resistance to propanil (50%) was most prevalent, followed by quinclorac (23%), imazethapyr (13%), and cyhalofop (3%). Multiple resistance increased with time, with 27% of accessions being multiple-resistant, mostly to propanil+quinclorac (12%). The parallel Resistance Demographics Survey tested resistance by species. Of the 264 accessions collected, 73% were junglerice, 14% were rough barnyardgrass, and 11% were barnyardgrass. Overall, this survey also showed resistance to propanil (53%) and quinclorac (28%) being most prevalent, with low frequencies of resistance to cyhalofop (12%) and imazethapyr (6%). Resistance to herbicides was less frequent with barnyardgrass (54%) and rough barnyardgrass (28%) than with junglerice (73%). Multiple resistance was most frequent with junglerice (33%) and least frequent with rough barnyardgrass (8%). Across both surveys, the resistance cases were clustered in the northeast and Grand Prairie regions of the state. Herbicide resistance among Echinochloa populations in rice fields is continuing to increase in frequency and complexity. This is a consequence of sequential selection with different major herbicide sites of action, starting with propanil followed by quinclorac and others.

Field research was conducted for 2 yr to evaluate response of corn and rice to simulated drift rates of a commercial premix of imazethapyr plus imazapyr [3:1 (w/w)]. Drift rates of the imazethapyr plus imazapyr premix represented 0.8, 1.6, 3.2, 6.3, and 12.5% of the usage rate of 63 g ai/ha (0.5, 1, 2, 4, and 7.9 g/ha, respectively). The imazethapyr plus imazapyr premix applied to six-leaf corn at 7.9 g/ha reduced height 11% compared with the nontreated control 7 days after treatment (DAT) but did not affect corn height 14 and 28 DAT. Corn yield was equivalent regardless of imazethapyr plus imazapyr rate and ranged from 10,200 to 11,500 kg/ha. At 28 DAT, rice height was reduced 12% when 7.9 g/ha of the imazethapyr plus imazapyr premix was applied early postemergence (EPOST) at two- to three-leaf and 14 and 5% when the imazethapyr plus imazapyr premix at 7.9 and 4 g/ha, respectively, was applied late postemergence (LPOST) at panicle differentiation. Reductions in mature rice height of 11 and 6% were observed when the imazethapyr plus imazapyr premix was applied LPOST at 7.9 and 4 g/ha, respectively, and a 5% reduction was observed for 7.9 g/ha of the imazethapyr plus imazapyr premix applied EPOST. Application of the imazethapyr plus imazapyr premix EPOST at 7.9 g/ha delayed heading in only 1 yr, but heading was delayed both years when applied LPOST. Rice yield was reduced 39 and 16% when the imazethapyr plus imazapyr premix was applied LPOST at 7.9 and 4 g/ha, respectively, compared with a 9% yield reduction for 7.9 g/ha applied EPOST.

Agricultural practices, other than herbicide use, can affect the rate of evolution of herbicide resistance in weeds. This study examined associations of farm management practices with the occurrence of herbicide (acetyl-CoA carboxylase or acetolactate synthase inhibitor)-resistant weeds, based upon a multi-year (2001 to 2003) random survey of 370 fields/growers from the Canadian Prairies. Herbicide-resistant weeds occurred in one-quarter of the surveyed fields. The primary herbicide-resistant weed species was wild oat, with lesser occurrence of green foxtail, kochia, common chickweed, spiny sowthistle, and redroot pigweed. The risk of weed resistance was greatest in fields with cereal-based rotations and least in fields with forage crops, fallow, or where three or more crop types were grown. Weed resistance risk also was greatest in conservation-tillage systems and particularly low soil disturbance no-tillage, possibly due to greater herbicide use or weed seed bank turnover. Large farms (> 400 ha) had a greater risk of weed resistance than smaller farms, although the reason for this association was unclear. The results of this study identify cropping system diversity as the foundation of proactive weed resistance management.