When velvetleaf plants from Mississippi and Minnesota populations were maintained in growth chambers with day/night temperatures of 29/23 C and photoperiods of 11, 12, 13, 14, and 15 h, flower buds and open flowers appeared first in the 12 h photoperiod. Buds and flowers appeared 2 to 4 d later at photoperiods of 11, 13, or 14 h. Increasing the photoperiod beyond 14 h to 15 h delayed bud appearance an additional 7 d in the MN plants and 12 d in the MS plants. Open flowers appeared four to five nodes higher, 10 d later at 15 than at 14 h in the MN plants and 20 d later in the MS plants. Vegetative shoot weight and fruit weight 73 d after emergence were greater in 13 h or longer photoperiods than at 11 or 12 h. In shorter photoperiods, MN plants produced more vegetative growth than MS plants, but the reverse occurred at longer photoperiods where MS plants were taller than MN plants. These growth differences occurred because earlier shifts in allocation to reproductive growth in MN plants limited their vegetative growth, particularly in the longest photoperiods. Differences in rate of reproductive development between populations were not evident until photoperiod exceeded 13 h. Reciprocal transfer of plants of the MS population between short and long photoperiods revealed the durations of the juvenile (pre-inductive), inductive, and post-inductive phases to be 3 to 5, 7 to 8 (short day) or 30 (long day), and 10 to 11 d, respectively. Differences in competitive ability among latitudinal biotypes of photoperiodically-sensitive weeds may depend on time of emergence in the field and consequent photoperiod exposure. Weed growth simulation models to be used in development of expert systems for weed management should take photoperiodic sensitivity into account.

The nonuniform spatial distribution of weeds complicates sampling, modeling, and management of weed populations. Principles of a rational approach to analysis of weed spatial distribution, combining classical and spatial statistics, are presented using data for cumulative emergence of common lambsquarters in no-tillage soybean fields in 1990 and 1993. Classical statistics, dispersion indices, mean/variance relationships, and frequency histograms confirmed that raw and loge-transformed data were not normally distributed, that populations were aggregated, and that large-scale trends in population means violated assumptions of spatial statistics. Detrending was accomplished by median polishing loge-transformed data and confirmed by evaluation of standardized residuals and frequency histograms. Detrended residuals were used to construct omni-directional and uni-directional semivariograms to describe the spatial structure of the populations. A spherical model fit to the data was verified by cross validation. Semivariograms showed that common lambsquarters density was spatially autocorrelated at distances to 16 m, with more than 30% of the variance in density due to distance between field locations. Comparisons of kriged estimates and their standard deviations with and without detrending indicated that estimates using detrended data were more appropriate and more precise. Kriged estimates of common lambsquarters density were used to draw contour maps of the populations.

The movement of sulfonylurea herbicide-resistant (R) kochia pollen was investigated in a spring barley field near Moscow, ID, using a Nelder plot design in 1991 and 1992. Each 61 m diameter plot had 16 rays spaced 22.5° apart and contained 211 kochia plants. There were 12 susceptible (S) plants and one R plant along each ray. The R and S plants were 1.5 m and 3.0 to 30.5 m from the center of the plot, respectively. Wind direction and speed in the 16 vectors, air and soil temperature, and rainfall were monitored continuously. Mature kochia seed was collected from individual plants, planted in the greenhouse, and sprayed with chlorsulfuron to test for resistant F1 progeny. Results from the 2-yr study showed outcrossing of R pollen onto S plants at rates up to 13.1% per plant 1.5 m from the R plants and declining to 1.4% per plant or less 29 m from the R plants. At least 35% of the total R x S crosses occurred in the direction of prevailing southeastward winds. Predicted percentages of R x S crosses per plant ranged from 0.16 to 1.29 at 1.5 m, and 0.00 to 0.06% at 29 m. Thus, resistant kochia pollen can spread the sulfonylurea-resistant trait at least 30 m during each growing season.

Tolerance of strawberry to Colletotrichum gloeosporioides f. sp. malvae, (C. g. malvae) a mycoherbicide for control of round-leaved mallow, was tested in the growth chamber and field in an area infested with round-leaved mallow. No visual infections of C. g. malvae were observed on inoculated strawberry plants in the growth chamber or field. However, C. g. malvae was reisolated from inoculated leaves and petioles from both situations, but no adverse effects were observed on strawberry plant development in the growth chamber or on berry yield in the field. Round-leaved mallow competition was detrimental to strawberry establishment, and berry yield was reduced by half compared to weed-free plots. In round-leaved mallow infested plots, treatment with C. g. malvae resulted in more vigorous strawberry plants, with berry production almost to the level of weed-free. Therefore, use of this mycoherbicide for control of round-leaved mallow in strawberries is a viable option that fits well in an integrated pest management system.

The effect of herbicide and urea-ammonium nitrate (UAN) combinations on winter wheat injury in absence of noncompetitive weeds and weed control during a winter wheat-fallow and a winter wheat-sorghum-fallow rotation were investigated. Winter wheat was planted at different dates to obtain different growth stages for spraying in the spring. Winter wheat produced greater grain yields when planted Sept. 15 or Sept. 25, 1987, 1988, and 1989 vs. Sept. 1 at North Platte, NE, while at Sidney, NE, grain yield was higher in wheat planted on Sept. 10 or Sept. 20, 1988, compared to Aug. 26. Spring-applied UAN increased grain yield on wheat planted Sept. 10 compared to no UAN in 1988–89 at Sidney, but not in 1987–88, while at North Platte, grain yields were not affected by UAN. At Sidney 2,4-D ester at 0.6 kg ae ha−1, 2,4-D amine plus dicamba at 0.3 plus 0.1 kg ae ha−1, metsulfuron at 0.007 kg ai ha−1 plus 0.25% nonionic surfactant (NIS), and metsulfuron plus 2,4-D ester at 0.007 plus 0.3 kg ha−1 plus NIS decreased grain yields compared to one handweeding. At North Platte in 1988–89, when UAN was applied with 2,4-D ester, 2,4-D amine plus dicamba, or metsulfuron plus 2,4-D plus NIS grain yields were reduced compared to the handweeded check on wheat planted Sept. 15. Occasionally, metsulfuron plus 2,4-D ester plus NIS treated wheat yielded less grain than metsulfuron plus NIS treated wheat. One or more herbicide treatments reduced wheat grain yields 4 of 15 application dates. Crop injury was related to growth stage and health of winter wheat when treatments were applied. Wheat under stress was more susceptible to herbicide damage than healthy wheat. Metsulfuron and metsulfuron plus 2,4-D controlled kochia, tumble thistle, and redroot pigweed better after wheat harvest than 2,4-D or 2,4-D plus dicamba at North Platte, but allowed summer annual grass weeds to grow. Yields of grain sorghum planted after a 10-mo fallow period were higher following winter wheat treated with three of four herbicides than the handweeded treatment.

A simulation model was developed to predict the population dynamics and economics of velvetleaf control in a corn-soybean rotation. Data compiled from the literature were used to parameterize the model for two situations, one in which velvetleaf was infected by a Verticillium spp. wilt and one without infection. Verticillium was assumed to have no effect on corn or soybean yield. In the absence of control, simulated seedbank densities of a Verticillium-infected velvetleaf population were 5 to 50 times lower than for an uninfected velvetleaf population. The model was used to evaluate a threshold weed management strategy under the assumption that velvetleaf was the only weed and bentazon the only herbicide available for its control. In the absence of Verticillium, an economic optimum threshold of 2.5 seedlings 100 m−2 afforded the highest economic returns after 20 yr of simulation. Simulations in which velvetleaf was infected in 8 out of 20 randomly assigned years indicated a 6% increase in annualized net return and an 11 % reduction in the number of years that control was necessary. Sensitivity analysis indicated the parameter estimates having the greatest impact on economic optimum threshold were seedling emergence and survival, maximum seed production, and herbicide efficacy. Under an economic optimum threshold of 2.5 seedlings 100 m−2, management practices that manipulate the most sensitive demographic processes increased annualized net return by up to 13% and reduced long-term herbicide use by up to 26%. Results demonstrate that combining an economic optimum threshold with alternative weed management strategies may increase economic return and reduce herbicide use.

The question of when to control weeds traditionally has been approached with the calculation of critical periods (CP) based on crop yields. The concept of economic critical period (ECP) and early (EEPT) and late (LEFT) economic period thresholds are presented as a comprehensive approach to answer the same question based on economic losses and costs of control. ECP is defined as the period when the benefit of controlling weeds is greater than its cost. EEPT and LEFT are the limits of the ECP and can be used to determine when first and last weed control measures should be performed. Calculation of EEPT accounts for the economic losses due to weed competition that occur between planting and postemergence weed control. In this way it is possible to better evaluate the economic feasibility of using preplant or preemergence control tactics. The EEPT for DCPA application is analyzed in the context of onion production in Colorado. The EEPT for DCPA application was calculated from an empirical regression model that assessed the impact of weed load and time of weed removal on onion yields. The EEPT was affected by control efficacy, weed-free yield, DCPA cost, and onion price. DCPA application was economically advisable in only one of 20 fields analyzed because of the tow DCPA efficacy (60%).

Wild poinsettia germination was at least 95% when exposed to solution pH of 2.5 to 10 or solution osmotic potential of up to −0.8 MPa. No germination was observed at pH 12 or osmotic potential of −1.5 MPa. Light was not required for germination. Seed germinated at temperatures ranging from 20 to 40 C with maximum germination of 97% at 35 C. Wild poinsettia had the highest emergence (80%) from seed placed 2 to 4 cm below the soil surface, while 20% of the seed planted at 14 cm emerged. Wild poinsettia germinated over a wide range of conditions, which explains why it is becoming an increasingly serious problem.

Experiments conducted in North Carolina and Virginia compared weed control, peanut yield, and net returns with systems using imazethapyr applied at various times and the regional standard treatment of paraquat applied at the ground-cracking stage of peanut (GC) followed by acifluorfen plus bentazon applied POST. Imazethapyr was applied PPI, PRE, GC, or POST at 70 g ae ha−1. Imazethapyr also was applied sequentially PPI plus GC, PPI plus POST, and PRE plus POST at 35 + 35 and 70 + 70 g ha−1. Late-season control of common ragweed and a mixture of entireleaf, ivyleaf, and pitted morningglories by the standard treatment ranged from 85 to 100%. Spurred anoda was controlled 80%, and common lambsquarters and prickly sida were controlled completely. Control of common lambsquarters, prickly sida, and morningglory by imazethapyr applied one or more times was similar to control by the standard. Control by imazethapyr exceeded that by the standard only for spurred anoda. The most effective time for applying imazethapyr varied by species and locations. Imazethapyr was equally effective on common lambsquarters and spurred anoda when applied PPI, PRE, or GC. Prickly sida and morningglory were controlled best when imazethapyr was applied PPI or PRE and GC, respectively. Common ragweed was controlled poorly with single applications of imazethapyr. Applying imazethapyr sequentially improved consistency of control across the range of species. In most cases, imazethapyr applied sequentially at 35 + 35 g ha−1 controlled all weeds as well as or better than when applied once at 70 g ha−1. Overall, imazethapyr at the registered rate of 70 g ha−1 was most effective when applied PPI at 35 g ha−1 followed by 35 g ha−1 at GC. Except for common ragweed, weed control with this treatment was similar to that by the standard. Peanut yield and net returns with this treatment were similar to those with the standard at three of four locations.

Drift of sulfonylurea and phenoxy herbicides from spring cereal fields to nearby spring pea and lentil crops was simulated by spraying pea and lentil with 2,4-D or the 2:1 commercial mixture of thifensulfuron and tribenuron at rates of 0, 0.33, 1, 3.3, or 10% of the use rates (X) for spring cereal crops approximately 3 and 5 wk after planting pea and lentil. 2,4-D had minimal inhibitory effects on both crops at all rates tested. Lentil was slightly more sensitive than pea to 10% X 2,4-D. Thifensulfuron:tribenuron had no effect on either crop at rates less than 3.3% X. Two weeks after application of thifensulfuron:tribenuron, 10% X, and to a lesser degree 3.3% X rates, caused newly emerged leaves to become chlorotic, reducing chlorophyll content 25 to 50%. These treatments also reduced net photosynthesis by 37% and reduced or halted growth of the main stem. Early formation of leaves was reduced, thus tripling light penetration through the canopy. Five to six weeks after application, 10% X thifensulfuron:tribenuron had, in some treatments, more than tripled branching in pea, more than quadrupled branching in lentil, and reduced biomass as much as 42%. Flowering and maturity were delayed. Plants recovered from stunting by thifensulfuron:tribenuron to varying degrees depending on environmental conditions, and final seed yield generally was reduced less than 25%. In controlled greenhouse experiments, rate response to thifensulfuron generally was similar to that observed in field experiments. Pea was stunted less at 30 C than at 10 C, whereas lentil was affected similarly at these temperatures. Overall, visual symptoms from thifensulfuron:tribenuron exposure were more pronounced in pea than in lentil and were detectable at levels substantially lower than those that affected final seed yields.

Dry bean producers have limited postemergence broadleaf weed control options. The purpose of this research was to determine whether bentazon increased dry edible bean tolerance to postemergence imazethapyr applications. Imazethapyr and bentazon were applied with a petroleum oil adjuvant to ‘Olathe’ pinto bean in the field and greenhouse. Imazethapyr injured pinto bean 7 DAT in the field and greenhouse. Chlorophyll a content, a quantitative measure of bean chlorosis, decreased compared to the untreated control following imazethapyr application. When 840 g ha−1 of bentazon was tank-mixed with 53 g ha−1 of imazethapyr, bean injury decreased and chlorophyll a increased compared to imazethapyr alone. Fifty three g ha−1 of imazethapyr delayed physiological maturity by 8 and 15 d compared to the untreated control in 1991 and 1992, respectively. Pinto bean seed yields were not reduced compared to the untreated control. When 840 g ha−1 of bentazon was tank-mixed with 53 g ha−1 of imazethapyr, maturity was not delayed. 14C-Imazethapyr absorption decreased by more than 40% and translocation of 14C from the treated leaf decreased by more than 50% when 14C-imazethapyr was tank-mixed with bentazon compared to 14C-imazethapyr alone. The addition of 20 mM Na-acetate inhibited absorption of 14C-imazethapyr, but did not inhibit translocation of 14C. The decreased absorption and translocation of imazethapyr when tank-mixed with bentazon likely accounts for the safening effect observed in greenhouse and field studies.

The effects of formulation on clomazone volatilization and transport through soil were studied. After 22 days of leaching under unsaturated flow in 49-cm long intact soil cores, greater clomazone movement was observed in Plainfield sand than in Cisne silt loam or Drummer silty clay loam soils. Soil clomazone concentrations resulting in injury to oats occurred throughout Plainfield soil cores but were restricted to the upper 14 cm of Cisne and Drummer soils. In addition, clomazone was detected in the leachate from Plainfield soil only. In a similar study with Plainfield sand cores, clomazone was less mobile than atrazine; encapsulation of the herbicides in starch granules did not affect clomazone movement but greatly decreased atrazine movement from the soil surface. Similarly, starch encapsulation did not affect bioavailability of clomazone but did reduce bioavailability of atrazine. In a laboratory study with continual air flow, volatilization of clomazone applied to the soil surface was reduced by encapsulation in starch and starch/clay granules. Clomazone volatilization was not affected by soil water content within a range of 33 to 1500 kPa water tension. Following soil saturation with water, clomazone volatilization from both liquid and granular formulations increased. Granule size appeared to have a greater impact than granule composition on clomazone volatilization.

Field studies were conducted over 3 yr in central Spain to investigate the tolerance of common vetch and oat to some preemergence herbicides and their effects on weed control, forage yields, and botanical composition of the forage mixture. Pendimethalin was the only herbicide that injured common vetch. Pronamide and pronamide plus diuron injured oat by affecting plant emergence. Prevalent weed species were fumitory, henbit, and wild buckwheat. All herbicides provided more than 90% control of fumitory and most herbicides except pronamide provided more than 90% control of henbit relative to the untreated check. Wild buckwheat stands were reduced by isoxaben (68%), linuron (40%), prometryn (69%), pronamide (86%), and pronamide plus diuron (61%). More than 90% control of prostrate knotweed was achieved with isoxaben, pronamide, and terbutryn. Pronamide and pronamide plus diuron reduced forage yields and increased vetch in the forage. The untreated vetch and oat monocrop treatments showed the competitive advantage of oat over vetch and weedy species.

Experiments were conducted from 1989 to 1991 on two silt loam and two clay soils to determine the effect of herbicides applied to the previous crop on growth and yield of rice. All herbicides were applied preplant-incorporated at recommended rates adjusted as needed for soil texture. Rice was planted the following year. Imazaquin, imazethapyr, alachlor, metolachlor, clomazone, trifluralin, and atrazine did not injure rice the year following application. Norflurazon was the only herbicide to injure rice on silt loam soils, with injury at one silt loam location in one of two years. Norflurazon and fluometuron residues caused rice injury on clay soils, and chlorimuron residues caused injury in one year on a day soil. This chlorimuron carryover injury was from August-planted soybean but did not occur from June-planted soybean. Norflurazon, fluometuron, and chlorimuron temporarily reduced rice dry matter early in the season. No herbicide reduced either rough rice or percent head rice yield on any of the soils.

Italian ryegrass interference in broccoli was measured in field experiments and the data fit a rectangular hyperbolic competition model. The model predicted 58% of broccoli yield loss related to Italian ryegrass density when pooled over three y. An economic threshold value of 4.9 Italian ryegrass plants m−1 of crop row was determined to be the density required to cause a 3.6% yield loss, equal to postemergence weed control costs. Italian ryegrass densities of 600 to 1000 plants m1 of broccoli row caused 100% yield loss.

The duration of imazaquin soil activity on corn, grain sorghum, rice, and cotton was studied from 1988 to 1990. Imazaquin applied at 0.035, 0.07, 0.14 (1× rate), and 0.28 kg ai ha−1 was incorporated into the soil, and rotational crops were planted at various times after application. Initial crop visual injury was directly related to imazaquin application rate. Cotton was most susceptible, followed by corn, grain sorghum, and rice. The carryover of imazaquin from soybeans to cotton, grain sorghum, and wheat was studied in separate experiments in Arkansas in 1987 to 1989. Wheat planted in the fall following soybean harvest was not affected by imazaquin residues. Grain sorghum planted the following spring was injured (16–35%), but there was no yield reduction. Cotton recovered from early injury (< 35%) in one year and did not recover from severe injury (> 50%) in the other, resulting in yield reduction.

The adsorption of atrazine and alachlor was studied on samples of three horizons from soils with different textures and organic carbon contents. Soils were equilibrated with five concentrations of atrazine and alachlor using batch techniques. Adsorption affinity for atrazine and alachlor was approximated by the Freundlich constant (Kf), distribution coefficient (Kd), and the normalized Kd based on organic carbon (Koc). Adsorption was not significantly correlated with soil depth, clay content, or organic carbon. Atrazine adsorption was a linear function of equilibrium concentration for nearly all soil horizons but was nonlinear in most horizons for alachlor. The extent of atrazine adsorption was greater in all horizons of the fine-textured soils (Kd = 1.5 to 5.5) compared to coarse-textured soils (Kd = 0.40 to 0.87). The same general trends with texture were not apparent for alachlor. Conversion of Kd to Koc failed to reduce the variability in the linear adsorption coefficient for atrazine and alachlor in the different soils of this study.

Field experiments were conducted in 1991 and 1992 to evaluate differential soybean cultivar tolerance to SAN 582H and metolachlor applied PRE, as influenced by soil moisture. Eighteen d after planting, a 3× rate of SAN 582H (3.0 kg ai ha−1) reduced plant populations of ‘Wright,’ ‘Bedford,’ and ‘Stonewall’ and metolachlor did not. At leaf drop, neither herbicide reduced crop height with optimum moisture. With excessive moisture, 3× rates of SAN 582H reduced plant height of Wright, Bedford, ‘Walters,’ ‘Young,’ ‘Brim,’ and ‘Bryan,’ and metolachlor reduced height of ‘Sharkey.’ The greatest yield reductions were due to excessive moisture, regardless of herbicide. Neither herbicide reduced yields with optimum moisture nor did a 1× rate with excessive moisture. However, with excessive moisture, a 3× rate of metolachlor (5.0 kg ai ha−1) reduced yield of Bedford by 470 kg ha−1, and the 3× rate of SAN 582H reduced yield of ‘Rhodes,’ Walters, and ‘Centennial’ more than 500 kg ha−1. Overall, soybean tolerance to both herbicides was good, but if used above recommended rates and with excessive moisture, injury can occur.

Experiments were conducted at Lexington and Quicksand, KY, in 1989 and 1990 to determine the effect of preplant incorporated clomazone on weed control, crop injury, and yield of broccoli, cauliflower, green cabbage, red cabbage, and pak choi. Oxyfluorfen and trifluralin were included as standard treatments. Greater than 80% weed control was observed at both locations with 0.8 kg ai ha−1 clomazone, with the exception of smooth pigweed at Lexington. All clomazone treatments caused crop injury 2 WAT. Injury was still evident 8 WAT at clomazone rates of 1.7 and 3.4 kg ha−1, but was minor with rates of 0.8 kg ha−1 or less on broccoli, cauliflower, and red cabbage. Injury was less at Quicksand in both years and seasons over all clomazone rates, most likely due to higher soil organic matter content Yields of broccoli and cauliflower treated with 0.8 kg ha−1 clomazone were similar to yields with oxyfluorfen on a 3% organic matter soil at Lexington in 1989. However, clomazone at 0.8 kg ha−1 at Lexington reduced 1989 spring yields of green cabbage, red cabbage, and pak choi and 1990 spring yields of all cole crops as compared to oxyfluorfen. Clomazone at 0.8 kg ha−1 at Quicksand reduced yield of green cabbage and pak choi in spring 1990 only on a 5.2% organic matter soil. Fall yields of broccoli and cauliflower in both years were not reduced by clomazone at 0.8 kg ha−1 or less at either location. Our studies indicated potential for utilization of clomazone on cole crops in higher organic matter soils, especially if some early season crop injury and occasional yield loss can be tolerated.

Soil-applied imazaquin or chlorimuron plus metribuzin reduced ivyleaf morningglory photosynthetic rate, leaf area, biomass, and seed production per plant compared to the nontreated control. Photosynthetic rate of ivyleaf morningglory was reduced a minimum of 53%, and total leaf area was reduced at least 67% with either herbicide, regardless of ivyleaf morningglory densities. Ivyleaf morningglory biomass and seed production were reduced at least 62% with either herbicide across all ivyleaf morningglory densities. Soybean leaf area, photosynthetic rate, and biomass were not affected by herbicide treatment or ivyleaf morningglory densities of 2, 4, 6, or 8 per m of row.