The differences in tolerance, morphology, and physiological response of diclofop-resistant and -susceptible wild oat biotypes collected from fields in Saskatchewan, Canada, were investigated under growth room and field conditions. Under herbicide-free conditions the resistant biotype had more upright leaves with about 12% less leaf area and 50% less leaf width than the susceptible biotype. A marked difference in the level of tolerance to diclofop was observed. Photosynthesis was initially significantly reduced in both biotypes after treatment with diclofop at the rate of 0.70 kg ai ha−1, but the resistant biotype was able to recover. Injury to the susceptible biotype was reduced by coating the seeds with 1,8-naphthalic anhydride. Differential foliar retention was not an important factor in selectivity of diclofop among the two biotypes.

Changes in dormancy and viability of freshly harvested seed of yellow nutsedge buried in the field, at depths of 5, 25, and 50 mm, were studied over a 2-yr period. The number of dormant, viable seed decreased most rapidly at 5 mm because more seed lost their dormancy and germinated (up to 30%) than at lower depths. Loss of viability through decay of seed appeared similar at all depths. No seedlings emerged from any depth. In a laboratory experiment, conducted in optimal soil moisture conditions, seedlings emerged from all seed that germinated, demonstrating that adequate soil moisture was critical for seedling establishment. This was substantiated in a second field experiment in which irrigation and mulching greatly increased seedling survival. The number of seedlings surviving in irrigated plots was 0.78% of seed sown and 0.03% in the rain-fed plots. The role of seed in the establishment of yellow nutsedge infestations is probably of little importance in dryland cropping areas despite the longevity and viability of the seed.

Field studies were conducted in 1987 and 1988 to evaluate the control and competitiveness of common lambsquarters in soybeans when present at four densities and when removed at four time intervals. Common lambsquarters were removed by hand or treated with a postemergence herbicide application of bentazon plus acifluorfen. Common lambsquarters could remain 10 weeks following soybean emergence before a 20% soybean yield reduction occurred when weeds were removed by hand. When treatment was attempted with postemergence herbicides, a 20% yield reduction occurred in 1987 if application was not made prior to 5 weeks after emergence. In 1988, all postemergence herbicide applications failed to control lambsquarters due to drought conditions. The postemergence herbicide's degree of control was reflected in the number of remaining lambsquarters plants. Common lambsquarters seed production and germination was not influenced by postemergence herbicide application. There was a strong correlation between dry weight of uncontrolled common lambsquarters plants and seed produced per plant.

Densities up to 100 downy brome m2 were established in winter wheat in southeastern Wyoming and west-central Kansas to quantify wheat yield loss from downy brome interference and to approximate economic threshold levels. A quadratic equation best described wheat yield loss as a function of weed density when downy brome emerged within 14 days after wheat emergence. Densities of 24, 40, and 65 downy brome m2 reduced wheat yield by 10, 15, and 20%, respectively. Wheat yield was not reduced when downy brome emerged 21 or more days later than wheat. Economic thresholds varied with changes in downy brome density, cost of control, wheat price, and potential wheat yield. In a greenhouse experiment, dry weight of 72-day-old wheat plants grown in association with downy brome was not affected by the distance between the weeds and wheat, whereas downy brome plant dry weight increased with increasing distance between the weeds and wheat.

Experiments were conducted in a growth chamber to determine the effects of planting depth on the developmental sequence of velvetleaf seedlings and the effect of cyanazine placement relative to velvetleaf planting depth on cyanazine toxicity. Velvetleaf seedling emergence was delayed when seeds were planted 6 cm deep compared to the 2- and 4-cm planting depths. Lateral roots did not appear until after the cotyledons had emerged and expanded. This was followed by secondary root emergence from the primary root which occurred before adventitious roots appeared from the hypocotyl for the 2- and 4-cm planting depths. The first true leaf did not unfold until after the secondary root system was well developed. The quantity of adventitious roots on the hypocotyl increased with increasing planting depth. Velvetleaf adventitious roots appeared to be involved in cyanazine toxicity when cyanazine was placed above the seed. Increasing planting depth increased the proportion of seedling absorptive tissue above compared to below the seed. This resulted in increased cyanazine exposure for the deeper planted velvetleaf seeds when herbicide was placed above the seed.

Two adjuvants, Triton 11 XE (Isoctyl phenyl polyethoxy ethanol) and urea were tank mixed with four herbicides, methabenzthiazuron [N-(benzothiazol-2-yl)N,N′-dimethyl urea] at 1.0 and 1.5 kg ai/ha, terbutryn [N-(1,1-dimethylethyl)-N′-ethyl-6-(methylthio)-1,3,5-triazine-2,4-diamine] at 0.50 and 0.75 kg/ha, metoxuron [N-3-chloro-4-methoxyphenyl)N,N-dimethylurea] at 1.0 and 1.75 kg/ha, and isoproturon [(N-(4-isopropylphenyl)-N,N-dimethylurea] at 0.50 and 0.75 kg/ha and applied to weeds in wheat. Metoxuron reduced weed dry matter by 90% over a 2-yr period. Isoproturon, terbutryn, and methabenzthiazuron reduced weed dry matter by 82%. Tank-mixing of Triton 11 XE at 0.5% (v/v) with metoxuron at 1.0 kg ai/ha, terbutryn at 0.5 kg/ha, isoproturon at 0.5 kg/ha, and methabenzthiazuron at 1.0 kg/ha reduced weed density and biomass by 52 to 67% and enhanced grain yield by 33% over herbicide alone. None of the herbicides had any significant interaction with adjuvants in terms of weed control and grain yield.

Efficacy of glyphosate on bulbous oatgrass, a creeping perennial grass, was investigated in greenhouse and growth chamber studies. Glyphosate at 1.2 and 2.5 kg ae ha−1 significantly reduced corm formation and corm viability. Glyphosate controlled bulbous oatgrass when applied at the 4- to 5- or 6- to 7-leaf stage. With 2.5 kg ha−1, a 24-h period between glyphosate application and removal of the shoots was sufficient to cause maximum reduction in regrowth. Total amount of 14C translocated out of leaves treated with radiolabeled glyphosate (1.0 kg ha−1 total glyphosate) did not differ among growth stages, but more 14C accumulated in the dormant corms when glyphosate was applied at the 2- to 3-leaf stage than at the 4- to 5- or 6- to 7-leaf stages. Lack of correlation between maximum plant injury and glyphosate accumulation in corms is not understood but may be related to differences in application method in the two studies.

Field experiments were conducted in 1985 and 1986 under eight environments to evaluate the population dynamics of a range of silverleaf nightshade densities and to measure the effects of those populations on cotton lint yield. Dry weed weights of silverleaf nightshade stands were influenced by growing conditions among years, but were positively related to initial densities as long as 2 yr after establishment. Stem numbers increased as initial densities and stand age increased. A negative linear relationship existed between cotton lint yield and weed biomass and between cotton lint yield and stem number from both 1- and 2-yr-old weed stands. For each 1 kg/10 m of row increase in dry weed weight from 1- and 2-yr-old stands, a 9 and 21% lint yield loss/ha was predicted, respectively. For each stem/10 m of row, a 0.35 and 0.31% yield loss was predicted, respectively. Late-planted cotton was less vulnerable to yield reductions by silverleaf nightshade; however, its yield potential was also less.

Texas panicum was grown in controlled-environment chambers in 16 day/night temperature regimes ranging from 19/11 to 34/26 C. After 33 days, maximum plant height, leaf area, and dry weight occurred at 34/26 C. The greatest number of tillers and leaves occurred at 34/16 C. Partitioning of plant biomass into stems increased with increasing night temperature, while partitioning into leaves tended to decrease. Partitioning of biomass into roots decreased with increasing night temperature at day temperatures of 19 or 24 C but not at 34 C. Plant dry matter production (DMP) was highly correlated with both leaf area duration (LAD) and net assimilation rate (NAR) during the first 2 weeks of growth. During the subsequent 3 weeks, DMP was highly correlated with LAD but not with NAR. The low-temperature threshold for growth of Texas panicum was approximately 11 C. Texas panicum attained 25% of its maximum growth at an average daily temperature of 21 C, 50% at 24 C, and 75% at 26 C. Texas panicum is unlikely to become a highly competitive weed outside the southern and southwestern United States.

Soil water from plots containing cotton, devil's-claw, cotton with devil's-claw, and bare soil was measured throughout the growing season using a neutron probe and related to weed interference with the crop. Volumetric water content throughout the soil profile to a depth of 180 cm did not differ among treatments before the 5th or 6th week after cotton emergence. Greater water depletion occurred early in the season in plots containing devil's-claw which corresponded to a period of rapid weed growth. In plots containing only cotton, the largest reduction in water content occurred later in the season during peak bloom and early boll formation. Soil water content at depths greater than 105 cm remained unchanged in all plots throughout the season. Interference from devil's-claw reduced cotton lint yield 96% in 1986 and 46% in 1987. Higher rainfall and reduced weed populations in 1987 reduced the impact of weed interference on cotton lint yield.

Suspension cultures of proso millet cells were treated with haloxyfop at different phases of growth. Treatment of 1-d cultures with 1 μM haloxyfop completely inhibited growth within 48 h. In contrast, 1 mM haloxyfop was required to elicit a similar response in 4-, 7-, or 10-d cultures. Calculated IC50 values indicated a 300-fold decrease in haloxyfop sensitivity during the period from 1 to 4 d. The observed changes in sensitivity to haloxyfop could not be attributed to changes in cell concentration during culture growth. In both 1-d and 4-d cultures, an initial rapid uptake of radiolabel was followed by a slow loss of radiolabel from cells. Almost all radioactivity extracted from 1-d and 4-d cells was present as the parent acid. Several radiolabeled compounds in addition to the parent acid were present in media. No major differences in the amounts of these materials were found between 1-d and 4-d media. Our results indicate that a special aspect of metabolism expressed during cell division is particularly sensitive to the herbicide.

Laboratory studies were conducted to determine the ability of the extenders dietholate and SC-0058 to prevent enhanced biodegradation of EPTC and butylate applied to Wisconsin soils with different histories of carbamothioate herbicide use. Enhanced EPTC and butylate biodegradation occurred in soils previously treated with those herbicides. Enhanced biodegradation of dietholate occurred on soils previously treated with that extender plus either EPTC or butylate. Enhanced dietholate biodegradation was observed when applied alone or in combination with butylate or EPTC. Application with dietholate prevented enhanced biodegradation of butylate but not EPTC even though enhanced biodegradation of dietholate was occurring. Enhanced biodegradation of SC-0058 did not occur. SC-0058 prevented enhanced EPTC and butylate biodegradation even in soils previously treated for three consecutive years with the respective herbicide plus SC-0058. Application of either 1.1 or 2.2 kg/ha SC-0058 plus 6.7 kg/ha EPTC provided equal or better wild proso millet control and sweet corn yields than applications of EPTC alone or EPTC plus dietholate.

Seed of 17 weed species that were buried at 2 and 15 cm at Fairbanks, AK, in 1984 were exhumed at 1-yr intervals and tested for germination and viability. Burial depth had a significant effect on seed viability of wild oats, bluejoint reedgrass, quackgrass, prostrate knotweed, and corn spurry with viability remaining higher at 15 than at 2 cm. The percent of viable foxtail barley and quackgrass seed that were dormant was greater at 15 than at 2 cm. Eleven of 17 species had greater than 6% seed viability 4.7 yr after burial. American dragonhead seed viability in 1989 was not significantly lower than in 1984. In contrast, viability of common hempnettle and quackgrass had dropped to zero by 2.7 and 3.7 years after burial, respectively. Foxtail barley and wild oats had less than 1% viable seed at the end of 4.7 yr. For most of the 17 species, a rectangular hyperbola model accounted for more of the variability in seed viability decline with time than did a negative logarithmic model.

Field, greenhouse, and laboratory studies were conducted to examine the effect of application timing on the activity of DPX-V9360 on rhizome johnsongrass. Field and greenhouse studies indicated that johnsongrass treated with postemergence applications of DPX-V9360 at late growth stages (>5 leaves) was controlled more effectively than when treated in early growth stages (<5 leaves). Johnsongrass control was optimized with split-postemergence applications (treatments applied at early and late growth stages) in field studies compared to a single postemergence application at either early or late growth stages. The pattern of translocation of 2-14C (pyrimidine)-labeled DPX-V9360 applied to a fully expanded johnsongrass leaf did not differ significantly between three different growth stages of 10-, 30-, and 60-cm height. Over 60% of the absorbed 14C remained in the treated leaf. Most of the translocated 14C moved out of the treated leaf within 3 days after application and distributed to the shoot in greater quantities than to the rhizomes. About 40% of 14C-DPX-V9360 applied to the leaf surfaces of a tolerant species (corn) or susceptible species (johnsongrass) was absorbed into the leaf. Corn metabolized over 90% of absorbed DPX-V9360 within 20 h, while there was no perceptible metabolism of DPX-V9360 in johnsongrass leaves after 24 h. Late growth stage and split-postemergence applications appear to provide more effective control than early growth stage applications because of better control of regrowth (new shoot emergence from rhizomes after application) and because tillering and plant emergence are more nearly complete at application time.

The effect of soil matric potential from −0.02 to <–1.5 MPa on Texas panicum growth in drainage lysimeters covered by an automatic rainfall shelter was measured in 1984, 1985, and 1986. Rate of tiller production was faster for plants established in April than June in 1984 and 1985, but not 1986. In 1985 and 1986, dry weight was greater, but total seed production was less for plants established in April than June. Irrigation when the soil matric potential was −0.02, −0.075, or −1.5 MPa did not affect rate of tiller production in 1984. Irrigation when the soil matric potential was −1.5 MPa decreased dry weight of plants but increased seed number per panicle compared to irrigation when the soil matric potential was −0.02 MPa in 1985 and 1986. Plants grown in lysimeters irrigated at soil matric potential <–1.5 MPa in 1985 and 1986 did not wilt at 8:00 a.m.; therefore, they were not watered after establishment. These nonirrigated plants averaged 0.9 and 0.4 kg dry weight and produced 92 200 and 16 100 seeds in 1985 and 1986, respectively.

The resistance of four populations of slender foxtail from England to chlorotoluron, pendimethalin, and the methyl ester of diclofop was studied in whole-plant spray assays. Two populations showed cross-resistance to all three herbicides while a third showed resistance to chlorotoluron only. Resistance, although not absolute, was sufficient to cause substantial reductions in herbicide performance at recommended field doses. Resistant (Peldon) and susceptible (Rothamsted) populations also were assessed for resistance to six dinitroaniline herbicides in a petri dish test in which the effects on shoot development were measured. The degree of resistance of the Peldon population varied considerably among herbicides, with greatest resistance to pendimethalin, whereas sensitivity to ethalfluralin, isopropalin, and trifluralin was similar to that of the Rothamsted standard. Differential response to butralin and oryzalin was smaller than to pendimethalin. The large difference between populations in their response to pendimethalin and trifluralin was confirmed in a pot experiment.

Field experiments were conducted from 1985 to 1987 to evaluate herbicide systems for minimum-tillage and conventional-tillage peanut production. While acceptable weed control could be achieved in both tillage systems, minimum-tillage systems generally had to be more herbicide intensive. Preemergence or preplant-incorporated within-the-row applications of either ethalfluralin or pendimethalin plus postemergence applications of paraquat and sethoxydim provided Texas panicum control equivalent to preplant-incorporated applications of ethalfluralin or pendimethalin. Early-postemergence paraquat applications improved Florida beggarweed and pitted morningglory control in conventional-tillage systems at least 15% compared to the same systems without paraquat Control of bristly starbur and sicklepod in conventional-tillage systems did not increase with paraquat application. Broadleaf weed control did not differ between tillage systems, except pitted morningglory control was lower in the minimum-tillage system. Conventional-tillage peanuts produced yields 800 to 1900 kg/ha higher, depending on herbicide system, and also provided greater net returns than minimum-tillage peanuts. The greater yield and net returns in conventional- versus minimum-tillage systems were not attributed to weed control or disease problems.

Field research to determine the influence of weed control systems in winter barley on subsequently no-till-planted grain sorghum was conducted from 1985 to 1987 in northwest Georgia. Herbicide treatments applied to fully tillered winter barley included 2,4-D, dicamba, metribuzin, oryzalin, oryzalin plus metribuzin, fluorochloridone, or thiameturon. None of these herbicides affected barley grain yields compared to nontreated plots. Treatments containing oryzalin reduced johnsongrass fresh weights in the subsequently planted grain sorghum. Oryzalin plus metribuzin, in 1 of 2 yr, resulted in higher seed yields of subsequently planted grain sorghum compared to the other treatments except for oryzalin used alone. Treatments containing paraquat plus propazine plus metolachlor applied to grain sorghum provided good weed control when averaged across barley treatments and resulted in the highest grain sorghum yield. No grain sorghum injury from herbicides used in the previous winter barley crop was observed.

Field experiments were conducted to evaluate competitive effects of johnsongrass and sicklepod on growth and yield of irrigated and nonirrigated soybeans. Johnsongrass reduced soybean growth early in the growing season. Sicklepod was competitive all season, but competitiveness was greatest during the soybean reproductive stage. Soybean yields were reduced 31% by sicklepod, 14% by johnsongrass, and 36% by both weeds growing together. Nonirrigated soybeans yielded less than irrigated, but percent yield reductions were similar. Johnsongrass dry matter production and seed yield were reduced more than sicklepod by soybean interference. Soybean and sicklepod interference reduced johnsongrass seed production 73 to 95%. Johnsongrass produced 245 to 1238 seeds per plant when interfering with soybeans and sicklepod. Sicklepod seeds per plant were reduced 6 to 31% by johnsongrass interference and 47 to 75% by soybeans and soybeans plus johnsongrass.

We examined effects of supplementary nitrogen on young Canada thistle stands in Alberta, Canada. Urea applied at 70 or 100 kg N ha–1 increased shoot population densities, mostly through increased root growth in the top 20 cm of soil rather than through released root bud dormancy. Nitrogen fertilization, therefore, may increase the severity of a Canada thistle infestation. In 1-yr-old stands, the number of emerged and unemerged root buds/m of root was higher near the soil surface in fertilized plots than in unfertilized plots, and no underground shoots were detected at the time of sampling. In 2-yr-old stands, more unemerged root buds/m of root were found at depths below 20 cm in fertilized plots than in unfertilized plots, and twice as many underground shoots/m of root occurred between 20 and 40 cm in fertilized plots as in unfertilized plots. However, no effect of nitrogen on the number of emerged root buds was detected. Nitrogen treatment increased shoot production only for those replanted root fragments from deeper than 60 cm in 1-yr-old stands and deeper than 160 cm in 2-yr-old stands.