The effects of chlorimuron as related to application timing were evaluated on sicklepod and soybean in the glasshouse. Plants were sprayed with chlorimuron at 8.8 g ai ha–1 at 14, 21, 28, and 35 days after planting (DAP). Early (14 DAP) chlorimuron application reduced sicklepod dry weight, leaf area, and water use to levels significantly less than those of plants treated at 28 and 35 DAP. Soybean dry weight and leaf area were not affected by chlorimuron. However, soybean water use was initially reduced at application timings of 28 DAP or earlier but recovered by 4 weeks after treatment (WAT). Sicklepod net assimilation rate (NAR) and relative growth rate (RGR) were reduced by chlorimuron regardless of application timing while soybean was not affected.

Diclofop–methyl and haloxyfop–methyl (0.001 to 10 μM) caused 9 to 61% inhibition of 14C–acetate incorporation into lipids in corn leaf segments within 1 h of herbicide treatment, while neither herbicide affected this process in bean leaf segments. The herbicides did not affect 14C-malonate incorporation into lipids in corn leaf segments. Diclofop-methyl and haloxyfop-methyl reduced 14C-acetate incorporation into polar lipids and triglycerides in corn while incorporation into sterols was increased. In vitro activity of acetyl-coenzyme A carboxylase (EC 6.4.1.2) was inhibited from 26 to 94% within 5 min of exposure to the herbicides (1 to 10 μM). Diclofop acid inhibited this enzyme activity more than did haloxyfop acid. Differences in field activity between diclofop-methyl and haloxyfop-methyl are not related to differential sensitivity of acetyl–coenzyme A carboxylase to the two herbicides.

Intraspecific and interspecific interference effects on growth, gas exchange, and water potential of wild oat and spring barley were measured under greenhouse conditions using a 1:1.06 barley to wild oat replacement series. Intraspecific barley interference affected barley growth more than interspecific wild oat interference. Interspecific wild oat interference with barley reduced wild oat growth more than intraspecific interference. Wild oat plant height surpassed barley plant height near barley anthesis. Growth and gas exchange of barley and wild oat responded the same to short-term water stress.

Field experiments were conducted during the winters of 1986–87 and 1987–88 at Haryana Agricultural University, Hisar, India to classify the ability of winter wheat cultivars to compete with wild oat. Wild oat reduced winter wheat grain yield by 17 to 62% depending upon cultivar. WH-147 and HD-2285 were the most competitive cultivars. Winter wheat dry matter accumulation and grain yield were negatively correlated with wild oat dry matter. A high number of tillers, particularly in HD-2009, WH-291, and S-308, did not always translate into grain yield advantage in wild oat-infested plots. Wheat height and dry matter accumulation per unit area during early crop growth were better characters than number of tillers for predicting the competitive ability of wheat cultivars to wild oat.

A biotype of the annual grass weed hare barley infesting an alfalfa field with a 24-yr history of the use of the bipyridylium herbicides paraquat and diquat, was investigated for resistance to these herbicides. Rates of up to 800 g ai ha–1 of each herbicide caused no mortality in the hare barley plants from this field. The same species, collected from an adjacent pasture field with no history of bipyridylium herbicide application, exhibited LD50's of 57 and 160 g ai ha–1 for paraquat and diquat, respectively. Tiller numbers and dry matter production in the biotype from the alfalfa field were not affected by the normal rate recommended for both herbicides. These results clearly show that hare barley from the alfalfa field is resistant to paraquat and diquat. Both biotypes were equally sensitive to fluazifop, glyphosate, and sethoxydim.

Density and spatial interference of barnyardgrass with ‘Lemont’ and ‘Newbonnet’ rice cultivars was investigated in 1987 and 1988. Barnyardgrass reduced grain yield of Lemont, a semidwarf cultivar, more than Newbonnet, a short-statured cultivar. Linear regression indicated that season-long interference of barnyardgrass at 1 to 20 plants m–2 reduced grain yields of Lemont and Newbonnet by 301 and 257 kg ha–1 per barnyardgrass plant, respectively. Grain yields of Lemont and Newbonnet were reduced by barnyardgrass densities to 40 plants m–2 but were not affected further at densities ≥40 plants m–2. Barnyardgrass reduced straw yield of Lemont more than of Newbonnet. Although barnyardgrass culms m–2 and straw yield increased as weed density increased, the weed produced more culms per plant at 2 and 5 m–2 than at 20 to 80 m–2. In a spatial interference experiment, Lemont grain yield reduction averaged 21% when plants were grown within 0 to 25 cm of a barnyardgrass plant group (group = 4 plants per 140 cm2). However, rice yields were not reduced when barnyardgrass plant groups were 25 to 50 or 50 to 100 cm away. Additional treatments included two barnyardgrass groups spaced 20, 40, 80, or 100 cm apart When Lemont grew between barnyardgrass groups spaced 20 or 40 cm apart, grain yields were reduced 40 and 27%, respectively, compared with weed-free rice. However, Lemont yield was not reduced when rice plants grew between weed plant groups spaced 100 cm apart.

Field experiments were conducted at Stuttgart, AR, from 1986 to 1988 to determine effects of season-long interference of red rice densities of 1, 2, 5, 10, 20, and 40 plants m–2 on ‘Lemont’ and ‘Newbonnet’ rice cultivars. This interference reduced straw dry weights of Newbonnet and Lemont by 100 and 130 kg ha–1 per red rice plant, and grain yields by 178 and 272 kg ha–1 per red rice plant, respectively. Grain yield reductions were due to decreases in panicle number and length, and in number of grains per panicle. Ten plants m–2 or more reduced height of Lemont, while 40 plants m−2 were required to reduce height of Newbonnet. Red rice at 10 plants m−2 or more reduced total milled and head rice yields of Lemont, but red rice did not affect these components for Newbonnet. Red rice interfered with rice, even at densities as low as two plants m−2, and interference was greater in Lemont, a semidwarf cultivar with mature plants 92 cm tall, than in Newbonnet, a conventional cultivar with mature plants 115 cm tall. Red rice produced more panicles m−2 and 31 to 64% greater straw dry weight when grown with Lemont than when grown with Newbonnet because red rice shaded Lemont more than Newbonnet. Red rice plants grew taller as red rice densities increased and also were taller when grown in Newbonnet than when grown in Lemont.

Wild oat reduced light penetration and growth of dwarf hard red spring wheat in field experiments performed under nonlimiting nitrogen and moisture conditions. Wild oat grew taller than wheat and had a greater portion of its canopy above 60 cm at maturity. Light penetration in a mixed canopy was similar to that in a monoculture wheat canopy when wild oat was clipped to the height of the wheat. A mathematical model was developed which accurately predicted the reduction in the growth rate of wheat from wild oat interference. The model also predicted that interference from wild oat was due to reduced leaf area of wheat at early growth stages and low wild oat densities, and reduced light penetration to wheat leaves at later growth stages and higher densities of wild oat.

A weed survey of 115 Nova Scotia lowbush blueberry fields was conducted during the summers of 1984 and 1985 to determine the most common and prevalent weeds associated with blueberry production. A total of 119 weed species were observed within the surveyed blueberry fields. The most common weed species were bunchberry, colonial bentgrass, poverty oatgrass, sheep sorrel, and false lily-of-the-valley. Weeds with low relative abundance in this study, but which may become more widespread, include three-toothed cinquefoil, Kentucky bluegrass, hair fescue, canker-root, and Canadian St. Johnswort.

Soils from long-term tillage plots at three locations in Ohio were sampled to determine composition and size of weed seed banks following 25 yr of continuous no-tillage, minimum-tillage, or conventional-tillage corn production. The same herbicide was applied across tillage treatments within each year and an untreated permanent grass sod was sampled for comparison. Seed numbers to a 15-cm depth were highest in the no-tillage treatment in the Crosby silt loam (77 800 m–2) and Wooster silt loam (8400 m–2) soils and in the grass sod (7400 m–2) in a Hoytville silty clay loam soil. Lowest seed numbers were found in conventional-tillage plots in the Wooster soil (400 m–2) and in minimum-tillage plots in the Crosby (2200 m–2) and Hoytville (400 m–2) soils. Concentration of seeds decreased with depth but the effect of tillage on seed depth was not consistent among soil types. Number of weed species was highest in permanent grass sod (10 to 18) and decreased as soil disturbance increased; weed populations were lowest in conventional tillage in the Hoytville soil. Common lambsquarters, pigweeds, and fall panicum were the most commonly found seeds in all soils. Diversity indices indicated that increased soil disturbance resulted in a decrease in species diversity. Weed populations the summer following soil sampling included common lambsquarters, pigweeds, fall panicum, and several species not detected in the seed bank.

Total weed density increased after 1 yr of no-tillage and after 2 yr of conventional tillage in a 4-yr experiment with repeated assignment of the same treatment to the same plots. Large crabgrass, goosegrass, and carpetweed densities were higher in the no-tillage compared with the conventional-tillage treatment in at least 1 yr whereas common lambsquarters density was greater in the conventional-tillage treatment the last year of the experiment. Within the no-tillage treatment, rye or hairy vetch residue reduced total weed density an average of 78% compared to the treatment without cover crop when cover crop biomass exceeded 300 g m–2 and when residue covered more than 90% of the soil. Goosegrass, stinkgrass, and carpetweed densities were reduced by cover crop residue in at least 1 yr whereas large crabgrass was unaffected. Common lambsquarters density increased where rye was grown as a cover crop prior to conventional tillage. Despite differences in weed density among treatments, weed biomass was equivalent in all treatments during the last 2 yr.

The effect of tillage systems on depth of emergence and densities of giant and green foxtail under different environmental and cropping conditions were evaluated from 1985 to 1987 at Arlington, Hancock, and Janesville, WI. Mean emergence depths in no-till were the shallowest, followed by chisel plow and conventional tillage at each location. At least 40% of the giant and green foxtail plants emerged from the upper 1 cm of soil in no-till compared to about 25% in chisel plow and less than 15% in conventional tillage. As many as 25% of the plants emerged from greater than 4 cm in conventional tillage compared to about 10% in chisel plow and less than 5% in no-till. Seedlings emerged from greater depths in a loamy sand than in a silt loam soil regardless of tillage system. At Arlington, green foxtail was the dominant species in conventional tillage, while giant foxtail dominated in chisel plow and no-till. Foxtail densities were greater in chisel plow and no-till than in conventional tillage at all three locations.

Controlled-environment experiments were conducted to determine the effects of soil temperatures of 5 to 20 C, seed depths of 0 to 6 cm, and above- or below-seed cyanazine placement on germination and seedling development of giant foxtail and velvetleaf. Giant foxtail did not germinate at 5 C and failed to emerge from 6 cm deep within 21 days at 10 C. Increasing soil temperature above 10 C increased giant foxtail germination and emergence. Velvetleaf germinated at 5 C but did not emerge within 21 days. Velvetleaf emerged within 21 days from soil depths of 2 to 6 cm at soil temperatures of 10, 15, and 20 C. Giant foxtail and velvetleaf seed germinated on a soil surface kept moist by mulch or frequent watering. Giant foxtail seedling survival was 100% after germination on the soil surface. Velvetleaf often failed to become established; only 28% of the velvetleaf that germinated at 20 C survived. Injury to giant foxtail by cyanazine increased with increasing soil temperatures and decreasing seed depths. Cyanazine placement above or below the seed did not have a consistent effect on giant foxtail injury. Cyanazine placed above the seed was more injurious to velvetleaf than placement below at 15 and 20 C. Differential responses of giant foxtail and velvetleaf seed germination and seedling survivability to initial seed depth appears to be a major factor in weed population shifts when tillage is reduced or eliminated.

Seed survival of jointed goatgrass over 2 yr was related to cyclical seasonal changes in ability to germinate, field emergence, and seasonal precipitation and air temperature. The ability of seed to germinate was studied by incubating spikelets under constant, defined incubation conditions after retrieving samples from the field every 2 weeks over 2 yr. Less than 8% of jointed goatgrass seed that were initially set outdoors survived beyond 2 yr when left undisturbed on the soil surface or buried 5 cm deep. Periods of greatest seed loss coincided with periods of peak emergence in fall (late September and October). The ability of seed to germinate also peaked in fall and preceded the period of greatest field emergence and greatest decreases in seed survival.

Effects of wild proso millet interference with irrigated corn were evaluated in Nebraska and Colorado over a 2-yr period. Corn yield reductions ranged from 13 to 22% from a wild proso millet density of 10 plants m–2. As density increased, corn yield reduction could be predicted with a rectangular hyperbola regression model. Ten wild proso millet plants m–2 growing with corn produced 4200 to 6200 seed m–2. Corn yields were reduced 10% at one location if wild proso millet removal was delayed 2 weeks after corn planting. If removal was further delayed until 6 weeks after corn planting, corn yield reductions at the two locations ranged from 16 to 28%.

Bermudagrass growing in a creeping bentgrass green is a severe problem where both grasses are grown. An experiment was conducted to determine the effects of multiple flurprimidol-herbicide applications on tolerance of bentgrass and on suppression of three bermudagrass cultivars in bentgrass. Application of 1.4 kg ai ha–1 flurprimidol on April 15 and 0.8 kg ha–1 repeated on June 1 injured creeping bentgrass 44% in 1988 and 22% in 1989. Bermudagrass was suppressed from the two flurprimidol applications until mid-June in 1988. Growth in 1989 was not influenced by number of applications since bermudagrass was suppressed only until late May. Application of 0.8 + 54 kg ai ha–1 flurprimidol plus siduron on April 15 and 0.4 + 27 kg ha–1 on June 1 injured creeping bentgrass 23 to 28% and suppressed bermudagrass until mid-July in 1988 and mid-June in 1989. Application of 0.8 + 1.7 kg ha–1 flurprimidol plus ethofumesate on April 15 and 0.4 + 0.8 kg ha–1 on June 1 injured creeping bentgrass 25 to 28% and effectively suppressed bermudagrass until late August in 1988 and late June in 1989. Application of 0.8 + 54 kg ha–1 flurprimidol plus siduron or 0.8 + 1.7 kg ha–1 flurprimidol plus ethofumesate applied in June or June and July to plots treated in April did not improve bermudagrass suppression. In most instances there was an inverse relationship between bermudagrass suppression and number of bermudagrass stolons. There were interactions between flurprimidol-herbicide treatments on bermudagrass cultivars on number of stolons but not on suppression.

The effects of the antidote CGA-154281, available soil water (ASW), and soil temperature on corn injury from preemergence applications of metolachlor were evaluated in field and growth chamber studies. In field studies, metolachlor at rates of 5.6, 8.4, and 11.2 kg ha–1 caused corn injury when there was sufficient ASW before corn emergence to activate the herbicide. Injury was prevented when CGA-154281 was applied with metolachlor (30:1, metolachlor:CGA-154281 by wt). The effects of surface-soil ASW, soil temperature, and CGA-154281 on corn tolerance to metolachlor were further evaluated in growth chamber studies. Corn injury from metolachlor was more severe when the surface soil was wet for 5 days immediately after herbicide treatment than when the surface-soil was dry. Corn injury from metolachlor also was greater when corn was grown under cool temperatures (21/13 C, day/night) than when grown under warm temperatures (30/21 C, day/night). CGA-154281 conferred protection against metolachlor injury regardless of surface soil ASW or growth temperature.

Field studies were conducted to evaluate the influence of herbicide application timing on weed control in no-till soybean production. Row spacing generally had no effect on weed control. Herbicide treatments containing chlorimuron plus metribuzin applied as many as 45 days prior to planting in 1988 and 1989 controlled broadleaf weeds throughout the growing season. Imazaquin applied 45 and 30 days prior to planting provided poor control of common cocklebur in 1989. Giant foxtail control was inconsistent with all herbicide treatments. Soybean yields subsequent to early preplant herbicide applications were greater than or equal to those in which applications were made at planting when late-season weed control was adequate. Herbicides applied preemergence did not control high densities of common lambsquarters in 1989.

Field experiments were conducted in 1988 and 1989 to evaluate imazethapyr for weed control in peanuts. Imazethapyr was applied PRE or POST at 3, 5, or 7 weeks after crop emergence (WAE) at 0.071 kg ai ha–1. Imazethapyr applied PRE controlled common lambsquarters 85%, prickly sida 92%, and a mixture of entireleaf, ivyleaf, pitted, and tall morningglory species 77%. Morningglory control was at least 91% with imazethapyr plus metolachlor PRE followed by imazethapyr plus 2,4–DB or imazethapyr plus acifluorfen at 3, 5, or 7 WAE. Yields from systems that included metolachlor plus imazethapyr PRE followed by imazethapyr plus acifluorfen, imazethapyr plus 2,4–DB, or acifluorfen plus 2,4–DB at 3 WAE were greater than yields from the handweeded check. All systems with imazethapyr plus metolachlor PRE followed by any POST treatment except imazethapyr plus acifluorfen 7 WAE provided net returns equivalent to the herbicide standard of metolachlor PRE and acifluorfen and bentazon plus 2,4–DB 3 WAE. All systems except imazethapyr PRE provided greater net returns than the handweeded weed–free check.

Field and laboratory investigations were conducted from 1983 through 1985 to evaluate morphological and environmental influences on the performance of imazaquin on 26 weed species common to the Mississippi Delta. Imazaquin applied preplant incorporated or preemergence at 140 g ai ha–1 controlled 16 species but controlled only five species at 140 g ha–1 foliar applied. Contour graphs accurately predicted imazaquin postemergence rates necessary to control a given weed species at a given time after emergence. Imazaquin applied preemergence with a sequential postemergence application controlled entireleaf morningglory and sicklepod better than a single soil application. Imazaquin at 140 g ha–1 applied preemergence required at least 50 mm of supplemental water following application to control sicklepod and was more phytotoxic to sicklepod grown at 25/10 C day/night temperatures than those grown at 32/22 C.