The nonionic surfactants Tween 20 [oxysorbic (20 POE) polyoxyethylene sorbitan monolaurate] and Tween 80 [oxysorbic (20 POE) polyoxyethylene sorbitan monooleate] at concentrations of 0.05, 0.1, 0.2, and 0.4% v/v stimulated germination of barnyardgrass [Echinochloa crus-galli (L.) Beauv. # ECHCG] seeds in petri dishes. Stimulation occurred under both 30/20 and 25/15 C diurnal temperature cycles whether or not light was supplied during the 8-h high-temperature portion of the cycle. Barnyardgrass responded most to treatment under environmental conditions in which untreated seeds germinated the least. A 0.1% surfactant concentration often stimulated germination of weed seeds as effectively as did higher concentrations. Germination of redroot pigweed [Amaranthus retroflexus L. # AMARE] seeds was inhibited by Tween 80 in the light under both the low- and high-temperature regimes and by Tween 20 at low temperature in the light. Germination of tumble pigweed (Amaranthus albus L. # AMAAL) seeds was inhibited by both surfactants in the high-temperature regime whether or not light was supplied. Common purslane (Portulaca oleracea L. # POROL) seeds were insensitive to treatment.

The adoption of conservation tillage systems has been challenged by concerns potential weed species shifts. A 9-yr study from 1988 to 1996 was conducted Delhi, Ontario, on a loamy sand soil to evaluate the effect of tillage systems (conventional [CT] and no-till [NT]), cover crop Secale cereale, and nitrogen (N) rate (0, 50, 100, 125, 150, and 200 kg N ha−1) on monocrop Zea mays L. (corn) yield and changes in the composition of the weed flora. CT consisted of spring moldboard plowing followed by cultivation with a tooth cultivator. Weed counts were taken in the last 3 yr of the study (1994, 1995, and 1996) prior to postemergence herbicide application and then again 2 to 3 wk after herbicide treatment. Composition or the weed flora was analyzed by canonical discriminant analysis (CDA). The relationship between weed density and tillage system was not consistent. Weed species composition differed between CT and NT systems. Chenopodium album and Amaranthus retroflexus were associated with CT and Digitaria sanguinalis with NT. N rate and cover crop did not affect weed density or species composition. Proper management of weeds with herbicides appeared to minimize any long-term effect on the weed flora resulting from varying N rates. Zea mays yields did not differ between CT and NT systems but were greater in both systems with a cover crop at the higher N rates. Disturbance caused by tillage was more important than N rate and cover crop as a mechanism influencing composition of the weed flora.

The relative sensitivity of various crops and weeds to celery root residue incorporated into soil was determined in greenhouse experiments. A range of concentrations was used to determine the percentage of celery residue needed to cause a 50% reduction in plant growth (I50). The most sensitive weed was spiny amaranth and the least sensitive was common purslane. The most sensitive crop was radish and the least sensitive was celery. Selectivity was calculated as crop I50/weed I50 and varied from a maximum of 13 for celery and spiny amaranth to a low of 0.4 for radish and common purslane. Selective weed suppression might be obtained when celery residue is present.

A screening method was used to characterize seed thermal responses of prostrate knotweed and common purslane, two important weeds invading wheat in the humid Pampa. Through this method, it was possible to detect thermal conditions that induce or break dormancy in both species. In addition, we were able to quantify changes in dormancy level in seed populations as a function of time of burial after dispersal, through changes in width of the thermal range within which germination can occur. Plotting the overlap of this thermal range and observed soil temperature throughout the year allowed the prediction of the seedling emergence period. This prediction was in agreement with observed seedling emergence in the field for both species, during 2 consecutive yr. From the analysis carried out under laboratory conditions, it was also possible to estimate required thermal time for germination of the nondormant fraction of the population and the base temperature above which thermal time is accumulated. The results obtained from this study are the basis for the formulation of seed germination models that predict not only the occurrence of seedling emergence in the field, but also the dynamics of germination within those periods.

In a semiarid Mediterranean site in central Spain, field experiments were conducted on a Calcic Haploxeralf (noncalcic brown soil), which had been managed with three crop rotations and two tillage systems (no-tillage and conventional tillage) since 1987. The crop rotations consisted of barley→vetch, barley→sunflower, and a barley monoculture. The study took place in two growing seasons (1992–1994) to assess the effects of management practices on the weed seedbank. During this period, spring weed control was not carried out in winter crops. In the no-tillage system, there was a significant increase in the number of seeds of different weed species: anacyclus, common purslane, corn poppy, knotted hedge-parsley, mouse-ear cress, spring whitlowgrass, tumble pigweed, venus-comb, and Veronica triphyllos. Conversely, the presence of prostrate knotweed and wild radish was highest in plots under conventional tillage. These results suggest large differences in the weed seedbank as a consequence of different soil conditions among tillage systems, but also the necessity of spring weed control when a no-tillage system is used. With regard to crop rotations, the number of seeds of knotted hedge-parsley, mouse-ear cress, and spring whitlowgrass was greater in the plots under the barley→vetch rotation. Common lambsquarters dominated in the plots under the barley→sunflower rotation, whereas venus-comb was the most frequent weed in the barley monoculture. Larger and more diverse weed populations developed in the barley→vetch rotation rather than in the barley→sunflower rotation or the barley monoculture.

We conducted several experiments on linuron-resistant and -susceptible Portulaca oleracea and on atrazine-resistant and -susceptible Chenopodium album to determine their immediate and long-term responses to photosynthesis-inhibiting herbicides. Several photosynthesis-inhibiting herbicides were used, and O2 evolution was measured with a Clark-type O2 electrode. Resistance ratios (RRs) for P. oleracea, based on O2 evolution inhibition, were 8 and > 6 for linuron and diuron, respectively; > 800 for atrazine; and > 20 for terbacil. Linuron-resistant P. oleracea was negatively cross-resistant to bentazon and pyridate (RR = 0.5 and 0.75, respectively). Time-course measurements of fresh weight, photosynthetic CO2 assimilation, and photochemical efficiency indicated that linuron and atrazine inhibited electron transport in susceptible (S) P. oleracea and C. album, ultimately resulting in death. Measurements of photochemical efficiency and CO2 assimilation of linuron-resistant P. oleracea treated with linuron indicated a transient injury from which plants recovered within 14 d. Recovery of linuron-resistant P. oleracea from atrazine injury was more rapid than from linuron injury for all measured variables. Atrazine-resistant C. album had no cross-resistance to linuron. Sequence analysis of the D1 protein revealed that linuron-resistant P. oleracea had a serine-to-threonine substitution at position 264.

Tolerance of direct-seeded pepper under clear plastic mulch to herbicides applied pre- and post-emergence was investigated in 1991 and 1992. Weed control of herbicides applied preemergence was also studied. Paprika cultivar ‘Buketen’ and canning cultivars ‘Piquillo de Lodosa’ and ‘Pico de Mendavia’ were seeded in the field in Zaragoza, Spain, in a loamy soil. Percent emergence, plant mortality after the postemergence treatment, dry weight of plants, and fruit weight were recorded. Pepper was tolerant to diphenamid and napropamide applied preemergence but these herbicides did not control broadleaf weeds. Clomazone, when applied preemergence resulted in chlorotic seedlings and at 2 kg ai/ha reduced the final stand, but was selective when applied at the 6 to 8-leaf stage of the crop. Pepper was tolerant to diethatyl-ethyl when applied preemergence. However, diethatyl-ethyl applied postemergence caused severe phytotoxicity symptoms, and when mixed with diphenamid, fruit ripeness in 1992 was delayed. Weed control during the crop emergence period was better when diethatyl-ethyl was used mixed with diphenamid or napropamide.

Replacement series studies were conducted under controlled conditions to determine the effect of phosphorus (P) rates and population densities on the competitiveness of smooth pigweed and common purslane with lettuce. Densities were 2, 4, and 8 plants per 113 cm2, whereas P rates were 0, 0.4, and 0.8 g PL−1 soil. A P-deficient Histosol (0.3 mg water-extractable P L−1 soil) was used. High P fertility enhanced the competitive ability of lettuce in smooth pigweed-lettuce mixtures. Smooth pigweed was not responsive to P rates. However, luxurious P consumption by smooth pigweed occurred, reducing the amount of the nutrient available for lettuce absorption. In common purslane-lettuce mixtures, the weed was responsive to P rates, increasing its competitive ability, whereas no increase in lettuce competitive ability was observed. Both weed species were more competitive than lettuce. Competition for P appears to be the main mechanism of common purslane interference on lettuce grown in low-P organic soils. Alternative fertilization strategies (i.e., banded applications) may reduce the effect of smooth pigweed on lettuce.

The seed composition in the upper 15-cm soil horizon was determined and correlated with weed seedlings growing with fieldbeans (Phaseolus vulgaris L. ‘Valley’). The total seed reservoir averaged 250 seed/kg of soil, and 19 species were represented. Seed occurring with the most frequency were redroot pigweed (Amaranthus retroflexus L. ♯ AMARE), common lambsquarters (Chenopodium album L. ♯ CHEAL), and common purslane (Portulaca oleracea L. ♯ POROL). Seed from these plants accounted for over 85% of the seed found. The number of barnyardgrass [Echinochloa crus-galli (L.) Beauv. ♯ECHCG], buffalobur (Solanum rostratum Dunal ♯ SOLCU), common lambsquarters, common purslane, and common sunflower (Helianthus annuus L. ♯ HELAN) seed in the soil was correlated with the number of plants growing in the field with fieldbeans. A correlation occurred between redroot pigweed, yellow foxtail [Setaria lutescens (Weigel.) Hubb. ♯ SETLU], and barnyardgrass growing in corn (Zea mays L.) fields in the fall of the year and plants growing in the field with fieldbeans the following year.

Patterns of weed emergence in upland and lowland soils were observed for several months after tillage by counting all emerged species. Forty to 50% of weed emergence occurred within 6 weeks of tillage on both sites. Significant emergence occurred within 3 weeks on both sites, but very little emergence occurred within 1 week in lowland soil. The data confirm the strong influence of soil tillage on weed emergence. They also suggest an influence of radiant energy on weed emergence. Peaks of weed emergence 6 or more weeks after tillage often occurred coincident with or soon after peaks of radiant energy.

Emergence periodicity of common purslane, hemp sesbania, horse purslane, prickly sida, spurred anoda, and velvetleaf was determined from March to October during 5 yr at Stoneville, MS, in the absence of reseeding. In early March of each year prior to emergence determinations, plots were not tilled or tilled to depths of 0, 5, 10, or 15 cm. Yearly emergence periodicity of the six weed species was not affected by tillage. In each year, weeds of all species emerged in identifiable flushes and very few emerged at other times during the growing season. Velvetleaf emerged mainly during the early growing season from March through May. Peak periods of emergence for the other species varied slightly over the 5 yr, but conformed generally to the early or the midseason peak period (June, July, and August) or both. In most years, common purslane, horse purslane, hemp sesbania, prickly sida, and spurred anoda emerged during the midseason period. Annual patterns of temperature fluctuations were similar during the 5 yr, but relatively warm soil temperatures during the early growing season of 1977 may have caused earlier emergence of velvetleaf, prickly sida, and hemp sesbania. Weekly water balance (rainfall minus open-pan evaporation) varied during March through May of each year but was negative during each June, July, and August. Apparently the main effect of a positive water balance during most of the year was to cause merger of the early and midseason peak emergence periods in 1979. Early and midseason emergence peaks were possibly due to different populations of seeds able to germinate in cooler soil temperatures of the early season and warmer soil temperatures of the midseason, respectively.

Weed emergence and viable weed seed numbers were determined in field plots during a 5-yr period where reseeding was prevented. Known numbers of seeds of seven weed species were added to the native seed population at the beginning of the study. Plots were nontilled or tilled to depths of 0, 5, 10, and 15 cm early in the spring of each year. Velvetleaf, spurred anoda, hemp sesbania, morningglory species, and pigweed species emergence was significantly greater from the nontilled plots during the first year after seeds were added to the native seed population. Tillage, regardless of depth, reduced weed emergence during the first year where seeds were added to the plots but had no effect on emergence from plots where no seeds were added. Tillage during the second through the fifth year did not affect emergence regardless of the addition of seeds. of the 5-yr emergence totals, 61 to 88% of all weeds in all plots emerged during the first year and 9 to 23% emerged during the second year. The 5-yr decline rate for emergence of all weeds was exponential. Viable seeds of prickly sida, spurges, and pigweeds in the nontilled plots declined from 590, 1531, and 4346 m−2, respectively, to zero by the third year. Tillage did not affect the decline. However, weed emergence in the field indicated that a few (1.0 to 5.6 m−2) seeds of those weeds were still viable after 3 yr. In nontilled plots, many recently added seeds were on or near the soil surface and germinated during the first year. Tillage moved many seeds to sites that were unfavorable for germination and emergence during the first year.

Weed seeds can require an exposure to light for induction of germination. Conducting tillage operations at night and thus preventing the photoinduction of germination has been proposed as a means to reduce weed emergence in agricultural systems. This research was conducted to evaluate night tillage as a weed management option and to determine which tillage operations have the greatest effect on light-mediated recruitment. Weed emergence was evaluated after conducting factorial combinations of day and night moldboard plowing and disking in the springtime from 1992 through 1995. The light environment during disking generally had a slightly greater effect on emergence than the light environment during plowing. Emergence of pigweed species and giant foxtail was, at most, 30 to 55% higher following day vs. night disking. Emergence of other weeds was not affected by the light environment during tillage. We conclude that night tillage may not be a viable approach to weed management due to insufficient reductions in weed emergence associated with night tillage and the high degree of variability in the recruitment response to light conditions during tillage.

A curved-linear Fresnel lens was investigated to test the effect of concentrated solar radiation on surface and buried weed seed. The lens produced a line focus 1 by 150 cm with a mean temperature of 309 C. A 20-s exposure to seed on the soil surface was 100% lethal to green foxtail, kochia, common lambsquarters, common purslane, and wild buckwheat. In a separate study, emergence from kochia and yellow foxtail seed was reduced 100% at 10 mm soil depth after 15 min in soil of 35 and 93 g/kg moisture and 20 to 40% at 178 g/kg moisture content. Concentrated solar radiation from a Fresnel lens has the greatest potential for affecting weed seed on the soil surface. A series of Fresnel lenses and/or larger lenses may be required for many practical field applications.

The influence of two weed management systems was determined on weed seed and seedling populations and on yields of cantaloupe (Cucumis melo L. var. reticulatus Naudin ‘Perlita′), bell pepper (Capsicum annuum L. var. grossum ‘Grande Rio 66′), cotton (Gossypium hirsutum L. ‘CP 3374′), onion (Allium cepa L. ‘1015Y′), and cabbage (Brassica oleracea, var. capitata L. 'Sanibel′) sequentially grown in two 3-yr cycles. Palmer amaranth (Amaranthus palmeri S. Wats. # AMAPA) did not exist initially, but hurricane-introduced seed populations increased to 1.1 billion/ha as seed populations of common purslane (Portulaca oleracea L. # POROL) decreased from 786 million/ha to 124 million/ha in the 6-yr period, without weeding or herbicide. Use of herbicides and handweeding reduced Palmer amaranth seed populations 98%, but 18 million/ha still remained after 6 yr. The use of herbicides and Palmer amaranth interference decreased the seed populations of common purslane by 84%, but handweeding was inefficient. Yields of all but the first crop of cantalouple were almost totally eliminated by season-long interference of Palmer amaranth. Savings with the utilization of herbicides rather than handweeding ranged from $62/ha for cotton to $4703/ha for bell pepper.

Imazethapyr was evaluated PRE and POST in five lettuce types and chicory under Florida field conditions. The relative sensitivity of leafy crop vigor (most sensitive to most tolerant) to imazethapyr PRE, based on 20% inhibition determined using regression analysis, was as follows: Boston > bibb > crisphead > romaine > leaf > escarole > endive. Leafy crop injury increased as the rate of imazethapyr applied POST increased, with all leafy crops responding in a similar manner. Surfactant addition increased imazethapyr phytotoxicity. Imazethapyr PRE treatments at 0.067 kg ai/ha provided greater than 80% control of livid amaranth, common purslane, flatsedge, and common lambsquarters. Imazethapyr POST at 0.067 kg/ha, with surfactant provided control greater than 85% of all weed species. Greater than 85% spiny amaranth control was provided by imazethapyr POST at 0.017 kg/ha. Use of surfactant with imazethapyr did not improve spiny amaranth control over imazethapyr with no surfactant. POST treatments did not decrease leafy crop yield compared with the hand-weeded check. Imazethapyr applied PRE reduced crop yield compared to the POST treatments and the hand-weeded control.

The efficacy of reduced rates of POST herbicides for weed control in sweet corn was evaluated over a three-year period in field studies conducted at the Horticultural Experiment Station, Simcoe, Ontario. POST applications of metolachlor/cyanazine, cyanazine/atrazine and metolachlor/atrazine at 1/2X and 1/4X of labelled rates controlled broadleaf weeds comparable to that of standard labelled rates. Control exceeded 94% each year. Annual grass control was reduced with below-label rates, especially for cyanazine/atrazine and metolachlor/atrazine. A 1/2X application of nicosulfuron/rimsulfuron, following the 1/4X rate of cyanazine/atrazine, significantly improved grass control, compared to cyanazine/atrazine applied alone. Below-label rates did not result in decreased sweet corn yields.

Transplanted cabbage was grown in conventional-tillage (100% cultivated) and strip-tillage (25% cultivated and 75% residue) production systems with various herbicide treatments to evaluate weed control and cabbage yield. DCPA at 9 kg ai/ha, napropamide at 1.1 kg ai/ha, oxyfluorfen at 0.3 kg ai/ha, and oxyfluorfen at 0.2 kg ai/ha plus napropamide at 1.1 kg ai/ha provided commercially acceptable (> 80%) control of carpetweed, swinecress, large crabgrass, redroot pigweed, and common lambsquarters under both tillage systems. However, only oxyfluorfen plus napropamide provided control of eclipta. At a second location, all herbicides controlled common purslane and barnyardgrass and all but DCPA controlled hairy galinsoga. Overall, cabbage yields were the same between conventional and strip-tillage systems, and among herbicide treatments when average cabbage head weights across tillage method were pooled.

Field studies were conducted in 1990 and 1991 at Davis and Five Points, CA to evaluate weed control with napropamide and hand-weeding and the effect on yield and profitability in transplanted bell pepper. Weed cover 8 wk after transplanting averaged less than 10% in plots hand-weeded biweekly for 6 wk after transplanting. Weed cover at harvest was less than 30% in plots hand-weeded at 8 wk after transplanting. Napropamide treatment reduced weed cover at 8 wk after transplanting an average of 34% compared to untreated plots, but differences declined to 14% at harvest. Time required to hand-weed plots was greater if the interval between weeding was 4 instead of 2 wk. The time needed to hand-weed plots was reduced from 38% to 71% by napropamide treatment. When weeds were excluded by hand-weeding for the entire season, bell pepper yielded 25 810 kg/ha and was 4% to 18% higher than other treatments. Net returns were greater for plots that were hand-weeded the entire season or when napropamide was combined with hand-weeding at 4 and 8 wk after transplanting, compared to other treatments.

Chloramben effectively controlled weeds in lettuce and endive grown on muck soils better than thiobencarb and chlorpropham. Chloramben at 1.1 kg ai/ha controlled two weeds economically important on muck soils, livid amaranth and oakleaf goosefoot. Chloramben applied preemergence at 1.1 kg/ha and higher caused early stunting of lettuce which the lettuce outgrew by harvest. Field-seeded endive was stunted severely at these same rates, severely reducing yield. Transplants improved tolerance of all crops. Effective use rates on muck soils were 25% to 50% of those typically used on mineral soils.