When herbicide concentration was constant, absorption of 14C-glyphosate increased with increasing droplet size (326 to 977 μm). Amount of 14C-glyphosate translocated away from the treated area, expressed as percent of absorbed, increased as droplet size decreased. Herbicide concentration of the droplet was more important than droplet number or droplet size in determining glyphosate absorption and translocation. Absorption and translocation increased with increasing herbicide concentration regardless of whether droplet size or number was altered in conjunction with herbicide concentration. This relationship explained why low spray volume (increased herbicide concentration) increased herbicide efficacy. The concentration gradient between droplet and leaf, rather than droplet coverage, was the primary mechanism responsible for the observed effect. Large droplets caused localized tissue injury, which may have caused decreased translocation.

Imazethapyr is a selective herbicide used to control a wide spectrum of weeds in several legume crops, including pea. The effect of imazethapyr on pea-Rhizobium symbiosis was studied through its effect on microorganism growth, microorganism nodulation ability and plant growth. Symbiotic plants were damaged by imazethapyr concentrations higher than 1.73 μM applied preemergence. The number of nodules per plant was affected more than nodule size, suggesting a direct imazethapyr effect on the nodule initiation rather than on nodule development. However, imazethapyr did not directly affect Rhizobium because doses higher than 0.34 mM were required to cause slight effects on Rhizobium growth in a defined medium. Also, nodulation ability of bacteria treated with imazethapyr was not affected. These results suggest that imazethapyr inhibits the growth of the symbiotic plant rather than having a direct effect on the bacteria. Accordingly, symbiotic plants were less affected than the corresponding nitrate-reducing pea plants.

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.

Growth chamber experiments evaluated the influence of ambient temperature and soil moisture on cotton and velvetleaf response to pyrithiobac. Additional studies determined the basis for observed plant responses to 14C pyrithiobac. Cotton injury from six times the normal dosage was < 20% at 2 wk for all temperatures and soil moistures. Pyrithiobac injured velvetleaf less at lower soil moistures. Both species absorbed more 14C-pyrithiobac at 30/28 or 35/33 C than at 25/23 C. Cotton absorbed more herbicide than velvetleaf at all temperatures and soil moistures. Velvetleaf translocated < 16% of absorbed 14C out of the treated leaf while cotton translocated < 3% of absorbed material. At warmer temperatures, velvetleaf translocated less 14C when soil was dry (–1.0 MPa) than when plants were watered to field capacity (–0.03 MPa). This decreased absorption and translocation may affect pyrithiobac activity on velvetleaf growing in dry soil. Translocation differences did not fully explain whole plant effects. The metabolism difference may account for cotton tolerance.

Protoplasts were isolated from quackgrass to investigate the influence of two surfactants and ammonium sulfate on glyphosate uptake. Without adjuvants, the uptake of glyphosate was not detectable. The uptake of the reference compound 2-aminoisobutyric acid was linear with time and was inhibited 60% in the presence of 10 μM carbonyl cyanide m-chlorophenylhydrazone. Glyphosate uptake increased in the presence of both surfactants and ammonium sulfate. After 0.5 h of incubation the internal glyphosate concentration was 37% of the external concentration (100 μM) in the presence of 0.025 g L−1 Ethomeen T/25, 21% in the presence of 0.25 g L−1 Atplus 201, 13% with 5 mM ammonium sulfate, and 20% with 50 mM ammonium sulfate. The surfactant treatments did not reduce either the viability or the internal pH (= 7.1) as judged by fluorescein diacetate staining or 5,5-dimethyloxazolidine-2,4-dione distribution, respectively. The treatments with ammonium sulfate slightly increased the internal pH. The concentration of Ethomeen T/25 required to disrupt the protoplasts was 0.075 g L−1; Atplus 201 did not disrupt the protoplasts at concentrations up to 10 g L−1.

A study was conducted to evaluate the response of common and giant ragweed to postemergence applications of imazethapyr using relative growth rate parameters. The relative growth rate was the same for untreated common and giant ragweed through the 21 d harvest interval. Relative growth rates of treated common and giant ragweed were 50% lower than the relative growth rates of untreated ragweeds 21 d after treatment. Between 21 and 56 d after treatment, the relative growth rate of common ragweed declined an additional 13%, while the relative growth rate of giant ragweed declined an additional 38%. The sharp continued decline in the relative growth rate of giant ragweed indicated plant death. The moderation and slight increase in the relative growth rate of common ragweed between 21 and 56 d corresponded with the initiation of lateral axillary buds and the regeneration of plant growth. Relative growth rate parameters identified differences in the response of common and giant ragweed to imazethapyr as early as 21 d after treatment. Relative growth rate demonstrated utility by objectively measuring differences in the growth response of these two weeds that are moderately susceptible to imazethapyr under laboratory conditions.

Greenhouse studies determined the influence of 14 adjuvants and desmedipham plus phenmedipham on velvetleaf control and sugarbeet injury by triflusulfuron. A second study determined if six adjuvants, or desmedipham plus phenmedipham, influenced 14C-triflusulfuron absorption by cotyledon or first true leaves of velvetleaf. Triflusulfuron applied twice at 4.4 g ai ha−1 (split application), alone or in combination with any adjuvant, did not reduce sugarbeet dry weight. Desmedipham plus phenmedipham applied twice at 370 g ai ha−1 (split application) reduced sugarbeet dry weight 3%. The addition of an adjuvant to desmedipham plus phenmedipham reduced sugarbeet dry weight 21 to 66% depending on the adjuvant. Sugarbeet injury was not greater when triflusulfuron was applied with any adjuvant (except urea ammonium nitrate (UAN)) plus desmedipham plus phenmedipham compared to desmedipham plus phenmedipham plus the respective adjuvant alone. Velvetleaf was most easily controlled in the cotyledon stage of growth. Velvetleaf control by triflusulfuron increased from 0 (no adjuvant) to 80% by the addition of Scoil or Sylgard 309 plus UAN. Cotyledonary leaves of velvetleaf absorbed 28% more 14C-triflusulfuron than first true leaves. Adjuvant increased 14C-triflusulfuron absorption by cotyledonary leaves from 15% (no adjuvant) to 48 to 90%. Desmedipham plus phenmedipham decreased velvetleaf control by triflusulfuron plus X-77 or X77 plus UAN compared to triflusulfuron plus the adjuvant alone. Velvetleaf control by triflusulfuron plus other adjuvants was not reduced by desmedipham plus phenmedipham. Absorption of 14C-triflusulfuron by cotyledonary and first true leaf velvetleaf decreased when desmedipham plus phenmedipham was added to triflusulfuron plus Dash, Dash plus UAN, X-77, X-77 plus UAN, and Sylgard 309 plus UAN.

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.

Accurate prediction of potential weed seedling density would allow growers to implement control measures more effectively and could help avoid inappropriate and over application of preemergence herbicides. We compared three methods for handling soil samples to predict potential weed seedling emergence in plow-disk and no-tillage corn: seedling emergence from greenhouse trays, emergence from intact cores, and seed extraction following sieving. Seedbank numbers were highest for common lambsquarters followed by annual grasses and redroot pigweed, and seed numbers were higher in no-tillage than plow-disk plots. Coefficients of variation typically exceeded 60%. Density of seedling emergence from cores was more similar to field density than was emergence from trays. The percent of seeds in the seedbank that emerged was commonly more than 90% for annual grasses and usually less than 20% for common lambsquarters. All methods gave equivalent and relatively poor predictions of field population density. Spearman rank correlation between predicted and actual populations ranged from low negative values for common lambsquarters in no-tillage to 0.60 for annual grass emergence from trays in 1991. No method consistently gave highest correlations in both years and both tillage systems. Seedling emergence from intact cores, evaluated for 4 yr in plow-disk and no-tillage soybean fields, was significantly correlated (rs = 0.15 to 0.68) with emergence in the field. Pooling data from three to five neighboring sample sites increased the correlation between core and field emergence densities.

A methodological approach to determine the optimum time to control weeds that integrates aspects of weed biology, weed-crop competition, and economics is presented. The approach is based on the concept of Time Density Equivalent: this is defined as the density of weed plants that germinate with the crop and compete until harvest that causes the same yield loss caused by a group of weeds with a given density, time of emergence, and time of removal. A model was developed that accounts for pattern of weed emergence and permits determination of timing of weed control that minimizes economic loss due to weeds emerging both before and after treatments. The outcomes of the model are presented with two examples: corn in competition with velvetleaf and soybean in competition with Amaranthus cruentus. For both crops, six different weed control strategies involving preemergence, chemical, and mechanical postemergence treatments are considered. The results obtained with the model are compared with the calculation of net margin based on assumptions of simultaneous emergence of crop and weeds and no effect of different times of control. Different control strategies are compared considering not only maximum net margin but also its dependence on time of control, because a strategy with a lower value of maximum net margin, but a flatter net margin curve, allows more flexibility of time of control.

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.

Rate of seedling emergence, relative growth rate (RGR), and phenological development were compared in several accessions of rigid ryegrass belonging to three distinct resistance classes. The aryloxyphenoxypropionate resistant (AOPP-R) class had a faster and less variable seedling emergence than the sulfonylurea resistant (SU-R) and susceptible (S) classes. However, even the fastest of the AOPP-R accessions was within the range of the S and SU-R classes. No significant differences were detected among the resistant classes in seed dormancy, RGR, and the rate of phenological development. The rate of spike emergence, irrespective of the resistance class, was related to the latitude of the origin of the accessions, suggesting adaptation to the local climates since introduction. Due to considerable variation among weed populations for most biological attributes, the need to include several R and S accessions, in studies similar to the one reported here, is of vital importance. Because of the means and variances of the three resistance classes, at least four accessions from each resistance class would have been required to detect the observed differences between emergence rates of the AOPP-R and S classes with a confidence of 95%.

Protocols for sampling weeds in fields generally consist of selecting a given number of quadrats of a certain size, randomly located in the field, and counting the number of weeds of each type within each quadrat. Such a procedure is appropriate if weeds are distributed randomly in the field. However, it has been documented that weeds tend to cluster in fields so that the spatial distribution can often be described by a negative binomial. This research was conducted to identify an appropriate scouting protocol for use when weed populations are clumped in such a manner. Binomial, censored, and presence/absence sampling plans were compared through simulated sampling from negative binomial distributions of varying degrees of clustering and varying mean weed densities. Plans were compared in terms of bias and root mean square error. Study results indicate that binomial and presence/absence sampling offer reasonable alternatives to traditional sampling methods, except when there is extreme clumping. There is a trade-off between sampling effort needed per quadrat and number of quadrats needed for the sample. Traditional methods require intensive weed counting in each quadrat sampled, whereas binomial and presence/absence sampling protocols require less effort per quadrat, but more quadrats sampled for comparable results. Censored sampling displayed no advantage over binomial sampling in terms of bias and root mean square error, and is somewhat more difficult to do.

Research was conducted to formulate a temperature-dependent population level model for johnsongrass seed germination and rhizome bud break. A nonlinear poikilotherm rate equation was used to describe development rate as a function of temperature, and a temperature-independent Weibull function was used to distribute development times for the population. Seed germination and initiation of rhizome bud break of johnsongrass were collected under constant temperature conditions to parameterize the model. Seed germination rate increased with temperature up to 36 C, then declined at 40 C. Rate of rhizome bud break increased with temperature up to 32 C, then rapidly decreased with further temperature increases. Rate of rhizome bud break was higher than for seed germination at temperatures of 32 C or below, but lower at higher temperatures. Time to first germination or bud break event was longer for seed than for rhizomes, but subsequent progression of development was higher for seed. A population level temperature-dependent model was developed by coupling the poikilotherm equation with the Weibull function. The model was validated against two independent seed germination and three independent rhizome bud germination data sets.

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.

A simulation model of rice-barnyardgrass competition for light was used for two management applications. First, simulations using 47 weather data sets from four locations in Asia were conducted to evaluate the influence of weather variation on single year economic threshold densities of barnyardgrass. Second, rapid leaf area expansion and leaf area index were evaluated as potential indicators of improved rice competitiveness and tolerance to barnyardgrass. Influence of weather variation on single year economic thresholds was small under the assumption that competition was for light only. Increasing early leaf area expansion rate reduced simulated barnyardgrass seed production and increased single year economic thresholds, suggesting that the use of competitive rice cultivars may reduce the need for chemical weed control. The model predicted that rice leaf area index 70 to 75 d after planting was a good indicator of early leaf area expansion rate.

The development of weed management systems requires accurate prediction of weed-crop competition. In this paper, simple regression models of crop yield losses based on weed density and weed leaf area are compared. In weed leaf area models, variations in the relative damage coefficient (q) were also analyzed. Finally, three simple methods to assess weed cover were compared: visual, photographic, and optic device assessment. Leaf area models were at least as accurate as weed density models. However, the generality of the leaf area models was restricted by changes in q, according to the date of leaf area evaluation and the year. Although all methods to assess weed cover correlated adequately with weed leaf area, visual estimates were the best to predict crop yield losses perhaps because very low levels of weed leaf area could be distinguished visually better than by other methods.

Ripgut brome has a quantitative response to vernalization in relation to flowering. In greenhouse studies, cold treatment (5 ± 2 C) of 2, 4, or 6 wk shortened the vegetative period, but longer exposure did not further decrease the time required to flower. Plants vernalized as imbibed seeds for 8 wk took 17 d to flower following transfer from cold treatment to the greenhouse. Unvernalized controls flowered 53 d after planting in the greenhouse. Greatest total seed dry weight and vegetative shoot dry weight were produced by unvernalized plants, whereas lengthening periods of vernalization from 2 to 8 wk decreased both parameters. The percent of total seed dry weight to total shoot dry weight was significantly greater for vernalized plants than unvernalized controls. In field studies, ripgut brome plants established in the fall flowered sooner after resumption of growth in the spring than those planted in the spring. Plants seeded after April failed to flower until the following spring.

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.

Research was conducted to formulate a temperature-dependent population-level model for rhizome johnsongrass flowering. A nonlinear poikilotherm rate equation was used to describe development as a function of temperature and a temperature-independent Weibull function was used to distribute development times for the population. Johnsongrass flowering data were collected under constant temperature conditions to parameterize the poikilotherm rate equation and Weibull function. Coupling the poikilotherm rate equation with the Weibull function resulted in a population level temperature-dependent model. The model was validated against independent field data sets. The model accurately predicted rhizome johnsongrass flowering from plants emerging in the spring. The model performed poorly for plants emerging in summer. Adjustments to the high-temperature inhibition parameter of the poikilotherm rate equation improved model performance in the summer without affecting spring predictions.