Greenhouse studies were conducted to evaluate halosulfuron tolerance of several squash and cucumber cultivars commonly grown in Georgia. There was an inverse linear relationship between squash plant biomass and rate of halosulfuron (r2 = 0.70 to 0.92). With the exception of ‘Supersett’, the slopes from regression of all squash cultivars were equivalent. The estimated amount of halosulfuron required to reduce growth by 20%, based on regression, ranged from 8.2 to 45 g ai/ha (for Supersett and ‘Dixie’, respectively). Squash plant height was also reduced by halosulfuron, though plants began to recover from the injury by the end of the study. There was no effect of halosulfuron rate on cucumber plant biomass or height. Cucumber cultivars appeared to be more tolerant to halosulfuron than did squash cultivars.

Selective placement studies were conducted under greenhouse conditions to determine the relative importance of root vs. foliar absorption of postemergence-applied CGA-362622 by torpedograss. All application methods were equally effective in reducing torpedograss foliage as measured 4 wk after treatment. However, foliar + soil and soil-only were more effective than foliar-only in suppressing regrowth at 10 wk after treatment. Foliar absorption by torpedograss and subsequent translocation was determined with radiotracer techniques. After 72 h, 29% of the applied CGA-362622 had been absorbed, and 2 and 7% of the amount applied had accumulated in developing rhizomes and roots, respectively. CGA-362622 was more readily absorbed and translocated by the root. Hydroponically grown plants were transferred to a hydroponic solution spiked with CGA-362622 at 200 ppb. After 6 h, whole plant concentration was 113.1 ng/plant. Only 56% of amount absorbed remained in the roots, the remainder having been translocated to other tissues. The youngest leaf and the immature rhizomes accumulated 2 and 15%, respectively. CGA-362622 soil adsorption was slightly influenced by CGA-362622 concentration and greatly influenced by soil pH. Average percent recovered in the soil solution (i.e., not absorbed) was 15.3 and 27.4% at pH 5.7 and 6.7, respectively. Soil mobility was also pH dependent. Soil solution and soil mobility data support the observation that soil application followed by root entry is more effective in delivering phytotoxic concentrations to the regenerative tissues of torpedograss than foliar application.

Field experiments were conducted at three locations in 2000 and 2001 to evaluate what effect soil characteristics have on sensitivity of two corn hybrids to soil-applied isoxaflutole plus flufenacet. Soil types were a Flanagan silt loam with 3.9% organic matter (OM), a Drummer silty clay loam with 5.0% OM, and a Cisne silt loam with 2.1% OM. Soil pH was adjusted to four target levels, < 6.0, 6.0 to 6.5, 6.6 to 7.0, and > 7.0, at each location. Isoxaflutole plus flufenacet treatments consisted of the recommended rate (1 ×) and two times (2 ×), and four times (4 ×) the recommended rates based on soil type. Additional treatments included 1 × and 2 × isoxaflutole plus flufenacet with 1.1 kg/ha atrazine and S-metolachor plus atrazine as a control treatment. Corn hybrid ‘Garst 8366’ (G8366) was more sensitive to soil-applied isoxaflutole plus flufenacet compared with ‘Garst 8600’ (G8600). Corn injury occurred only at two of three locations. Addition of atrazine to the 1 × and 2 × isoxaflutole plus flufenacet treatments showed a trend (P = 0.15) toward increased injury and reduced yield only for the G8366 on the Cisne soil. The 4 × rate of the premixture reduced yield 9 to 49% at two locations. Yields were reduced at the low-OM (2.1%) location, regardless of hybrid or application rate. At two locations, there was a negative linear relationship between soil pH and corn grain yield, particularly at the 4 × rate. Corn visual injury and yield reductions were greatest from herbicide applications to low-OM soils with high pH levels.

Field studies were conducted at 35 sites throughout the north-central United States in 1998 and 1999 to determine the effect of postemergence glyphosate application timing on weed control and grain yield in glyphosate-resistant corn. Glyphosate was applied at various timings based on the height of the most dominant weed species. Weed control and corn grain yields were considerably more variable when glyphosate was applied only once. The most effective and consistent season-long annual grass and broadleaf weed control occurred when a single glyphosate application was delayed until weeds were 15 cm or taller. Two glyphosate applications provided more consistent weed control when weeds were 10 cm tall or less and higher corn grain yields when weeds were 5 cm tall or less, compared with a single application. Weed control averaged at least 94 and 97% across all sites in 1998 and 1999, respectively, with two glyphosate applications but was occasionally less than 70% because of late emergence of annual grass and Amaranthus spp. or reduced control of Ipomoea spp. With a single application of glyphosate, corn grain yield was most often reduced when the application was delayed until weeds were 23 cm or taller. Averaged across all sites in 1998 and 1999, corn grain yields from a single glyphosate application at the 5-, 10-, 15-, 23-, and 30-cm timings were 93, 94, 93, 91, and 79% of the weed-free control, respectively. There was a significant effect of herbicide treatment on corn grain yield in 23 of the 35 sites when weed reinfestation was prevented with a second glyphosate application. When weed reinfestation was prevented, corn grain yield at the 5-, 10-, and 15-cm application timings was 101, 97, and 93% of the weed-free control, respectively, averaged across all sites. Results of this study suggested that the optimum timing for initial glyphosate application to avoid corn grain yield loss was when weeds were less than 10 cm in height, no more than 23 d after corn planting, and when corn growth was not more advanced than the V4 stage.

The classification of herbicides by site of action, published in 1997, has been revised. The classification system uses a numbering system for a herbicide's site of action, chemical family, and common name. Regulatory agencies in the United States and Canada have published labeling guidelines based on the classification to aid in herbicide resistance management.

Determination of the active growth state of weeds may lead to more reliable predictions of herbicide efficacy. Experiments were conducted at Lacombe and Lethbridge, Alberta, Canada, from 1996 to 1998 to determine if wild oat leaf extension (growth) rate could be used to predict the efficacy of imazamethabenz and ICIA 0604. As expected, wild oat control increased and wild oat biomass decreased with increasing imazamethabenz and ICIA 0604 rates. Mean wild oat growth rates ranged from 6 to 44 mm over a 24-h time interval. Wild oat control at 25% of the recommended doses of ICIA 0604 or imazamethabenz increased as wild oat growth rate increased. However, wild oat growth rate did not influence herbicide efficacy at higher herbicide rates. Regression analysis confirmed that wild oat control at the lowest application rates increased 6 or 14% for every 10 mm of wild oat growth during the 24 h preceding herbicide application of ICIA 0604 or imazamethabenz, respectively. Covariance analysis confirmed the influence of wild oat growth rate on wild oat control by imazamethabenz but not by ICIA 0604. Monocot leaf extension rates may be useful for predicting herbicide efficacy in integrated weed management or decision support systems.

The effect of CGA-362622 on annual grass control in cotton by clethodim and fluazifop-P and crop response was determined in field studies. CGA-362622 applied postemergence at 5.3 g ai/ha injured cotton up to 34% 1 wk after treatment (WAT). Injury was similar when crop oil concentrate (COC) and nonionic surfactant (NIS) were included. Adding fluazifop-P, but not clethodim, to CGA-362622 increased crop injury. Cotton recovered by 3 WAT. CGA-362622 mixed with fluazifop-P at 210 g ai/ha reduced broadleaf signalgrass and large crabgrass control 60 to 80%. Control was similar with COC or NIS in the mixture. Increasing the fluazifop-P rate to 315 g/ha did not improve control. CGA-362622 mixed with clethodim at 105 g ai/ha reduced control 65 to 80% with NIS and 40 to 65% with COC. Increasing the clethodim rate to 175 g/ha, especially with COC, substantially increased control. However, control was still less than that with clethodim at 105 g/ha. Prometryn plus MSMA postemergence-directed 3 wk after gramincide and CGA-362622 application increased control. Cotton yield was similar with clethodim alone and clethodim at 175 g/ha plus CGA-362622. Yield was reduced 29% by CGA-362622 mixed with fluazifop-P at 350 g/ha. Control was reduced by CGA-362622 applied 1 d before or after clethodim but not 5 d before or after clethodim. Control by fluazifop-P was reduced by CGA-362622 applied 1 or 5 d before or 1 d after the graminicide. Under greenhouse conditions, CGA-362622 and pyrithiobac mixed with clethodim, fluazifop-P, quizalofop-P, or sethoxydim reduced control of large crabgrass. Greater antagonism was noted with CGA-362622 than with pyrithiobac, and with fluazifop-P and quizalofop-P than with clethodim or sethoxydim.

Corn injury from isoxaflutole has been reported in sites across the corn belt, and variation in tolerance to isoxaflutole has been observed among corn hybrids. Greenhouse trials were conducted to study the variation and the inheritance of isoxaflutole tolerance in corn. Inbred lines from two families were treated with isoxaflutole applied preemergence, at four times the typical use rate, to study plant response. Plant height of corn inbreds treated with isoxaflutole compared with the untreated control ranged from 47 to 96% among eight lines within the B73 family and from 54 to 97% among six lines within the C103 family. One tolerant and one sensitive inbred line were selected from each family. The inbred lines were crossed in the field to produce four F1 hybrids. The four hybrids were treated with isoxaflutole in the greenhouse, and the tolerance of each hybrid was compared with the parent inbred lines. Data indicate that sensitivity to isoxaflutole is a heritable trait that is mediated by nuclear genes. Tolerance of corn hybrids to isoxaflutole can be predicted from the response of the parent inbreds.

Common ragweed is an annual weed problem after winter wheat harvest in southwest Michigan. Although an interseeded cover crop of red clover is known to reduce weed populations in winter wheat stubble, the most effective rates and cultivars for weed suppression under Michigan conditions have not been identified. Three red clover cultivars were planted in March at three seeding rates in established winter wheat; after wheat harvest, a section of each plot was mowed to mimic forage harvest of clover. The experiment was repeated in 2 yr. Mowing significantly reduced common ragweed biomass each year. All cultivars and seeding rates were equally effective at significantly reducing common ragweed biomass in each year, despite the variation among years, cultivars, seeding rates, and mowing treatments in production of clover biomass.

Persistence of microencapsulated (ME) and emulsifiable concentrate (EC) formulations of alachlor and acetochlor, as well as EC formulations of metolachlor, s-metolachlor, dimethenamid, and flufenacet were studied using a bioassay based on root response of oat Kassandra grown in soil. Flufenacet was the most persistent of the herbicides, but biologically available residues were not detected at 0- to 10-cm soil depth 50 d after any herbicide treatment. All herbicides applied preemergence (PRE) in field trials gave good to excellent control of redroot pigweed, black nightshade, barnyardgrass, and green foxtail but only partial control of jimsonweed. Furthermore, when they were applied postemergence (POST) in mixture with atrazine in the field, the control obtained for black nightshade, common lambsquarters, common purslane, and redroot pigweed was excellent, but good to excellent for bristly foxtail and green foxtail. Their efficacy against yellow foxtail and large crabgrass was fair to good. Generally, the EC-alachlor and EC-acetochlor applied POST in mixture with atrazine gave better control of grasses than did their ME-formulations. None of the herbicide treatments showed any phytotoxic effect on corn, and all of them produced corn yield greater than that of weedy control but slightly lower than that of the weed-free control. Flufenacet, s-metolachlor, and dimethenamid were suitable alternatives to acetochlor, alachlor, and metolachlor.

Imazethapyr-based weed control treatments were evaluated in water-seeded imidazolinone-resistant (IR) rice in Louisiana in 2000 and 2001. Treatments included imazethapyr applied to soil before seedling flood (surface [SURF] applied) or no soil application (no SURF), followed by imazethapyr applied postemergence (POST). Averaged over SURF application treatments (SURF, no SURF), all imazethapyr mixtures increased control of barnyardgrass at 14 and 42 d after POST treatment (DAT) compared with imazethapyr alone. Treatments containing imazethapyr-applied SURF followed by POST treatments controlled red rice 90 to 96% at 21 DAT. Alligatorweed and hemp sesbania control increased with POST mixtures compared with imazethapyr-only treatments at 35 DAT. Rice yield increased with imazethapyr applied SURF compared with no SURF application, averaged over POST applications. Other herbicides will be required to control troublesome weeds such as hemp sesbania and alligatorweed in an IR rice production system.

Herbicide-resistant cultivars may improve some aspects of weed management in wheat. However, negative consequences such as gene transfer among species, increased development of resistant weeds, or less effective volunteer wheat control may result from their use. Therefore, we reviewed literature on volunteer (self-sown) wheat seedling emergence and seed longevity in soil for insight in managing herbicide-resistant wheat. Data from classical burial studies suggested that wheat seeds were short-lived in soil, persisting less than 1 yr. Yet, in field studies, volunteer wheat seedlings were still emerging 16 mo after harvest; occasionally, seedlings have been observed 2 yr after harvest. Volunteer wheat emergence was extremely variable; causes of the variability are numerous and include genotypic, environmental, and production factors. This variability makes it difficult to predict volunteer wheat infestations in future years. Diverse cropping systems will enable producers to accrue the benefits of herbicide-resistant cultivars and yet still manage wheat volunteers, minimize gene flow by pollen, and avoid transfer of herbicide resistance. In regions where alternative crops are not viable, a key concern will be controlling volunteers and gene transfer in the next wheat crop.

Advances in biotechnology within the last decade have caused dramatic changes in agricultural production systems. In Mississippi, over 60% of the soybean hectarage was planted to glyphosate-resistant soybean in the 2001 production season. One advantage of glyphosate-resistant systems may be that fields with fewer weeds at harvest result in less foreign material and lower seed moisture due to less foreign material. Research was conducted to assess the foreign matter and moisture content of representative glyphosate-resistant and conventional soybean by evaluating elevator receipts collected from soybean producers in the southern and midwestern United States. A survey of growers using glyphosate-resistant and conventional soybean was conducted during the fall and winter of 2000. Copies of elevator receipts were collected to compare the foreign matter percentage and moisture content of soybean in both categories. A total of 16,535 ha were represented, of which 13,903 were from glyphosate-resistant soybean and 2,632 were from conventional soybean. Average foreign matter content from the glyphosate-resistant soybean was 1.9%, compared with 2.5% from the conventional soybean. Thus, the glyphosate-resistant program reduced foreign matter, an indication of reduced weed seed and trash contained in the sample or improved harvest efficiency. No difference was noted in seed moisture content between the two systems.

Experiments were conducted in Virginia in 2000 and 2001 to investigate responses of winter wheat and diclofop-sensitive and -resistant Italian ryegrass to the experimental herbicide mixture AE F130060 03 and crop safener AE F107892 applied postemergence. AE F130060 03 at 15 or 18 g ai/ha with or without methylated seed oil controlled both diclofop-sensitive and -resistant Italian ryegrass 82 to 99% and reduced inflorescence emergence 59 to 98%. Although AE F130060 03 controlled existing Italian ryegrass 4 wk after any application, timing of application influenced late-season Italian ryegrass control and inflorescence emergence. Applications to two- to three-leaf Italian ryegrass resulted in greater emergence of Italian ryegrass after application than applications made to two- to three-tiller or four- to five-tiller Italian ryegrass. Wheat injury by AE F130060 03 was greater than injury from diclofop, but wheat appeared to fully recover; and yields from AE F130060 03–treated wheat were similar to yields of diclofop-treated wheat and at least 21% greater than yields from nontreated wheat. In greenhouse experiments, differential growth responses between diclofop-sensitive and -resistant Italian ryegrass occurred after AE F130060 03 application at normal (15 g ai/ha) and below-normal application rates. When rates were increased beyond normal application rates, growth responses were similar between diclofop-sensitive and -resistant Italian ryegrass.

Field trials were conducted in Georgia and South Carolina in 2000 and 2001 to evaluate the effects of glyphosate, diflubenzuron, and boron combinations on crop injury and soybean yield. No interactions among glyphosate, diflubenzuron, and boron were observed. Both glyphosate and boron caused significant foliar injury (chlorosis and leaf burn) ranging from 7 to 10% in one of three locations. Diflubenzuron did not cause significant soybean injury at any location. Soybean injury symptoms observed in these experiments were transient and did not negatively affect yield.

Red rice has long been a troublesome, conspecific weed of cultivated rice. Rice varieties carrying certain herbicide-resistant traits acquired through genetic modification (herbicide-resistant varieties) now offer new options for red rice control. In concert with this innovation is the risk of gene flow, which can result in the transfer of that specific herbicide resistance to red rice and thus render this weed control measure ineffective. Gene flow in concept is simple, however, the parameters that determine the establishment of a new trait in a weed population are complex. Cross-pollination to make hybrid seed and the subsequent fate of those hybrid families in the general weed population are some of the biological factors that influence gene flow between red rice and cultivated rice. Natural outcrossing among rice plants is generally low. Most of the pollen dispersal studies published to date indicated that rice × rice outcrossing rates were less than 1.0%. Numerous reports summarized in this study suggest that outcrossing rates between rice and red rice can be highly variable but usually are similar to or lower than this level. However, once hybrids form, they may introgress into a red rice population within only a few generations. If hybrid seed families are to persist and establish herbicide-resistant red rice populations, they must successfully compete in the crop–weed complex. The ability to survive a herbicide applied to a herbicide-resistant rice variety would be a strong selective advantage for these hybrid families. Thus, the well-established principles of weed resistance management appear to be relevant for herbicide-resistant crop systems and should be used in combination with practices to minimize coincident flowering to mitigate the potential impact of gene flow from herbicide-resistant rice into red rice. For the rice–red rice crop–weed complex, there are both biological factors and agricultural practices that can work together to preserve these new weed control options.

Field studies were conducted to evaluate weed control, tuber yield, gross return, economic return on investment (EROI), and net return in glyphosate-resistant ‘Ranger Russet’ potato in 2000 and 2001 at the University of Idaho Aberdeen Research and Extension Center near Aberdeen, ID. Three types of weed control programs were evaluated: a total glyphosate program of single or sequential applications (TGLY), tank mixtures of glyphosate and residual herbicides applied early postemergence (GLY + RES EPOST), and residual preemergence herbicides followed by (fb) a late postemergence glyphosate application (RES PRE fb LPOST GLY). A standard rimsulfuron + metribuzin + nonionic surfactant EPOST treatment was included for comparison. The standard EPOST treatment and all glyphosate-containing treatments controlled hairy nightshade 88 to 99%. RES PRE fb LPOST GLY treatments improved hairy nightshade control compared with the RES PRE components applied alone. All herbicide treatments controlled kochia 87 to 99% and green foxtail 87 to 100%. Redroot pigweed and common lambsquarters were controlled ≥85 and ≥89%, respectively, by all herbicide treatments except a single EPOST application of glyphosate at 420 g ae/ha. Depending on the year, sequential applications of glyphosate, GLY + RES EPOST, or RES PRE fb GLY LPOST treatments controlled weeds better than single EPOST glyphosate applications. Single LPOST glyphosate applications generally controlled kochia, redroot pigweed, common lambsquarters, and green foxtail better than single EPOST applications. However, single EPOST glyphosate applications controlled hairy nightshade better than a single LPOST application of glyphosate at 420 g/ha. RES PRE fb GLY LPOST treatments improved redroot pigweed, common lambsquarters, and green foxtail control, compared with the RES PRE components applied alone, depending on the RES PRE component and the year. Sequential applications of glyphosate at 840 g ae/ha and the standard nonglyphosate EPOST, GLY + RES EPOST, and RES PRE fb GLY LPOST treatments generally provided similar weed control. No crop injury was observed as a result of any herbicide treatment. Sequential applications of glyphosate at 840 g/ha had better tuber yields and economic returns than a single EPOST or LPOST application of glyphosate at 420 g/ha or a single LPOST application of glyphosate at 840 g/ha. A single EPOST application of glyphosate at 420 g/ha had lower tuber yields and economic returns than a single EPOST application of glyphosate at 840 g/ha. The RES PRE alone treatments, except metribuzin + pendimethalin, had similar tuber yields, EROI, and net returns as sequential applications of glyphosate at 840 g/ha. Glyphosate + rimsulfuron resulted in lower tuber yields than sequential applications of glyphosate at 840 g/ha, whereas EROI and net returns were similar. All other combinations of glyphosate and residual herbicides except glyphosate + pendimethalin EPOST, had similar tuber yields, EROI, and net returns as sequential applications of glyphosate at 840 g/ha.

Interference between weeds and crops is a key topic in undergraduate weed science courses. A laboratory exercise was developed at Iowa State University to actively demonstrate how small-seeded weeds can compete with large-seeded crops despite the initial seedling size disadvantage. Spring wheat and wild mustard were grown in pots in monoculture and in competition with each other. One set of plants was harvested at 1 wk after planting and another at 6 wk after planting. Relative growth rates (RGR) were calculated for the 5-wk period using the classical approach of plant growth analysis. The results from four semesters were analyzed to determine whether the experiment was meeting its intended outcomes. It was successful in this regard. In each of the four semesters, wild mustard had a lower initial dry weight and a greater RGR than did wheat. Students were required to write a scientific paper using the experimental results after completing a series of active-learning exercises. Assessment by students suggested that the experiment, active-learning exercises, and writing assignment were valuable activities.

Two field experiments were conducted to evaluate the reduction of torpedograss canopy by multiple split applications of quinclorac applied postemergence (POST) to bermudagrass golf course roughs in Florida. In one experiment, quinclorac treatments were reapplied for a second year to the same plots, followed by biomass harvest, to evaluate the reduction of torpedograss rhizomes. Quinclorac sprayed at 1.68 kg/ha/yr visually reduced torpedograss canopy to a varying extent, depending on the number of split applications. The most effective treatment, 0.42 kg/ha quinclorac applied four times each year for 2 yr, reduced torpedograss canopy to 10% compared with 86% torpedograss canopy in the untreated plots, and reduced torpedograss dry weight to 1,570 kg/ha compared with 8,010 kg/ha in the untreated plots. After reapplication for 2 yr with the commercially labeled treatment, quinclorac at 0.84 kg/ha applied twice per year, torpedograss canopy was reduced to 45% and dry weight to 4,640 kg/ha. Visual evaluation of canopy was too optimistic in representing the herbicidal control of torpedograss by quinclorac because torpedograss regrew from rhizomes, and canopy was a relatively small part of the plant. In plots not chemically treated, pachymorph rhizomes were 63%, leptomorph rhizomes were 24%, and leaves were only 13% of the total dry weight of torpedograss.

“One Queen Artemisia, as old stories tell, When deprived of her husband she loved so well, In respect for the love and affection he show'd her, She reduc'd him to dust and she drank up the powder.”
Another on the said Occasion (lines 1 to 4), Robert Burns (1759 to 1796).

Biennial wormwood (Artemisia biennis Willd.) is an aggressive and prolific seed-producing plant of the Asteraceae. The genus Artemisia belongs to the Anthemidaeae, or mayweed, tribe and includes various annual, biennial, and perennial herbs and shrubs that are often aromatic and bitter (Hall and Clements 1923; McArthur and Welch 1986). There is little consensus about the exact number of species within the genus Artemisia (Stebbins 1974); estimates include “over 100” (Munz and Keck 1968), “nearly 300” (Kelsey and Shafizadeh 1979), “346” sensu stricto (Ling 1994), “about 400” (Greger 1982), and “more than 400” (Polyakov 1995) species.