Longspine sandbur is a troublesome weed infesting corn in the Great Plains. However, herbicides are now available to control this species. This study characterized longspine sandbur ecology in irrigated corn to aid producers in integrating herbicides into their production systems. Longspine sandbur began emerging May 25, and by June 15, 84% of the seasonal emergence had occurred. Plant development was related to cumulative growing degree days. Seeds were viable early in longspine sandbur's development, with 20% of seeds viable by heading. Producers can minimize seed production of longspine sandbur in field borders by mowing plants at the boot stage. Bur production per plant was related to time of emergence, with seedlings emerging in late May producing 1,120 burs per plant. Seedlings emerging 4 wk later produced 84% fewer burs. Controlling longspine sandbur before 4 wk of interference prevented loss of corn grain yield.

Wild carrot is becoming a serious weed problem in Michigan continuous no-tillage crop production. Greenhouse and field research was conducted to identify effective management strategies for wild carrot control in no-tillage cropping systems. In the greenhouse, PRE applications of acetochlor plus dichlormid (5.8:1 w:w), cyanazine, linuron plus chlorimuron (18:1), and metribuzin plus chlorimuron (10:1) and POST applications of bentazon, CGA-152005, clopyralid, cyanazine, and MON 12000 provided the greatest control of wild carrot seedlings. In the field, PRE and POST treatments containing chlorimuron consistently controlled overwintered wild carrot greater than 71% at 30 d after the POST application in no-tillage soybean. Atrazine, MON 12000, nicosulfuron, and primisulfuron applied POST consistently controlled overwintered wild carrot greater than 78% at 30 DAT in no-tillage corn. Glyphosate at 0.84 or 1.68 kg ae/ha applied in October to established wild carrot provided greater than 74% control the following spring. Early preplant (EPP) applications of glyphosate at 0.84 kg/ha in no-tillage soybean gave 95 and 24% control of overwintered wild carrot in St. Clair and Lenawee Counties, MI, respectively, at 70 DAT. PRE applications of glyphosate at 0.84 kg/ha controlled overwintered wild carrot less than 69% at 58 DAT in no-tillage corn and soybean. Fall applications of 2,4-D ester at 1.12 kg ae/ha provided 18 and 88% control of wild carrot the following spring in Clinton and Lenawee Counties, respectively. EPP applications of 2,4-D ester at 1.12 kg/ha provided 7 and 72% control of overwintered wild carrot in St. Clair and Lenawee Counties, respectively.

Seven field experiments were conducted in Oklahoma to compare efficacy and wheat response to currently registered cheat suppression or control herbicide treatments. Chlorsulfuron + metsulfuron premix (5:1 w/w) at 26 g ai/ha applied PRE controlled cheat 20 to 61%, increased wheat grain yields at two of seven locations, and decreased dockage due to cheat at five of seven locations. Chlorsulfuron + metsulfuron at 21 g/ha tank-mixed with metribuzin at 210 g/ha, applied early fall POST, controlled cheat 36 to 98% and increased wheat yield at four of seven locations. Metribuzin applied POST in the fall at 420 g/ha controlled cheat 56 to 98% and increased wheat yields at five of seven locations. Both POST treatments decreased dockage at all locations.

Five biotypes of common cocklebur that were not controlled with acetolactate synthase (ALS)-inhibiting herbicides were tested in greenhouse and laboratory studies to determine the magnitude of resistance and cross-resistance to four ALS-inhibiting herbicides. In vivo inhibition of ALS was also evaluated. Based on phytotoxicity, all five ALS-resistant biotypes of common cocklebur were > 390 times more resistant than the susceptible biotype to imazethapyr. However, only four of these biotypes were also resistant to another imidazolinone, imazaquin. Two biotypes were cross-resistant to the sulfonylurea, chlorimuron, and the triazolopyrimidine sulfonanilide, NAF-75. One biotype demonstrated intermediate susceptibility to imazaquin, chlorimuron, and NAF-75. In all cases, the resistance exhibited at the whole plant level was associated with an insensitive ALS.

Field studies were conducted at three sites in 1993 and 1994 to evaluate weed control, crop response, and economic returns to 0.5× and 1× postemergence rates of chlorimuron, chlorimuron plus quizalofop, bentazon plus acifluorfen, fluazifop plus fenoxaprop, imazethapyr, and imazethapyr plus clethodim in no-till narrow-row soybean production. Common lambsquarters, ivyleaf morningglory, and common ragweed were controlled equally with 0.5× rates applied early postemergence and 1× rates applied mid-postemergence. Control of giant foxtail, large crabgrass, and common cocklebur was generally greater with 1× rates mid-postemergence than with 0.5× rates early postemergence. Soybean yields were equivalent with 0.5× and 1× rates. Soybean yields and net income were highest with 1× and 0.5× rates of chlorimuron plus quizalofop and imazethapyr plus clethodim, and metolachlor preemergence (PRE) followed by 0.5× bentazon plus acifluorfen.

Diclofop at 840 g ai/ha, fenoxaprop at 90 g ai/ha, and imazamethabenz at 530 g ai/ha fall-applied controlled wild oat 96, 99, and 95% and increased wheat grain yields 26, 29, and 24%, respectively. These herbicides controlled wild oat over a wider range of growth stages than current labels indicate. The same treatments applied in March were less effective for wild oat control and did not increase wheat yield.

Additive series experiments were conducted under greenhouse conditions to determine the effect of season-long interference of different initial population densities of purple nutsedge on the shoot dry weight and fruit yield of tomato and bell pepper. Purple nutsedge densities up to 200 plants/m2 linearly reduced shoot dry weight at flowering and fruit yield of both crops as weed density increased. Both variables were directly correlated, and for each percentage unit of tomato shoot dry weight loss at flowering, fruit yield was reduced 1.24 units, whereas for bell pepper this relationship was 1 to 2.01. Total shoot and tuber biomass of purple nutsedge increased as density increased. The presence of either crop caused a decline in the total shoot dry weight accumulation of purple nutsedge, with tomato producing a higher degree of loss than bell pepper to the weed. Fruit yield losses due to purple nutsedge interference reached 44% for tomato and 32% for bell pepper.

Many upland pastures and forest meadows in the western United States contain significant infestations of yellow hawkweed and oxeye daisy. Documentation of infestations is necessary in order to plan and assess control tactics. Previous work with an airborne charge coupled device (CCD) with spectral filters indicated that flowering yellow hawkweed with at least 30% cover was detectable at 1 m resolution. A single image of a large area may not capture all plants in the flowering phase and multiple images are costly. The objective of this paper was to assess the accuracy of images recorded at different phenological stages. We compared three methods of classification: unsupervised classification of a three principal component analysis image, supervised classification of a three principal component analysis image, and supervised classification of a composited image consisting of four bands and normalized difference near infrared (NIR)/red band. Regardless of the classification method, images of yellow hawkweed and oxeye daisy in full bloom had lower classification error than at early bloom or post bloom. The percent error for yellow hawkweed classification was about twice as high at post bloom as at full bloom, but varied slightly depending on the method of classification and cover class. The ability to detect discrete colonies of yellow hawkweed was not affected by phenological stage, but the ability to measure the area of each cluster differed among stages. Less than one-third fo the pixels classified as yellow hawkweed or oxeye daisy in the early bloom image remained in the same class in the full bloom image. About half the pixels in the full bloom image remained in the 90 to 100% cover class at the post bloom image. Seasonal growth of the grasses masked some yellow hawkweed and oxeye daisy plants, and accounted for differences in classification among phenological stages.

Field studies were conducted from 1993 to 1995 to evaluate MON-12051 for turfgrass tolerance and control of yellow and purple nutsedges. The availability of herbicides for selective control of these weeds in turfgrass is limited. A sulfonylurea compound, MON-12051, has recently been developed for selective control of the nutsedges in turfgrass. When MON-12051 was applied at 0.07 to 0.14 kg ai/ha, the injury to Kentucky bluegrass, tall fescue, bermudagrass, and zoysiagrass was slight, with a maximum of 10% injury. At these rates, MON-12051 outperformed both bentazon and imazaquin in controlling yellow and purple nutsedges. Averaged over all tests 6 wk after treatment, yellow nutsedge control with MON-12051 was 83%. Control averaged 44% during the same period when treated with bentazon, whether applied once at 2.24 kg ai/ha or twice at 1.12 kg ai/ha. Purple nutsedge control averaged 96% when treated with MON-12051 in Kentucky bluegrass, while control was 42% with imazaquin applied at 0.19 and 0.43 kg ai/ha.

Field studies were conducted between 1990 and 1992 to determine tolerance of English pea at different growth stages to bentazon (0.56 kg ai/ha) and paraquat (0.14 kg ai/ha), applied alone and in combination. Initial crop injury with paraquat and paraquat plus bentazon averaged 37 and 21%, respectively. Crop injury dissipated by 21 d after treatment. Foliar injury at the different growth stages was equivalent but yields of peas treated at the six-node stage were reduced in two of four trials. Applying paraquat plus bentazon at the earlier growth stages consistently resulted in pea yields equivalent to those of plots receiving trifluralin plus handweeding or bentazon alone. Greenhouse studies were conducted to determine the effect of paraquat plus bentazon on control of selected weeds common to pea production fields. Redroot pigweed, common purslane, and common lambsquarters mortalities were 100, 100, and 90%, respectively, with the bentazon plus paraquat combination. However, barnyardgrass and large crabgrass mortalities decreased 88 and 91%, respectively, when bentazon was added, compared to paraquat alone.

Field experiments were conducted in Louisiana to evaluate the effectiveness of tillage, tillage plus herbicide, and herbicide-only fallow programs in controlling bermudagrass. Tillage of fallow fields three times at 3- to 4-wk intervals between row formation in late May and planting in late September or early October reduced bermudagrass infestations in the newly planted crop in December 44 to 51%. When the final tillage operation was replaced by glyphosate postemergence at either 2.24 or 3.36 kg ai/ha in late August, bermudagrass infestation level in December on the surface of the newly planted rows was reduced to below 20%. Two applications of glyphosate at 3.36 kg/ha were more effective than tillage only and as effective as tillage twice followed by glyphosate in reducing bermudagrass infestation levels in the newly planted crop. Application of atrazine at 3.36 kg ai/ha, imazapyr at 0.68 kg ai/ha, metribuzin at 1.68 kg ai/ha, or sulfometuron at 0.14 and 0.28 kg ai/ha to newly formed rows in late May of the fallow period did not reduce the need for tillage or an application of glyphosate during the fallow period to control bermudagrass, but imazapyr at 1.12 kg/ha did. Sugarcane shoot populations in December of the fallow year were similar for the treatment programs despite differences in bermudagrass infestation levels, which ranged from 11 to 94%. However, sugarcane stalk number and height and cane and sugar yield in the plant-cane crop the following year were highest when glyphosate was applied to bermudagrass as two applications either alone or following the application of atrazine or metribuzin, when tillage was followed with one application of glyphosate, and in 1 yr when imazapyr alone was applied to newly formed rows.

A 3-year study was conducted in irrigated potato to compare weed control efficacy and economics of hilling plus one or two cultivations with the standard treatment of hilling plus a preemergence application of pendimethalin plus metribuzin. Trials were conducted under both weedy and weed-free conditions. Common lambsquarters and redroot pigweed were present in weedy treatments each year; significant populations of hairy nightshade and green foxtail also were present one year. When weed populations were low or moderate (< 45 plants/m2), total weed biomass in the hilling plus one cultivation treatment was reduced 98 to 99% relative to the weedy control, and U.S. No. 1 tuber yields were equal to the standard treatment. However, when weed populations were high (145 plants/m2), hilling plus one cultivation and hilling plus two cultivations provided only 30% and 61% reduction in weed biomass, respectively, and U.S. No. 1 tuber yields were 35% and 13% less, respectively, than the standard treatment. The standard treatment provided 99% reduction in total weed biomass each year, and yields were equal to the weed-free, hill plus no cultivation control. Net return was $37 to $100/ha higher for the hilling plus one cultivation treatment compared to the standard treatment when weed populations were moderate or low, but was $808/ha less than the standard treatment when weed populations were high.

A plant-weighing procedure was used to measure the total mass of spray mixture intercepted by small whole corn and cotton plants. Mixtures of water and water plus crop oil concentrate or spreader–sticker were applied at spray volumes of 280 to 28,000 L/ha. The plants were weighed before and after passing under the spray and leaf areas, and shoot fresh and dry weights for each plant were measured. Spray deposition increased with spray volumes but not proportionally. Corn plants were larger than cotton plants and retained more spray per plant; however, cotton retained more spray per unit leaf area. The two adjuvants had similar effects on deposition, tending to increase it in corn and decrease it in cotton.

Field experiments were conducted from 1989 to 1992 in Balcarce, Tandil, and San Cayetano, all in Buenos Aires province (Argentina), to evaluate clethodim, fenoxaprop-P-ethyl, fluazifop-P, haloxyfop, propaquizafop, and quizalofop-P-ethyl for bermudagrass control in sunflower, soybean, and potato. The highest levels of bermudagrass control 60 days after treatment (DAT) were generally obtained with fenoxaprop-P-ethyl, haloxyfop, propaquizafop, and quizalofop-P-ethyl. Late season bermudagrass control, based on biomass of stolons and rhizomes, suggested that the most consistent treatments in potato and sunflower were haloxyfop at 180 g ai/ha and quizalofop-P-ethyl at 54 g ai/ha. In soybean, clethodim at 336 g ai/ha, haloxyfop at 180 g/ha, propaquizafop at 100 g ai/ha, and quizalofop-P-ethyl at 54 g/ha provided the greatest reductions in bermudagrass biomass. All of the postemergence (POST) graminicide treatments increased crop yields when compared to a nontreated check. However, little or no difference in yield was obtained when herbicide rate was increased.

Field experiments were conducted at Kalispell, MT, to determine the optimum adjuvant combination for wild oat control in peppermint with quizalofop. Quizalofop was applied to four- and eight-leaf wild oat plants at 20 and 50 g ai/ha with either a nonionic surfactant (NIS) or methylated seed oil (MSO) alone or in combination with 28% urea ammonium nitrate (UAN) liquid fertilizer. Differences among adjuvants were most apparent when quizalofop was applied at the lowest rate. MSO was more effective than NIS for enhancing quizalofop activity. Quizalofop efficacy with both adjuvants increased when applied with UAN. Greater than 90% wild oat control was obtained with the lowest rate when applied with MSO plus UAN to four-leaf wild oat plants. These results demonstrate the potential to improve the consistency of weed control as well as reduce postemergence herbicide rates when applied with the proper adjuvant combination.

Quinclorac is a systemic herbicide registered to control annual grass and broadleaf weeds in rice. Field research was conducted to determine the most effective quinclorac treatment for leafy spurge control. Leafy spurge control 12 mo after treatment (MAT) averaged 38 and 77% when quinclorac was applied at 0.56 and 1.7 kg ae/ha, respectively. Quinclorac applied with the crop-oil-based adjuvant Scoil provided similar leafy spurge control compared to quinclorac applied alone. Quinclorac plus Scoil applied with picloram provided better leafy spurge control than quinclorac plus Scoil alone. The optimum date for fall treatment with quinclorac is broad. Quinclorac at 1.1 kg/ha plus Scoil at 2.3 L/ha applied on September 1, September 15, October 1, and October 15 provided similar leafy spurge control and averaged 56% 12 MAT.

Greenhouse and field experiments were conducted to assess the effectiveness of herbicide tank mixtures for control of sulfonylurea (SU)-resistant and -susceptible kochia biotypes. Kochia control at three growth stages was evaluated for fluroxypyr, dicamba, 2,4-D, bromoxynil, and bromoxynil plus MCPA. Control decreased as kochia size increased, especially at less than full labeled herbicide rates. Treatments containing bromoxynil provided best overall control in the greenhouse but were less effective on larger kochia plants in the field. Dicamba and fluroxypyr provided similar, effective kochia control both in greenhouse and field experiments. 2,4-D did not provide acceptable kochia control regardless of formulation, rate, or application timing. All kochia biotypes responded similarly to non-SU herbicides. In the field, inclusion of an SU herbicide in the treatment generally resulted in no increased R-kochia control above that provided by the non-SU herbicide. R-kochia population dynamics can affect selection of alternative control strategies.

Field experiments were conducted at Jay and Marianna, FL in 1988 and 1989 to determine the effects of sicklepod, Florida beggarweed, and common cocklebur density on chlorothalonil deposition to peanut foliage, peanut foliar disease incidence, and peanut yield. At a density of four weed plants per 8 m of row, Florida beggarweed and sicklepod reduced chlorothalonil deposition on peanut foliage by 20%, while common cocklebur reduced fungicide deposition by 34%. At the same density, incidence of the foliar diseases early leaf spot and late leaf spot increased 10% with Florida beggarweed, 14% with sicklepod, and 20% with common cocklebur compared with weed-free peanut. The predicted peanut yield loss from a weed density of four plants per 8 m was 16 to 19% for Florida beggarweed, 23 to 25% for sicklepod, and 31 to 39% for common cocklebur. Weed biomass increased with increasing weed density.

A field study was conducted on tolerance of four seashore paspalum cultivars to postemergence (POST) herbicides in Georgia during 1995 and 1996. Quinclorac (0.8 kg/ha) and MON 12000 (0.07 kg/ha) at recommended rates were the only POST herbicides that did not affect the performance of any seashore paspalum cultivar (≥ 89%). When the rates were increased to three times recommended, the quality of ‘AP 10’ (≥ 96%) and ‘PI 28960’ (93%) cultivars was not affected, but the quality of ‘HI 25’ (≥ 84%) and ‘K-7’ (77%) was lower when compared with respective untreated plots. HI 25 recovered within 4 wk, but K-7 required 6 wk or longer. Dicamba at 1.7 kg/ha affected the quality of K-7 (69%) more at 2 wk than the other cultivars (85 to 92%). Diclofop, imazaquin, and 2,4-D plus mecoprop plus dicamba significantly reduced the quality of all cultivars 2 wk after treatment (WAT). The quality of all cultivars was similar at 2 WAT with diclofop at 1.1 kg/ha (79 to 84%). However, when treated with imazaquin at 0.4 kg/ha, the quality of K-7 (65%) was lower than AP 10 (85%), HI 25 (77%), and PI 28960 (81%), and when treated with 2,4-D plus mecoprop plus dicamba at 1.1 + 0.6 + 0.1 kg/ha, the quality of AP 10 (75%), PI 28960 (72%), and K-7 (57%) was lower than HI 25 (87%). POST applications of quinclorac, dicamba, and MON 12000 were safe to use for weed control in paspalum cultivars, whereas the safety of diclofop and imazaquin was marginal, and 2,4-D plus mecoprop plus dicamba was unacceptable.

The objective of this research was to evaluate the influence of different initial weed densities on weed control effectiveness in no-tillage corn at two locations in North Carolina during 1994 and 1995. Different weed densities were established over a 4-year period (1989–1992) by using various weed management strategies. Resultant density levels were estimated and used to establish high and low weed density plots. Treatments applied were PRE, POST, at-planting, and an untreated control. Weed density estimates were made 37 and 57 DAP. Common lambsquarters at low densities (20 to 40 weeds/m2) was controlled with the cover crop alone, but common lambsquarters at high densities (150 to 170 weeds/m2) and redroot pigweed at any density were not controlled. POST herbicides reduced weed densities as well as the PRE herbicides, regardless of initial weed densities.