Different Saskatchewan soils were studied to determine their populations of 2,4-D [(2,4-dichlorophenoxy) acetic acid] degraders and gross aerobic bacteria. Soil from Indian Head field plots receiving relatively high field rates (1.12 or 1.68 kg/ha) of 2,4-D amine or ester applied annually over the previous 32 yr contained the highest population of degraders, ranging from 103 to 104 degraders/g soil. Control plots recorded lower populations of degraders (2.5 ± 0.5 × 103 degraders/g soil). Five soils from around the province were also studied, and a relationship was found between the rate of breakdown of 14C-2,4-D in the soil and the number of degraders. Another soil from a different site at Indian Head, but of a similar type to that in the field plots, displayed a much slower rate of 2,4-D degradation and also a much lower degrader population (7 ± 2 degraders/g soil) when compared to the other four soils (4 × 102 to 9 × 102 degraders/g soil). The ratio of degraders to the total bacterial flora varied in all studies between 1:6.5 × 102 and 1:3 × 105. When 1030 soil isolates were screened for their ability to degrade 2,4-D using an agar-block extraction technique, none possessed this feature. This would support the hypothesis that the ratio of 2,4-D degraders to the total microbial flora was indeed large (at least 1:103).

Characteristics of glyphosate [N-(phosphonomethyl)glycine] uptake for as long as 3 days by plant cells were studied using suspension-cultured potato (Solanum tuberosum L. cv. ‘Red Pontiac’ and S. brevidens Phil.) cells. An experimental system was developed wherein uptake was assayed in a reduced salt medium that minimized cell growth yet maintained cell viability. Glyphosate uptake was nonlinear over time; an immediate rapid uptake phase was followed by uptake at a decreased rate that continued through 72 h. Glyphosate uptake was the same in both an experimental uptake medium (reduced salt and sucrose, no hormones) and in the complete growth medium used for routine cell growth. The pattern of glyphosate uptake was similar with both Red Pontiac and S. brevidens cells, though uptake was greater in Red Pontiac cells. Glyphosate uptake was not inhibited by 1 mM CaSO4, MgSO4, MnSO4, or ZnSO4. Rather, uptake was inhibited by the absence of salts at 8 h, and ZnSO4 and MnSO4 increased uptake. Similarly, increasing concentrations (0 to 5 mM) of CaSO4 and CaCl2 did not inhibit uptake, whereas the absence of either salt of Ca2+ decreased uptake. Increasing concentrations of (NH4)2 SO4 (0 to 150 mM) did not affect glyphosate uptake, but (NH4)2 SO4 at 50 mM and above was toxic to the cells.

The impact of a weevil (Rhinocyllus conicus Froel.) (Coleoptera: Curculionidae) on Carduus thistles in Virginia was evaluated in terms of seed reduction and viability during 1973 and 1974. Musk thistle (Carduus nutans L.) seed production from terminal and first lateral heads was reduced by 10% in 1973 and 75% in 1974 due to larval feeding of R. conicus. The greater seed reduction in 1974 was caused by heavy infestation (45% in 1973, 70% in 1974); there was a five-fold increase in the number of weevil pupation chambers per head over the previous year. Total seed production per plant decreased by 35 to 36% in both years despite increased plant vigor due to better growing conditions in 1974. Larval feeding reduced viability of mature musk thistle seeds. Although individual heads of plumeless thistle (Carduus acanthoides L.) were frequently destroyed by larval feeding, seed production from early heads of this thistle decreased by only 5% in 1973 and 4% in 1974 due to low rates of infestation (9% in 1973, 5% in 1974). Total seed reduction per plant for each of the two years was less than 0.2%. Infested heads of plumeless thistle did not produce sufficient seeds for evaluation of the weevil's impact on seed viability.

The effects of four densities of annual broadleaf weeds and annual grass weeds and an equal combination of the two on the yields of sugar beet (Beta vulgaris L.) roots and tops, percent sucrose and total solids, average diameter of beet roots, and total plot yields were studied during 1964 and 1965. Broadleaf weeds were more competitive than annual grass weeds and the effects of an equal combination were intermediate. Without cultivation, all weed densities reduced the yield of beet roots below the weed-free plots. Twenty broadleaf weeds/sq ft were required to reduce yields when plots were cultivated and weeds were limited to a 10-in band over the beet row. The effects on the yield of beet tops, average root diameter, and total plot yield were similar to the effects on beet root yield. The components of total plot yield are examined. No weed density had a real effect on beet stand, percent total solids, or percent sucrose of the beet roots.

Recent investigations on the use of 2-chloro-N,N-diallylacetamide (CDAA), 2-chloroallyl diethyldithiocarbamate (CDEC), and ethyl N,N-di-n-propylthiolcarbamate (EPTC) for weed control have stressed soil incorporation rather than the more conventional application to the soil surface. By this treatment loss of the herbicides as vapors may be reduced, the risk of crop injury lessened, and weed control improved. Although both pre-planting and pre-emergence soil incorporation have been investigated, pre-planting treatments appear most promising. In many weed-crop situations the results obtained with EPTC have been especially promising.

Aqueous extracts of quackgrass [Agropyron repens (L.) Beauv. # AGRRE] shoots and rhizomes inhibited seed germination and root growth of alfalfa (Medicago sativa L. ‘Vernal’), soybean (Glycine max (L.) Merr. ‘Corsoy 79’], navybean (Phaseolus vulgaris L. ‘Seafarer’), and curly cress (Lepidium sativum L.) at concentrations of less than 2.5 mg dried extract/ml. Extracts of quackgrass shoots were generally more inhibitory than extracts of rhizomes. Root and shoot dry weights of snapbeans (Phaseolus vulgaris L. ‘Bush Blue Lake’) grown under sterile conditions were reduced by aqueous extracts of shoots. Root systems were stunted and necrotic and lacked root hairs. The growth of Rhizobium species was not influenced by the presence of 40 or 80 mg/ml concentrations of extracts of shoots or rhizomes. Quackgrass may inhibit indirectly the legume-Rhizobium symbiosis by inhibiting root hair formation rather than directly inhibiting Rhizobium growth. The presence of soil microorganisms was not necessary for the development of quackgrass toxicity in soil or agar. Soil microorganisms reduced toxicity of quackgrass residues in soil.

Growth chamber studies were conducted to examine the soil residual properties of DPX-A7881, a new sulfonylurea herbicide. The phytotoxic residue levels in the soil were determined by a lentil radicle bioassay. The duration of activity was prolonged in soil adjusted to pH 7.6 and 8.1 relative to more acidic levels. The rate of breakdown in the soil was enhanced with increased temperature and soil moisture content; a significant temperature by moisture interaction was noted over the duration of the incubation period. The dissipation of DPX-A7881 in soil obeyed first-order kinetics in both studies. An accelerated rate of breakdown in unsterilized versus sterilized soil (pH 7.6) indicated that microbial degradation was an important factor affecting the persistence in alkaline soils. Herbicide residues in the soil caused a reduction in taproot length and number of primary lateral roots of canola seedlings 15 days after planting but there were no other morphological effects observed on the root. The secondary laterals, however, had generally recovered by this time.

Annual bluegrass is the most problematic winter annual weed in managed turfgrass. Acetolactate synthase (ALS)-inhibiting herbicides are effective for annual bluegrass control, but reliance on this mode of action can select for herbicide-resistant biotypes. Two annual bluegrass biotypes not controlled with ALS-inhibiting herbicides were reported at golf courses in South Carolina and Georgia. Research was initiated at Clemson University to verify the level of resistance of these biotypes to ALS inhibitors. Two ALS-susceptible (S) and suspected resistant (SCr, GAr) annual bluegrass biotypes were established in a greenhouse. Dose-response experiments were conducted on mature annual bluegrass plants using trifloxysulfuron, foramsulfuron, and bispyribac-sodium, all ALS-inhibiting herbicides. Additionally, a rapid diagnostic ALS activity assay was optimized and conducted using the same herbicides. For dose-response experiments, the rate of herbicide that reduced shoot biomass 50% (I50) values for the S biotypes were 13.6 g ai ha−1 for trifloxysulfuron, 7.0 g ai ha−1 for foramsulfuron, and 38.3 g ai ha−1 for bispyribac-sodium. Fifty percent shoot biomass reduction was not observed in either the SCr or GAr biotypes at eight times the labeled field rate of all ALS-inhibiting herbicides tested. For in vivo tests of ALS activity, the SCr biotype yielded I50 (concentration of herbicide that reduced ALS activity 50%) values 3,650, 3,290, and 13 times the S biotypes following treatment with trifloxysulfuron, foramsulfuron, and bispyribac-sodium, respectively. Similarly, I50 values for the GAr biotype were 316, 140, and 64 times greater than the S biotypes following the same herbicide treatments. This research indicates high levels of annual bluegrass resistance to multiple ALS-inhibiting herbicides in South Carolina and Georgia. Future research should focus on the mechanisms of ALS resistance in these annual bluegrass biotypes as well as alternative options for control not targeting the ALS enzyme.

Silky windgrass and annual bluegrass are among the most troublesome weeds in northern European winter crops, while problems with rattail fescue have been especially linked to direct-drilling practices. This study investigated the germination patterns of silky windgrass, annual bluegrass, and rattail fescue in multiple water potentials and temperature regimes. Temperature and water potential effects were similar between silky windgrass and rattail fescue, but differed from annual bluegrass. The three grass weeds were able to germinate under low water potential (−1.0 MPa), although water potentials ≤−0.25 MPa strongly delayed their germination. Silky windgrass and rattail fescue seeds were able to germinate at 1 C, while the minimum temperature for annual bluegrass germination was 5 C. Germination of silky windgrass and rattail fescue was very similar across temperature and water potentials, which implies similar emergence flushes under field conditions, allowing management interventions to follow the same scheme.

The fungal plant pathogen Microsphaeropsis amaranthi is virulent against a number of key weeds in the Amaranthaceae, including common waterhemp, and is under investigation as a bioherbicide. Common waterhemp has become a key weed in midwestern crop production systems and is a good target for a bioherbicide that could be integrated into weed management systems. We investigated the direct effects of a range of chemical herbicides and adjuvants upon conidia of M. amaranthi and found that many herbicides and most adjuvants were strongly inhibitory to germination. On the other hand, M. amaranthi was compatible with a selection of postemergence herbicides commonly used in midwestern weed management systems, including carfentrazone, chloransulam, and imazethapyr. Most glyphosate products suppressed or abolished germination of M. amaranthi conidia, but by testing adjuvants commonly used in glyphosate products and technical-grade glyphosate salts, it was revealed that this inhibition was due to formulation additives and not the active ingredient. When glyphosate and conidia of M. amaranthi were sprayed onto common waterhemp seedlings, the herbicide predisposed plants to infection by M. amaranthi. When M. amaranthi was applied 1 to 3 d after glyphosate, the glyphosate rate required to control common waterhemp was reduced by half. Similar results were observed on clones propagated from a common waterhemp plant resistant to glyphosate. When M. amaranthi was applied to seedlings 2 d before glyphosate, the efficacy of the herbicide was reduced. These findings demonstrate that positive interactions between herbicides and M. amaranthi exist but reveal practical difficulties that may limit the integration of the strategy in the field.

Aerial applications of 20% tebuthiuron {N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N,N-dimethylurea} pellets at 2.2 kg/ha (ai) in the spring effectively controlled post oak (Quercus stellata Wangenh.), blackjack oak (Q. marilandica Muenchh.), water oak (Q. nigra L.), yaupon (Ilex vomitoria Ait.), winged elm (Ulmus alata Michx.), downy hawthorn (Crataegus mollis Scheele), gum bumelia [Bumelia lanuginosa (Michx.) Pers.], and willow baccharis (Baccharis salicina Torr. & Gray) in the Post Oak Savannah of Texas. Tree huckleberry (Vaccinium arboreum Marsh.), black hickory (Carya texana Buckl.), honeylocust (Gleditsia triachanthos L.) and cedar elm (Ulmus crassifolia Nutt.) were partially controlled by 2.2 kg/ha of tebuthiuron. Canopies of buckbrush (Symphoricarpos orbiculatus Moench), southern dewberry (Rubus trivialis Michx.), American beautyberry (Callicarpa americana L.), and persimmon (Diospyros virginiana L.) were reduced for about two growing seasons after which the woody species recovered. American beautyberry appeared to increase in abundance by the third growing season after control of the other woody species. Honey mesquite [Prosopis juliflora (Swartz) DC. var. glandulosa (Torr.) Cockerell], eastern redcedar (Juniperus virginiana L.), and Texas persimmon (Diospyros texana Scheele) were not controlled by tebuthiuron at 2.2 or 4.4 kg/ha, and saw greenbrier (Similax bona-nox L.) and peppervine [Ampelopsis arborea (L.) Koehne] apparently increased following application of tebuthiuron.

A 2-yr field study was conducted to determine effects of posttreatment irrigation timing on pendimethalin efficacy and dissipation in turfgrass. Factors investigated included herbicide rate, formulation, and the interval between pendimethalin application and the initial posttreatment irrigation. Plots received an initial posttreatment irrigation of 1.25 cm 0, 7, 14, 21, and 28 d after treatment. Pendimethalin efficacy on smooth crabgrass was evaluated, and turfgrass foliage and the upper 2.5-cm layer of soil were periodically assayed for pendimethalin residues. Pendimethalin 1.71% granular provided better weed control than pendimethalin 60% wettable powder at all rates, irrigation events, and years. Efficacy of granular pendimethalin was not affected by a delay in posttreatment irrigation, whereas efficacy of pendimethalin in the wettable powder formulation was reduced when irrigation was applied later than the day of treatment. Chromatographic analyses indicated that an average of 54% of the applied pendimethalin (wettable powder formulation) was retained on turfgrass foliage immediately after treatment, compared to 9% for the granular formulation. Soil residue analyses confirmed that a greater proportion of applied pendimethalin reached the soil surface immediately after treatment in the granular formulation than in the wettable power formulation.

Populations of rigid ryegrass suspected of resistance to trifluralin due to control failures exhibited varying levels of susceptibility to trifluralin, with 15 out of 17 populations deemed resistant (>20% plant survival). Detailed dose–response studies were conducted on one highly resistant field-evolved population (SLR74), one known multiply resistant population (SLR31), and one susceptible population (VLR1). On the basis of the dose required to kill 50% of treated plants (LD50), SLR74 had 15-fold greater resistance than VLR1, whereas, the multiply resistant SLR31 had 10-fold greater resistance than VLR1. Similarly, on the basis of dose required to reduce shoot biomass by 50% (GR50), SLR74 had 17-fold greater resistance than VLR1, and SLR31 had 8-fold greater resistance than VLR1. Sequencing of the α-tubulin gene from resistant plants of different populations confirmed the presence of a previously known goosegrass mutation causing an amino acid substitution at position 239 from threonine to isoleucine in resistant population SLR74. This mutation was also found in 4 out of 5 individuals in another highly resistant population TR2 and in 3 out of 5 individuals of TR4. An amino acid substitution from valine to phenylalanine at position 202 was also observed in TR4 (3 out of 5 plants) and TR2 (1 out of 5 plants). There was no target-site mutation identified in SLR31. This study documents the first known case of field-evolved target-site resistance to dinitroaniline herbicides in a population of rigid ryegrass.

Dicamba (3,6-dichloro-o-anisic acid), 2,4-D [(2,4-dichlorophenoxy)acetic acid], and picloram (4-amino-3,5,6-trichloropicolinic acid) were applied as simulated spray drift to sugarbeets at several growth stages. When applied at early growth stages 2,4-D tended to decrease root yield but decreased purity and extractable sucrose content by as much as 54% when applied at later growth stages. Dicamba at 0.14 kg ae/ha and 2,4-D at 0.28 kg ae/ha decreased extractable sucrose/ha and tended to decrease root yield at harvest while picloram at 0.028 kg ae/ha did not significantly reduce root yield or sucrose content at harvest compared to the control. All rates of 2,4-D from 0.035 to 0.28 kg ae/ha increased sucrose losses during post-harvest storage at 5 C and 95% relative humidity. Dicamba and picloram at 0.14 and 0.028 kg ae/ha, respectively, caused similar storage losses. Sugarbeets that are inadvertently exposed to 2,4-D, dicamba, or picloram spray drift during the growing season should be processed immediately after harvest.

The tolerance of broad bean (Vicia faba L. Moruna and Alameda) to glyphosate [N-(phosphonomethyl)glycine] was studied under field conditions. Chlorosis, rolling of apical leaves, and reduced growth were the typical symptoms caused by glyphosate on broad bean plants. Visual phytotoxicity from glyphosate at 60 and 120 g ae/ha was very slight. Moderate or severe crop injury was caused by glyphosate at 360 and 600 g ae/ha, when applied before blooming. Seed yields were 2, 4, 9, 23, and 47% lower than the control after glyphosate applications at 60, 120, 240, 360, and 600 g ae/ha, respectively, when averaged over stages of growth and locations. Broad bean was more susceptible to glyphosate at the vegetative stage, when the plants had five to seven nodes, and less susceptible at the late-flowering and pod-bearing stage. Double applications of glyphosate at 120 g ae/ha reduced seed yield more than 10% compared with similar treatments at 60 g ae/ha, regardless of growth stage. Glyphosate decreased seed yields mainly by reducing the number of pods per plant, but germination, fiber content, and gross energy of the seeds were not altered.

Rhizomes of quackgrass [Agropyron repens (L.) Beauv.] and johnsongrass [Sorghum halepense (L.) Pers.] were buried at various depths in the soil in the fall of 1972 and 1973, then removed at different times for viability tests. Quackgrass rhizomes retained viability at 2 cm where the coldest temperatures were −17 C. Johnsongrass rhizomes survived the winter at depths of 20 cm or more, and were killed at temperatures below about −9 C. Quackgrass rhizomes had a higher concentration of lipids and higher proportions of unsaturated fatty acids than did johnsongrass rhizomes. This difference may contribute to the superior cold tolerance of the quackgrass rhizomes.

An experiment was conducted at two locations in Georgia and two locations in North Carolina during 1994 and 1995 to evaluate weed management in conventional-tillage bromoxynil-resistant Gossypium hirsutum L. (cotton). The weed management systems evaluated included different combinations of fluometuron preemergence (PRE), bromoxynil or bromoxynil plus MSMA early postemergence (EPOST), bromoxynil postemergence (POST), and cyanazine plus MSMA late post-directed (LAYBY). Fluometuron PRE improved control of Acanthospermum hisptdium DC. (bristly starbur), Cassia occidentalis L. (coffee senna), Chenopodium album L. (common lambsquarters), Desmodium tortuosum (Sw.) DC. (Florida beggarweed), Sida spinosa L. (prickly sida), Jacquemontia tamnifolia (L.) Griseb. (smallflower morningglory), and Anoda cristata (L.) Schlecht. (spurred anoda), compared to system that did not use fluometuron PRE. It also improved G. hirsutum yields at three four locations. Bromoxynil-containing systems provided better weed control and higher G. hirsutum yields than systems without bromoxynil. Bromoxynil EPOST controlled A. hispidium, C. occidentalis, C. album, D. tortuosum, S. spinosa, J. tamnifolia, and A. cristata. Control of these species was frequently improved by a second application of bromoxynil POST. Bromoxynil EPOST, POST, or EPOST plus POST did not control Senna obtusifolia (L.) Irwin and Barneby (sicklepod), but the addition of MSMA to bromoxynil EPOST improved S. obtusifolia control. Control of all dicotyledonous weeds was improved by a LAYBY treatment of cyanazine plus MSMA, and yields were improved at three of four locations with this treatment. Gossypium hirsutum was not injured by POST treatments of bromoxynil, and only temporary injury resulted from POST treatments of MSMA.

The effect of ethyl N,N-di-n-propylthiolcarbamate (EPTC) on photosynthesis was determined by C14O2 fixation by red kidney bean, Phaseolus vulgaris L., leaves following root application of this herbicide in a time-course study at various concentrations of the herbicide. EPTC inhibited C14O2 fixation at relatively high concentrations and long exposures of the herbicide, but did not alter the relative distribution of C14 in the various labeled compounds. The effect of EPTC on the respiration of excised embryos of corn and mung bean (Phaseolus aureus Roxb.) following 24 or 48 hr exposures to the herbicide. When the data were expressed on a fresh-weight basis, it indicated that EPTC stimulated respiration, but the same data expressed on an embryo basis did not show this stimulation. EPTC inhibited the growth and the concurrent increase in fresh weight of these embryos. The effect of EPTC on oxidative phosphorylation was determined with cucumber mitochondria. EPTC markedly inhibited both phosphorus uptake and oxygen consumption. Phosphorus esterification was more sentitive than oxygen utilization: this gave a reduced P/O ratio or an uncoupling effect. However, since the concentration of EPTC required to produce these effects was quite high, the physiological significance of the results is questionable.

Two propagules, axillary tubers of American pondweed and subterranean tubers of floating-leaf pondweed, previously undescribed, are reported. (Results for the first 4 yr of an 8 yr study of the yields of propagules certain aquatic plants grown are reported.) Single propagules of were planted April 1 in California or May 1 in Montana and harvested at the end of the growing season. Two types of propagules were studied, seeds and specialized asexual disseminules. (The latter included subterranean and axillary tubers, axillary buds, and stem segments.) The yields of seeds ranged from 0 with three species to over 2,000,000 with horned pondweed. One culture of sago pondweed formed over 36,000 subterranean tubers. Over 49,000 and 23,000 axillary propagules were produced by leafy and curlyleaf pondweeds, respectively.

August, September, and October were the most productive months for the development of propagules. The period varied with each species.

Studies of effects of weed competition on crop yields, and of procedures used in screening herbicides as to effectiveness for weed eradication, often require the planting of weed seeds under field or greenhouse conditions. Because of the many factors that contribute to delayed germination, field and greenhouse stands of weeds obtained in this way are often irregular and slow in developing. Although methods for obtaining laboratory germination of weed seeds have been worked out for many species the methods are not always applicable for field planting, particularly where removal of seed coats or use of alternations of widely different temperatures are advocated.