Littleseed canarygrass is a troublesome grass weed in wheat fields in Iran. Predicting weed emergence dynamics can help farmers more effectively control weeds. In this work, four nonlinear regression models (beta, three-piece segmented, two-piece segmented, and modified Malo's exponential sine) were compared to describe the cardinal temperatures for the germination of littleseed canarygrass. Two replicated experiments were performed with the same temperatures. An iterative optimization method was used to calibrate the models and different statistical indices (mean absolute error [MAE], coefficient of determination [R2], intercept and slope of the regression equation of predicted vs. observed hours to germination) were applied to compare their performance. The three-piece segmented model was the best model to predict the germination rate (R2 = 0.99, MAE = 0.20 d, and coefficient of variation 1.01 to 4.06%). Based on the model outputs, the base, the lower optimum, the upper optimum, and the maximum temperatures for the germination of littleseed canarygrass were estimated to be 4.69, 22.60, 29.62, and 38.13 C, respectively. The thermal time required to reach 10, 50, and 90% germination was 31.98, 39.26 and 45.55 degree-days, respectively. The cardinal temperatures depended on the model used for their estimation. Overall, the three-piece segmented model was better suited than the other models to estimate the cardinal temperatures for the germination of littleseed canarygrass.

Leaf development and expansion are important factors in determining the outcome of crop-weed interference. The comparative effects of temperature and corn canopy-induced shading on the rate of leaf appearance (RLA) of redroot pigweed were quantified in this study. Growth cabinet results indicated a linear increase in RLA with increased temperature. Weed RLA was predicted utilizing both this function and field temperature data. The ratio of observed to predicted RLA of redroot pigweed grown in field experiments decreased in 1990 and 1991 as shading increased with increased corn density and delayed weed planting date. Results indicated that RLA is substantially affected by canopy-induced shading in addition to temperature.

Cogongrass [Imperata cylindrica (L.) Beauv.] was introduced into the Mobile Bay area of Alabama in 1911 or 1912. During the ensuing 60 years this rhizomatous weedy grass has spread into most of the southern portion of the state. Cogongrass has become a problem in agronomic and horticultural crops which are not intensively cultivated. Several currently available herbicides show promise of control.

The molecular basis of resistance to tribenuron-methyl, an acetolactate synthase (ALS)–inhibiting herbicide was investigated in four resistant (R) and three susceptible (S) flixweed populations. The resistance level in the R populations was assessed in whole-plant pot experiments in a greenhouse, and resistance indices ranged from 723 to 1422. The ALS genes of the three S populations and four R populations were cloned and sequenced, and the full coding sequence of the ALS gene of flixweed was 2,004 bp. The sequences of the ALS genes of the three S populations collected from Shaanxi, Gansu, and Tianjin were identical. Comparison of the ALS gene sequences of the S and R populations with Arabidopsis revealed that proline at position 197 of the ALS gene was substituted by leucine in R population SSX-2, by alanine in R population SSX-3, and by serine in R populations TJ-2 and GS-2. In another study of two R flixweed populations from Hebei and Shaanxi, resistance was also related to mutation at position 197 of the ALS gene. Both studies confirmed tribenuron-methyl resistance in flixweed in China, with the resistance mechanism being conferred by specific ALS point mutations at amino acid position 197.

2,4-D [(2,4-dichlorophenoxy)acetie acid] and dalapon (2,2-dichloropropionic acid) were applied to a natural stand of annual weeds at a time near flowering to determine effects on seed production and the dormancy and viability of seeds produced. At rates of 0.6 and 1.1 kg/ha, 2,4-D reduced, respectively the seed production of common lambsquarters (Chenopodium album L.) 99 and 99%, redroot pigweed (Amaranthus retroflexus L.) 77 and 84%, and jimsonweed (Datura stramonium L.) 64 and 100%, while giant foxtail (Setaria faberi Herrm) seed production was increased to 307 and 381% of the control, respectively. Dalapon at rates of 2.2 and 4.5 kg/ha reduced respectively seed production of giant foxtail 100 and 100%, and jimsonweed 100 and 91%. Before and after overwinter burial in the soil, common lambsquarters seeds from plants treated with 4.5 kg/ha dalapon were less dormant than control seeds. After overwintering, redroot pigweed seeds from dalapon-treated plants were less dormant than controls, and more seeds survived the winter burial. Common lambsquarters and redroot pigweed seeds from plants treated with 1.1 kg/ha 2,4-D were more dormant than control seeds before overwintering,’ while giant foxtail seeds from 2,4-D treated plants were less dormant than controls after overwintering. Viability of seeds produced by herbicide-treated plants, as determined by germination in KCN, was not greatly different from control seeds. Treatment with 2,4-D or dalapon resulted in the production of common lambsquarters seeds which produced seedlings about half as vigorous as controls. Jimsonweed seedlings grown from seeds from 2,4-D-treated plants showed phenoxy herbicide injury symptoms.

Metolachlor has been evaluated both as a herbicide for use in cultivated oats (Avena sativa L.) and for its potential as an alternative herbicide for the control of herbicide-resistant rigid ryegrass. Eight herbicide-resistant and two susceptible biotypes of rigid ryegrass were tested for their susceptibility to metolachlor. Response to metolachlor was determined both in soil and an agar germination medium. The LD50 for metolachlor in agar for a susceptible biotype (VLR1) was 0.13 μM. Five biotypes, SLR5 (6.9 fold), SLR31 (5.2 fold), SLR10 (2.5 fold), NLR12 (2.1 fold) and VLR69 (1.9 fold), were cross-resistant to metolachlor when compared with VLR1. Relative response of the biotypes was similar in both soil and agar, validating the use of an agar germination test to determine the susceptibility of rigid ryegrass biotypes to metolachlor. Biotypes cross-resistant to metolachlor also were cross-resistant to alachlor (SLR5 6.7 fold, SLR31 5.9 fold, SLR10 2.4 fold, and VLR69 1.6 fold with the LD50 for VLR1 being 0.49 μM) and propachlor (SLR57.2 fold, SLR31 7.2 fold, SLR10 3.0 fold and VLR69 2.5 fold with the LD50 for VLR1 being 0.47 μM) indicating that cross-resistance extends to other members of the chloroacetamide group. Cross-resistance to chloroacetamides was observed in biotypes that previously had been reported as cross-resistant to other herbicides. In contrast, biotypes with limited herbicide histories were generally not cross-resistant to metolachlor. These results indicate that there is a high probability of chloroacetamide cross-resistance in populations of herbicide-resistant rigid ryegrass.

The growth of barley (Hordeum distichum L., var. Esperanza) seedlings in nutrient solution was reduced by purple nutsedge (Cyperus rotundus L.) plants growing in connected vials and by addition of ethanolic extract of purple nutsedge. Aqueous and ethanolic extracts from subterranean parts of purple nutsedge reduced the germination and root elongation of barley; stimulations of barley root elongation were obtained with diluted aqueous extract. The germination of barley, mustard (Brassica nigra L., var. Alsace), and cotton (Gossypium hirsutum L., var. Acala 1517) was reduced in soil which had been infested previously with purple nutsedge. Radicle elongation of barley was inhibited by ethanolic extract of soil previously infested. Chromatographic studies indicated that the inhibitors are phenolic substances.

Field and laboratory experiments were conducted over a 3-yr period to evaluate the persistence of several dinitroaniline herbicides. Incorporation of these herbicides was delayed for 1, 3, and 7 days after application. Herbicidal activity was assessed by a sorghum bioassay, and by GLC analysis of soil samples. Herbicides least affected by a 2-day delay in incorporation were oryzalin (3,5-dinitro-N4,N4-dipropylsulfanilamide, nitralin 4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline, butralin 4-(1,1-dimethylethyl)-N-(1-methylpropyl)-2,6-dinitrobenzenamine, dinitramine (N4, N4-diethyl-α,α,α-trifluoro-3,5-dinitrotoluene-2,4-diamine), fluchloralin N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)aniline, and penoxalin N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine. Of the herbicides evaluated, trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-P-toluidine) and profluralin N-(cyclopropylmethyl)-α,α,α-trifluoro-2,6-dinitro-N-propyl-p-toluidine, were the least persistent after a 1-day incorporation delay. Losses of nitralin, butralin, dinitramine, fluchloralin, penoxalin, trifluralin, profluralin, benefin (N-butyl-N-ethyl-α,α,α-trifluoro-2,6-dinitro-p-toluidine), and isopropalin (2,6-dinitro-N, N-dipropylcumidine) increased dramatically when incorporation was delayed 3 or more days. Persistence on the soil fur-face under field conditions was demonstrated the most by oryzalin. After incorporation, dinitramine was least persistent. The effects of temperature in UV light and dark on loss of dinitroaniline herbicides were demonstrated in the laboratory by applying the herbicide to TLC plates of soil. While herbicide losses did occur in 24 h of UV light, differences in herbicide losses were not significantly different between butralin, penoxalin, profluralin, trifluralin, isopropalin, oryzalin, nitralin, fluchloralin, and benefin. Dinitramine was most affected by the UV light. Butralin, oryzalin, nitralin, isopropalin, and dinitramine were least affected by 45 C in the dark with losses of 2 to 10% in 24 h. Profluralin, benefin, and trifluralin were found most volatile with losses of 35%, 25%, and 18%, in 24 h respectively. With increasing moisture levels, no significant increased losses were detected for butralin, nitralin, oryzalin, penoxalin, and isopropalin at the greater moisture contents. There was a moderate increase in the loss of dinitramine with increased soil moisture. AT 33% initial soil moisture, fluchloralin, benefin, and trifluralin were rapidly lost (75 to 94% in 24 h at 30 C). The trifluoro-methyl group was a common structural component where moisture affected the compound's loss. Application of the intermediate to very volatile dinitroanilines to moist soil surfaces without soil incorporation as soon as possible is not feasible unless higher rates of herbicide are applied.

WEEDSIM is a bioeconomic decision aid for management of annual weeds in corn and soybean. It was field-tested for 4 yr in Minnesota. The decision aid has two categories of management recommendations: soil-applied plus postemergence (PRE+), based on estimated weed seedbank composition and density; and postemergence (POST), based upon observed weed seedling composition and density. Weed densities, weed control, herbicide use, environmental impact of herbicide use, weed management costs, crop yields, and economic returns that resulted from PRE+ and POST recommendations were compared to those associated with herbicide management systems (HERB) that were standard for the region. After 4 yr of applying WEEDSIM recommendations to the same plots, there were no increases in annual weed densities (seedbanks, seedlings, established plants, or seed production) or decreases in weed control or crop (soybean, rotation corn, and continuous corn) yields, compared to HERB. WEEDSIM recommendations resulted in average annual herbicide applications of 1.1 kg ai ha−1 for PRE+ and 1.0 kg ai ha−1 for POST, compared to 3.5 kg ai ha−1 for HERB. Environmental impact indices associated with PRE+, POST, and HERB were 0.75, 0.71, and 0.54, with the lowest value indicating greater environmental risk than the two higher values. Similarly, average weed management costs were $24, $33, and $77 ha−1 for PRE+, POST, and HERB, respectively. Based on crop prices of $94 Mg−1 for corn and $220 Mg-1 for soybean, the average gross margins over weed control costs were higher for PRE+ ($509 ha−1) and POST ($522 ha−1) than for HERB ($455 ha−1). In general, WEEDSIM appeared to make management recommendations that adequately controlled weeds, maintained crop yields, reduced herbicide use, decreased environmental risk, lowered weed management costs, and increased gross margins over weed control costs compared to the use of herbicides standard for the region.

A chromatographic procedure for detecting 4-amino-3,5,6-tricholorpicolinic acid (picloram) residue to a sensitivity of approximately 0.05 ppm in soils is described. Picloram is extracted from soil as the free acid and methylated with boron trifluride-methanol reagent. Data indicate that the amount of herbicide which actually reaches the soil may only be 10% of that applied to dense stands of oak. Rainfall was an important factor in determining the movement and persistence of picloram. Picloram was detected to a depth of 24 in within 6 weeks after application. No detectable residues were found in the upper 24 in of soil after one year with rates as high as 8 lb/A.

Glyphosate-tolerant spring wheat currently is being developed and most likely will be the first major genetically engineered crop to be marketed and grown in several areas of the northern Great Plains of the United States. The public has expressed concerns about environmental risks from glyphosate-tolerant wheat. Replacement of traditional herbicide active ingredients with glyphosate in a glyphosate-tolerant spring wheat system may alter ecological risks associated with weed management. The objective of this study was to use a Tier 1 quantitative risk assessment methodology to compare ecological risks for 16 herbicide active ingredients used in spring wheat. The herbicide active ingredients included 2,4-D, bromoxynil, clodinafop, clopyralid, dicamba, fenoxaprop, flucarbazone, glyphosate, MCPA, metsulfuron, thifensulfuron, tralkoxydim, triallate, triasulfuron, tribenuron, and trifluralin. We compared the relative risks of these herbicides to glyphosate to provide an indication of the effect of glyphosate when it is used in a glyphosate-tolerant spring wheat system. Ecological receptors and effects evaluated were avian (acute dietary risk), wild mammal (acute dietary risk), aquatic vertebrates (acute risk), aquatic invertebrates (acute risk), aquatic plants (acute risk), nontarget terrestrial plants (seedling emergence and vegetative vigor), and groundwater exposure. Ecological risks were assessed by integrating toxicity and exposure, primarily using the risk quotient method. Ecological risks for the 15 herbicides relative to glyphosate were highly variable. For risks to duckweed, green algae, groundwater, and nontarget plant seedling emergence, glyphosate had less relative risk than most other active ingredients. The differences in relative risks were most pronounced when glyphosate was compared with herbicides currently widely used on spring wheat.

The primary goal of this research was to determine the inheritance of cross-resistance to several groups of auxinic herbicides through classical genetic approaches using auxinic herbicide–resistant (R) and –susceptible (S) wild mustard biotypes obtained from western Canada. F1 progeny were raised from crosses between homozygous auxinic herbicide–R and –S wild mustard parental lines. The F1 and F2 populations were assessed for picloram (pyridine group) and 2,4-D (phenoxyalkanoic group) resistance or susceptibility. Analyses of the F1 as well as the F2 progeny indicate that a single dominant gene confers the resistance to picloram and 2,4-D similar to an earlier report of dicamba-based (benzoic acid group) resistance in this wild mustard biotype. Furthermore, analyses of backcross progeny in this species indicate that resistance to all three auxinic herbicides, i.e., picloram, dicamba, and 2,4-D, is determined by closely linked genetic loci. With this information on inheritance of resistance to several auxinic herbicide families, the R biotype of wild mustard offers an excellent system to isolate and characterize the auxinic herbicide–resistance gene.

An integrated pest management (IPM) program is a systems approach that requires an understanding of the overall agroecosystem. The interaction between different pest categories (weeds, insects, pathogens, nematodes), as well as their management practices, should be examined, understood, and taken into consideration in the design of IPM systems. Several studies have shown that plant pathogens are affected by other pests and their management practices. Herbicide application has often been cited as an example of a management practice that affects plant pathogens and disease development in various cropping systems. The activity of herbicides can extend beyond their target organisms and inhibit spore germination or mycelial growth, alter the level of phytoalexins, or interfere with other physiological processes in plants. This paper summarizes the published reports on direct effects of herbicides on plant disease and provides insights for future research on this aspect. Examples are drawn from some common agricultural herbicides and adjuvants. The discussion on various findings on herbicide and crop diseases re-emphasizes the fact that pest management in agriculture requires a systems approach. Although it is difficult to come to a common consensus on the effect of these chemicals on crop diseases or pathogens, this paper provides an overview of several interactions between herbicides and crop diseases and pathogens in various cropping systems. Knowledge of such interactions can help in the design of IPM systems.

Site-specific weed management recommendations require knowledge of weed species, density, and location in the field. This study compared several sampling techniques to estimate weed density and distribution in two 65-ha no-till Zea mays–Glycine max rotation fields in eastern South Dakota. The most common weeds (Setaria viridis, Setaria glauca, Cirsium arvense, Ambrosia artemisiifolia, and Polygonum pensylvanicum) were counted by species in 0.1-m2 areas on a 15- by 30-m (1,352 points in each field) or 30- by 30-m (676 points in each field) grid pattern, and points were georeferenced and data spatially analyzed. Using different sampling approaches, weed populations were estimated by resampling the original data set. The average density for each technique was calculated and compared with the average field density calculated from the all-point data. All weeds had skewed population distributions with more than 60% of sampling points lacking the specific weed, but very high densities (i.e., > 100 plants m−2) were also observed. More than 300 random samples were required to estimate densities within 20% of the all-point means about 60% of the time. Sampling requirement increased as average density decreased. The W pattern produced average species densities that often were similar to the field averages, but information on patch location was absent. Weed counts taken on the 15- by 30-m grid were dependent spatially and weed contour maps were developed. Kriged maps presented both density and location of weed patches and could be used to establish management zones. However, grid-sampling production fields on a small enough scale to obtain spatially dependent data may have limited usefulness because of time, cost, and labor constraints.

Corn (Zea mays L.) in the two to three-leaf stage grown 18 to 21 days in a growth chamber under cold, wet conditions was injured by postemergence application of 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) plus emulsifiable phytobland oil. Injury was most severe when these plants were kept under cold, wet conditions for 48 hr after the herbicidal spray was applied, followed by exposure to high light intensity and high temperature. Under these growth chamber conditions, approximately 50% of the atrazine-treated plants died. Since wet foliage before and after application increased foliar penetration and low temperature decreased the rate of detoxication to peptide conjugates, atrazine accumulated under cold, wet conditions. This accumulation of foliarly-absorbed atrazine and the “weakened” conditions of the plants grown under the stress conditions is believed to be responsible for the injury to corn. Hydroxylation and the dihydroxybenzoxazin-3-one content in the roots were reduced at low temperature, but it is unlikely that this contributed to the death of the corn.

Regression analysis of clomazone adsorption on 19 soils and sediments yielded a positive correlation and linear relationship between distribution adsorption constants (Kd) and soil organic carbon content. The agreement of the experimental mean Koc with Koc values predicted from the octanol-water partition coefficient and water solubility of clomazone indicated hydrophobic bonding was one mechanism responsible for adsorption. Kaolinite and montmorillonite had a strong affinity for clomazone, and adsorption increased with the charge of the cation on the clay exchange sites. Adsorption was not correlated with the clay content of soils and sediments, indicating that organic coatings on clay in soils may block adsorption sites on clay surfaces. The adsorption of clomazone on some soils and sediments with high clay contents was greater than predicted based on organic carbon content alone, however, indicating clay contributed to adsorption on these samples.

Postemergence applications or a combination of preemergence and postemergence treatments in double cropped soybeans [Glycine max (L.) Merr. ‘Ransom’] resulted in higher soybean yields than preemergence applications. Preemergence-treated plots were 98% weed free early in the growing season; however, weeds emerged later and reduced yields. Weeds had to be controlled in soybeans for 90% of the growing season to avoid yield loss. Soybean yields were higher under no-till than conventionally tilled management in two of three years and tended to be higher during the third year. Distribution and timing of rainfall were more important in determining soybean yield than the total amount received during the growing season.

Relationships between production of cotton (Gossypium birsutum L. ‘Westburn M’) and full-season interference from tumble pigweed (Amaranthus albus L. ♯ AMAAL) at densities ranging from 0 to 64 plants/10 m of row were measured in three replicated experiments in the field. Dry weight of harvested tumble pigweed increased by 0.149 to 0.402 kg/plot for each additional weed/10 m of row. Intraspecific competition among tumble pigweed plants was observed at the higher weed densities. Cotton plant height was reduced in all three experiments at the 32 or 64 weed densities, but harvesting difficulties were not encountered in these studies. The threshold density where initial lint yield reductions occurred ranged from 4 to 16 tumble pigweed plants/10 m of row in the three experiments. Lint yields were reduced from 8 to 11 kg/ha for each additional tumble pigweed plant/10 m of row. Interference from tumble pigweed did not significantly affect cotton fiber length, uniformity, strength, or micronaire of hand-harvested bolls.

The competitiveness of annual weeds in irrigated sweet Spanish onions (Allium cepa L.) was studied at North Platte, Nebraska, during 1969 and 1970. Weeds allowed to grow in the row for 2, 4, 6, and 8 weeks after onion emergence reduced onion yields 20, 20, 40, and 65%, respectively. When plots were kept weed-free until onion emergence and 2, 4, 6, 8, 10, and 12 weeks after emergence, onion yields were reduced 100, 99, 87, 75, 46, 25, and 5%, respectively. Redroot pigweed (Amaranthus retroflexus L.), kochia (Kochia scoparia L.), and grass weeds accounted for 54, 21, and 21%, respectively, of the total weed yield.

EPTC (S-ethyl dipropylthiocarbamate) metabolism in corn (Zea mays L. ‘inbred B37’) was studied utilizing carbonyl-14C-EPTC and propyl-1-14C-EPTC. Two radioactive compounds were found in the organic-soluble fraction, namely, EPTC and EPTC-sulfoxide. Five metabolites were found in the water-soluble fraction. Amino acid analysis indicated that one water-soluble metabolite was a glutathione conjugate [S-(N,N-dipropylcarbamyl)glutathione] and that three other metabolites were probably degradation products of the glutathione conjugate. It was determined that EPTC-sulfoxide reacts chemically with reduced glutathione, yielding the conjugate. There was no detectable glutathione S-transferase enzyme activity associated with the sulfoxide cleavage.