Experiments were conducted to examine the influence of adjuvant, growth stage, air temperature, and soil moisture on nicosulfuron activity, absorption, and distribution in quackgrass. Foliar absorption of 14C-nicosulfuron by quackgrass was greater in one-leaf than five-leaf plants. Total translocation of 14C-nicosulfuron was similar regardless of growth stage. However, nicosulfuron was more phytotoxic to three- and five-leaf than one-leaf quackgrass. Of the absorbed 14C, at least 71 % remained as 14C-nicosulfuron 168 h after application in three-leaf plants. 14C-Nicosulfuron absorption, translocation, and accumulation increased with increasing temperatures and was independent of soil moisture. Under adequate soil moisture conditions (-0.03 MPa), nicosulfuron activity was not influenced by temperature. Nicosulfuron efficacy was not influenced by soil moisture at a cool (11/6 C) temperature. As plant moisture stress increased, due to low soil moisture and increasing air temperature, nicosulfuron efficacy declined. Differences in nicosulfuron efficacy due to growth stage or environmental conditions could not be explained by differential 14C recovery, absorption, translocation, or accumulation. The addition of 28% urea-ammonium nitrate liquid fertilizer (UAN) to 14C-nico-sulfuron plus petroleum oil adjuvant (POA) provided greater 14C absorption than POA alone. Translocation of 14C with POA alone 168 h after treatment (HAT) was similar to POA plus UAN 24 HAT. However, 168 HAT there was no difference in the total amount of radiolabelled material translocated among the adjuvants.

Experiments were conducted to evaluate the occurrence and significance of release of herbicide through subterranean parts of nicosulfuron-treated johnsongrass. In a bioassay, the rooting medium of johnsongrass treated foliarly with 50 or 100 μg nicosulfuron plant−1 was inhibitory to the radicle elongation of sorghum and corn indicating the increased toxicity of the rooting medium of nicosulfuron-treated johnsongrass. The study with 14C-nicosulfuron indicated a basipetal translocation of foliarly applied nicosulfuron in johnsongrass to its roots/rhizomes and also into the rooting medium. By 30 DAT, around 23% of the 14C-label absorbed by johnsongrass was found exuded into the rooting medium. Radiochromatogram scans of thin layer chromatography plates of rooting medium indicated unmetabolized nicosulfuron as the major 14C-labeled compound (56%). The study also revealed a subsequent uptake of exuded 14C by corn roots sharing the medium. On the whole, the amount of 14C-label recovered from untreated corn amounted to 4.3% of the total applied to johnsongrass. In another experiment, the presence of nicosulfuron in the rooting medium was detrimental to corn growth. Reductions in corn growth occurred at concentrations of 10−8 M nicosulfuron or greater in the rooting medium. The sensitivity of corn to root uptake was attributed to greater accumulation of nicosulfuron at a faster rate in the growing parts.

Tribenuron antagonized diclofop control of wild oat in greenhouse studies. Wild oat shoot fresh weight was reduced 79% when plants were treated with diclofop at 1.12 kg ha−1, but only 46% when treated with diclofop at 1.12 kg ha−1plus tribenuron at 18 g ha−1. Increasing diclofop rate increased control of wild oat but did not overcome the antagonism. Separating the applications of diclofop and tribenuron by as little as 12 s reduced the antagonism by 40%, indicating the importance of close proximity of the two herbicides on the leaf surface for the antagonistic response. The inhibitory effect of diclofop at its target site (acetylCoA carboxylase) was not reduced by tribenuron. Metabolism of 14C-diclofop by wild oat leaves was not altered quantitatively or qualitatively by tribenuron. Tribenuron had no effect on spray retention or absorption of 14C-diclofop by wild oat leaves, and only slightly decreased the total amount of radiolabel translocated out of the treated zone on the leaf. However, tribenuron decreased basipetal translocation of diclofop from the treated zone by approximately 20%. The ability of tribenuron to reduce basipetal translocation of diclofop to meristematic regions of wild oat may be a factor in antagonism.

Laboratory experiments were conducted to identify adjuvants that improve absorption of imazethapyr, 2,4-D amine, and picloram by leafy spurge. Adjuvants (0.25% v/v) included crop oil concentrate (COC), methylated seed oil (MSO), nonionic surfactant (NIS), organosilicones (Silwet L-77®, Sylgard® 309, Silwet® 408), 3:1 mixtures of acetylinic diol ethoxylates (ADE40, ADE65, ADE85) with Silwet L-77, ammonium sulfate (2.5 kg ha−1), and 28% urea ammonium nitrate (UAN, 2.5% v/v). Adjuvants were combined with 14C-herbicide and commercially formulated herbicide product. Leaves were harvested 2 DAT, rinsed with 10% aqueous methanol to remove surface deposits of herbicide, and dipped in 9:1 hexane:acetone to solubilize cuticular waxes. Imazethapyr absorption increased by 38 to 68% when UAN was combined with COC, NIS, or MSO. Total absorption of imazethapyr plus COC, MSO, or NIS exceeded 86% 2 DAT when UAN was added. Urea ammonium nitrate reduced the amount of imazethapyr associated with the cuticular wax by 2.0%. Imazethapyr absorption was similar on both the abaxial and adaxial leaf surface when UAN was not added; however, 12% more imazethapyr was absorbed from the abaxial leaf surface than from the adaxial leaf surface when UAN was combined with Sylgard 309. Uptake of 2,4-D ranged from 54 to 78% and was greatest with Silwet 408 and 3:1 mixture of ADE40: Silwet L-77. Picloram absorption ranged from 3 to 19%. Buffering picloram treatment solutions to pH 7 and including 2.5 kg ha-1 ammonium sulfate increased picloram absorption to 37%.

The objectives of this study were to determine the inheritance of aryloxyphenoxypropionate (APP) resistance in the wild oat population UM33 and to determine the genetic relationship between resistance in UM33 and another population, UM1, which has a different cross-resistance pattern. Reciprocal crosses were made between UM33 and a susceptible population UM5, and between UM33 and UM1. Initial screenings of F1 and F2 Is populations derived from crosses between UM33 and UM5 were conducted over a range of fenoxaprop-P rates to determine a discriminatory dosage. F2 populations and F2-derived F3 families were then screened at this dosage (1200 g ai ha−1) to determine segregation patterns. Results from reciprocal UM33 x UM5 F1 dose-response experiments, and F2 and F2-derived F3 segregation experiments indicated that UM33 resistance to fenoxaprop-P was governed by a single, partially dominant nuclear gene system. To determine if resistance in UM1 and UM33 results from alterations at the same gene locus, 584 F2 plants derived from reciprocal UM33 x UM1 crosses were screened with 150 g ha−1 fenoxaprop-P. This dosage was sufficient to kill susceptible plants (UM5), but was not sufficient to kill plants with a resistance allele from either parent. None of the treated F2 plants exhibited injury or death, indicating that UM1 and UM33 resistance genes did not segregate independently. From this it was concluded that resistance in both populations is encoded at the same gene locus.

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.

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.

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.

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.

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.

Biological fitness and negative cross-resistance to other herbicides may be an important factor in managing triazine-resistant common lambsquarters. Greenhouse experiments examined the sensitivity of a resistant and a susceptible biotype to foliarly-applied bentazon, bromoxynil, dicamba, pyridate, and thifensulfuron. The noncompetitive vigor of triazine-resistant and susceptible common lambsquarters also was compared by growing plants in individual containers and harvesting them periodically throughout their vegetative period and at reproductive maturity. In the herbicide susceptibility study, 11 kg ai ha−1 atrazine had no effect on the growth of the resistant biotype, while it reduced susceptible common lambsquarters’ biomass by up to 68%. Estimated I50 values indicated the resistant biotype exhibited between 36 and 79% greater susceptibility to bentazon, bromoxynil, dicamba, and pyridate than did the susceptible one, while both responded similarly to thifensulfuron. In growth studies, the susceptible biotype achieved greater height, leaf area, and plant dry weight than the resistant population for the majority of harvest dates; however, values equalized between biotypes as the plants reached maturity. These experiments suggest that alternative management programs that exploit reduced fitness and increased herbicide susceptibility in triazine-resistant common lambsquarters could be developed. However, further studies are needed to determine whether these results have application for the management of triazine-resistant weeds in the field.

Experiments were conducted to establish low temperature thresholds for initiation of bud sprouting in dormant vegetative propagules of johnsongrass, purple nutsedge, and yellow nutsedge, and seed germination in cotton. Weed propagule sprouting and cotton seed germination responses to temperature were determined in a series of experiments conducted on a temperature gradient bar. Four calculated indices were used to quantify germination: mean percent germination per day, reciprocal median response time, and two versions of germination rate index. Data were analyzed as a series of regressions of germination indices against temperature. Maximum and minimum temperatures for germination were derived directly from the regressions and compared among species. Yellow nutsedge had the lowest temperature threshold (6 C), while the other species had low temperature thresholds of 11 to 12 C. Upper temperature thresholds were similar among species and ranged from 42 to 44 C. The lower low temperature threshold of yellow nutsedge sprouting compared to those for cotton, johnsongrass, and purple nutsedge suggests that early establishment by yellow nutsedge is an important factor in competitiveness in mixtures of these species. The results presented here suggest that application of principles derived from studies in seed biology might advance our understanding and ability to manage perennial weeds.

Studies were conducted from 1989 to 1993 in continuous no-tillage and moldboard plow corn fields to describe rates of velvetleaf seed predation with time and with seed density, and to identify principal seed predators. Rates of seed loss from the soil surface averaged 1 to 57% day−1 and were equivalent in the two tillage systems. Predator populations were the same in no-tillage and moldboard plow fields. The predation rate was generally low in winter months, increased in mid-summer, and declined in late summer. In 2 of the 4 yr, predation increased in October and November. The predation rate was described by an exponential decay function of seed density, with high rates of seed loss at low densities and leveling off to a nearly constant level at densities above 600 seeds m−2. Predation was highest where seed access was not restricted, and exclosures of 6.5 and 1.6 cm2 reduced predation up to 15 and 52%, respectively. Mice were important predators in the field. In laboratory feeding studies, the carabid beetle Amara cupreolata, the slugs Arion subfuscus and Deroceras reticulatum, and cutworms (Agrotis ipsilon) consumed imbibed velvetleaf seeds. Amara cupreolata and A. subfuscus were the only predators to damage unimbibed velvetleaf seeds.

Field experiments were conducted using entireleaf morningglory seed collected from areas of adaptation throughout the United States to determine whether biotypes or ecotypes exist and whether differences in susceptibility to acifluorfen exist. Initiation of first flower for the entireleaf morningglory populations ranged from 63 to 81 d after emergence. The interval between emergence and initiation of first flower decreased 2.8 d for each increase in degree of latitude from which the seed was collected. Plants originating from southern latitudes remained in the vegetative phase longer and tended to produce more total dry-weight biomass than plants originating from northern latitudes. Thus, ecotypes do exist for entireleaf morningglory because of adaptation to a specific environment. The adaptation allows ecotypes to utilize the length of the growing season associated with the area of origin. Trichome density on the adaxial leaf surface ranged from 147 to 206 cm−2 across the ecotypes and was not correlated with latitude or differences in acifluorfen susceptibility. Ecotypes differed in susceptibility to acifluorfen, but resistance among ecotypes was not evident.

The influence of weeds on crop yield is not only dependent on weed-related factors such as density and time of emergence, but also on environmental and management factors that affect both the weed and crop through time. This study was undertaken to develop the first physiologically based dry bean model that would account for the influence of weed competition. The specific objective was to develop a model that would account for the influence of weed competition on crop yield, and to use this model to test the hypothesis that crop yield losses resulted from competition for photosynthetically active radiation (PAR). To this end, a model that simulated the growth and development of dry bean was developed. The model performed daily calculations and simulated the phenology, leaf area expansion, dry matter production and distribution, and grain yield of dry bean based on weather and management information, but assumed adequate water and nutrients. The model was calibrated without weed competition at two locations and yr, and for these situations, adequately described the growth and development of the crop. Simulations were then run for five common ragweed densities and two times of emergence. Common ragweed leaf area was read into the model from input files and used to simulate weed shading. Shading of the dry bean canopy by common ragweed accounted for about 50 to 70% of the yield losses observed in field studies when weeds emerged with the crop. Weed shading did not account for the yield reduction measured from weeds that emerged at the second trifoliate stage of crop growth. The agreement between model predictions and field studies was consistent with the hypothesis that competition for PAR was a principal factor in weed-crop interaction. The ability to account for differences in weed densities, management, and environmental conditions suggested that modeling was a useful tool for evaluating the interaction among weeds and crops.

An intensive survey of two farmer-managed corn and soybean fields in eastern Nebraska was conducted to investigate parametric sequential sampling of weed seedling populations using a multistage procedure to estimate k, of the negative binomial distribution. k is a nonspatial aggregation parameter related to the variance at a given mean value. Mean weed seedling density ranged from 0.18 to 3.11 plants 0.38 m−2 (linear meter of crop row) based on 806 sampling locations. The average value of k, derived from 200 multistage estimation procedures, ranged from 0.17 to 0.32. A sequential sampling plan was developed with the goal of estimating the mean with a coefficient of variation (CV) of 10, 20, 30, and 40% of the sample mean. A sampling plan was also constructed to estimate the mean within a specified distance H of the true mean (H(x̄)= 0.10, 0.50 and 1.0 plants 0.38 m−2) with 80, 85, and 90% confidence. Estimating mean weed seedling density within a specified CV of the true mean CV(x̄) using parametric sequential sampling techniques was superior to estimating the mean within a specified distance (H(x̄)) of the true mean when considering the frequency of sampling and probability of error, especially at intermediate k values. At a k: value of 0.32 and 0.25, the difference between the actual CV(x̄) obtained from sampling and the CV(x̄) specified by the sampler was minimal. However, the accuracy of weed seedling density estimates was reduced with decreasing k values below 0.25, especially as the specified CV(x̄) increased.

Field experiments were established in Greece from 1990 to 1993 to study the effect of littleseed and short-spiked canarygrass density on wheat and barley yield, as well as the influence of littleseed canarygrass removal time on wheat yield. Competitive ability of both canarygrass species in wheat was similar, though littleseed canarygrass showed faster growth rate and formed more panicles than short-spiked canarygrass. Grain yield of wheat was not significantly affected by the presence of 76 plants m−2 of either canarygrass species, but it was reduced by 36 to 39% by the presence of 304 canarygrass plants m−2. None of the canarygrass species at any density had any effect on grain yield of wheat when cold and dry conditions prevailed during the early stages of growth. Grain yield of barley was not affected by any density of either canarygrass species. Growth and consequently panicle number of both canarygrass species were severely reduced by the interference of barley. The littleseed canarygrass removal time study indicated that emergence of this weed was completed by the middle of February. Grain yield of wheat grown with 150 littleseed canarygrass plants m−2 was not affected even when these plants were removed early in April. However, the presence of these canarygrass plants until harvest reduced grain yield of wheat by 23 to 28%.

Greenhouse experiments were conducted to assess the storage of nonstructural carbohydrates in the stems and roots of honey mesquite and their mobilization and utilization following defoliation by hand or clopyralid. All leaves were removed by hand from one group of plants while leaving stems intact. Another group received a foliar spray of clopyralid at a sublethal acid equivalent rate of 0.07 kg ha−1. Samplings at treatment on day 0 and at 14, 28, 42, and 58 d after treatment measured leaf area; dry weight of leaves, stems, and roots; and grams and percent total nonstructural carbohydrates in stems and roots. Controls accumulated dry weight and grams total nonstructural carbohydrates in stems and roots, while maintaining a constant percent total nonstructural carbohydrates throughout the experiment. Percent and grams total nonstructural carbohydrates in stems and roots of hand defoliated plants decreased by about one-half from day 0 to 14 as new leaf growth occurred, but increased and returned to pretreatment levels on day 28. A similiar, though slower, pattern of decline and recovery of total nonstructural carbohydrates in stems and roots occurred in response to defoliation by clopyralid. These results showed that the nonstructural carbohydrates in stems and roots of honey mesquite were important sources of carbohydrates to support new leaf regrowth following defoliation by hand or herbicide.

Laboratory studies were conducted to determine the effect of various factors on germination of tropical soda apple. Tropical soda apple seed were found to be moderately photoblastic with 30% germination occurring in the dark. Germination increased in response to green (545 nm) and red light (650 nm) to 75 and 66%, respectively, indicating phytochrome regulation. No germination occurred in response to blue (450 nm) and far-red light (750 nm). Germination increased from 4 to 64% between 10 to 30 C, but no germination was found at 5 and 40 C. Maximum germination occurred at 30 C. A second degree polynomial (Y= −48.7 + 7.9X – 0.2X2, R2 = 0.76) best fit the data. Seed germinated over a broad range of pH (2 to 14) with the highest germination occurring at pH 8 and no germination occurring at pH 2 and 14 (Y = −63.3 + 30.5X – 1.9X2, R2 = 0.95). Seed germination declined as osmotic potential increased from 0 to −1.0 MPa (Y = 76.7 – 12.9X + 5.2X2, R2 = 0.87). Seedling emergence was maximum from depth of 3 to 6 cm, but no seedling emergence occurred when seed were planted below 12 cm (Y = 10.2 + 17.8X – 1.6X2, R2 = 0.93). Optimum germination occurred at a depth of 5.6 cm. Mechanical and sulfuric acid scarification increased the rate of germination but not the overall percentage. Tap water or hot water increased the rate of germination 26% and KNO3, GA3, or ethephon increased it 53%. Tropical soda apple seed germinated in response to variable environmental and edaphic conditions which could allow establishment in diverse ecosystems.

Wild poinsettia is a serious weed in several crops, including peanut, grown in the southeastern United States. A study was conducted over 3 yr at Jay, FL, to characterize the growth and development of wild poinsettia grown from seed collected at Plains, GA; Marianna, FL; and Baton Rouge, LA. Seedlings from each selection were transplanted to the field and were grown either alone or in competition with peanut. Observations made throughout the growing season indicated that the Louisiana selection flowered later, grew to a larger size, produced more leaf area and biomass, and caused greater light attenuation and peanut yield reduction than the other two selections. The Georgia selection produced the smallest plants, least leaf area and biomass, and was least competitive with peanut. The Florida selection was intermediate for these parameters. Wild poinsettia dry biomass production was reduced by 78 to 83% when grown with peanut compared with monoculture wild poinsettia.