To broaden the spectrum of herbicides useful in potato production, the bxn gene for bromoxynil resistance, which encodes a nitrilase specific for bromoxynil, was introduced into ‘Lemhi Russet’ potato by Agrobacterium tumefaciens-mediated transformation. In GR50 studies, transformed potato clones were at least 70-fold more resistant to bromoxynil than the untransformed control. Resistance was due to rapid metabolism of bromoxynil to 3,5-dibromo-4-hydroxybenzoic acid, followed by conjugation to polar compounds. In yield trials, the best performing transgenic clones had total tuber yields equal to the untreated, untransformed control, but U.S. No. 1 tuber yields were 15 to 30% lower than the untreated, untransformed control. Tubers from three out of four transgenic clones had specific gravities, percent solids, and fry color similar to or better than the untreated, untransformed control. The data suggest that lower U.S. No. 1 yields in the transgenic clones were due to somaclonal variation and that expression of the bxn transgene had no consistent, detrimental effect on internal tuber quality.

A shattercane biotype resistant to primisulfuron was identified during greenhouse evaluations of seed obtained from southeastern and south-central Nebraska fields previously treated with primisulfuron or nicosulfuron a minimum of 3 consecutive yr. Absorption, translocation, metabolism, and acetolactate synthase (ALS) assay experiments were conducted to determine resistance mechanism(s) by comparing ALS-susceptible forage sorghum (ROX) to resistant shattercane (RS). The ROX had 10 and 12% greater 14C absorption than RS 24 and 96 hours after treatment (HAT), respectively. Absorption of 14C increased over time for both ROX and RS, while 14C translocation from the treated leaf to the shoots and roots was similar for ROX and RS. Unmetabolized primisulfuron accounted for > 80% of the radioactivity recovered 24 h after application for both ROX and RS. The ROX and RS had similar ALS-specific activities and total protein concentrations. Km values for RS and ROX were 10.4 and 5.8 mM pyruvate, respectively. The ALS isolated from RS was less sensitive than ROX to inhibition by primisulfuron. The I50 values for RS and ROX were 231 and 0.025 μM primisulfuron, respectively. The mechanism of primisulfuron resistance in this RS biotype is an altered ALS with decreased sensitivity to primisulfuron.

Repeated use of propanil to control barnyardgrass (BYG) and other weeds in rice has led to the development of propanil-resistant barnyardgrass (R-BYG). R-BYG possesses elevated aryl acylamidase activity levels, which cause rapid metabolism of propanil analogous to propanil degradation in rice. The current screening method for determining propanil resistance in BYG requires about 10 mo. The present study examined the use of chlorophyll fluorescence as a more rapid method to identify propanil resistance in BYG soon after it is suspected. Chlorophyll fluorescence data from excised BYG leaf tissue (R-BYG and susceptible-BYG [S-BYG]; 13- to 41-d-old) exposed to 100 μM propanil for 2 h indicated a 95 to 100% inhibition of electron transport (photosynthesis inhibition) in both R- and S-BYG. However, when incubated in water in the dark for 22 h after the initial 2-h treatment, metabolism in R-BYG was sufficient to reduce levels of absorbed propanil and facilitate recovery. Lack of metabolism of propanil prevented recovery in S-BYG, thus allowing the two biotypes to be distinguished easily by the chlorophyll fluorescence assay. Further studies using this 2-h exposure to 100 μM propanil followed by a 22-h recovery period evaluated several assay parameters. A longer recovery time and the effects of various propanil concentrations were also evaluated. A herbicide dose-response curve showed the greatest difference in photosynthesis inhibition for both biotypes at about 100 μM propanil, but both biotypes were inhibited > 95% when treated with 400 μM propanil. Inhibition of photosynthesis in both biotypes was greatest when the recovery incubation temperature was 35 C compared to 20, 25, and 30 C. Fluorescence data from harvested tissue stored moist in plastic bags at 23 C (to simulate shipment) showed that biotypes could be differentiated even when received as late as 4 d after harvest. Thus, samples can be harvested from the field soon after propanil failure and resistance or susceptibility to propanil determined after only a few days. This technique can greatly reduce the time, space, and labor currently required to determine propanil resistance in BYG.

Inheritance of resistance to diclofop was studied in three wild oat biotypes (designated B, C., and H) from the Willamette Valley of Oregon. Cultivated oat (cultivar ‘Monida’) was crossed, including reciprocals, to three wild oat biotypes. Leaves of each F1 plant were spotted with diclofop as a nondestructive test for resistance or susceptibility. All F1 hybrids were resistant, indicating that resistance is dominant and is under nuclear control. The F2 plants where Monida was the maternal parent were screened with diclofop, and F2 plants of the Monida/C cross were screened with fenoxaprop because the parent C biotype was resistant to fenoxaprop. At lower doses, a 3:1 (R:S) segregation ratio in F2 was observed and at higher doses a 1:3 (R:S) segregation ratio was often observed. The F2:3 families segregated in a 1:2:1 (all resistant : segregating resistant and susceptible all susceptible) ratio when treated with a 1.1-kg ae ha−1 dose of diclofop. This confirms that resistance to diclofop in the B, C, and H biotypes is primarily under monogenic control, with resistance being dominant to susceptibility at lower herbicide doses. At increased doses, susceptibility becomes dominant. Knowledge of the inheritance of resistance may help in the development of containment measures to prevent the spread of herbicide-resistance genes.

Field studies were conducted in 1989 and 1990 at St-Augustin, Quebec, Canada, to determine the economic threshold density of quackgrass in potato. Potato yield losses due to quackgrass interference increased with quackgrass population density. Potato yield ranged from 33 to 73% in 1989, and from 19 to 44% in 1990. The relationship between potato yield losses and quackgrass densities was described by a rectangular hyperbolic function. Dry weight of quackgrass proved to be the best predictor of potato yield loss compared to shoot number. The interference thresholds for 10% potato yield loss amounted to 25 shoots m−2 or 20 g total dry biomass m−2. The economic threshold varied between 0.04 and 2 shoots m−2 or 0.0165 and 1.5 g total dry biomass m−2, depending on the variables considered.

Density and spatial arrangement (rectangularity) effects on the competitive relationships, yield performance, and dynamics in canopy dominance of winter wheat and Italian ryegrass were evaluated using two addition series experiments. In experiment 1, combinations of six densities of each species formed the treatment matrix of addition series. In experiment 2, each species was tested at four densities and three rectangularities (RE) of winter wheat. In monocultures, crop density (plants per square meter) explained 82 to 85% of the total variation in the per-plant biomass of winter wheat in experiment 1. In mixtures of crop and weed, initial wheat density (N1) and initial ryegrass density (N2) and interaction of N1 and N2 explained 74 to 80% of the total variation in the per-plant biomass of winter wheat and 68 to 79% of Italian ryegrass in experiment 1. Intraspecific competition was apparent between 15 and 90 days after emergence (DAE) in winter wheat and between 90 and 170 DAE in Italian ryegrass. In mixtures, RE influenced plant size of Italian ryegrass up to 50 DAE only. Maximum winter wheat intraspecific competition occurred at 170 DAE, but maximum interspecific competition occurred during reproductive stages in mixtures. High RE increased seed yield, seed size, and harvest index of winter wheat and reduced biomass of Italian ryegrass. Grain yield of winter wheat was reduced up to 92% by competition from ryegrass. Even nine ryegrass plants in 100 winter wheat plants m−2 reduced winter wheat grain yield by 33%. However, the extent of loss in winter wheat grain yield was less in RE 16 (wider spacing) than in RE 1 (square planting) or 4 (close row spacing). Winter wheat was the stronger competitor during vegetative stages, but Italian ryegrass became the stronger competitor during the reproductive stages of development. Winter wheat leaves dominated at the top canopy during the vegetative stage, but ryegrass dominated at the top canopy during the reproductive stages. In the top canopy of mixtures at 200 DAE, the leaf area indices (LAI) of ryegrass was 6.6 times greater than winter wheat at RE 1 compared to only 1.6 times at RE 16. Greater LAI of Italian ryegrass in the top canopy reduced photosynthetically active radiation available to winter wheat by 68% at booting stage.

Medusahead is an invasive annual grass that, once established, severely affects range-land productivity and stability. Medusahead builds large seedbanks in the litter and on the soil surface. Effective weed control of medusahead involves either inhibiting germination from the seedbank, eliminating the seedbank, or enhancing germination so that plants are available for control. The purpose of this study was to determine the influence of nitrogen enrichment, immobilization, and nitrification inhibition treatments in the field on the size and germination status of medusahead seedbanks. The germination status of medusahead seeds in seedbanks was determined by periodically collecting field samples of surface soil and litter and bioassaying them in greenhouse emergence tests. Control seedbanks had increased seedling emergence with KNO3 or GA3 enrichment of the bioassay substrate. The combination of these two materials increased emergence. Nitrogen enrichment increased seedling establishment in the field. Carbon enrichment in the field decreased seedling establishment and increased medusahead seeds in seedbanks. Nitrapyrin treatment decreased medusahead in the field similar to carbon enrichment. In comparison to the control or other treatments, GA3 enrichment was not as effective in increasing emergence from nitrapyrin-treated bioassay samples. The combination of carbon and nitrapyrin treatments was very effective in eliminating medusahead emergence in the field, but in wetter years, it never completely eliminated medusahead seedling recruitment and subsequent reproduction. These treatments have promise for influencing succession in medusahead infestations if an adapted perennial species, capable of competing under low nitrogen levels, becomes available.

The relative genetic similarity of 37 wild oat samples was determined using amplified fragment length polymorphism (AFLP) analysis. These data were compared with the herbicide-resistance characteristics of each sample to determine the importance of mutation and gene flow in the spread of Acetyl-CoA Carboxylase (ACCase) inhibitor resistance. There was a strong association between the genotypic clustering of samples, their herbicide-resistance characteristics, and their field of origin. Up to eight separate patches in a field were genetically similar, confirming that gene flow by seed movement is important in the distribution of resistant (R) wild oat seed. A greater emphasis on field scouting and treatment of wild oat patches could reduce the spread of resistance within fields. Samples collected from different fields were also found to be genetically similar, indicating that R wild oat was spread between fields. Improved sanitation of tillage and harvesting equipment and the use of certified seed could limit such seed movement.

Populations of horseweed and Virginia pepperweed resistant to the herbicide paraquat were found in several fruit orchards in Essex County, Ontario, Canada, where paraquat had been applied four to five times per year for at least 10 yr. Dose–response experiments were conducted in the greenhouse for each species, using 2-to 4-mo-old plants from populations that had received repeated exposure to paraquat and those that had not. The paraquat dose that reduced shoot dry weight by 50% (ED50) was estimated for each population. Resistant populations of horseweed had ED50 values 25 to 35 times higher than susceptible populations. A resistant population of Virginia pepperweed had an ED50 10 times higher than susceptible populations.

PALWEED:WHEAT is a bioeconomic decision model for determining profit-maximizing postemergence herbicide treatments for winter wheat in the Washington–Idaho Palouse region. PALWEED:WHEAT performed relatively well economically in 2 yr of on-farm field tests. However, the model was less sensitive than desired in prescribing postemergence broadleaved herbicides in the presence of high densities of broadleaved weed seedlings. Therefore, PALWEED:WHEAT was revised in response to the field testing. This paper compares the revised model's agronomic and economic performance to the original model in computer simulations. The revised model, PALWEED:WHEAT II, differs from the original model in several respects: (1) exponential functions replace linear functions in predicting weed survival, (2) preplant application of a nonselective herbicide is entered as an exogenous binary variable, (3) separate indices of broadleaved and grass competition are substituted for an aggregate weed competition index in the wheat yield function, (4) hyperbolic replaces logistic functional representation of weed damage to wheat yield, and (5) separate models are estimated for winter wheat after spring dry pea and for winter wheat in all examined crop rotation positions. In simulations including a variety of agronomic and economic conditions, PALWEED:WHEAT II recommended postemergence herbicide types and rates that consistently complied with agronomic and economic theory. Furthermore, the revised model, especially when estimated from the relevant wheat after pea data set, was markedly more balanced in recommending both broadleaved and grass herbicides in response to observed densities of both weed groups. The substantial change in herbicide recommendations in response to changes in model functional specifications following field testing confirms the importance of field testing and revision of bioeconomic decision models.

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.

Palmleaf morningglory (IPOWR) and pitted morningglory (IPOLA) were compared in field and greenhouse tests under simulated rice field conditions to quantify flood tolerance characteristics of the two species. IPOWR survived a 13-cm flood in the field if the flood was applied 12 days after planting (DAP) or later. In contrast, IPOLA did not survive floods applied before 19 DAP. Only IPOWR survived a 5-cm flood applied 5 DAP in the field. In the greenhouse, both species were unable to emerge from under 1 cm of soil when flooded greater than 1 cm deep. Although both species emerged from water-saturated soil, IPOWR emerged earlier and produced more dry weight than did IPOLA. Emergence from the water surface of 2-cm-tall IPOWR was better than that of IPOLA when submerged in water up to 7 cm deep, and total dry weight (as a percent of a nonflooded control) was two to four times greater for IPOWR. Even a 5-cm flood reduced leaf area, height, dry weight, and photosynthesis of IPOLA more than that of IPOWR. At 20 C and low oxygen (1%), germination percentage of IPOWR was about four times higher than IPOLA. Germination, growth, and certain physiological processes of IPOWR are more tolerant than those of IPOLA to moderately deep floods (and low soil oxygen levels). Thus, IPOWR is potentially a greater weed problem than IPOLA in rice fields. The fact that neither species was capable of emerging from soil under even shallow flood depths helps explain why morningglories are prevalent only in shallow or water-saturated areas on rice field levees.

Ridge truncation methods affected weed populations, weed control, and corn yields in a ridge-tillage production system. Ridges were left undisturbed, partially truncated, deeply truncated, or destroyed by full-width tillage. The effect of truncation on weed densities and control (visual estimates of biomass reduction) varied by weed species, herbicide treatment, and time of observation. Without herbicide, deep truncation resulted in the lowest densities of all species except giant foxtail. Band application of herbicide controlled all weed species as well as broadcast application following deep truncation. When herbicides were broadcast, weed control was usually not affected by ridge truncation. Ridge truncation often increased corn yields compared with no truncation due to better weed control. While deep truncation often resulted in lower weed populations following herbicide treatment than shallow truncation or disking, differences were not great enough to affect corn yields. When ridges were truncated, broadcast herbicide did not increase corn yields compared with banded herbicide.

The influence of soil moisture on the competitive relationship between yellow nutsedge and cotton was determined in an additive competition experiment where main plots were irrigation regimes and subplots were nutsedge densities. Increasing nutsedge density caused a significant linear decrease in seed cotton yield. Regression analysis indicated that approximately 9 (1993), 12 (1994), or 37 (1995) kg ha−1 of seed cotton yield was lost for each additional initial nutsedge tuber per meter of crop row. Both postharvest stem biomass, an indicator of total aboveground biomass, and seed cotton yield increased with increasing soil moisture in all 3 yr of the study. Increasing yellow nutsedge density reduced midseason cotton height in both wet and dry soil moisture regimes but only slightly reduced postharvest stem biomass in the wet treatment in 1995. There was an interaction between soil moisture regime and yellow nutsedge density only with respect to midseason cotton leaf water potentials. Soil moisture availability did not interact with the effect of yellow nutsedge density with respect to end of the season stem biomass or seed cotton yield.

Results from the “Potato Ecosystem Project,” a cropping systems study in northern Maine, were used to test the hypothesis that greater reliance on organic nutrient sources and less reliance on synthetic fertilizer sources can benefit weed management efforts. ‘Atlantic’ potato was grown in a 2-yr rotation within a factorial arrangement of three pest management systems, two soil management systems, and both rotation entry points. Weed control in the conventional (CONV) pest management system relied on full rates of herbicides, whereas the biointensive (BIO) system relied exclusively on cultivation. The reduced input (RI) pest management system relied on cultivation in 1991 and 1992 and on 50% of standard herbicide rates plus cultivation from 1993 to 1995. The two soil management systems, unamended (barley/red clover rotation crop; 1× synthetic fertilizer for potato) and amended (pea/oat/hairy vetch green manure rotation crop; manure, compost, and 0.5× synthetic fertilizer for potato) contrasted practices typical for the region to those designed to achieve rapid improvements in soil quality. Midseason weed biomass in potato was dominated by common lambsquarters. In 1991 and 1992, weed biomass in potato was least in the CONV system and did not differ between the RI and BIO systems. In 1993, weeds in both RI and CONV potatoes were effectively suppressed below the level measured in the BIO system. Soil management had no effect on weed biomass from 1991 to 1993 but became an important factor affecting weeds in the BIO system in 1994 and 1995. Weed biomass was 77% lower in 1994 and 72% lower in 1995 in the amended soil management system than in the unamended system. No significant yield loss due to weeds was detected in the 1994 BIO system, but in 1995 yield loss due to weeds was 37% in the unamended system compared to 12% in the amended system. Soil management effects on weeds in the 1994 BIO pest management system carried through to the following season's germinable seed bank. Density of germinable common lambsquarters seed (0 to 10 cm soil depth) in the 1995 BIO system was 4,082 m−2 in the unamended soil management system compared to 1,280 m−2 in the amended soil management system. We suggest that organic amendments and green manure promote a potato crop better able to compete with weeds and that these inputs be considered as potentially important components of integrated weed management systems that have minimal reliance on herbicides.

A hammer mill removed most of the lemmas, paleas, and pericarps from cheat florets. Typically, the cuticular layer of the testa was the only remaining intact layer, and damage to the embryos and endosperm was severe. A roller mill disrupted tissue organization of lemmas, paleas, and outer layers of the caryopses primarily at the cuts. Large gaps between the aleurone layer and testa, between testa and pericarp, and between the scutellum and endosperm were created. In the field, germination of mill-damaged florets was reduced, and florets exhibited progressive degradation the longer they were buried. Nematodes and fungi penetrated the cuticular layer of mill-damaged seed. Attaching a hammer mill or a roller mill to a grain combine to treat cheat seed before it is returned to the field could provide a novel method of cheat control.

Field studies were conducted at Goiânia, GO, Brazil, on an Oxisol (clayey, kaolinitic, isothermic, Typic Haplustox) and at Jussara, GO, Brazil, on an Oxisol (loamy sand, kaolinitic, isothermic, Typic Haplustox) during 1995 and 1996 to determine the carryover effect of fomesafen, imazamox, and acifluorfen, applied to edible bean, on rotational crops (maize, sorghum, rice, and millet) and to estimate the level of soil residues under Brazilian Savanna conditions. Averaged across locale, year, and rate, fomesafen dissipation time (DT50) (37.5 d) was longer than acifluorfen (27.5 d) and imazamox (25.9 d). For both locations, soil herbicide persistence (average of herbicides) was longer in 1995 than in 1996. This was due to higher soil moisture content in 1996. The sensitivity of rotational crops to fomesafen and imazamox residues was, in decreasing order: sorghum, corn, millet, and rice, and for acifluorfen: sorghum, corn, rice, and millet. The period between herbicide application and rotational crop planting (PAP) varied in agreement with the sensitivity of rotational crops to herbicide residues in soil and the persistence of the herbicide. Considering both location and year, the PAP for fomesafen (250 g ai ha−1) ranged from 69 to 132 d for corn, 114 to 179 d for sorghum, 29 to 95 d for rice, and 52 to 111 d for millet; the PAP for imazamox (40 g ai ha−1) ranged from 68 to 111 d for corn, 78 to 139 d for sorghum, 25 to 75 d for rice, and 40 to 102 d for millet; and the PAP for acifluorfen (170 g ai ha−1) ranged from 56 to 89 d for corn, 96 to 139 d for sorghum, 61 to 95 d for rice, and 43 to 82 d for millet.

The effect of herbicides applied to V3 soybeans on race 3 soybean cyst nematode (SCN) reproduction and glyceollin production in roots was measured. Soybeans were treated postemergence with 1 × and 2 × rates of herbicides plus adjuvants or with adjuvants alone, and SCN development was measured. Acifluorfen, bentazon, lactofen, crop oil concentrate (COC), and nonionic surfactant (NIS) applications reduced SCN egg population densities 50 to 60% compared with the untreated control 4 and 8 wk after application. The SCN reproduction on plants treated with fluazifop-P, sethoxydim, and imazethapyr was similar to the untreated control. Crop oil concentrate or NIS applications alone were as effective as acifluorfen, bentazon, or lactofen applications for reducing SCN reproduction. However, no additive effect of adjuvant-herbicide combinations was observed, nor did herbicide rate affect SCN reproduction. Treatments reduced SCN reproduction only when applied to soybeans and had no effect on SCN reproduction when applied directly to the soil. No treatment stimulated SCN reproduction relative to the untreated control. Soybeans treated with COC, NIS, acifluorfen, and bentazon also had more glyceollin detected than the untreated control. Herbicide-induced glyceollin production may have increased the resistance of soybean to SCN.

Experiments were conducted in the field and greenhouse to evaluate the effects of cultivar and sulfentrazone rate on soybean injury and yield from soil-applied sulfentrazone and to determine soybean inheritance of sulfentrazone tolerance. Excessive rainfall and cool growing conditions in 1996 enhanced sulfentrazone injury and caused yield reduction in the susceptible cultivars ‘KS4895’ and ‘Hutcheson’ at 0.56 kg ai ha−1 and KS4895 at 0.42 kg ha−1. Yields of tolerant cultivars ‘Deltapine 3478’ and ‘Manokin’ were not reduced. Sulfentrazone at 0.42 kg ha−1 reduced soybean stand 17 and 35% for tolerant and susceptible cultivars, respectively. Field and greenhouse inheritance studies were conducted on plant families developed from two crosses: Manokin (tolerant) by ‘Asgrow A4715’ (susceptible) and ‘Northrup King S59–60’ (tolerant) by KS4895 (susceptible). The sulfentrazone tolerance of Manokin and Northrup King S59–60 appeared to be controlled by a single gene, with tolerance being dominant to susceptibility. The allelic relationship of tolerance from these two sources needs to be determined.