The mode of action of quinclorac was investigated in broadleaf and grass species. Quinclorac induced characteristic auxinlike symptoms in broadleaf species but not in susceptible grasses. In susceptible grasses, quinclorac caused necrotic bands near the zones of elongation in shoots and roots. Electrolyte leakage was induced by quinclorac in smooth crabgrass and other susceptible grasses but not in tolerant grass or susceptible broadleaf species. In smooth crabgrass, increased electrolyte leakage and reduced fresh weight were rate dependent, and initially specific to young tissues. An inhibitory effect on elongation in the youngest leaf of smooth crabgrass and in primary roots of corn was detected 6 and 3 h after quinclorac treatment, respectively. Electrolyte leakage required more than 12 and 6 h in the leaf and root, respectively. Depolarization of corn root cell membrane potential was not observed in a 6-h treatment period. Results presented here provide additional evidence that quinclorac activity differs between susceptible broadleaf and grass species. In addition, the action of quinclorac appears to be similar in both shoot and root tissues of susceptible grasses. It is proposed that quinclorac-induced electrolyte leakage in susceptible grasses is a secondary response and that the primary mechanism of action involves inhibition of an as yet unknown metabolic process associated with cell expansion.

Factors accounting for the differential response of five quackgrass biotypes to primisulfuron were compared. Previous field and greenhouse experiments had identified a biotype from Montpelier, OH, as being less susceptible to primisulfuron than biotypes from Amherst, MA; Mahnomen, MN; Fargo, ND; and State College, PA. Differences in shoot, root, and rhizome fresh weights and rhizome bud numbers did not account for the reduced susceptibility of the Montpelier, OH, biotype to primisulfuron. Spray retention was lower in biotypes from OH and MA compared to the others. Absorption of 14C following 14C-primisulfuron application was greater in the Amherst, MA, biotype compared to the others. Only 29% of absorbed 14C was present as primisulfuron in the Montpelier, OH, biotype versus 41 to 45% in the other biotypes 72 h after application. The amount of primisulfuron present in the Montpelier, OH, biotype 72 h after application was less than half the amount present in the other four biotypes due to reduced spray retention and faster metabolism. These factors combined explain lower susceptibility of the Montpelier, OH, biotype to primisulfuron.

A laboratory study was conducted to determine the effects of two adjuvants and temperatures at time of treatment on efficacy, absorption, and translocation of thidiazuron defoliant on cotton. Five days after treatment at 30/21 C day/night temperatures, leaf drop was 17% with no adjuvant, 37% with addition of crop oil concentrate, 40% with ammonium sulfate, and 75% with two adjuvants combined. At 21/13 C day/night temperatures, there was less than 10% leaf drop with all treatments. At 10 d after treatment, leaf drop was not different among treatments at the high or low temperatures. Shoot regrowth at high and low temperature was reduced 55 to 60% with addition of both adjuvants and 44 to 50% with each adjuvant or with no adjuvant when compared to plants defoliated by hand. Absorption of 14C-thidiazuron was not affected by variations in temperature during the time of treatment but was affected by adjuvants. With no adjuvants, absorption was 7 to 10%. With 1.25% by vol crop oil concentrate, absorption was 33 to 46%. Addition of ammonium sulfate resulted in 18 to 19% absorption, and the combination of ammonium sulfate and crop oil concentrate increased absorption to 65 to 68%. There was no movement of radiolabel away from treated leaves as determined by autoradiographs of treated plants.

Studies were conducted to examine the micromorphology of leaf surfaces of coca and nova plants and to determine which spray diluents provided optimum coverage of leaf surfaces. Adaxial leaf surfaces of both species were covered with a smooth layer of polygonal-shaped epidermal cells which were covered with wax crystals. Abaxial leaf surfaces were composed only of papillae cells and the guard and subsidiary cells surrounding stomata. Abaxial leaf surfaces of both species were also covered with wax crystals. Average wax weights were 28 to 53 μg cm−2. Water droplets containing an organosilicone surfactant at 0.1% by vol spread better on leaf surfaces that did other adjuvants. Droplets of several different petroleum-based oils spread much better on leaf surfaces than either water droplets with adjuvants or droplets of soybean or cottonseed oil.

Winter wheat cultivars that are competitive with weeds help control weeds in crop rotations. Ten winter wheat cultivars were evaluated for interference with summer annual grasses in the wheat and the subsequent grain sorghum crop in a winter wheat-ecofallow sorghum-fallow rotation in which there are two 10–mo fallow periods and two crops in 3 yr during 1983 to 1987. The medium–tall (100 to 109 cm tall) and medium–statured (90 to 99 cm tall) winter wheat cultivars (‘Buckskin’, ‘Siouxland’, ‘Lancota’, ‘Centurk 78’, and ‘Brule’) were more competitive than medium-short (80 to 89 cm tall) and short (68 to 79 cm tall) cultivars (‘Eagle’, ‘Homestead’, ‘Colt’, ‘Vona’, and ‘TAM 101’). Atrazine plus paraquat was applied to all cultivars 30 d after wheat harvest. When grain sorghum was planted in areas previously seeded with medium–tall and medium-statured winter wheat, summer annual grass weed biomass in sorghum was 61% less than in grain sorghum seeded into areas previously planted with medium-short and short wheat cultivars. Use of pendimethalin plus 2,4–D in winter wheat and glyphosate plus alachlor in grain sorghum eliminated differences in summer annual grass weed density and weed biomass among wheat cultivars. Sorghum grain yields were improved 7% when herbicides were used in the winter wheat and sorghum but value of the increase was less than cost of herbicides. Substituting less costly herbicides for herbicides used in this study still would not have been enough to pay for cost of herbicides for five cultivars. Grain sorghum grown on weed–free stubble of medium–tall and medium–statured winter wheat produced more grain than grain sorghum grown after medium–short and short-statured winter wheat by 5%. Volunteer wheat density during the fallow period following grain sorghum was lower in areas originally seeded to Centurk 78 and Siouxland wheat while volunteer wheat density was higher in areas planted to Homestead and Vona.

Field experiments were conducted to determine the effect of a short-term spring-seeded smother plant on corn development and weed control. Yellow mustard was managed to provide interference durations of 2,4,6, or 8 wk, and maximum height of 10 or 20 cm. Three yellow mustard planting patterns and eight seeding rates were evaluated during 1989 and 1990 at St. Paul and Rosemount, MN. Yellow mustard seeded at 2120 seeds m−2 with an interference duration of 4 wk and a maximum height of 10 cm decreased corn yield 17% and reduced weed dry weight 4 wk after yellow mustard emergence an average of 66%. Yellow mustard with a 2-wk interference duration did not reduce weed dry weight. Yellow mustard seeded at 2120 seeds m−2 with a 6- or 8-wk life cycle and 10-cm height reduced weed dry weight at the end of the interference period an average of 82% but delayed corn silk emergence an average of 5.3 d and reduced average grain yield 19%. Increasing yellow mustard height from 10 to 20 cm delayed corn silk emergence and reduced grain yield but did not decrease weed dry weight. Yellow mustard with an interference duration of 4 wk and a maximum height of 10 cm, seeded over the corn row at 530 seeds m−2, reduced weed dry weight 4 wk after mustard emergence an average of 51%, and resulted in an average corn grain yield reduction of 4%, compared with corn grown in monoculture averaged over weedy and weed-free treatments. These results suggest that it may be possible to develop spring-seeded smother plants that reduce weed biomass up to 80% but have only a small impact on corn yield.

Alternaria crassa, a mycoherbicide for jimsonweed control, was tested for differential effects on growth, infectivity, and phenolic metabolism of two field-collected jimsonweed varieties. Seeds from field-grown red- and green-stemmed jimsonweed varieties were grown in the greenhouse and spores were applied in aqueous solutions. Infection studies showed no significant differences between the two weed varieties when tested at various growth stages from cotyledonary to 8–leaf stage at 2.5 × 104 spores ml−1. Spore dose response tests with A. crassa spores (6.25 to 100 x 103 spores ml−1) showed no significant varietal differences in disease ratings. Extractable phenylalanine ammonia-lyase activity increased nearly equally (2- to 3-fold above control) in both varieties (8- to 9-leaf stage) 48 to 72 h after treatment with 5 × 104 spores ml−1. The only significant differences between treated varieties occurred 72 h after treatment. Exposure to A. crassa lowered total ethanol-soluble phenolic levels in both varieties 72 h after treatment. Spectrophotometric scans (400 to 700 nm) of ethanol extracts (72 h after treatment) indicated that infection caused some absorbance increases, especially in red-stemmed jimsonweed. Anthocyanin content was about 11-fold higher in red-stemmed than green-stemmed jimsonweed, and A. crassa treatment slightly increased the anthocyanin absorption peak of the red-stemmed variety.

Seed germination, one aspect of fitness, of chlorsulfuron-resistant (R) and -susceptible (S) kochia biotypes collected in North Dakota and Kansas was compared at 8, 18, and 28 C. Cumulative germination, characterized for each biotype using a Weibull function, was different at 8 and 18 C but not at 28 C. The Kansas R biotype reached 50% and maximum germination 70 and 300 h before Kansas S biotype at 8 C and 12 and 100 h before the S biotype at 18 C, respectively. The North Dakota R biotype reached 50% and maximum germination 12 and 100 h before the North Dakota S biotype at 8 C, respectively, and they were not different at 18 C. The resistance trait affects the cumulative germination process of kochia and may affect resistant-weed management strategies implemented early in the growing season when temperatures are lowest.

Laboratory experiments were carried out under aseptic conditions to study the effect of nitrogen applied as ammonium nitrate at 0, 20,40,60,80 and 100 ppm (wt per vol) in either distilled H2O or modified Hoagland's solution on germination and radicle length of branched broomrape in association with growing seedlings of different crops. As nitrogen rate increased, seed germination and radicle length of branched broomrape decreased linearly in flax, lentil, pepper, tomato, and wheat in modified Hoagland's solution. Similar responses to nitrogen application were found with distilled water. The parasite's seed germination was considerably lower in the media with wheat than with other crops, and lower still in the absence of crop seedings.

Separate field experiments were conducted for cotton and soybean in 1990 and 1991 to determine the influence of planting date on yield loss due to interspecific interference from entireleaf morningglory and sicklepod and to determine the relative competitiveness of each weed species. Percent soybean yield loss due to weed interference increased as planting date was delayed from early May to early June. Averaged over weed species, yield losses from 1.7 weeds m−1 row were 10, 18, and 20% for soybeans planted in early May, mid-May, and early June, respectively. Yield loss from 6.7 weeds m−1 row were 17, 31, and 35% at the early May, mid-May, and early June planting dates, respectively. Percent seed cotton yield losses averaged over weed species in 1990 were 33 and 28% for the early May and early June planting dates, respectively, at 1.7 weeds m−1 and 50% for both planting dates at weed densities of 6.7 plants m−1. The only experimental factor that significantly affected seed cotton yield in 1991 was weed density. Unlike soybeans, planting date had little effect on weed interference in cotton. Entireleaf morningglory was more competitive than sicklepod in both crops. Results suggest that selection of optimum soy bean planting dates may be a viable means of reducing losses due to weed interference.

Growth chamber and greenhouse studies were conducted to evaluate effects of long-term low-light intensity on wild oat control with imazamethabenz and fenoxaprop. Seventy percent shading imposed during the entire experimental period resulted in enhanced activities for both herbicides applied at early and later growth stages. Such shading also reduced wild oat regrowth following application of imazamethabenz and fenoxaprop. When applied to plants exposed to 70 to 90% prespraying shading, both herbicides had phytotoxicity similar to, or better, than plants grown under continuous shading. Postspraying shading has less effect on herbicidal activity than prespraying shading or prolonged shading, especially with imazamethabenz. Full-light treatment more adversely affected fenoxaprop activity than imazamethabenz activity.

Potato response to simulated drift of imazamethabenz, imazethapyr, or imazapyr was evaluated in field studies conducted near Aberdeen, ID, in 1989 and 1990. Herbicides were applied at rates corresponding to 0.02, 0.1, and 0.5 times the typical use rate (x rate) of each herbicide. Simulated drift treatments were applied at potato emergence, tuber initiation, or tuber bulking. Foliar injury symptoms varied with herbicide, rate, and potato growth stage at the time of application, but were generally most severe with imazapyr and least severe with imazamethabenz. Yield losses also varied with herbicide, rate, and potato growth stage at the time of application, but generally were greater when drift occurred at tuber initiation or tuber bulking than at potato emergence. Losses in U.S. Number 1 (highest quality) yield were greater than losses in total yield, indicating that tuber quality was affected more by the herbicides than was tuber biomass accumulation. U.S. Number 1 yield was not reduced by simulated drift of imazamethabenz at 0.02x but was reduced 25% by the 0.1x rate applied at tuber bulking. At the 0.5x rate, U.S. Number 1 yield was reduced 36, 85, or 92% when drift occurred at potato emergence, tuber initiation, or tuber bulking, respectively. For imazethapyr, U.S. Number 1 yield losses ranged from 0 to 68% for the 0.02x rate; 19 to 98% for the 0.lx rate; and 64 to 100% with the 0.5x rate, depending on potato growth stage at the time of application. Losses were least when drift occurred at potato emergence. Simulated drift of imazapyr at the 0.02x rate applied as potatoes were emerging reduced U.S. Number 1 yield 79%. All other imazapyr treatments caused ≥99% loss in U.S. Number 1 yield.

Greenhouse experiments were conducted to determine the effects of drought stress length and relief on velvetleaf growth and bentazon efficacy with crop oil concentrate (COC) and 28% urea ammonium nitrate (UAN). Drought stress caused leaf water and osmotic potentials to decline linearly over time. Leaf water potential ΨL) declined 0.02 and 0.08 MPa day−1 for velvetleaf subjected to −0.03 and −0.4 MPa soil water potential (ΨP), respectively. Osmotic potential (ΨO) declined 0.02 and 0.21 MPa day−1 for velvetleaf subjected to −0.03 and −0.4 MPa ΨP respectively. Bentazon did not reduce drought-stressed compared to non-drought-stressed velvetleaf dry weight 21 d after treatment with either COC or no adjuvant but did decrease dry weight with UAN. Reduced velvetleaf dry wights in drought-stressed velvetleaf compared to non-drought-stressed velvetleaf could not be accounted for by increased bentazon penetration in the drought-stressed plants. Rewatering velvetleaf subjected to 7 d of −0.4 MPA ΨP caused them to be more sensitive to bentazon than velvetleaf subjected to −0.03 MPa ΨP except when UAN was the adjuvant.

Greenhouse experiments were conducted to evaluate the effect of carbon chain length and percent ethoxylation of primary linear ethoxylated alcohols on postemergence control of johnsongrass and quackgrass with primisulfuron. Control of johnsongrass and quackgrass was greatest with primisulfuron plus the experimental ethoxylated alcohols 1412-60 and 1412-70 which had chain lengths of 14 and 12 carbon atoms (60:40 ratio) and 60 or 70% ethoxylation. Control of these two weed species decreased when primisulfuron was applied with ethoxylated linear alcohols of similar chain lengths but decreased percent ethoxylation or with alcohols of similar percent ethoxylation but decreased carbon chain length. Quackgrass and johnsongrass control with primisulfuron plus 1412-60 or 1412-70 was equal to or greater than the control obtained with primisulfuron plus the commercial adjuvants of the nonionic, organosilicone, sunflower methyl ester, or crop oil concentrate types. The experimental adjuvant 1412-70 provided equal or greater uptake and translocation of 14C-primisulfuron in johnsongrass and quackgrass compared to four commercial adjuvants.

Surface microrelief and agrichemical placement influence agrichemical movement through soil. This study determined if anhydrous ammonia shank placement influenced atrazine movement in a ridge tillage system. Atrazine and Br− were broadcast over the plot or banded on the ridge while N fertilizer was Injected into the ridge or valley. Rainfall (17 cm) was applied and leachate was collected incrementally from grid lysimeters placed 75 cm below the soil surface. Amounts of atrazine and Br− in leachate and postrainfall soil atrazine distribution were determined. The injection slot created by fertilizer application influenced chemical movement. When atrazine and the fertilizer injection slot overlapped, atrazine movement through the soil was enhanced. These data suggest that herbicide applications and fertilizer injection slots that are not covered should not overlap due to increased potential for movement.

Laboratory batch equilibrium studies were conducted to evaluate the sorption-desorption behavior of imazethapyr and its major plant metabolite, 5-hydroxyimazethapyr, in three Minnesota soils. Sorption of both compounds on all soils was low, and pH did not significantly influence sorption in the range of 4,8 to 7.1. Less 5-hydroxyimazethapyr was sorbed than imazethapyr on the same soil. Once sorbed, both compounds were only partially desorbable from all soils. Significant hysteresis and formation of nonextractable residues indicate that the small amount of chemical sorbed is bound to selective surfaces with strong bonds. Webster clay loam had greater irreversibility than Waukegen silt loam and Estherville sandy loam soil. The hysteresis observed in desorption may be responsible for the difference between mobility estimations made from laboratory sorption studies and the limited mobility observed in field experiments.

Laboratory and growth room experiments were conducted to determine effects of CGA-133205, oxabetrinil, and flurazole on grain sorghum hybrids ‘DeKalb 46’ and ‘DeKalb 48’ after the seed were treated with the herbicide safeners and stored for up to 24 wk at various relative humidity levels. Each safener treatment had an adverse effect on grain sorghum seed as reflected by fewer normal seedlings at 4 d when averaged over storage time and relative humidity levels. Flurazole and oxabetrinil increased the number of ungerminated seed of both hybrids, and 0.4 and 0.8 g ai kg–1 CGA-133205 increased ungerminated seed of DeKalb 46. At ≥ 8 wk after storage, a reduction in germination was observed for seed treated with flurazole or oxabetrinil compared to seed stored without safener. This reduction also occurred with 0.4 and 0.8 g kg–1 CGA-133205, but only at 24 wk after storage. Each safener treatment minimized reduction in seedling shoot fresh weight by herbicides compared to the no-safener check through 24 wk after storage.

Previous efforts to model crop yield loss from multiple weed species constructed competitive indices based on yield loss from individual weed species. Our model uses a multispecies modification of Cousens’ rectangular hyperbolic yield function to estimate a nonlinear competitive index for weed-crop interference. Results from 13 Minnesota and Wisconsin data sets provide measures of the relative competitiveness of mixed green and yellow foxtails, common lambsquarters, redroot pigweed, velvetleaf, and several other weed species. Competition coefficient estimates are stable over years, but not locations.

Field experiments were conducted over a 4-yr period, 1989 to 1992, in southern Manitoba to investigate potato (5. tuberosum) response to simulated drift of the dimethylamine salt of dicamba, 2.8 to 22.2 g ai ha–1; alkanolamine salt of clopyralid, 4 to 32 g ai ha–1; and tribenuron, 0.15 to 1.2 g ai ha–1. Rate ranges investigated corresponded to 2 to 16% of the recommended field rate for each herbicide. Dicamba at 22.2 g ai ha–1, clopyralid at 32 g ai ha–1, and tribenuron at 1.2 g ai ha–1 reduced total tuber yields 40, 29, and 41%, respectively, and marketable yields 74, 75, and 53%, respectively. Average weight of marketable-size tubers was unaffected by any treatment in any year. No tuber malformations were observed. Tubers produced on treated plants were planted the year following treatment to investigate the effects of simulated drift on subsequent tuber regrowth. Injury symptoms were evident for each herbicide in 2 out of 3 yr, but neither total nor marketable yields were affected.

The high near-infrared reflectance (0.76 to 0.90 μm) of Big Bend loco and Wooton loco contributed significantly to their orange-red and red image tonal responses, respectively, on color-infrared aerial photographs making them distinguishable from associated vegetation and soil. Big Bend loco could also be distinguished on color-infrared and near-infrared black-and-white video imagery where it had distinct red and whitish tonal responses, respectively. Computer analyses of photographic and videographic images showed that Big Bend loco and Wooton loco populations could be quantified from other landscape features. A global positioning system was integrated with the video imagery that permitted latitude-longitude coordinates to appear on each image. The latitude-longitude data were integrated with a geographical information system to map Big Bend loco populations.