Greenhouse and laboratory experiments were conducted to determine if the synergistic injury to sulfonylurea-tolerant soybean (STS soybean) caused by applications of thifensulfuron plus imazethapyr involves an interaction between the herbicides at the ALS enzyme. In vitro ALS from STS soybean (Asgrow 3200) was 10 times more tolerant to thifensulfuron than ALS from non-STS soybean (Williams 82). In vivo ALS was 30% of maximum activity 6 HAT in STS soybean treated with 4.4 g ha−1 thifensulfuron, but 0% of maximum in similarly treated non-STS soybean. Increasing thifensulfuron from 4.4 to 141 g ha−1 did not increase in vivo ALS inhibition in STS soybean, indicating a highly resistant ALS isozyme. Both cultivars responded similarly to imazethapyr in terms of visual injury, in vitro ALS inhibition, and in vivo ALS inhibition. Soybean injury and in vivo ALS inhibition increased with imazethapyr rate in both cultivars. The relationship between imazethapyr inhibition of ALS in vivo and soybean injury was curvilinear. In vitro ALS assays revealed no synergistic interaction between thifensulfuron and imazethapyr at the enzyme level. In vivo ALS activity was reduced to 7 and 10% of controls 24 and 48 HAT by the tank mixture compared to 56 and 84% for thifensulfuron alone and 20 and 28% for imazethapyr alone. These data show that the synergistic increase in soybean injury caused by tank mixing thifensulfuron with imazethapyr results from the cumulative inhibition of ALS, not a synergistic inhibition of the enzyme.

Greenhouse and laboratory experiments were conducted to determine the basis of imazethapyr tolerance in cowpea. Cowpea required more than 700 g ha−1 imazethapyr to reduce shoot growth 50% while corn required 70 g ha−1 to reduce growth 50% in greenhouse experiments. Extractable acetohydroxy acid synthase activity from cowpea 24 h after foliar imazethapyr treatment was only 20% of that of untreated cowpea but increased to control levels 72 h after treatment (HAT). The unifoliate leaves of cowpea absorbed 25% of the 14C-imazethapyr applied 72 HAT. Movement of 14C-imazethapyr out of the treated zone increased gradually to approximately 47% of the absorbed 14C 168 HAT. Parent imazethapyr decreased from 41 to 27% of the recovered 14C between 4 and 24 HAT, and continued to decrease to 14% by 168 HAT. Polar metabolites increased from 27 to 55% of the recovered 14C from 24 to 168 HAT. The polar metabolites of imazethapyr included hydroxyimazethapyr and conjugates of hydroxyimazethapyr. Tolerance of cowpea to imazethapyr is based on the plants ability to metabolize the herbicide to nontoxic metabolites.

Laboratory experiments were conducted to determine the mechanism of resistance to diclofop in two wild oat biotypes (designated ‘B’ and ‘C’ biotypes) from the Willamette Valley of Oregon. Resistance could not be attributed to differential absorption, translocation, or metabolism of diclofop. Resistance was not correlated with membrane plasmalemma repolarization following diclofop acid treatment. Compared to a susceptible (’S') wild oat biotype, acetyl CoA carboxylase from the B and C biotypes showed a 10.3 and 4.5 fold increase in the level of resistance, respectively, to diclofop acid. Cross-resistance to fenoxaprop acid was 5.5 and 7.3 times higher in the B and C biotypes, respectively than the S biotype. Correlation between resistance at the whole plant level and at the ACCase level was good for diclofop and fenoxaprop in the B biotype. For the C biotype, this correlation was not as good. Possible reasons for the discrepancy are given.

Poisonous plants and noxious weeds are often chemically examined to determine concentrations of secondary metabolites which are responsible for their toxic or biological activity. This study examined sample size requirements and sample methods necessary to quantify accurately the concentrations of individual and total toxic alkaloids in two tall larkspur populations. A high performance liquid chromatography analytical method was utilized to determine toxic alkaloid concentrations in all leaves from three individual plant stems and leaves from the remaining stems (remainder) from each of 50 plants in each population. To obtain high precision in quantifying toxic alkaloids in the larkspur populations (within 2.5 to 5% of the population mean, 0.95 confidence), very large numbers of samples (>50–200) were required. However, lower precision (within 10% of the population mean, 0.90–0.95 confidence) required only 20 samples. Similarly, testing parameters relating to toxin concentration in tall larkspur populations within 5 or 10% of the population mean also required hundreds of samples at power levels of 0.95 and α-levels of 0.05. Relaxing power and α-level requirements to 0.80 and 0.1 respectively, reduced sample size to about 30. The means obtained by four different sampling methods were similar (P>0.05). Alkaloid concentrations in leaf samples from single stems were highly correlated to whole-plant leaf (remainder) samples (r2≥0.76), indicating that harvesting leaves from single stems provided representative samples of the entire plant. The results indicate the difficulty in obtaining accurate information about toxins in poisonous plant populations for risk assessment by livestock producers or extension agents and demonstrate the necessity for efficient analytical methodology. Researchers evaluating concentrations of plant compounds in other weeds or toxic plants should consider variability, sampling procedure, and sample size before experiments begin.

The incidence of weed resistance to acetolactate synthase (ALS) inhibiting herbicides has increased in the United States. In 1993, a population of ALS-resistant common waterhemp was discovered after two confirmed applications of an imidazolinone herbicide. Following another imazethapyr application in the glasshouse, the resistant biotype demonstrated 130-fold resistance to imazethapyr at the whole plant level. The concentration of imazethapyr required to inhibit the ALS activity by 50% was 520 times greater for the resistant biotype than the susceptible. Plants also demonstrated cross-resistance to the sulfonylureas, chlorimuron and thifensulfuron, at the whole plant and enzyme levels. This particular discovery is of concern due to the low number of applications of the selection agent (imazaquin 1989, imazethapyr 1992, and imazethapyr in the greenhouse) and the high degree of cross-resistance eliminating several options for weed control.

White light-grown seedlings of smallseed dodder were cyclically irradiated from above with red light for 4 h followed by red plus far-red for 4 h. Tracings of initially nearly vertical, white light-grown seedlings were made from time-lapsed video images after four cycles of red light followed by red plus far-red. Stems of seedlings cycled into red light were negatively gravitropic and stems of seedlings then cycled into red plus far-red became photoreversibly, positively gravitropic. Observations suggest gravitropic sign reversal in smallseed dodder is regulated by photochrome.

Sulfentrazone was foliar applied at 34 and 56 g ai ha−1 alone or in combination with surfactants to soybean cultivars Hutcheson and Centennial and to sicklepod, coffee senna, smallflower morningglory, velvetleaf, and yellow nutsedge. The most sensitive weeds, including coffee senna, smallflower morningglory, and velvetleaf, were severely injured by the lowest rate when sulfentrazone was applied with surfactants. Sulfentrazone provided the highest control of yellow nutsedge with X-77. Soybeans were not severely injured by sulfentrazone applied alone, but 55% foliar injury occurred when the herbicide was applied with X-77. However, the seedlings were not killed. Sicklepod was the most tolerant of the weeds tested. In the absence of surfactants, the order of radiolabeled sulfentrazone absorption by the foliage was Centennial (5.8%) = Hutcheson (8.5%) = coffee senna (10.4%) < yellow nutsedge (17.0%) < velvetleaf (22.3%) = smallflower morningglory (24%). Sicklepod leaves did not retain droplets containing sulfentrazone when no surfactant was used. Species with the highest foliar absorption also showed the greatest phytotoxic response to the herbicide. Addition of surfactants to the spray mixture enhanced the foliar absorption and overall phytotoxicity of sulfentrazone in the weeds. An inverse relationship was detected between the foliar absorption of sulfentrazone without surfactants and the amount of cuticular wax present on the leaves. No such correlation was observed when surfactants were used. Thus, surfactants overcame the barrier to absorption imposed by the cuticular wax and, under these conditions, selectivity apparently became dependent upon species-specific cellular tolerance to sulfentrazone.

Individual barnyardgrass plants were grown in the absence of competition in a common garden environment. Cohorts were initiated in mid-March, late April, early June, mid-July, mid-August, and mid-September. Plants within a cohort varied from prostrate to upright. Early and late cohorts required slightly longer to achieve flowering and seed shatter than those initiated in late spring and early summer. The onset and development of senescence varied by as much as 5 wk between individuals within a cohort, and senescence progressed more rapidly for plants in later cohorts. Longest tiller length per plant averaged 150 cm and did not differ by cohort, but within cohort variation was approximately 33%. The number of tillers per plant declined with increasing delay in cohort initiation; within cohort variation exceeded 40%. Leaf numbers decreased from more than 10,000 per plant to less than 400 per plant with increasing delay in cohort initiation date. Individuals within a cohort had more than two-fold variation in leaf numbers. The number of inflorescences decreased from more than 4500 per plant to less than 100 with increasing delay in cohort initiation. Mean inflorescence length and frequency of different inflorescence lengths per plant varied between plants within a cohort. Vegetative biomass exceeded 3000 g for many plants in the early cohorts, and decreased to less than 25 g per plant for some individuals in the September cohort. More than two-fold variation in biomass occurred between plants within a cohort. Plasticity in morphology and phenology may contribute to the success of barnyardgrass as a weed.

Soil profile distribution of a single simulated seed dispersal event after tillage followed a nonlinear model. Approximately 40% of recovered simulated seed were found within 4 cm of the surface following chisel plowing and disking, and density declined steadily with depth to 20 cm. Moldboard plowing placed 50 to 60% of simulated seed to a depth of 11 to 16 cm. Simulated seed were evenly distributed to 8 cm depth by disking and did not differ regardless of whether plots were chisel or moldboard plowed prior to seed dispersal. More than 90% of seed remained within 2 cm of the surface with no-tillage. Simulated seed distribution was the same after a tillage rotation of chisel plowing, disking, moldboard plowing, and disking regardless if the dispersal event occurred prior to chisel or moldboard plowing. Two years of the tillage rotation and two dispersal events resulted in a distribution pattern greatly influenced by the most recent major tillage of chisel or moldboard plowing. Seedling emergence of common milkweed averaged 80% for seed buried 0.5 to 4 cm deep and emergence was negligible for seed on the surface or buried 7 cm in soil in pots in a growth chamber.

We examined the relationship between temperature and emergence of yellow nutsedge tubers to generate predictive models for the arid southwestern United States. Field experiments were conducted in California and Arizona to obtain phenological and temperature data needed to generate degree-day models. The effect of air temperature on emergence was tested with available programs using four methods to calculate degree-days (single sine, double sine, single triangle, and double triangle). Separate models were tested for each genotype examined (Arizona source and California source) as no one model was a good predictor of emergence when data were pooled. Results indicate that there is year to year variation in model accuracy but predictions of date of emergence can be made to within 2 d of actual emergence. This information can be used to schedule cultivations to reduce early yellow nutsedge competition in the field.

Biotypes of common cocklebur were identified, which were assumed to differ only with respect to sensitivity to MSMA. Water use of these biotypes when grown as isolated plants in a growth chamber was determined with stem flow gauges. The MSMA-tolerant biotype exhibited less whole-plant sap flow at media moisture levels above 17%. Furthermore, only in the tolerant biotype did sap flow differ between the upper and lower canopy; the upper canopy had greater sap flow. Field experiments were conducted to compare growth parameters, water use and competitive ability toward cotton of the MSMA-sensitive and -tolerant biotypes. Sensitive biotype plants were taller and had greater leaf area than those of the tolerant biotype; dry weight was equivalent. The tolerant biotype used less water and reduced seed cotton yield more than the sensitive biotype.

Soybean and ivyleaf morningglory were grown alone and together to quantify water use and growth after treatment with soil-applied chlorimuron plus metribuzin, imazaquin, or no herbicide. Soybean water consumption was 22 ml pot−1 day−1 or more when grown alone or with another soybean, regardless of herbicide treatment. Ivyleaf morningglory grown alone or with another ivyleaf morningglory consumed ≥ 32 ml water pot−1 day−1 when no herbicide was applied. Treatment with chlorimuron plus metribuzin or imazaquin reduced ivyleaf morningglory water consumption to ≤ 4 ml pot−1 day−1. Water use was reduced with either herbicide when soybean and ivyleaf morningglory were grown together, indicating little or no water consumption by ivyleaf morningglory. Ivyleaf morningglory height, root length, leaf area, and dry matter were reduced by imazaquin or chlorimuron plus metribuzin. Soybean leaf area was also reduced by herbicide treatment or the presence of another plant. Soybean biomass was not reduced in any planting combination, except when two soybean were grown together and treated with imazaquin. Nontreated ivyleaf morningglory partitioned more biomass into the shoots than roots. Ivyleaf morningglory responded to herbicide treatment by partitioning more biomass into roots.

Field studies evaluated response of soybean to Rhizoctonia foliar blight (RFB) disease in combination with varying densities of common cocklebur, hemp sesbania, or johnsongrass. Soybean plants at both V10 and R1 growth stages were not inoculated or inoculated with suspensions containing equal concentrations of Rhizoctonia solani AG-1 IA and IB mycelia. Intensity of RFB was rated weekly beginning at V1 soybean growth stage, and data were used to determine area under disease progress curves. Intensity of RFB was greater in 1993 than in 1994. When averaged across weed species and weed densities, soybean yield in 1993 was reduced 18% in plots inoculated with R. solani compared with those not inoculated. Intensity of RFB, however, did not differ between inoculated and noninoculated plots in 1994. Interactions between R. solani and weed density for RFB intensity and yield were not significant either year. Soybean yields in 1994, however, were reduced by hemp sesbania and johnsongrass in inoculated plots. Soybean maturity was delayed both years when hemp sesbania was present.

Repeated use of the preemergence herbicide triallate has selected for wild oat populations that are resistant (R) to field use rates. Field collections and an inbred R line were shown in greenhouse and petri dish dose response experiments to be 6- to 20-fold more tolerant to triallate than susceptible (S) lines. R populations and the inbred line were also resistant (8-fold) to the related thiocarbamate herbicide diallate, as well as to the chemically unrelated postemergence herbicide difenzoquat (60-fold). 14C-triallate uptake and translocation patterns were similar between R and S lines for the first 24 h after application. However, translocation of radioactivity was more rapid in S lines than R lines from 24 through 60 h after application. 14C-difenzoquat uptake was the same in R and S lines 12 h after application, but was 10 to 20% higher in R lines than S lines by 24 through 96 h after application. Similarly, translocation of radioactivity after 14C-difenzoquat application was 7 to 14% greater in R than S lines after 12 h, although translocated radioactivity amounts were not significantly different between R and S lines. The relatively minor differences in triallate and difenzoquat uptake and translocation patterns between R and S lines are most likely not of sufficient magnitude to explain the observed resistance levels.

The relative competitiveness of triazine-resistant (TR) and -susceptible (TS) common waterhemp, as a component of fitness, was assessed using TR and TS biotypes collected from the same field in Fillmore County, Nebraska. Competition between biotypes was evaluated under greenhouse conditions using a set of replacement series experiments with five ratios of TR:TS (1:0, 3:1, 1:1, 1:3, and 0:4) and four plant densities (50, 100, 150, and 300 plants m−2). No difference in final individual plant weight between TR and TS plants was observed in the absence of interbiotypic competition at all plant densities examined. Interbiotypic competition was indicated by ANOVA of plant weight and modified relative crowding coefficients (RCC) for plant weight, height, and volume. Replacement series diagrams showed TS plants were more competitive than TR plants, as measured by plant weight at densities of 50, 100, and 150 plants m−2. TS and TR plants were equally competitive at 300 plants m−2. Individual TS plant weight for 50, 100, and 150 plants m−2 was greatest at the 3:1 ratio (TR:TS) and decreased as the proportion of susceptible plants increased. Individual TR plant weight for 50, 100, and 150 plants m−2 was greatest at the 1:0 ratio and decreased as the TR proportion decreased. Lack of interbiotypic competition at 300 plants m−2 and decreasing TS plant weight with increasing ratios suggest intrabiotypic competition. RCC values for volume and height were calculated weekly during the experiment and indicated the competitive advantage of TS plants was constant from transplanting through harvest.

Ivyleaf morningglory weed-free period, weedy period, and density studies were conducted in 1993 and 1994 to determine the effects of soil-applied imazaquin and chlorimuron plus metribuzin on ivyleaf morningglory interference in soybean. A weed-free period of 4 wk was needed to maintain soybean yield equivalent to weed-free season long when herbicides were not used, but treatment with either chlorimuron plus metribuzin or imazaquin maintained yield comparable to weed-free at all intervals. Ivyleaf morningglory biomass was reduced by both herbicides across all weed-free periods. The period nontreated ivyleaf morningglory plants were tolerated without yield loss was only 2 wk, but this interval was extended to 8 wk after treatment with chlorimuron plus metribuzin or imazaquin. Without herbicides, soybean yield was reduced by four ivyleaf morningglory per m of row, but the highest population evaluated in this study (eight per m of row) did not reduce soybean yield after treatment with chlorimuron plus metribuzin or imazaquin. Ivyleaf morningglory biomass was reduced at all densities after treatment with either herbicide.

Narrower corn row widths, higher crop densities, and interrow cultivation may be used as part of integrated weed management (IWM). During a three-year study, we tested whether these factors affected corn growth, development and grain yield at final harvest, and weed biomass when weeds were late-emerging (after the three-leaf stage of corn). Increasing corn density from 7 to 10 plants m−2 or decreasing row width from 75 to 50 cm significantly increased corn leaf area index (LAI), and reduced photosynthetic photon flux density (PPFD) available for a mixture of weed species located below the corn canopy. Narrower rows and higher corn density significantly reduced biomass of late-emerging weeds. Corn yields increased significantly (10 to 15%) only when narrower rows were used. Intraspecific competition between corn plants in the higher density significantly reduced early corn growth and offset any gain in yield from reduced weed competition. In comparison to plots where late-emerging weeds grew uncontrolled, interrow cultivation did not decrease biomass of late-emerging weeds, hence did not increase corn yield. We recommend using narrower row widths to reduce weed competition and increase corn yield. Increased corn densities had no net yield benefit. Early-season weed management is crucial as the later-emerging weeds were less important in terms of their effect on yield.

The growth and seed return of auxin herbicide resistant (R) wild mustard was compared to that of a susceptible (S) biotype in wheat in the field. In the absence of herbicide, the S biotype accumulated shoot dry matter more quickly than the R biotype throughout most of the growing season. However, in only one of the two years did the S biotype set substantially more seed than the R biotype (3120 versus 2520 seeds plant−1). The recommended dosage of 2,4-D for wild mustard control (420 g ai ha−1) killed all S plants in both years of the study, and severely inhibited growth and seed return of R plants. Shoot dry matter accumulation and seed return of treated R plants were reduced 75 to 90% compared to the untreated control. However, at a density of 20 plants m−2 R seed return was still very high; 9000 and 5700 seeds m−2 in 1992 and 1993, respectively. The recommended dosage of dicamba (300 g ha−1) did not inhibit the growth and seed return of either S or R wild mustard to the same extent as 2,4-D. Dicamba at 300 g ha−1 reduced S shoot dry matter and seed return 80 to 90%, while R shoot dry matter and seed return was reduced 60 to 65%. The results of this study indicate a very high selection pressure for R wild mustard at recommended dosages of 2,4-D. Despite a high selection pressure, and considering the long history of phenoxy herbicide usage on the Prairies, the relatively rare occurrence of phenoxy herbicide resistant weeds implies that the frequency of resistant individuals is very low. From a mathematical model it was determined that the frequency of R wild mustard in an unselected population may be in the order of 10−30.

Experiments were conducted from 1990 to 1994 to investigate the density-dependent effects of rhizome and seedling johnsongrass on the magnitude of full-season interference in field corn. Corn grain yield was decreased as rhizome johnsongrass density increased. A maximum density of 3 rhizome johnsongrass plants 9.8 m−1 of row was considered critical to avoid yield losses above 5% of full-season weed-free corn. Interference of johnsongrass plants originating from seed and growing at densities up to 128 plants 9.8 m−1 of row did not affect grain yield. Seedling johnsongrass plants growing at a constant density of 9.8 plants m−1 of row did not affect yield from corn planted at five densities.

This study documented an undescribed aspect of medusahead germination behavior. Fibrous hairs were consistently observed growing from the coleorhizal surface of germinating medusahead seeds. Three medusahead accessions were investigated to determine temperature effects on coleorhizal hair growth and germination. Coleorhizal hair emergence at 8 C was first noted at 72 h. By 24 h at 18 C, 15 to 74% of seeds had hairs, and at 28 C, hairs were visible on 75 to 94% of seeds. Germination at 18 and 28 C was 80% at 96 h; germination at 8 C did not exceed 80% until 168 h. Less than 4% of germinated seeds were without coleorhizal hairs. Appearance of coleorhizal hairs confirmed that germinating medusahead embryos were active at least 72 h before germination was complete. These data affirm coleorhizal hairs are a normal part of medusahead germination biology. Coleorhizal hairs probably aid early phases of medusahead seedling establishment.