The adsorption of naphthalene by a Bosket loam soil was studied at 6.5, 15, 25, and 37C. Because the solubility and vapor pressure of naphthalene are precisely known over this temperature range, a complete thermodynamic description of the differences between standard vapor, crystal, solution, and soil-adsorbed states is possible. The results indicate that enthalpies and entropies of the solution and surface standard states are quite similar (though very different from the vapor state) resulting in a near-zero standard enthalpy difference and a small standard entropy increase for adsorption from solution. Considering naphthalene as a model for herbicides and as a probe into the nature of the soil surface, the results emphasize the importance of the unique structured nature of water acting as a solvent, and the competition of water for adsorption sites.

The effects of temperature, relative humidity (RH) and light on absorption and translocation of 14C-glyphosate [N-(phosphonomethyl)glycine] in common bermudagrass [Cynodon dactylon (L.) Pers. ‘Common’] and the effect of temperature and stage of growth on glyphosate activity in horsenettle (Solanum carolinense L.) were determined. 14C-glyphosate was used to evaluate the absorption and translocation of foliar-applied herbicide in bermudagrass plants grown at 22 or 31 C and 35 or 80% RH and absorption in excised leaf tips and stem sections. Autoradiography of bermudagrass indicated that downward translocation of 14C was extensive with large accumulations in new roots and rapidly growing stolons. RH of 35% decreased 14C-glyphosate translocation to the untreated foliage of plants grown at 31 C as compared to 80% RH. RH of 35% also decreased the translocation of 14C to the rhizomes and roots of plants grown at 22 C. The absorption of 14C was greater in excised leaf tips than stem sections and was greater in the light than in the dark. Horsenettle plants grown at 32 C had shoots which were more rapidly killed by glyphosate but had more regrowth than plants treated at 13 C. Shoots of plants treated while blooming were also more rapidly killed than those treated in the prebloom stage of growth, but the regrowth was not significantly different.

The uptake of herbicides by soybean (Glycine max (L.) Merr.) seed from 14C-labeled aqueous solutions of 3-amino-2,5-dichlorobenzoic acid (amiben), 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine), 3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea (linuron), S-ethyl dipropylthiocarbamate (EPTC), and isopropyl m-chlorocarbanilate (chlorpropham) was determined by measuring reduction in the concentration of the herbicides in the soaking solution. There was a direct proportional relationship between uptake and concentration. Raising the temperature from 10 to 30 C increased uptake of herbicides. The greatest increase occurred from 10 to 20 C. After 48 hr at 30 C, 39% of the amiben, 61% of the atrazine, 65% of the EPTC, 83% of the linuron, and 92% of the chlorpropham in the original solution was absorbed by the seed. Uptake rates were similar in living and dead seed. Uptake was reversible with amiben but not with chlorpropham. Absorption of herbicides could not be associated with absorption of water except during the first few hours when the seed were rapidly imbibing water. Ratios of seed to solution herbicide concentrations all exceeded 1:1 after 48 hr and with chlorpropham, reached 68:1. The uptake of herbicides by seed was largely a physical process that required seed hydration, but herbicide uptake continued after water uptake ceased.

Field experiments were conducted to study the feasibility of using several dinitroaniline herbicides for the selective control of johnsongrass [Sorghum halepense (L.) Pers.] from seed and rhizomes in soybeans [Glycine max (L.) Merr. ‘Bragg’]. The herbicides were incorporated into the soil with two disk cultivations immediately after application. These were trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) at 0.6 to 2.2 kg/ha, nitralin [4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline] at 0.6 to 2.2 kg/ha, dinitramine (N4,N4-diethyl-α,α,α,-trifluoro-3,5-dinitrotoluene-2,4-diamine) at 0.4 to 1.5 kg/ha, fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)aniline] at 0.6 to 2.2 kg/ha, profluralin [N-(cyclopropyl-methyl)-α,α,α-trifluoro-2,6-dinitro-N-propyl-p-toluidine] at 0.8 to 3.4 kg/ha, butralin [4-(1,1-dimethylethyl)-N-(1-methylpropyl)-2,6-dinitrobenzenamine] at 1.7 to 6.7 kg/ha, AC-92390 (N-sec-butyl-2,6-dinitro-3,4-xylidine) at 0.8 to 3.4 kg/ha, and AN-56477 [N,N-di(2-chloroethyl)-4-methyl-2,6-dinitroaniline] at 2.2 to 4.5 kg/ha. On Bosket sandy loam soil, the best average johnsongrass control over a 2-yr period was obtained following profluralin at 1.7 kg/ha and butralin at 3.4 kg/ha. These treatments also resulted in highest average soybean yields. On Sharkey clay soil, profluralin at 3.4 kg/ha and butralin at 6.7 kg/ha resulted in maximum johnsongrass control and soybean yields. Immediate incorporation of profluralin and butralin into both soils for 2 successive years effectively controlled johnsongrass from rhizomes without soybean injury and with greatly increased soybean yields. Trifluralin, nitralin, and fluchloralin also provided acceptable johnsongrass control within individual experiments, and greatly increased soybean yields.

Results from identical experiments conducted at Headland, AL, and Plains, GA, from 1980 through 1982 show insecticide treatment had little effect on soybean [Glycine max (L.) Merr. ‘Coker 237′] growth and morphology. Maximum insecticide applications increased soybean seed weight in two of five trials. Soybeans maintained free of sicklepod (Cassia obtusifolia L. ♯3 CASOB) for 4 weeks after emergence produced yields equal to those receiving season-long control in all trials, and 2-week control was equal to season-long maintenance in three trials. Length of weed interference-free maintenance did not affect soybean height. The number of pods per plant and seed weight were decreased when there was no control. Sicklepod shoot fresh weight and numbers decreased as the weed-free period increased from 0 weeks through the season. Row spacing had no effect on soybean height or seed size; however, the number of pods per plant was higher in 80- than in 40-cm rows. Row spacing influenced yield in only one trial where 20-cm rows outyielded 40-cm rows. A significant interaction occurred between the weed-free period and row spacing in two trials. Soybeans in 20-cm rows outyielded those in 40- and 80-cm rows when sicklepod was not controlled (i.e., 0 weeks interference-free maintenance).

Managing weed infestations in a spatially precise manner requires accurate and cost-effective weed identification techniques. The goal of our research was to quantify the accuracy of continuous weed presence–absence maps and assess how management based on those maps may affect producer net returns. Each continuous sampled map covered the entire field and contained vector polygons labeled as either wild oat presence or wild oat absence. The accuracy of the continuous wild oat maps at each sampling time was determined from georeferenced quadrats of wild oat densities. The accuracy of the continuous wild oat seedling maps ranged from 48.3 to 87.1% among the six site-years. The accuracy of the wild oat seedling maps improved by at least 8% when a 10-m buffer was included around areas mapped as wild oat presence. The accuracy of continuous wild oat panicle maps from the combine at harvest ranged from 65.8 to 90.9% among the six site-years. The variation in accuracy for the wild oat seedling maps among sites was greater than the accuracy of the panicle maps. Net returns ($ ha−1) for four site-years were calculated and compared for four possible weed management approaches on each field. A site-specific herbicide application to areas mapped as wild oat presence always generated higher net returns than a herbicide application over the entire field for four sites. A site-specific herbicide application to areas mapped as wild oat presence plus a surrounding 10-m buffer area only resulted in a higher net return in one of the 12 site-years compared with a site-specific herbicide application without the 10-m buffer. This site had the lowest (48.3%) wild oat seedling map accuracy, and uncontrolled wild oat had a high-yield effect. This research indicates that using a continuous weed sampling method based on presence or absence for site-specific herbicide application can be profitable over a herbicide application to the entire field, even with the associated technology cost and seedling map errors.

Chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carboxyl]benzenesulfonamide} has recently been registered for the control of several broadleaf weeds in spring wheat (Triticum aestivum L.), durum wheat (Triticum durum Desf.), and barley (Hordeum vulgare L.) in western Canada. Residue carryover of this herbicide can cause injury to subsequent sensitive crops. A bioassay technique based on growth of pregerminated corn roots was used to detect levels of chlorsulfuron as low as 0.125 ppb in three Saskatchewan soils. Confidence levels of 95% were established. These confidence bands allowed the detection of chlorsulfuron levels within a minimum and maximum variation of 3.1 and 11.5% of root development.

R-25788 (N,N-diallyl-2,2-dichloroacetamide) and H-31866 [N-allyl-N-(3,3-dichloroallyl)dichloroacetamide] were more effective than CDAA (N,N-diallyl-2-chloroacetamide) in preventing yield reductions to corn (Zea mays L. ‘NB-611’) from alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] or metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] in the greenhouse. A CGA-43089 [α-(cyanomethoximino)-benzacetonitrile] seed treatment (1.25 g/kg) was more effective than a R-25788 tank mix in preventing yield reductions to grain sorghum [Sorghum bicolor (L.) Moench ‘G-623 GBR’] from alachlor or metolachlor in the greenhouse. Absorption of 14C-alachlor by sorghum seedlings grown in petri dishes, and absorption, translocation, and metabolism of 14C-metolachlor by sorghum seedlings grown in soil, were not affected by CGA-43089 seed treatment. Forage sorghum [Sorghum bicolor (L.) Moench ‘Rox Orange’] was used to simulate shatter cane [Sorghum bicolor (L.) Moench] in field plots. In the absence of Rox Orange, alachlor and metolachlor reduced sorghum grain yields. This yield reduction was prevented by a CGA-43089 seed treatment, but not by a R-25788 tank mix with herbicides. In plots seeded with 10,000 Rox Orange seed/57 m2, grain yields of sorghum increased as alachlor or metolachlor plus CGA-43089 rates increased. There was no grain yield response to any herbicide treatment in plots seeded with 50,000 Rox Orange seed/57 m2.

Responses of the vegetative growth of cogongrass [Imperata cylindrica (L.) Beauv.] to temperature and photoperiod were studied in plants grown from stem and rhizome propagules collected from plants originating at four locations in southern Alabama and Mississippi. The plants were grown in four controlled-environment chambers with day/night temperature regimes of 23/17 C and 29/23 C, photoperiods of 12 h and 16 h, and photosynthetic photon flux densities (PPFD) of 650 μE(einsteins)m−2s−1. After 87 days, heights, leaf areas, total dry weights, rhizome numbers, and rhizome dry weights were significantly greater in plants grown at 29/23 C than at 23/17 C. Total dry weights, rhizome numbers, and rhizome dry weights were greater at the 16-h photoperiod than at 12 h. Plants grown from propagules from the McNeil, Mississippi population, which originated from an introduction from the Philippines, were significantly smaller than plants grown from propagules collected from populations thought to have originated from an introduction of cogongrass from Japan. Mathematical analysis of the growth data showed that the observed differences in dry matter production were more closely related to differences in leaf area duration than to differences in net assimilation rate.

The absorption, translocation, metabolism, and retention of an isopropylamine formulation of glyphosate [N-(phosphonomethyl)glycine] in the presence and absence of the commercial dimethylamine salt and isooctyl ester formulations of 2,4-D [(2,4-dichlorophenoxy)acetic acid], or the commercial isooctyl ester formulation of bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) was investigated in barley (Hordeum vulgare L. 'Summit′). In all treatments, absorption of 14C-glyphosate continued for up to 24 h, but no further absorption occurred with a further 48 h of treatment. A considerable reduction in absorption and translocation of 14C-glyphosate occurred when it was applied to the leaf surface in a mixture with the commercial formulation of either 2,4-D amine, 2,4-D ester, or bromoxynil. When a 14C-glyphosate solution was spotted on the leaf adjacent to drops of the commercial formulation of either 2,4-D amine, 2,4-D ester, or bromoxynil, no reduction in absorption or translocation of 14C-glyphosate occurred. The herbicide components of the 2,4-D amine and bromoxynil formulations were solely responsible for reduced absorption and translocation of 14C-glyphosate. No 14C-glyphosate metabolites were detected in barley following treatment with 14C-glyphosate either alone or mixed with the commercial formulations of 2,4-D amine or bromoxynil. No 14C-glyphosate complexes were detected in mixtures of the commercial formulations of 2,4-D amine, 2,4-D ester, or bromoxynil with 14C-glyphosate. Barley leaves retained less glyphosate when it was spray-applied together with the commercial formulation or the solvent system of 2,4-D amine.

The effect of Canada thistle on major yield components of spring wheat was characterized using path coefficient analysis, which provides insight concerning which yield components are most sensitive to Canada thistle competition. Increasing Canada thistle density decreased wheat stand in each of three years. Canada thistle also reduced spikes per plant and seed per spike to varying extents depending on year, but Canada thistle had comparatively little effect on wheat seed weight per 1000 seed. In the path coefficient model proposed, Canada thistle was assumed to reduce yield by directly reducing wheat yield components and through them, indirectly reducing yield. Path coefficient correlation analysis showed that Canada thistle reduced spring wheat yield chiefly by indirect effects of decreasing wheat density, the earliest formed yield component. Canada thistle reduced wheat density which, in turn, reduced wheat yield. These data suggest that Canada thistle must be controlled either before or shortly after wheat emergence if detrimental effects on wheat yield are to be minimized.

After 45 weeks, between 5 and 8% of the applied radioactivity from 14C-chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} applied to small sandy loam plots was recovered as chlorsulfuron and about 15% as 2-chlorobenzenesulfonamide. After 95 weeks, 2% of the original radioactivity remained as chlorsulfuron and 20% as the sulfonamide. The persistence of formulated chlorsulfuron was investigated using small field plots at two locations for three successive years. Between 3 and 16% of the herbicide applied during May was detected in the top 10-cm soil layers the following May using a corn-root (Zea mays L.) bioassay procedure.

Multiple weed species in the field combine to cause yield losses and can be described using one of several empirical models. Field studies were conducted to compare observed corn yield loss caused by common sunflower and shattercane populations with predicted yield losses modeled using a multiple species rectangular hyperbola model, an additive model, or the yield loss model in the decision support system, WeedSOFT, and to derive competitive indices for common sunflower and shattercane. Common sunflower and shattercane emerged with corn and selected densities established in field experiments at Scandia and Rossville, KS, between 2000 and 2002. The multiple species rectangular hyperbola model fit pooled data from three of five location–years with a predicted maximum corn yield loss of 60%. Initial slope parameter estimate for common sunflower was 49.2 and 4.2% for shattercane. A ratio of these estimates indicated that common sunflower was 11 times more competitive than shattercane. When common sunflower was assigned a competitive index (CI) value of 10, shattercane CI was 0.9. Predicted yield losses modeled for separate common sunflower or shattercane populations were additive when compared with observed yield losses caused by low-density mixed populations of common sunflower (0 to 0.5 plants m−2) and shattercane (0 to 4 plants m−2). However, a ratio of estimates of these models indicated that common sunflower was only four times as competitive as shattercane, with a CI of 2.5 for shattercane. The yield loss model in WeedSOFT underpredicted the same corn losses by 7.5%. Clearly, both the CI for shattercane and the yield loss model in WeedSOFT need to be reevaluated, and the multiple species rectangular hyperbola model is proposed.

Weed control strategies for rice (Oryza sativa L.) and soybean [Glycine max (L.) Merr.] emphasize integration of cultural and chemical practices (16, 30, 31). Numerous herbicides are registered and used as preplant, preemergence, or post-emergence applications for control of weeds in rice and soybean (3, 16, 25). Although these herbicide treatments control most of the problem weeds in rice and soybean, most fail to control northern jointvetch [Aeschynomene virginica (L.) B.S.P. # AESVI] in either crop. The two most effective herbicides, 2,4,5-T [2-(2,4,5-trichlorophenoxy) acetic acid] and silvex [2-(2,4,5-trichlorophenoxy) propanoic acid], for control of northern jointvetch in rice have been under the U.S. Environmental Protection Agency's (EPA) Rebuttal Presumption Against Registration (RPAR) process which may ultimately cancel the use of both herbicides in rice (50). New, effective control strategies are needed for control of northern jointvetch in rice and soybean.

Two new varieties of foxtails, Setaria viridis var. robusta alba Schreiber and Setaria viridis var. robusta-purpurea Schreiber are described. As their names imply, they differ from each other only in the color of their bristles but differ from Setaria viridis (L.) Beauv. and Setaria viridis var. major (Gaud.) Posp. in their robust growth habit, morphology, and physiological responses. A key distinguishes these from other species of similar habit and habitat.

Glyphosate-resistant populations of Conyza canadensis have been spreading at a rapid rate in Ontario, Canada, since first being documented in 2010. Determining the genetic relationship among existing Ontario populations is necessary to understand the spread and selection of the resistant biotypes. The objectives of this study were to: (1) characterize the genetic variation of C. canadensis accessions from the province of Ontario using simple sequence repeat (SSR) markers and (2) investigate the molecular mechanism (s) conferring resistance in these accessions. Ninety-eight C. canadensis accessions were genotyped using 8 SSR markers. Germinable accessions were challenged with glyphosate to determine their dose response, and the sequences of 5-enolpyruvylshikimate-3-phosphate synthase genes 1 and 2 were obtained. Results indicate that a majority of glyphosate-resistant accessions from Ontario possessed a proline to serine substitution at position 106, which has previously been reported to confer glyphosate resistance in other crop and weed species. Accessions possessing this substitution demonstrated notably higher levels of resistance than non–target site resistant (NTSR) accessions from within or outside the growing region and were observed to form a subpopulation genetically distinct from geographically proximate glyphosate-susceptible and NTSR accessions. Although it is unclear whether other non–target site resistance mechanisms are contributing to the levels of resistance observed in target-site resistant accessions, these results indicate that, at a minimum, selection for Pro-106-Ser has occurred in addition to selection for non–target site resistance and has significantly enhanced the levels of resistance to glyphosate in C. canadensis accessions from Ontario.

Field experiments were conducted to evaluate competitive effects of johnsongrass and sicklepod on growth and yield of irrigated and nonirrigated soybeans. Johnsongrass reduced soybean growth early in the growing season. Sicklepod was competitive all season, but competitiveness was greatest during the soybean reproductive stage. Soybean yields were reduced 31% by sicklepod, 14% by johnsongrass, and 36% by both weeds growing together. Nonirrigated soybeans yielded less than irrigated, but percent yield reductions were similar. Johnsongrass dry matter production and seed yield were reduced more than sicklepod by soybean interference. Soybean and sicklepod interference reduced johnsongrass seed production 73 to 95%. Johnsongrass produced 245 to 1238 seeds per plant when interfering with soybeans and sicklepod. Sicklepod seeds per plant were reduced 6 to 31% by johnsongrass interference and 47 to 75% by soybeans and soybeans plus johnsongrass.

Application of EPTC (S-ethyl dipropylthiocarbamate) at 3.4 and 4.5 kg/ha in 1972 increased root rot severity and reduced navy bean (Phaseolus vulgaris L.) yields in soil with an artificial root rot [Fusarium solani (Mart.) Appel and Wr. f. Phaseoli (Burk.) Snyd. and Hans.] inoculum level. EPTC at 3.4 kg/ha and chloroamben (3-amino-2,5-dichlorobenzoic acid) at 3.4 kg/ha in 1973 caused the greatest root rot severity and greatest decrease in yield. Dinoseb (2-sec-butyl-4,6-dinitrophenol) and fluorodifen (p-nitrophenyl α,α,α-trifluoro-2-nitro-p-tolyl ether) at 5 kg/ha interacted with root rot less than the other herbicides assayed. In the growth chamber at 23 C, EPTC at 3.4 kg/ha and alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl) acetanilide] at 2.8 kg/ha applied to navy bean grown in soil infested with F. solani reduced plant growth. Root rot injury was most severe in plants treated with EPTC at 20 C ambient temperature. Navy bean growth was reduced as the level of F. solani chlamydospore inoculum increased in the presence of EPTC.

An air-assist spray nozzle was constructed to apply herbicides in oil or water at volumes as low as 2.3 L/ha. The spray was metered through a disc orifice inside the nozzle and forced into an airstream which atomized the liquid. The nozzle produced 16 to 54 spray droplets/cm2 with air pressures from 3.4 to 62 kPa from a 2-mm outlet using a lightweight paraffinic oil. The number of spray droplet deposits produced with a paraffinic oil at 34 kPa increased from 7 droplets/cm2 with a 0.8-mm outlet to 48 droplets/cm2 with a 2.4-mm outlet. The nozzle was designed primarily to apply herbicides in oil at 2.3 to 4.7 L/ha, but it has the capability of applying 150 L/ha of oil or water.

Studies were conducted to determine the effect of interference between ladysthumb and cotton on plant growth and productivity. Ladysthumb remained shorter than cotton until at least 70 d after cotton planting. However, ladysthumb grew over twice as tall as cotton and, depending on plant density, produced between 179 and 681 g dry biomass per plant by cotton harvest. Ladysthumb biomass per plant was not affected by weed density when grown with cotton. When grown alone, ladysthumb produced over 2,000 g dry biomass per plant, which was over four times greater than biomass produced by plants grown with cotton. Cotton lint yield decreased between 0.7 and 0.9 kg ha−1 with each gram increase in weed dry biomass per meter of the row. The relationship between ladysthumb density and cotton percent yield loss was described by the rectangular hyperbola model with the asymptote (coefficient a) constrained to 100% maximum yield loss. The estimated coefficient i (yield loss per unit density as density approaches zero) was 35 ± 5 and 14 ± 2 in 1998 and 2000, respectively. Ladysthumb seed production was also described by the hyperbolic function. Estimated seed production at 1 plant m−1 of cotton row was 33,000 and 47,000 seed m−2 in 1998 and 2000, respectively.