The outstanding success of 2,4-D as a selective herbicide stems from the great susceptibility of many broadleaved (dicotyledonous) weeds to the compound. Applications of 2,4-D which will eliminate many dicots will leave most grass crops relatively unaffected. Several suggestions that have been made for the differences in susceptibility between grasses and broadleaved weeds, including differences in foliar absorption, restricted translocation of the compound in grasses, and differences in detoxification of the 2,4-D molecule.

The relative competitive ability of common lambsquarters and giant foxtail in mixed weed–corn communities was characterized in 1998 and 1999 using empirical models that described late-season weed biomass on the basis of weed density, early-season relative leaf area, or early-season relative shoot volume. Competition coefficients estimated from weed density were inconsistent between years because they indicated that giant foxtail was more competitive than common lambsquarters in 1998 but that common lambsquarters was more competitive than giant foxtail in 1999. In contrast, the competition coefficients based on relative leaf area and relative volume were consistent between years. Competition coefficients estimated from relative leaf area indicated that giant foxtail was more competitive than common lambsquarters in each year. Competition coefficients estimated from weed relative volume indicated that the relative competitive ability of each weed species was similar in each year. Weed relative competitive abilities were characterized further by describing the mechanisms of competition related to shoot height and width growth. Giant foxtail was taller than common lambsquarters shortly after emergence each year, but plasticity of common lambsquarters growth resulted in reduced height differential between the weed species over time. Even so, giant foxtail was taller than common lambsquarters at physiological maturity each year. Coefficients that described the ability of each weed species to crowd neighbors indicated that giant foxtail shoot width was affected more by increased common lambsquarters density and proportion than was common lambsquarters shoot width by giant foxtail. The greater ability of common lambsquarters to crowd neighbors relative to giant foxtail was attributed to the greater leaf area density (LAD) of common lambsquarters compared with that of giant foxtail. Although characterization of shoot height, width, LAD, and biomass elucidated in part the mechanisms of competition between these species, models that accounted for differences in early-season relative plant size were consistent between years, indicating that giant foxtail was equally or more competitive than common lambsquarters in corn.

Alachlor [2-chloro-2′,6′-diethyl-N(methoxymethyl)-acetanilide], propachlor (2-chloro-N-isopropylacetanilide) and prynachlor [2-chloro-N-(1-methyl-2-propynyl)acetanilide], inhibited gibberellic acid (GA3),-induced production of protease and α-amylase in deembryonated barley (Hordeum vulgare L, ‘Schuyler’) seed. Production of protease was more sensitive to these herbicides than α-amylase. The degree of inhibition of protease and α-amylase production caused by alachlor was equivalent to that caused by cycloheximide, puromycin and actinomycin-D, known protein and nucleic acid synthesis inhibitors. Higher GA3 concentrations reversed the inhibition of protease and α-amylase synthesis caused by alachlor but did not reduce the effect of alachlor on barley seed germination and growth. It is concluded that the inhibition of protease and α-amylase production by alachlor is perhaps only one of several effects on early seed germination and seedling development.

Understanding patterns of weed seedling emergence within a growing season and over years is important to develop models to predict optimum timing of weed management practices. A study was conducted in a field with no previous velvetleaf infestations to describe emergence patterns following seed burial at three depths in two tillage systems. Freshly harvested velvetleaf seeds were planted 0, 2, and 6 cm deep in moldboard plowed (MP) and no-tillage (NT) corn stubble in October 1990. Velvetleaf seedling emergence was monitored over the following 4 yr in continuous corn. Emergence was higher in NT than in MP plots throughout 4 yr of observation. The first growing season following seeding, emerged seedlings represented 9.3 to 15.8% of the seeds sown in NT, compared with 0.1 to 0.8% of seeds sown in MP. After four growing seasons, emerged seedlings were 12.5 to 25% of seeds sown in NT but only 6 to 7.4 % of seeds sown in MP. Emergence was consistently higher from the 0-cm depth than from the 6-cm depth in NT, but seeding depth did not influence emergence in MP due to mixing of the soil during tillage. Velvetleaf emergence was related to growing degree days (base 7.5 C), with greater consistency in NT than in MP. Averaged over years and planting depths, 50% velvetleaf emergence occurred within 8 and 13 d of the predicted date in NT and MP systems, respectively.

A cooperative study to evaluate the accuracy and precision of bioassays was carried out among investigators at several locations for 3 years. Oats (Avena sativa L., var. Cimarron) seedlings were used as indicator plants for soil-applied 2,4-bis-(isopropylamino)-6-methylthio-s-triazine [prometryne]. Assay results varied a great deal initially, and ranged from 147% below to 234% above the prometryne actually applied. The adoption of uniform conditions and procedures greatly increased the uniformity of the determinations between locations to a range of from 37% low to 0% high in prometryne applied. Chemical analyses of the soils ranged from 13% low to 20% high. Measurements of herbicidal activity included dry and fresh plant weights, plant height, visual injury ratings, and plant water use. The average concentration of prometryne in the soil as estimated with several types of measurements gave a much better measure of prometryne content than any one measurement alone. Prometryne rapidly and greatly affected the utilization of water by oats seedlings and careful measurement of water use showed potential of being a good early indicator of prometryne activity.

Seedlings of 19 plant species were grown in soils containing 0 to 1,000 ppm of O,O-diisopropyl phosphorodithioate S-ester with N-(2-mercaptoethyl)benzenesulfonamide (bensulide) or 0 to 100 ppm of a,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) to determine phytotoxicity. Standard growth curves were constructed. The concentration of bensulide required to reduce the growth of shoots 50% (GR50) ranged from 1 ppm with barnyardgrass (Echinochloa crusgalli (L.) Beauv.) to more than 1,000 ppm with carrot (Daucus carota L., var. Long Imperator). Several plant species were more sensitive to bensulide in a Hidalgo sandy loam (pH 8.1) than in other soils including a Delfina loamy fine sand (pH 6.0). The concentration of trifluralin required to reduce the growth of shoots 50% (GR50) was not markedly affected by soil texture and ranged from 0.1 ppm with German millet (Setaria italica (L.) Beauv.) to more than 100 ppm with Long Imperator carrot. The standard growth curves have been used to quantify herbicidal activities in field soils.

In the Midwestern United States, winter hilling, consisting of two tillage activities per year, is required in vinifera-grape vineyards for winter protection. However, this practice often leads to severe soil erosion and pesticide offsite movement. The effectiveness of wheat straw mulch as a replacement for soil mounding was investigated as a way of providing winter protection and to mitigate pesticide leaching and runoff. A laboratory experiment was conducted where simazine was applied to wheat straw or bare soil and then followed by simulated rainfalls. When compared with bare soil, straw reduced simazine leaching and runoff by 40 and 68%, respectively. Adsorption or interception, or both, of simazine by straw were responsible for this effect. Additionally, straw reduced soil erosion by 95% and would largely reduce simazine runoff associated with sediment displacement. The first simulated rainfall contributed 70 and 34% of total simazine runoff from bare soil and straw, respectively. In conclusion, mulching with straw during winter months to provide winter protection could be an effective practice for controlling simazine offsite movement and soil erosion in vinifera vineyards.

Imazamox is an imidazolinone herbicide used to control many grasses and broadleaf weeds in leguminous crops such as soybean, alfalfa, and dry bean; however, imazamox cannot be used on red lentil due to unacceptable injury. Studies were conducted to compare imazamox absorption, translocation, and metabolism in red lentil and dry bean to determine if any or all of these factors contributed to differential crop sensitivity. Radiolabeled imazamox was applied to three young red lentil leaves and the youngest, fully expanded dry bean trifoliolate leaf. Absorption, translocation, and metabolism were followed over a 96-h time course. Red lentil had more rapid absorption compared to dry beans with 64 and 54% of the applied dose absorbed 12 h after treatment (HAT), respectively. Maximum absorption was also greater in red lentil than dry bean, 79 and 61%, respectively. Translocation out of the treated leaf was significantly higher in red lentil compared with dry bean, 16 and 0.5%, respectively, at 96 HAT. Translocation was greater to red lentil roots compared to shoots, 9 and 7%, respectively, at 96 HAT. In dry bean only 14% of applied 14C-imazamox remained intact 24 HAT, while 79% of the radioactivity was imazamox in red lentil 24 HAT. Both species metabolized the herbicide to more polar metabolites. The inherent sensitivity of aceolactate synthase (ALS) from dry bean and red lentil was also evaluated. ALS from both species had similar I50 values for imazamox, 7.2 and 8.2 µM, respectively. The combined effects of increased imazamox absorption and reduced imazamox metabolism are the major contributors to differential selectivity between dry bean and red lentil. Rapid imazamox metabolism in dry bean significantly limited herbicide translocation out of the treated leaf.

The herbicidal action of sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} on corn (Zea mays L. ‘Goldencrossbantam’) was investigated in preemergence and postemergence experiments and hydroponic culture. Soil-applied sethoxydim did not inhibit corn germination. Leaves failed to emerge through the coleoptile with a high herbicide rate (1.6 kg ai/ha), but chlorotic leaves emerged at a lower rate (0.8 kg ai/ha). Growth of corn seedlings treated with a foliar application of 0.2 kg ai/ha was inhibited within 1 day after treatment, but at a lower application rate (0.05 kg ai/ha) growth continued with chlorotic zones on newly expanding leaves. Over the concentration range 3 × 10-5 to 1 × 10-6 M, sethoxydim inhibited the growth of primary roots of corn in hydroponic culture within 24 h. Cytological investigation showed that sethoxydim inhibited cell division but did not interfere with mitosis.

Asiatic dayflower has recently become a troublesome weed in eastern Iowa. This weed demonstrates an extended emergence period and there is anecdotal evidence of glyphosate tolerance. Thus, Asiatic dayflower is difficult to manage in glyphosate-resistant (GR) corn and soybean. Greenhouse experiments were conducted to evaluate the response of Asiatic dayflower to glyphosate applied at different rates and growth stages. Field research was conducted in 2005 and 2006 to evaluate different herbicides for Asiatic dayflower control in soybean. PRE herbicides were applied at planting and POST herbicides were applied 21 and 42 d after planting (DAP). In addition, shikimate accumulation in response to glyphosate was compared among Asiatic dayflower and GR and non-GR corn and soybean. Under greenhouse conditions, a single application of glyphosate (0.84 kg ae ha−1) did not control Asiatic dayflower. Only the highest rate evaluated, 13.44 kg ae ha−1 (16X), was lethal to Asiatic dayflower. Even when applied at an early growth stage (two leaves) and using high rates (3.36 kg ae ha−1), glyphosate controlled Asiatic dayflower just 28%. In the field, metribuzin and KIH-485 controlled Asiatic dayflower 80 and 73%, respectively. Early POST applications (21 DAP) of cloransulam or lactofen controlled Asiatic dayflower 80 and 67%, respectively. A single glyphosate application of 0.86 kg ae ha−1 controlled Asiatic dayflower approximately 50%. Glyphosate-treated Asiatic dayflower and non-GR corn and soybeans accumulated shikimate after application. GR corn and soybeans did not accumulate shikimate in response to glyphosate. Twenty-one days after treatment, all the non-GR soybean and corn plants died; however, Asiatic dayflower plants survived.

Field experiments were conducted in 1999 and 2000 to determine if (1) seed predation of redroot pigweed plants occurred in agricultural fields, (2) corn-cropping patterns could be manipulated to influence the quantity of weed seed predation, and (3) alterations in corn canopies affected the microenvironment, possibly influencing predator populations. Corn was planted in standard (75 cm) or narrow (37.5 cm) rows with corn population densities ranging from low to very high (2.5 to 10 plants m−2). The extent of seed predation occurring on terminal weed inflorescences in the treatments was evaluated. Predation levels of redroot pigweed were highly variable spatially and temporally. Coleophora lineapulvella Chambers (Lepidoptera: Coleophoridae) was the dominant predator of redroot pigweed seed. Seed predation was higher in 2000 than in 1999 (P < 0.05). On average, C. lineapulvella larvae attacked 11% of the inflorescences in 2000 and 3% of inflorescences in 1999. The proportion of damaged seeds per attacked inflorescence was as high as 93% in 2000 but only 42% in 1999. Row spacing and corn density did not affect levels of weed seed predation (P > 0.05). But canopies of closely spaced corn increased shading to redroot pigweed plants growing below the canopy, consequently decreasing total weed biomass and seed production.

Perennial pepperweed (Lepidium latifolium L. # LEPLA) is a serious weed of meadows and riparian habitats in the intermountain area of the western United States. Germination of seeds of this species was tested at 55 constant- and alternating-temperature regimes. No significant differences were found in mean germination at a range of temperatures varying from 0 to 40 C for seedlots collected at the same site in different years or at different locations in the same year. Optimum germination not significantly (P = 0.01) lower than the maximum observed occurred most frequently at 5/40 C (5 C for 16 h and 40 C for 8 h), and 10/40 and 15/40 C. Germination was enhanced by alternating regimes with low night temperatures (0, 2, or 5 C) and high day temperatures (35 and 40 C), in comparison with more moderate constant-temperature regimes.

Field experiments were conducted to determine the effectiveness of the herbicide antidote R-25788 (N,N-diallyl-2,2-dichloroacetamide) in protecting corn (Zea mays L.) cultivars from EPTC (S-ethyl dipropylthiocarbamate) injury. Preplant-incorporated applications of EPTC, with and without R-25788, were made to a fine sandy loam in 1976 and a loamy sand soil in 1977. EPTC at 6.7 kg/ha was 20 to 30% more injurious to the susceptible cultivars grown in sandy, lower organic-matter soil than in fine sandy loam. R-25788 did not decrease the weed-control efficacy of EPTC. The cultivar ‘TXS114’ was highly tolerant to EPTC at 6.7 kg/ha with or without antidote protection. Most of the other EPTC-susceptible cultivars showed significant stand improvement when R-25788 was used in combination with EPTC. R-25788 did not alleviate EPTC phytotoxicity toward ‘XL55’ and ‘XL379’ cultivars. EPTC applied in combination with R-25788 showed no, or only, slight injury to all other cultivars.

The effect of timing of preemergence application of N,N-dimethyl-2,2-diphenylacetamide (diphenamid) on phytotoxicity to pepper (Capsicum annuum L.), efficacy of weed control, and damping-off incidence was studied in greenhouse and field experiments. Delay of application from day of sowing to close to emergence reduced phytotoxicity to pepper while only partially reducing herbicidal action. Addition of a mixture of the contact nonresidual herbicides 6,7-dihydrodipyrido[1,2-a:2′,1′-c]pyrazinediium ion (diquat) and 1,1′-dimethyl-4-4′-bipyridinium ion (paraquat) to a late preemergence application of diphenamid resulted in control of weed seedlings which emerged after sowing of pepper and significantly increased the yield. The increase of Rhizoctonia damping-off incidence due to diphenamid was also reduced by delayed application.

Greenhouse and field studies showed that α,α,α,-trifluro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) gave better control of wild cane [Sorghum bicolor (L.) Moench] when incorporated in the soil above rather than below the wild cane seed. Of four herbicides studied in field experiments, trifluralin gave the best control of wild cane in soybeans (Glycine max L., var. Clark) followed by S-propyl dipropylthiocarbamate (vernolate). Herbicides were more effective than cultivation in controlling wild cane, but neither alone was adequate. A combination of cultivation plus herbicides markedly increased soybean yields on wild cane infested areas. These results show that an economical crop of soybeans can be produced on wild cane infested land.

In crop/weed interactions a method is needed to reduce the complex data derived from interference research to a simple form which can be utilized immediately. A response surface technique was utilized in analyses involving densities, duration of interference, interference abilities of different species, and weed growth rates. At high weed densities interfering for long durations, multispecies interference is not additive but at low densities additiveness is approached. Soybean [Glycine max (L.) Merr.] yield loss from multispecies weed interference was calculated by subtracting the percent yield loss from the most competitive weed and then repeating the same procedure for remaining weeds. Interference data for weed species in which fewer data were available were determined by linear interpolation for densities of more comprehensively researched, closely related species. Polynominal regression curves were utilized to predict weed age and future weed size from weed height. These analyses can produce information to assist soybean producers in recognizing economically detrimental threshold levels of weed infestations which require the initiation of control measures.

Field experiments were conducted at Frederick, MD, in 1985 and 1986 to determine effects of weed-interference periods and insects on bell peppers. Weed-interference periods of approximately 40 and 60 d reduced both bell pepper fruit number and weight 10 and 50%, respectively. Bell pepper foliage weight was reduced 10 and 50% with approximately 20- and 50-day weed-interference periods. In 1985 and 1986, insect populations were low with an average of 10 and 3% of the pepper fruit infested, respectively. Most infested fruit was damaged by European corn borer. No significant differences in insect infestation of fruit as related to time of weed-interference periods were noted.

Annual herbicide treatments are usually necessary for acceptable control of grassy weeds in turfgrasses and it is important that repeated treatment do not injure the turf. Seven herbicides were applied annually for 4 yr at recommended and three times recommended rates to ‘Emerald’, ‘Meyer’, and ‘Matrella’ zoysia (Zoysia spp.) and three herbicides were applied annually for 3 yr at similar rates to Tifway′, ‘Tifgreen’, ‘Tifdwarf’ and ‘Ormond’ bermudagrass [Cynodon dactylon (L.) Pers.]. Herbicide treatments delayed early spring green-up of Emerald and Matrella slightly more than Meyer zoysia when compared with respective untreated checks. Four annual treatments of napropamide [2-(α-naphthoxy)-N,N-diethylpropionamide] and profluralin [N-(cyclopropylmethyl)-a,a,a-trifluoro-2, 6-dinitro-N-propyl-p-toluidine] reduced the cover of Meyer more than Emerald or Matrella when final ratings were made. The 10.1 kg/ha rate of prosulfalin {N-[[4-(dipropylamino)-3,5-dinitrophenyl] sulfonyl]-S,S-dimethylsulfilimine} and benefin (N-butyl-N-ethyl-α,α,α-trifluoro-2,6-dinitro-p-toluidine) also reduced the cover of Meyer. Profluralin at 10.1 kg/ha was the only treatment that reduced the stand of Matrella and none of the treatments reduced the stand of Emerald. The 10.1 kg/ha treatments of profluralin and prosulfalin reduced the stand of Tifgreen, Tifdwarf, and Ormond more than Tifway bermudagrass. None of the herbicide treatments reduced the stand of Tifway.

Mefluidide {N-[2,4-dimethyl-5-[[(trifluoromethyl) sulfonyl]amino] phenyl] acetamide} at concentrations of 2.9 × 10-4 M and above retarded the growth of corn (Zea mays L. ‘Pioneer 3369A’) grown in nutrient culture. Dwarfing was accompanied by some plant distortion and chlorosis of the bases of those leaves formed soon after treatment. The chlorosis was associated with decreased chlorophyll a content. Chlorophyll b content was not affected. The growth-retardant effects of mefluidide could not be reversed by applications of gibberellic acid. Using cucumber (Cucumis sativus L. ‘Ashley’) cotyledon discs, a low level of mefluidide (2.9 × 10-5 M) increased the incorporation of 14C from leucine into protein, while a high concentration (2.9 × 10-4 M) reduced incorporation. Neither concentration affected the absorption of 14C-leucine.

A field study was established in southern Iowa in 1994 to study seasonal and long-term weed population dynamics on land being brought back into production after 8 yr as part of the conservation reserve program (CRP). The study was a split-plot design with four replications; two tillage regimes, two crop rotations, and three herbicide application methods were used. Even though the tillage regime did not influence individual weed population density throughout the study, the no-till (NT) regime had more weeds compared to conventional tillage (CT). However, when weeds were grouped into categories, tillage influenced broadleaf weeds in 1994 and 1996 and total weeds in 1995. Plots under the NT regime had an average of 46 broadleaf weeds m−2 compared to 27 in CT in 1994, with Amaranthus rudis Sauer (common waterhemp) being the most prevalent. NT had a total of 186 weeds m−2 compared to 125 m−2 weeds in CT in 1995; however, in 1996, CT plots had 184 weeds m−2 compared to 121 m−2 in the NT regime. Except for broadleaf weeds in 1994, crop rotation did not influence the number of weeds, and herbicide application methods had the greatest effect on weed populations. Overall, weed populations were greater in 1997, 1996, and 1995 than in 1994 for all herbicide application methods. The no-herbicide treatment had the highest number of weeds throughout the study. The total number of weeds in band and broadcast treatments averaged 41 and 26 m−2 in 1994; 96 and 24 m−2 in 1995; 96 and 12 m−2 in 1996; and 109 and 95 m−2 in 1997. The use of broadcast herbicides in NT should be recommended for land coming out of CRP. Regardless of the herbicide application method or crop rotation, CT plots had better yields for both Zea mays L. (corn) and Glycine max L. (soybean). Glycine max had a better stand compared to Z. mays in the first year, indicating that a rotation starting with G. max might be preferred in the land coming out of CRP.