Hemp dogbane is sensitive to fluroxypyr and tolerant to clopyralid. Absorption, translocation, and metabolism of clopyralid and fluroxypyr were studied in hemp dogbane to determine if differences in these processes could be responsible for differential sensitivity. In addition, the effect of growth stage on herbicide absorption and translocation was evaluated. The 14C-herbicides were applied to the adaxial side of a single leaf located near the midpoint of hydroponically cultured plants. Uptake of fluroxypyr was more rapid than clopyralid. At 72 h after treatment (HAT), fluroxypyr and clopyralid absorption was 62 and 38%, respectively. Clopyralid was much more mobile than fluroxypyr, with 75% of the absorbed 14C from 14C-clopyralid recovered outside the treated leaf compared to only 45% for fluroxypyr 72 HAT. Relative to fluroxypyr, a higher percentage of 14C-clopyralid recovered outside the treated leaf translocated acropetally, especially when plants were treated during the vegetative stage. Treatment during the early reproductive stage increased basipetal and reduced acropetal translocation relative to the vegetative stage. Neither herbicide was metabolized rapidly. Approximately 60 and 90% of the recovered 14C was attributable to unaltered fluroxypyr and clopyralid, respectively, 72 HAT. Some differences in absorption, translocation, and metabolism between clopyralid and fluroxypyr exist, but they cannot fully account for differential sensitivity of hemp dogbane to these two herbicides. Differences in activity at the target site may be responsible for differential activity of these herbicides on hemp dogbane.

When buds on the rhizome of quackgrass [Agropyron repens (L.) Beauv.] were released from apical dominance either by increasing the nitrogen supply to the parent shoot (from 5.25 to 210 ppm) or by raising the humidity around the rhizome (from 55 to 100%), the growth response of the buds was closely correlated with their uptake of foliar-applied 14C-labeled glyphosate [N-(phosphonomethyl] glycine]. The 14C level in the buds, expressed on a dry-weight basis, was greatest in the youngest, most rapidly growing bud at the apical node and decreased in successively older buds along the rhizome. A similar gradient was shown by the 14C content of the associated rhizome nodes. The high-humidity treatment also increased the total amount of 14C that was translocated into the rhizome, whereas increasing the nitrogen supply, while promoting 14C uptake by the buds, markedly reduced the amount in the rhizome nodes and in other parts of the plant. This nitrogen-induced reduction in translocation was associated with a reduction of about 30% in uptake of the herbicide by the treated leaves.

A spatially explicit metapopulation neighborhood model was extended to encompass the seed bank dynamics of the annual weed Alopecurus myosuroides Huds. (blackgrass), growing in U.K. winter wheat crops established after noninversion tillage. The effects of the initial spatial pattern of infestation, herbicide, and combine harvesting on seed bank densities and on expected yield losses of the infested crop were studied within a 634 m2 area. In the absence of herbicide, all seed bank populations were large and typical of values in the literature; those with patchy initial distributions spread quickly over the entire field. The effects of intraspecific competition ensured that even after 10 yr, the average seed bank density from three patterns with the same initial density consistently retained the same rank order: the initially uniform pattern consistently ranked largest; a patchily distributed moderate infestation ranked next; a more patchily distributed heavy infestation ranked least. Expected grain yield losses sometimes exceeded 40%. With the introduction of a herbicide, seed densities declined exponentially, but the rank order with regard to spatial pattern remained. Relatively dense patches occasionally persisted for longer than 10 yr. The economic threshold for treatment was achieved within 2 and 5 yr, depending on the infestation pattern, but the model predicted that it would take many more than 10 yr before infestations could be completely eliminated. The effects of harvesting by combine were modeled. In all cases, the speed of the spread of infestation along a row appeared to be largely due to the combine rather than to natural dispersal or other cultivation practices. The proportion of seeds removed by the combine was an important determinant of the number of years required to drive the population below the economic threshold and of the probability of long-term eradication of the weed metapopulation.

Tall fescue (Festuca arundinacea Schreb. ‘Kentucky 31′) drilled into a pasture infested with broomsedge (Andropogon virginicus L.) and fertilized with N, P, and K, eliminated broomsedge after 4 years. In a second experiment, fertilization with N, P, and K increased the density of Kentucky bluegrass (Poa pratensis L.), and broomsedge was gradually reduced over a 5-year period. Fertilization with P and K caused the density of legumes and grass to increase and broomsedge was nearly eliminated after 5 years. In both experiments the old growth of broomsedge was mowed each winter and cattle were then able to graze the new growth of broomsedge. Grazing may have aided in the elimination of broomsedge.

The amount of free, unaltered (2,4-dichlorophenoxy) acetic acid (2,4-D) in resistant and susceptible plant species 1, 4, and 8 days after treatment was determined by three procedures. Centrifugation and chromatography of plant homogenates was a more reliable assay than trichloroacetic acid (TCA) precipitation or dialysis procedures. The foliar penetration of 14C-2-4-D and radioactivity which moved from roots into the growth media following foliar application of 14C-2,4-D varied from one plant species to another, but no general correlations with 2,4-D resistance was observed. The resistant burcucumber (Sicyos angulatus L.) and oats (Avena sativa L.) were not fatally injured primarily because unaltered 2,4-D was immobilized in the treated leaves and unaltered, free 2,4-D was reduced to nontoxic concentrations. The 2,4-D in susceptible cocklebur (Xanthium sp.) remained largely as free and mobile 2,4-D, and the treated plants were near death 8 days after treatment.

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.

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.

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.

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.

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.

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.

Sixteen field experiments were conducted across Oklahoma to evaluate the effects of MON 37500 time of application on cheat control, and winter wheat injury and yield. Winter wheat injury from MON 37500 applied preemergence (PRE) was slight and was influenced by cumulative precipitation for 10 d after application. Winter wheat injury was more frequent with early vs. late postemergence (POST) applications and was influenced by wheat growth stage and mean, high, and low air temperatures before and after application. Cheat control averaged 75% (n = 16 treatments with four to six replicates) when applied PRE and 88% (n = 126 treatments with four to six replicates) when applied POST. Cheat control from MON 37500 applied POST declined with increasing cheat growth stage at application and with decreasing mean diurnal low temperatures 0 to 14 d and 0 to 21 d before application. MON 37500 applied PRE increased yields 52 and 66% compared with the untreated control in Year 1 and Year 2, averaged over eight experiments each year. MON 37500 applied POST increased wheat yields 68 to 69% compared with the untreated check in Year 1 and Year 2, when averaged over all applications in eight experiments each year. Wheat yields were greater from fall POST applications than from late-winter applications.