Experiments were conducted to determine the influence of nitrogen (N) fertility and winter wheat cultivars on weed infestations in a winter wheat-ecofallow sorghum-fallow rotation near North Platte, NE. Centurk 78 and Lancota winter wheat suppressed density and growth of barnyardgrass and green foxtail significantly more than Eagle winter wheat before and after wheat harvest. Increasing N rates applied to winter wheat decreased annual grass weed population and weed yields. However, 67 and 101 kg N ha−1 reduced winter wheat grain yields compared to 34 kg N ha−1. Plots treated at 2.8 plus 0.3 kg ai ha−1 of atrazine plus paraquat 31 d after wheat harvest had more barnyardgrass before grain sorghum planting in 1983 than plots treated 17 d after wheat harvest but the reverse was true for green foxtail after grain sorghum emergence in 1984. Increasing N rates from 34 kg ha−1 to 67 and 101 kg ha−1 in the previous wheat crop decreased weed density before and after grain sorghum planting. There was no advantage in weed control in the grain sorghum from applying N to winter wheat in the fall vs. spring.

Field experiments were conducted in 1996 and 1997 to evaluate the efficacy of combined cultural and biological weed control for management of Canada thistle in conservation tillage soybean production. For cultural control, we used a highly weed-competitive soybean variety (cv. ‘Kato’). The biological control agent was the phytopathogenic bacterium Pseudomonas syringae pv. tagetis (PST). The application of PST reduced Canada thistle survivorship, height growth, and seed production, although these reductions were usually less than those resulting from bentazon application. Only bentazon application resulted in significant reduction of biomass of Canada thistle plants that survived all season. These results suggest the value of PST for management of Canada thistle in conservation tillage systems due to its negative effects on survival, growth, and reproduction. However, the weed-competitive soybean variety did not affect Canada thistle performance differently than a less competitive variety used for comparison, and there was no indication of synergy between the effects of the two control methods.

The leaching of α,α,α-trifluoro-2,6-dinitro-N,N,-dipropyl-p-toluidine (trifluralin) and 3,5-dinitro-N4,N4-dipropylsulfanilamide (oryzalin) with and without surfactants was studied in soil columns. In dry soil without surfactants leaching of trifluralin was less than that of oryzalin and did not amount to more than a few centimeters despite the use of light soil and large amounts of herbicide. In wet soil trifluralin leached more than twice as deep as in dry soil with the same amount of water. The addition of surfactants at a concentration of 2% of the spray volume increased both depth of water penetration and herbicide movement into the soil. The most pronounced effect of surfactants was the increase in movement of trifluralin in dry soil, but they also increased trifluralin movement in wet soil. There was no direct relationship between the ability of the surfactants to improve water penetration and their effects on leaching of the two herbicides. It was therefore concluded that increased leaching was due to an effect of surfactants on the sorption-desorption balance rather than on improved penetration of water. The improved movement of trifluralin into the soil as a result of surfactant action was also evident from another experiment in which surfactants increased the herbicidal activity of trifluralin sprayed preemergence on the soil surface.

Field studies were conducted from 1995 to 1997 near Tifton, GA, to determine the benefits of stale seedbed weed control in cucumber. Three stale seedbed management systems—(1) power till stale seedbeds twice (2 ×), (2) glyphosate application immediately after planting, and (3) combination system of stale seedbeds power tilled once 2 wk prior to planting followed by glyphosate application immediately after planting cucumber—were evaluated as main plots. Subplots were weed management systems after planting cucumber: intensive, basic, and cultivation alone. Weed densities were generally greater in 1996 and 1997 than in 1995. Yellow nutsedge was the overall predominant species in 1995 (46 plants m−2), with Florida pusley being the predominant species in 1996 and 1997, at 80 and 124 plants m−2, respectively. Generally, stale seedbeds shallow tilled 2 × had fewer weeds and greater cucumber yields than stale seedbeds treated with glyphosate. Glyphosate did not adequately control emerged Florida pusley on stale seedbeds, resulting in reduced cucumber yield. Clomazone preemergence and bentazon/halosulfuron postemergence were used for broadleaf weed control in the intensive weed management system. These herbicides injured cucumber plants, delayed maturity, and reduced yield. Based on our results, stale seedbeds shallow tilled 2 × can be integrated into cucumber production and provide effective cultural weed control. Furthermore, these systems will replace the need for potentially injurious herbicides.

The effect of quackgrass [Agropyron repens (L.) Beauv.] density and duration of interference on soybeans [Glycine max (L.) Merr.] was investigated in two studies. In the density study, the average reduction in soybean yield by quackgrass densities of 520 and 910 shoots/m2 was 19 and 55%, respectively. Quackgrass densities of 95 and 160 shoots/m2 did not significantly reduce soybean yields. The number of branches that produced pods, the number of pods and seeds per plant, and soybean height were reduced by the two highest densities. In 1980, quackgrass densities of 265 and 595 shoots/m2 reduced soybean leaf area and plant dry weight. Leaf area partitioning (LAP), leaf weight partitioning (LWP), and net assimilation rate (NAR) of soybeans were reduced by a quackgrass density of 595 shoots/m2. In the duration study, interference by a natural stand of quackgrass for 6 weeks, 8 weeks, and full-season decreased soybean yield an average of 11, 23, and 33%, respectively. Results from the duration study suggest that a selective postemergence herbicide for the control of quackgrass in soybeans would be beneficial if available.

Teliospores of Sphacelotheca holci Jack. [= S. cruenta (Kuhn.) Potter] infected johnsongrass [Sorghum halepense (L.) Pers. # SORHA) systemically after inoculation of cut stems in greenhouse studies. A preliminary host range test in the greenhouse revealed that no infection occurred on four varieties of sorghum (S. vulgare L.) and three cultivars of sudangrass (S. Sudanese L.). Infected johnsongrass plants grown in the greenhouse exhibited reduced vegetative growth and altered morphological characteristics compared to healthy johnsongrass plants. The aboveground biomass and lateral rhizome expansion of smutted johnsongrass field transplants were significantly lower than that of uninfected plants. Significant correlations between fraction of johnsongrass stems infected and plant weight, and basal area of plants were observed in field studies. Corn (Zea mays L.) and alfalfa (Medicago sativa L.) grown in competition with johnsongrass, and johnsongrass infected with S. holci reduced the aboveground biomass and rhizome length of johnsongrass plants significantly, compared to that of uninfected johnsongrass plants grown alone in greenhouse studies. Uninfected johnsongrass plants also exhibited higher plant weight than infected plants when both were grown in competition with alfalfa, regardless of the alfalfa planting density in the greenhouse or when measured after 3 months in field trials.

In two field experiments over a 4-yr period, 2,4-D amine applied up to four times per year from late April to early June at 2.5, 10, and 25 ppmw produced a wide range of symptoms on ‘Concord’ grapes. When 2,4-D symptoms in July were severe, yields were reduced by as much as 85%. However, when 2,4-D symptoms in July were slight (less than 2 on a 0 to 6 visual scale), grape yields were not reduced. A second-order polynomial described well (R2 = 0.92) the relationship between symptoms and grape yield. Symptoms of 2,4-D on grape leaves did not persist beyond the year of treatment, although grape yields did not recover fully until the second year after the last 2,4-D treatment In the first year of 2,4-D treatments, low grape yields were correlated with reduced weight and number of berries per cluster. In the second and third year of treatment, low grape yields were correlated with fewer clusters per shoot in addition to lower cluster weight and fewer berries per cluster. When 2,4-D was applied one year only, soluble solids in the grape juice were reduced as much as 9%, although reductions were not related to severity of foliar symptoms. When 2,4-D was applied for three consecutive years, soluble solids in the juice were consistently higher in grapes with moderate to severe 2,4-D symptoms. There were no consistent effects of 2,4-D on juice color or titratable acidity. The pH of fresh juice was increased slightly by 2,4-D with the maximum being 10% after 3 yr of consecutive treatments. In the first subsequent non-treatment year after 3 yr of 2,4-D treatments, juice quality was similar to the quality of nontreated grapes. At a detection limit of 0.05 ppmw, 2,4-D was not detected in the harvested grapes. Based on the results of these experiments, grapes with slight 2,4-D symptoms would not be expected to have reduced yields or reduced juice quality.

Field experiments were conducted to study the competitive ability of green foxtail (Setaria viridis (L.) Beauv.) with wheat (Triticum aestivum L.) and the feasibility of using a,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) for the selective control of green foxtail in wheat. Rates of 0.56 to 0.84 kg/ha of trifluralin gave 90 to 100% control of foxtail when growing in competition with wheat. Wheat tolerated rates of trifluralin up to 2.24 kg/ha, the maximum rate used, without injury or reduction in crop yield, when the herbicide was applied and incorporated to a shallow depth 1 day after seeding. A semidwarf and a tall variety of wheat were not affected by trifluralin treatments. Green foxtail had a relatively low competitive ability, and wheat plants suppressed foxtail to a marked degree under normal growing conditions. However, suppressed plants still produced sufficient seed to insure a dense infestation of foxtail plants in the succeeding year. Further, when wheat was grown under less competitive situations, green foxtail resulted in significant reductions in crop yield. It was concluded that green foxtail is an “opportunist-type” weed.

The use of winter cover crops is beneficial to agriculture. We have tried to design a cover crop that self-destructs in response to an environmental cue, thereby eliminating the use of herbicides and tillage to remove the cover crop in late spring. Here, this novel concept is tested in a model system. The onset of summer brings with it elevated temperatures. Using this as the environmental cue, a self-destruction cassette was designed and tested in tobacco. A heat-shock–responsive promoter was used to direct expression of the ribonuclease Barnase. Because Barnase is extremely toxic to cells, it was necessary to coexpress its inhibitor, Barstar, whose expression was under the control of the CaMV 35S promoter. The wild-type and two mutated Barnase genes, one missense and one translation attenuated, were tested. Our results indicated that the translation-attenuated version of the Barnase gene was most effective in causing heat-shock–regulated plant death. Analysis of the T2 progeny of a transgenic plant carrying this Barnase mutant showed that the Barnase gene expression was sixfold higher in heat-shock–treated plants compared with untreated plants. This level of Barnase gene expression was sufficient to kill transgenic plants.

From 1976 to 1981, emergence date, height, and dry weight per plant were taken for eight weed species grown at Bushland, TX, at two stand densities. Height and dry weight per plant were related to precipitation and degree-days for air and soil temperatures using base temperatures of 10, 15.5, or 21 C. Kochia [Kochia scoparia (L.) Schrad. ♯ KCHSC] emerged early in March and was followed by poison suckleya [Suckleya suckleyana (Torr.) Rydb. ♯ SUKSU], sunflower (Helianthus annuus L. ♯ HELAN), and common cocklebur (Xanthium pensylvanicum Wallr. ♯ XANPE) in early April. Buffalobu(Solanum rostratum Dunal. ♯ SOLCU), puncturevine (Tribulus terrestris L. ♯ TRBTE), lanceleaf sage (Salvia reflexa Hornem. ♯SALRE), and barnyardgrass (Echinochloa crus-galli L. Beauv. ♯ECHCG) emerged in late April. The weeds began to increase in height during May but plants did not start increasing in weight until June. Kochia, common cocklebur, and sunflower grew tallest, produced most dry matter, made most efficient use of water, and weighed most per plant. Heat unit accumulations were strongly correlated with plant height.

All land uses in eastern and southeastern Nebraska were infested to some extent with hemp dogbane (Apocynum cannabinum L.). The highest infestations were observed in oats (Avena sativa L.) and soybeans [Glycine max (L.) Merr.] and the lowest infestations were in alfalfa (Medicago sativa L.), pastures, and winter wheat (Triticum aestivum L.). Yield reductions from hemp dogbane infestations ranged from 0 to 10% in corn (Zea mays L.), 28 to 41% in soybeans, and 37 to 45% in sorghum [Sorghum bicolor (L.) Moench]. Emergence of hemp dogbane from crown roots occurred when the soil temperature was 17 to 19 C, during April in 1977 and 1978. Plants attained the bud stage within 4 to 7 weeks after emergence. Early flower, full bloom, and pod initiation occurred subsequently at about 1 week intervals. Seeds produced were first viable 10 weeks after full bloom. Root activity or regenerative capacity as measured by length and number of new shoots and roots produced at monthly intervals in the germinator showed a cyclic pattern. The highest activity occurred in the spring and late fall and lowest activity in summer and early fall. Protein levels in the roots ranged from 7 to 9% in the fall and spring to 4 to 5% during the summer. Percentage total nonstructural carbohydrates (TNC) ranged from 20 to 31% in lateral roots and 32 to 53% in crown roots, but there was not a consistent cyclic pattern of percentage TNC during the growing season.

Site-specific fertilizer application is of primary interest among those investigating site-specific crop management. Site-specific soil moisture levels may be associated with the relative success of certain weed species, and weed competition can be affected by fertilizer application. A field study was conducted to investigate how landscape position–based site-specific nitrogen fertilizer application would influence wild oat interference in spring wheat. Wild oat was allowed to grow in spring wheat at foot and knoll landscape positions in the presence or absence of 80 to 90 kg ha−1 of spring preseed broadcast nitrogen fertilizer. In all three site–years of the study, landscape position did not affect wild oat competitiveness in wheat or relative wild oat biomass, and there were no significant landscape position by nitrogen fertilizer interactions for these variables. The lack of landscape position effect may be attributed to the lack of substantive differences in soil characteristics between landscape positions at the sites used in this study and to normal to above-average seasonal precipitation levels in all three site–years. Wild oat competitiveness in wheat was significantly greater in the presence of nitrogen fertilizer for all three site–years. The results of this study suggest that, when there are not great differences in typical soil characteristics between landscape positions and precipitation levels are normal or above normal, landscape-based site-specific nitrogen fertilizer application does not affect wild oat competitiveness in spring wheat, but preseed spring broadcast nitrogen fertilizer does make wild oat more competitive in spring wheat.

Saflufenacil/dimethenamid-P is a relatively new prepackaged herbicide mixture that has the potential to provide enhanced weed control in soybean when tank-mixed with reduced doses of imazethapyr. Six field experiments were conducted over a 3-yr period (2011, 2012, and 2013) near Ridgetown and Exeter, Ontario, Canada, to determine the dose of imazethapyr, applied PRE, that must be added to saflufenacil/dimethenamid-P (245 g ai ha−1) to provide effective weed control in soybean. The predicted dose of imazethapyr PRE for 80% control of common lambsquarters, common ragweed, green foxtail, and velvetleaf 8 wk after soybean emergence (WAE) was 66, 180, 137, and 48 g ai ha−1, respectively. In contrast, when tank-mixed with saflufenacil/dimethenamid-P (245 g ha−1), the dose of imazethapyr PRE needed for 80% control of common lambsquarters, common ragweed, green foxtail, and velvetleaf was reduced to 11, 80, 48, and 18 g ha−1, respectively. The control of common lambsquarters, common ragweed, green foxtail, and velvetleaf was improved by 21, 23, 34, and 27%, respectively when saflufenacil/dimethenamid-P (245 g ha−1) was added to imazethapyr PRE. Imazethapyr at 104 g ha−1 resulted in soybean yield that was 95% of the weed-free control; however, when tank-mixed with saflufenacil/dimethenamid-P (245 g ha−1) only 54 g ha−1 of imazethapyr was required for the same yield level. Based on this study, PRE application of saflufenacil/dimethenamid-P with reduced doses of imazethapyr has the potential to improve soybean yield and provide acceptable weed control (≥ 80%); however, the extent that imazethapyr dose can be reduced is dependent upon weed community composition.

Many acid herbicides must be methylated before analysis by gas-liquid chromatography. Several reagents were evaluated for the esterification of substituted acid herbicides. Esterification by the different methods was determined on 2,3,6-trichlorophenylacetic acid [fenac], 2,3,6-trichlorobenzoic acid [2,3,6-TBA], 4-amino-3,5,6-trichloropicolinic acid [picloram], 2-methoxy-3,6-dichlorobenzoic acid [dicamba], and 3,6-dichlorosalicylic acid, a possible metabolite of dicamba. The effect of substituent size and position is discussed in relation to esterification. The best methylating reagent for acid herbicides was diazomethane although it is toxic, explosive, and more costly than the other methods. BF3-methanol was equally as good when the compound was not sterically hindered. Water interfered with the esterification efficiency of all reagents. Diazomethane esterified the ammonium salt but not the sodium salt of the acids used.

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.

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 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.

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.

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.

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.