Gas chromatographic and thermoelectric methods were combined in a quantitative study of absorption and movement of 4-amino-3,5,6-trichloropicolinic acid (picloram) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in bean plants in various intensities of moisture stress.

Foliar absorption of the herbicides studies was unaffected by extreme moisture stress. Herbicidal movement in the plant was markedly reduced, however. Distribution patterns were affected only when stress was permitted to progress to the wilting point. Picloram was more mobile than 2,4,5-T at all stresses.

Germination responses of redroot pigweed (Amaranthus retroflexus L.) seeds to temperature, water potential, atmospheric ethylene and carbon dioxide concentrations, light, and nitrate ion were examined individually. Seeds kept in dry storage at −20 C and tested within 2 yr of harvest germinated at 35 C (12 to 25%) or 39.5 C (40 to 65%), but only 0 to 2% at 30 C and below. Germination at 35 C was prevented by water potentials below −4 bars. When seeds were kept in dry storage at 24 to 28 C, afterripening became evident within 2 months. After storage at this temperature for 4 yr, seeds showed 38% germination at 14 C, 40% at 35 C and −8 bars water potential, and over 90% under more favorable conditions. Ethylene (1 to 100 ppmv) or continuous light enhanced germination at 30 C regardless of degrees of afterripening, although the ethylene effect was most dramatic in nonafterripened seeds. Ethylene at 100 ppmv caused 40% germination in these seeds, compared to 1% for controls. Neither carbon dioxide (0.001 to 4.5% v/v) nor dissolved potassium nitrate (0.02 to 0.2% w/v) influenced germination. These results are discussed in relation to environmental factors influencing field emergence of redroot pigweed.

Laboratory, greenhouse, and field experiments were conducted in 1972 and 1973 on the efficacy of controlling yellow nutsedge (Cyperus esculentus L.) in soybeans [Glycine max (L.) Merr.] with bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-(4)3H-one 2,2-dioxide], glyphosate [N-(phosphonomethyl)glycine], and perfluidone [1,1,1-trifluoro-N-[2-methyl-4-(phenylsulfonyl)phenyl] methanesulfonamide]. Young nutsedge plants (four- to six-leaf stage) were more susceptible to foliar sprays of the three herbicides than were older (six- to eight-leaf stage) plants. Foliar application of bentazon resulted in slow acropetal translocation of the herbicide, but killed the parent tubers. Good coverage of the foliage by bentazon sprays is essential, because the bentazon frequently kills only the foliage contacted by the spray. Applied as a postemergence broadcast spray, glyphosate injured soybeans severely at rates higher than 0.3 kg/ha, but did not satisfactorily control yellow nutsedge at rates less than 2.2 kg/ha. Perfluidone was active on yellow nutsedge when applied to either soil or foliage; yellow nutsedge was controlled best with preplant incorporated treatments at 4.5 kg/ha. Applications of 4.5 kg/ha perfluidone in the field significantly injured soybeans and reduced yields.

Chemical herbicides are without question the most effective immediate solution to most weed problems. They are not, however, the only option or necessarily the best for all weed control situations (23, 24, 34). Recent commercialization of two biological herbicides illustrates another weed control technology with potential (24, 27, 44, 50). Abbott Laboratories marketed the fungus Phytophthora palmivora Butler in 1981 for control of strangler vine (Morrenia odorata Lindl.) in Florida citrus groves. The Upjohn Company will market the fungus (Colletotrichum gloeosporioides f. sp. aeschynomene (c.g.a.) in 1982 for control of northern jointvetch [Aeschynomene virginica (L.) B.S.P.] in Arkansas rice (Oryza sativa L.) and soybean [Glycine max (L.) Merr.] fields (45).

Interference within a mixture of equal densities of common lambsquarters (Chenopodium album L.), kochia [Kochia scoparia (L.) Schrad.], and redroot pigweed (Amaranthus retroflexus L.) in sugarbeets (Beta vulgaris L. ‘Mono Hy D2’) was determined in a 3-yr field study. Yield of sugarbeet roots and sucrose per hectare decreased as intensity of competition from equal populations of these three weeds increased. At densities of 6, 12, 18, and 24 broadleaf weeds per 30 m of row, root yields were reduced 13, 24, 33, and 39%, respectively. Sucrose yields were reduced similarly. Fewer than three weeds per 30 m of row did not significantly reduce root yield. Reduction in root yield (Y) of sugarbeets caused by specific densities (X) of the three broadleaf weeds was predicted by using the linear equation Y = 1.64 + 1.88 X. The actual reductions in yield were always less than the predicted reductions when this equation was tested against 36 weed control systems because the competitive ability of broadleaf weeds that were treated with herbicides, but not killed, was suppressed during the growing season. By harvest, broadleaf weeds present in weed-control-system plots weighed an average of 75 to 85% less than broadleaf weeds present in nontreated plots.

Propanil is an acylanilide herbicide introduced in the early 1960s to control dicotyledonous weeds and grasses, including Echinochloa species in cultivated rice. Since then, propanil has been used extensively in rice production in the United States and in several other countries. Propanil is an inhibitor of photosystem II, but rice is tolerant to propanil because of the presence of a high level of aryl acylamidase that catalytically degrades the compound to nonphytotoxic products, i.e., 3,4-dichloroaniline and propionic acid. About 10 yr ago, biotypes of barnyardgrass and junglerice were discovered to be resistant to propanil. The resistance mechanism of these two biotypes has been shown to be elevated levels of aryl acylamidase activity. Various strategies to combat propanil resistance and to more fully understand the biochemistry involved in this resistance have been investigated. These include studies on the interactions of herbicides and other chemicals with propanil, rotation of rice with other crops (consequently the use of other herbicide modes of action), and use of alternative herbicides in rice. Certain compounds, including some organophosphate insecticides, are potent inhibitors of aryl acylamidase, which can act as synergists with propanil to increase phytotoxicity. Another compound that lacks insecticidal or herbicidal activity, PPG-124, has been commercialized as a herbicide synergist for propanil. These chemical and biochemical interactions and other factors involved in propanil-resistant Echinochloa weeds are presented and discussed.

The root-parasitic broomrape species cause severe damage to field and vegetable crops worldwide. This study evaluated the relationship between small broomrape development and temperature with red clover as a host plant. Red clover plants were grown in soil artificially infested with small broomrape seed in temperature-controlled growth chambers. Parasite development was quantified at 48 different accumulated growing degree days (GDD). Small broomrape parasitism and temperature were strongly related. Small broomrape tubercle initiation was delayed by low temperature. Tubercle development initiated at about 750 GDD and peaked at about 1,100 GDD. Small broomrape biomass accumulation correlated with the increase in tubercle number over time. Parasitism stages were divided into lag, log, and maximum phases that were strongly related to GDD. Development of a predictive system for parasitism growth stage is needed to allow precise herbicide application for effective control before small broomrape shoot emergence.

Most of the evidence for the production of toxic substances, by one plant, that affect the growth and survival of another, is circumstantial, according to a review by Bonner (5). There is direct evidence that the leaves of Artemisia absinthium L. (3), Encelia farinosa A. Gray (6), and Thamnosma montana Torr. & Frem. (2), produce substances which are toxic to other plants. A substance has been isolated from the roots of guayule which is toxic to guayule but which is unimportant agriculturally because this substance remains only a short time in the soil (4). Also dead brome grass roots have been found to be toxic to brome seedlings (1); however the evidence here is indirect (5). Le Tourneau et al. (13) have shown also that water extracts from tissues of many crops and weeds inhibit germination of wheat.

Harvest weed seed control (HWSC) is an Australian innovation, developed to target high proportions of weed seed retained at crop maturity by many major weed species. There is the potential, however, that a reduction in the average height of retained seed is an adaptation to the long-term use of HWSC practices. With the aim of examining the distribution of rigid ryegrass (Lolium rigidum Gaudin) seed through crop canopies, a survey of Australian wheat (Triticum aestivum L.) fields was conducted at crop maturity. Nine sites with medium to long-term HWSC use were specifically included to examine the influence of HWSC use on seed retention height. During the 2013 wheat harvest, L. rigidum and wheat plant samples were collected at five heights downward through the crop canopy (40, 30, 20, 10, and 0 cm above ground level) in 71 wheat fields. Increased crop competition resulted in higher proportions of L. rigidum seed in the upper crop canopy (>40 cm). The increase in plant height is likely a shade-intolerance response of L. rigidum plants attempting to capture more light. This plant attribute creates the opportunity to use crop competition to improve HWSC efficacy by increasing the average height of seed retention. Crop competition can, therefore, have a double impact by reducing overall L. rigidum seed production and increasing seed retention height. Examining the distribution of wheat biomass and L. rigidum seed through the crop canopy, we determined that reducing harvest height for HWSC considerably increased the collection of L. rigidum seed (25%) but to a lesser extent wheat crop biomass (14%). Comparison of + and − HWSC use at nine locations found no evidence of adaptation to this form of weed control following 5 to 10 yr of use. Although the potential for resistance to HWSC remains, these results indicate that this will not readily occur in the field.

Dicamba (dimethylamine salt of 2-methoxy-3,6-dichlorobenzoic acid) followed slightly behind the forward penetration of added water in both loam and sandy loam soils. Dicamba persisted longer than 2,4-D butyl ester in autoclaved and not-autoclaved loam and sandy loam soils. Tartary buckwheat, Fagopyrum tataricum (L.) Gaertn. planted at intervals in the autoclaved soils showed no injury from the 2,4-D at 16 oz/A after 4 weeks but was completely killed by dicamba at 8 oz/A after 12 weeks of incubation. In the not-autoclaved soils there was no visible effect on the buckwheat plants from 2,4-D after 2 weeks but nearly all of the plants were killed by dicamba after 12 weeks of incubation. Oxygen consumption in the loam soil was greatly reduced by dicamba at concentrations of 10,000 ppm or higher. Similar concentrations of 2,4-D butyl ester had only a slight and sometimes no effect on oxygen uptake. These concentrations are well in excess of those suggested for selective weed control in grain crops.

Weeds are often the major biological constraint to growing legume crops successfully, and an understanding of the critical period of weed control (CPWC) is important for developing environmentally sustainable weed management practices to prevent unacceptable yield loss. Therefore, we carried out two field experiments to identify the CPWC for two grain legume crops traditionally grown in Mediterranean areas: chickpea and faba bean. The experiments were conducted at two sites both located in the Sicilian inland (Italy). In chickpea, when weeds were left to compete with the crop for the whole cycle, the grain yield reduction was on average about 85% of the weed-free yield, whereas in faba bean the reduction was less severe (on average about 60% of the weed-free yield). The onset of the CPWC at a 5% yield loss level varied by species, occurring later in faba bean than in chickpea (on average, 261 and 428 growing degree days after emergence for chickpea and faba bean, respectively). In both species, the end of the CPWC occurred at the early full-flowering stage when the canopy of each crop enclosed the interrow space. On the whole, the CPWC at a 5% yield loss level ranged from 50 to 69 d in chickpea and from 28 to 33 d in faba bean. The results highlight the fact that faba bean has a higher competitive ability against weeds than chickpea. This could be attributable both to more vigorous early growth and to the plant's greater height, both factors related to a greater shading ability and, consequently, to a better ability to suppress weeds.

The influence of wheat straw cover, tillage, and irrigation on metribuzin and alachlor movement and dissipation in an Appling coarse sandy loam (Typic Hapludult) and the influence of soil depth on their rate of dissipation were measured in field and laboratory experiments conducted in 1987 and 1988. Overall, metribuzin moved more than alachlor in both years. Alachlor movement was greater in tilled plots compared to no-till plots in 1988. Metribuzin movement was greater in no-till plots in 1987. Straw cover had little effect on the movement of alachlor, but the presence of 2800 kg ha−1 of straw on the soil surface increased the downward movement of metribuzin in both years compared to soil with no straw cover. The rate of alachlor dissipation in the soil was faster in straw-covered and no-till plots in both years and in the high irrigation level in 1988. Metribuzin dissipation was not affected by any of these factors. Alachlor rate of dissipation did not differ among depths in the field; however, dissipation differed in the laboratory in the order 0 to 20 cm > 20 to 45 cm = 45 to 68 cm. Metribuzin dissipated faster at the 8-cm depth in the field compared to the 58-cm depth. In the laboratory, metribuzin dissipation rates followed the order 0 to 20 cm > 20 to 45 cm > 45 to 68 cm. At the greatest depth, the dissipation of metribuzin was faster in the field than in the laboratory.

Goatweed seeds require light for germination, with optimum germination occurring at 25/20 or 30/25 C day/night temperatures. Germination was 8.5% 7 days after incubation at 20/15 C. At 15/10 C, no germination occurred, irrespective of the light treatment. Chilling at 4 C for 10 days prior to incubation at 30/25 C did not induce germination of dark-incubated seeds and had no effect on germination of light-incubated seeds. A photoperiod of at least 1 h daily was required to induce germination. Maximum germination, however, occurred with a 9-h photoperiod. Light intensity as low as 25 μE·m–2·s–1 induced germination of goatweed seeds. Germination percentage increased with an increase in light intensity. Treatment with 1 mM gibberellic acid or 10 or 100 mM sodium nitrate or ammonium nitrate solution was effective in enhancing germination of light-incubated seeds, but not in inducing germination of dark-incubated seeds. Ammonium nitrate at 10 and 100 mM was more effective than sodium nitrate at the same concentrations in enhancing germination of goatweed seeds in the light. Ammonium chloride did not increase percent germination over that of seeds incubated in distilled water in the light. Germination percentage was maximum at pH 10 and at osmotic potentials 0.0 MPa.

The survival of two Rhizobium japonicum strains was determined in soils treated with 13 herbicides used in soybeans [Glycine max (L.) Merr.]. Survival in soils treated with concentrations equivalent to or 10-fold greater than those from field applications at recommended rates was compared to survival in untreated soil under controlled temperature and moisture conditions. Nine of the herbicides had transitory or no effects, regardless of rate. Acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid}, bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide], fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)benzenamine], and dinoseb [2-(1-methylpropyl)-4,6-dinitrophenol] consistently reduced survival of one or both strains at rates 10-fold greater than recommended. At rates of application equivalent to suggested field use rates, effects on survival were reduced or eliminated. Reductions in survival attributable to herbicides were in some cases comparable to those obtained by a weekly wetting and drying of the soil, indicating that herbicide effects on R. japonicum may be equivalent in magnitude to effects due to environmental stress. The herbicides were not sufficiently toxic to reduce R. japonicum populations in soil to levels that would be likely to affect nodulation.

Secale cereale is a serious weed problem in winter Triticum aestivum–producing regions. The interference relationships and economic thresholds of S. cereale in winter T. aestivum in Colorado, Kansas, Nebraska, and Wyoming were determined over 4 yr. Winter T. aestivum density was held constant at recommended planting densities for each site. Target S. cereale densities were 0, 5, 10, 25, 50, or 100 plants m−2. Secale cereale–winter T. aestivum interference relationships across locations and years were determined using a negative hyperbolic yield loss function. Two parameters—I, which represents the percent yield loss as S. cereale density approaches zero, and A, the maximum percent yield loss as S. cereale density increases—were estimated for each data set using nonlinear regression. Parameter I was more stable among years within locations than among locations within years, whereas maximum percentage yield loss was more stable across locations and years. Environmental conditions appeared to have a role in the stability of these relationships. Parameter estimates for I and A were incorporated into a second model to determine economic thresholds. On average, threshold values were between 4 and 5 S. cereale plants m−2; however, the large variation in these threshold values signifies considerable risk in making economic weed management decisions based upon these values.

High levels of cover-crop residue can suppress weed emergence and also can intercept preemergence herbicides and potentially reduce their effectiveness. This research was conducted in continuous no-tillage corn to compare the effect of residue from a hairy vetch cover crop with that of background crop residue on the soil solution concentration of atrazine and metolachlor and on the emergence of weeds with and without herbicide treatment. In a 3-yr field experiment, 5-cm-deep soil samples were taken and the weed density measured in paired microplots with and without herbicide at approximately weekly intervals after application of atrazine and metolachlor. High levels of residue were present in both treatments; the percentage of soil covered by residue ranged from 91 to 99 in the no–cover-crop treatment and from 99 to 100 in the hairy vetch treatment. Initial metolachlor concentration was lower and degradation rate higher in two of the 3 yr with a hairy vetch cover crop than without a cover crop. Cover-crop treatment had little effect on atrazine concentration or degradation. Annual grass weeds (predominantly fall panicum) were the major species in this field. Hairy vetch alone reduced grass emergence by 50 to 90%, and preemergence herbicides alone reduced emergence by 72 to 93% compared with the treatment without cover crop and herbicide. The combination of preemergence herbicides with hairy vetch provided only 24 to 61% control of grass weeds compared with control by hairy vetch alone and 23 to 52% compared with control by herbicide alone, suggesting an antagonism probably resulting from reduced metolachlor concentration by hairy vetch residue. Metolachlor with hairy vetch delayed emergence of weeds and reduced the concentration of metolachlor required to prevent emergence initiation compared with metolachlor without a cover crop.

Field studies determined flowering pattern, seed production, and somatic polymorphism of pitted morningglory, common cocklebur, and sicklepod under interspecific and no interference with soybean. Flowering patterns differed between years but were similar under both interferences. Interspecific interference reduced flower and seed numbers 85 to 99%. Rainfall had little effect on flowering, but photoperiod and temperature responses were evident for all weed species. The production equations for seed number per plant from dry weight were quadratic for pitted morningglory and common cocklebur but linear for sicklepod. All species showed somatic polymorphism; however, differences in seed germination and viability were not consistent between years.

Common ragweed was the most difficult to control of the species present and its control appeared to have the greatest effect on peanut yield. Preplant-incorporated (PPI) treatments provided no control of common ragweed but ethalfluralin plus vernolate PPI provided better than 90% control of yellow nutsedge and nearly 50% control of morningglory species. Single applications of paraquat at 0.14 kg ai ha−1 following PPI applications of ethalfluralin or ethalfluralin plus vernolate provided less than 75% common ragweed control. Sequential applications of paraquat applied 1 and 3 weeks after peanut emergence (1 + 3 WAE) provided at least 81% common ragweed control. Peanut yield with ethalfluralin plus vernolate PPI followed by paraquat 1 WAE (4400 kg ha−1) was equivalent to the handweeded yield (4470 kg ha−1). Yields were not significantly less with the same PPI application followed by paraquat 1 + 3 WAE (3730 kg ha−1) or by acifluorfen plus bentazon 3 WAE (3730 kg ha−1), and ethalfluralin PPI followed by paraquat 1 + 3 WAE (3740 kg ha−1). Ethalfluralin plus vernolate PPI and paraquat 1 WAE provided the highest net returns of $1370 ha−1.

Field margin weeds may contribute to the invasion and persistence of weeds in arable fields. Experimental studies of this hypothesis, however, have been inconclusive. We examined the role of field margin weed populations with a spatially explicit simulation model of Canada thistle population dynamics. We measured the contribution of field margin populations to weed pressure in the field across a wide range of parameter values and compared the weed control value of efforts applied to the field margin to that of similar efforts applied to the field. Under most combinations of parameter values, field margin weeds contributed little to weed pressure in the field, suggesting that controlling field margin weeds may often be of little value. Two conditions appeared to be necessary for field margin weeds to influence weed population dynamics within the field: the presence of unoccupied weed habitat, which increased the importance of dispersal to population growth, and high dispersal rates of field margin weeds relative to field weeds, which increased the relative contribution of field margin weeds to dispersal.

Large crabgrass, redroot pigweed, and hairy galinsoga are three important weed species in bell pepper and other crops in the northeastern United States. Field experiments were conducted in 1998 and 1999 to determine the influence of density and relative emergence time of the three weed species on bell pepper fruit yield. Densities of 0, 1, 2, 4, 8, 16, and 32 plants m−1 row were established for each weed species from naturally occurring weed populations. The effects of relative emergence time were studied by investigating the different yield responses to weeds emerging at two different times: 3 d or 2 wk after transplanting of pepper. Both weed density and relative emergence time affected pepper yield loss. The relative competitive ability of weed species varied between years. Large crabgrass was the most competitive species in 1998 and the measure of yield loss at low weed densities, I, was estimated to be 34% on the basis of the nonlinear hyperbolic equation. Redroot pigweed was most competitive in 1999 with an estimate of 88% for I. Hairy galinsoga was the least competitive weed in both years. Maximum yield loss under 32 plants m−1 row ranged from 19% with late-emerging hairy galinsoga in 1998 to 99% with early-emerging redroot pigweed in 1999. A new equation was proposed to characterize the relation between yield loss and weed pressure by expanding the nonlinear hyperbolic equation to include a parameter to account for the change of maximum yield loss with emergence time. The expanded equation generally provided a more accurate prediction of yield loss. In addition, several models are introduced to describe both the effects of density and relative emergence time of multiple weed species on crop yield. Generally these models provided an adequate fit of the data and a good description of the competitive ability of the mixed population.