The ability of the pesticide root zone model (PRZM) and the groundwater-loading effects of agricultural management systems (GLEAMS) model to predict movement of two herbicides in soil was evaluated using site-specific environmental data from sites in three states. Predictions of herbicide movement with site-specific data were compared to predictions using more generalized database soil and pesticide data within each model. Field experiments examined fluometuron and norflurazon movement in three soils representative of the cotton-growing regions of the southeastern United States. In comparing the use of site-specific vs. database values, the small increase in accuracy using site-specific inputs would not justify the large cost to obtain the data. The databases for each model gave predictions similar to those using the site-specific numbers. Both the PRZM and the GLEAMS model had similar accuracy levels in predicting the presence of fluometuron or norflurazon present in three surface soils, although each model tended to overpredict movement and total herbicide concentration, especially at lower herbicide concentrations. At higher herbicide concentrations, prediction accuracy was less than that probably needed to predict agronomically relevant herbicide concentrations in surface soils.

Field trials were conducted in 1996 and 1997 to determine the influence of glufosinate and glyphosate application rates, application timings, and interrow cultivation on weed control and corn yield. Glufosinate-ammonium rates ranged from 0.18 to 0.41 kg ai/ha, while rates for the isopropylamine salt of glyphosate ranged from 0.21 to 0.84 kg ae/ha. Increasing rates of glufosinate and glyphosate often improved weed control. Control of many of the weed species was improved by delaying herbicide application timing. Weed control was most consistent from late postemergence (LPOST) applications of glufosinate at 0.41 kg ai/ha or glyphosate at 0.84 kg ae/ha. Corn yields were reduced due to incomplete weed control when the lowest rate of glufosinate was applied. Weed control from early postemergence (EPOST) glufosinate and glyphosate applications followed by cultivation was similar to weed control from LPOST glufosinate and glyphosate applications without cultivation. Interrow cultivation following glufosinate or glyphosate application did not affect corn yield.

Field studies were conducted from 1990 through 1997 to evaluate the long-term effect of 2- and 3-yr rotations on the control of downy brome, jointed goatgrass, and feral rye in winter wheat. At the completion of the study, jointed goatgrass and feral rye densities averaged 8 plants/m2 and < 0.1 plant/m2 for the 2- and 3-yr rotations, respectively. Downy brome densities averaged < 0.5 plant/m2 for both the 2- and 3-yr rotations, with no treatment differences observed. Winter annual grasses were not eradicated after two cycles of the 3-yr rotations, but weed densities were reduced 10-fold compared to densities after one cycle and more than 100-fold compared with the 2-yr rotations. Wheat grain contamination with dockage and foreign material followed a similar trend. The 3-yr rotations were economically competitive with 2-yr rotations and provided superior control of the winter annual grass weeds.

An experiment was conducted at five locations in North Carolina to compare management systems utilizing glufosinate applied postemergence (POST) in glufosinate-resistant corn with standard systems of metolachlor plus atrazine preemergence (PRE) or nicosulfuron plus atrazine POST Glufosinate alone and both standard systems controlled common ragweed and prickly sida at least 98%, whereas sicklepod control was < 20% late in the season. Late-season control of common lambsquarters, smooth pigweed, pitted morningglory, and tall morningglory was generally less with glufosinate alone than with the standard systems. However, late-season control of common lambsquarters, smooth pigweed, pitted morningglory, tall morningglory, and sicklepod by mixtures of glufosinate plus atrazine was at least 99, 100, 89, 93, and 81%, respectively, and was equal to or greater than control by either standard. Broadleaf signalgrass, large crabgrass, and fall panicum were controlled similarly by glufosinate and the standards. Goosegrass control by glufosinate was similar to control by nicosulfuron plus atrazine, but it was less than control by metolachlor plus atrazine. Metolachlor applied PRE or atrazine mixed with glufosinate increased goosegrass control to that achieved with metolachlor plus atrazine. Mixing atrazine with glufosinate did not affect fall panicum control, but metolachlor PRE followed by glufosinate controlled fall panicum as well as the standards. Cultivation or ametryn applied at layby increased control when PRE or POST herbicides alone controlled weeds less than about 90%. Ametryn was generally more effective than cultivation. Glufosinate POST followed by ametryn at layby controlled sicklepod > 90% and other species > 95% late in the season. Corn yield and net returns were similar in the glufosinate and standard systems.

Based on bermudagrass ground cover 2 wk prior to sugarcane planting, tillage plus glyphosate applied postemergence sequentially at 3.4 followed by 2.2 kg ai/ha or a single application at 3.4 kg/ha during the summer fallow period was more effective than tillage alone. Effectiveness of tillage was enhanced when less rainfall was received during the summer fallow period the first year. Rainfall of less than 1 cm 20 d after preemergence application of sulfometuron at 0.2 kg ai/ha in June resulted in 100% bermudagrass ground cover the first year compared with 37% the second year with 15 cm of rainfall during the same period. Terbacil applied after sugarcane planting and metribuzin applied in February resulted in bermudagrass ground cover in May or June of 62% (experiment 1) and 2% (experiment 2) when sulfometuron was used during the fallow period, but no more than 5% when terbacil and metribuzin followed glyphosate plus tillage or tillage alone. In most cases, bermudagrass ground cover at that time was greater when the same glyphosate/tillage treatments were followed by atrazine after planting and pendimethalin plus atrazine in February compared with terbacil after planting and metribuzin in February. When after-planting and February herbicide treatments were applied, sugarcane stalk population, height, and yield each was equivalent regardless of the previous fallow treatment.

Field experiments were conducted in 1989 and 1990 to evaluate seasonal effects of herbicide application, smutgrass (Sporobolus indicus) growth stage, and herbicide rate on smutgrass control. The experimental design was a split–split plot, with season (late spring, midsummer, and fall) as the whole plot, physiological stage at application (uncut, 15-, and 30-cm regrowth) the subplot, and hexazinone rate (0.0, 0.56, 0.84, and 1.12 kg ai/ha) as the sub-subplot treatment. Whole plots were arranged in three randomized complete blocks. Smutgrass control for the mean of the hexazinone treatments (1989 and 1990) was 95%. Application of 0.56 kg/ha hexazinone during late spring, midsummer, and fall seasons provided 77, 92, and 92% smutgrass control, respectively. Generally, the influence of plant stage at time of hexazinone application had no effect (P > 0.05) on smutgrass control. At 20 d after treatment (DAT), hexazinone had injured bahiagrass (Paspalum notatum) plants by yellowing them, but the plants reverted to their usual dark green color by 40 DAT. Total nonstructural carbohydrates were lower for the 30-cm regrowth than for the uncut plants, regardless of season; however, 30-cm regrowth had little effect on smutgrass control.

Greenhouse and laboratory studies were conducted to examine certain characteristics of RPA 201772 and how they may affect its utility. 14C-RPA 201772 was used to determine the stability of RPA 201772 in various pH spray solutions over time. After 24 h, degradation of RPA 201772 was 20, 36, and 93% in spray solutions adjusted to pH 4.0, 7.0, and 10.0, respectively. The major metabolite was diketonitrile (DKN), which is herbicidally active. In addition, 9 and 15% of the RPA 201772 had degraded into an inactive benzoic acid derivative at pH 7.0 and 10.0, respectively. The differences in herbicidal activity of RPA 201772 and its metabolite DKN were also evaluated. Corn seeds and velvetleaf seeds readily imbibed RPA 201772, but only low levels of DKN were imbibed. Preemergence (applications of RPA 201772 and DKN were equally active on corn and velvetleaf. Further studies determined that the principal sites of uptake of RPA 201772 by corn was roots and seeds for four different corn hybrids. Another study determined that light was required for RPA 201772 activity. Corn shoots from seeds germinated under 14 h of light exhibited rate-dependent injury, while corn germinated and grown in the darkness was not injured.

Certain salts in the spray carrier can antagonize sethoxydim, while other salts overcome the antagonism or increase the herbicide's efficacy. Sethoxydim responses to salts have not been consistent, and the inconsistency is not completely understood. Experiments were conducted in the greenhouse to determine sethoxydim phytotoxicity to oat as influenced by salts in the spray carrier and by air temperature at and immediately after application as they alter the spray deposit characteristics. Thick amorphous spray deposits were related to ineffective sethoxydim-petroleum oil adjuvant treatments applied alone at 10 C or with bentazon at 10 or 25 C. Also, thin deposits with salt residues were related to ineffective sethoxydim treatments, as when sodium bicarbonate was present. Ammonium sulfate reduced or overcame antagonism of sethoxydim phytotoxicity caused by low temperature, bentazon, and sodium bicarbonate and gave thin deposits having close contact with the leaf epicuticular surface but without obvious salt deposits. UV light antagonism of sethoxydim phytotoxicity was different depending on temperature at and shortly after application and depending on salts in the spray mixture. The results indicate that salts in a bentazon spray carrier are in part antagonistic because of their effect on final spray deposit.

Observations in a citrus grove indicated that hairy beggarticks numbers were higher in plots receiving thiazopyr alone or thiazopyr with oxyfluorfen than in nontreated plots. At 120 days after treatment (DAT), approximately 90% more hairy beggarticks plants had emerged in plots receiving thiazopyr at 0.56 kg ai/ha than in nontreated plots. A tank mix of thiazopyr plus oxyfluorfen, each at 0.56 kg/ha, resulted in a 55% increase in hairy beggarticks number at 120 DAT. Greenhouse studies using seeds collected from sites with or without herbicide history produced similar results. Oxyfluorfen at 0.28 kg/ha stimulated seed germination but injured seedlings by 47% or more at 36 DAT when applied at higher rates. Thiazopyr applied at 0.56 kg/ha antagonized the preemergence activity of oxyfluorfen at 0.28 or 0.56 kg/ha. Thiazopyr at 1.12 kg/ha stimulated hairy beggarticks germination by 37% compared with nontreated containers.

Growth-regulating characteristics of trinexapac-ethyl, paclobutrazol, and flurprimidol were investigated on ‘Tifway’ bermudagrass and four turfgrass weeds (large crabgrass, goosegrass, bahiagrass, and purple nutsedge). All treatments reduced the height of Tifway bermudagrass (25 to 37%) and large crabgrass (14 to 34%), but the height of purple nutsedge, goosegrass, and bahiagrass was not consistently changed. In 1995, all rates of trinexapac-ethyl reduced clipping weights of Tifway bermudagrass (70 to 87%) and bahiagrass (53 to 59%) more than large crabgrass, goosegrass, or purple nutsedge (< 39%). Trinexapac-ethyl (0.4 kg ai/ha) and paclobutrazol (1.1 kg ai/ha) applications increased both large crabgrass clipping weight and canopy visible density. Increased large crabgrass density was attributed to greater tiller numbers with trinexapac-ethyl (60 tillers/pot) and paclobutrazol (63 tillers/pot) compared to untreated large crabgrass plants (53 tillers/pot). Weed growth was not suppressed as much as Tifway bermudagrass growth; therefore, turfgrass stands may be adversely affected by plant growth regulator use under weedy conditions.

The dissipation of four commonly used soil-applied herbicides was examined in a standardized field experiment in three southern states (Kentucky, Mississippi, and Tennessee). Averaged over the three soils and 2 yr, the relative order of increasing half-life defined as time for 50% disappearance in days (DT50) was acetochlor (6.3 d) = alachlor (6.3 d) = SAN 582 (7.3 d) < metolachlor (13.7 d). Metolachlor was the most persistent in the soil surface, and this could potentially translate into greater duration of weed control into the growing season. All examined herbicides had a DT50 that averaged less than 14 d in all states in both years, so full-season weed control of susceptible species would not be expected. Rapid herbicide degradation was encouraged in these field sites by adequate to excessive soil moisture and warm temperatures throughout the sampling interval. The soils also were light textured, and the lower adsorption of the herbicide allowed for degradation ease and perhaps leaching below the sampling depth.

Field experiments were conducted during 1996 and 1998 to determine the effect of atrazine, bentazon, atrazine + bentazon, the amine salt of 2,4-D, and bromoxynil on broadleaf signalgrass and barnyardgrass control when applied in mixture with sethoxydim. Herbicide combinations were applied with crop oil concentrate, crop oil concentrate + ammonium sulfate, or BCH 815. Bentazon and atrazine + bentazon reduced broadleaf signalgrass and barnyardgrass control by sethoxydim. Bromoxynil reduced barnyardgrass control but had no affect on broadleaf signalgrass control. Including ammonium sulfate with crop oil concentrate or substituting BCH 815 for crop oil concentrate increased barnyardgrass and broadleaf signalgrass control by sethoxydim when applied with bentazon. Ammonium sulfate and BCH 815 increased barnyardgrass control when sethoxydim was applied with bromoxynil but did not affect control when sethoxydim was applied with atrazine + bentazon. Atrazine and the ammonium salt of 2,4-D did not reduce barnyardgrass and broadleaf signalgrass control by sethoxydim regardless of adjuvant.

A 3-yr field study was conducted in peanut in which weed control systems with varying levels of herbicides, cultivation inputs, or both were supplemented with mowing to remove weeds extending above the crop canopy. Highest yield and net returns were consistently obtained with the highest level of herbicide/cultivation inputs, and mowing was of no value. However, mowing was consistently beneficial to yield and net return when the only other weed control inputs were paraquat and 2,4-DB applied at 4 wk after planting (WAP) and cultivation at 10 WAP. In the absence of any other weed control inputs, mowing was detrimental in a drought stress year, but beneficial in years with near-normal growing conditions. Although disease incidence was aggravated by the lack of weed control inputs, it was generally independent of mowing.

A 4-yr field experiment was conducted to determine the merits of combining cultural and chemical controls to manage foxtail barley in reduced-tillage systems. Factors studied were crop row spacing, seeding rate, and application rate and timing of glyphosate within a spring wheat-flax cropping sequence. Glyphosate applied preseeding at 400 or 800 g/ha killed foxtail barley seedlings but only suppressed established perennial plants. Glyphosate applied postharvest at 800 g/ha killed 60 to 70% of established plants. Combinations of preseeding and postharvest glyphosate gave the greatest reductions in foxtail barley biomass and seed production and resulted in the greatest increases in crop yield. Including flax in the rotation allowed use of grass herbicides such as quizalofop or sethoxydim that effectively controlled foxtail barley seedlings and provided some suppression of perennial plants. An increase in wheat seeding rate from 75 to 115 kg/ha reduced foxtail barley growth and increased wheat yield in 3 of 4 yr. Increasing the flax seeding rate from 40 to 80 kg/ha or reducing wheat and flax row spacing from 30 to 20 cm provided little benefit in managing foxtail barley or increasing crop yield. A multiyear approach combining agronomic practices and timely use of herbicides should allow growers to effectively manage foxtail barley in annual cropping systems using conservation tillage.

Currently, there are no effective herbicides registered for the control of hoary cress (Cardaria draba) in alfalfa (Medicago sativa) or alfalfa/grass mixtures. Due to this lack of control options, field experiments were conducted near St. Ignatius, MT, from 1995 through 1997 to determine the optimum imazethapyr rate and adjuvant combination for hoary cress control in alfalfa/orchardgrass (Dactylis glomerata) pastures. Imazethapyr was applied in the spring to rosette stage hoary cress plants at 33, 67, and 100 g ai/ha with either a nonionic surfactant or methylated seed oil alone or combined with 28% urea ammonium nitrate (UAN) liquid fertilizer. Hoary cress control was poorest when imazethapyr was applied at 33 g/ha, yet provided a minimum of 80% control. Imazethapyr applied at 67 and 100 g/ha provided greater than 90% control during the year of application regardless of surfactant or UAN additions. None of the treatments controlled hoary cress the year following application. Alfalfa yield was unaffected by imazethapyr applications. First-cutting orchardgrass yield decreased as imazethapyr rate increased, but total seasonal forage yield was unaffected.

Field experiments were conducted at Dekalb, IL, in 1996 and 1997 to determine the optimum application timing and rate of cloransulam for giant ragweed (Ambrosia trifida) control in soybean (Glycine max). Cloransulam treatments included preplant incorporated (PPI) and preemergence (PRE) applications of 35 g ai/ha and early postemergence (EPOST), postemergence (POST), and late postemergence (LPOST) applications of 18 or 27 g ai/ha. Cloransulam applied at 18 g/ha was also combined with lactofen at 70 g ai/ha at each POST application timing. At 60 d after LPOST, cloransulam applied PPI provided 68% giant ragweed control in 1996 and 1997 compared to PRE applications, which provided 95 and 25% giant ragweed control, respectively. The reduction in giant ragweed control with PRE applications of cloransulam in 1997 was likely due to insufficient rainfall for activation. Cloransulam applied at 18 g/ha EPOST provided 87 and 88% giant ragweed control, respectively, in 1996 and 1997. Cloransulam applied POST provided 97 and 82% giant ragweed control, respectively, in successive years. Delaying cloransulam application until LPOST reduced giant ragweed control to 53 and 47%, respectively, in 1996 and 1997 compared to EPOST and POST. At EPOST and POST application timings, increasing the rate of cloransulam to 27 g/ha or adding lactofen did not improve giant ragweed control. However, giant ragweed control was improved by at least 20% by increasing the rate of cloransulam to 27 g/ha at LPOST. Similarly, applying cloransulam in combination with lactofen improved giant ragweed control by at least 15% at LPOST.

A study was conducted in 1996 and 1997 near Rohwer, AR, to evaluate weed control with standard herbicide programs in nontransgenic and glyphosate-resistant soybean (Glycine max) compared with glyphosate-based herbicide programs in glyphosate-resistant soybean. Total postemergence (POST) programs containing 0.84 kg ae/ha glyphosate 14 d after emergence (DAE) followed by 0.42 kg/ha at 42 DAE controlled pitted and entireleaf morningglory (Ipomoea lacunosa and I. hederacea var. integriuscula) 80 to 83%. This control was lower than with any other weed control program in the study. All glyphosate containing herbicide programs controlled barnyardgrass (Echinochloa crus-galli) at least 94%. Soybean injury was 8% or less for all herbicide treatments except for ‘Hartz 5088 Roundup Ready’ treated with pendimethalin plus chlorimuron and metribuzin preplant incorporated (PPI) followed by glyphosate or acifluorfen plus bentazon 42 DAE. All weed control and soybean injury ratings were similar at 24 d after the 42 DAE application (DAA) when compared with the 10 DAA rating. Two weed control programs with ‘Northrup King S-59-60’ soybean using standard herbicide programs yielded 3,015 and 3,070 kg/ha, and net returns were $92.20/ha and $122.82/ha, respectively. Two production systems containing the Hartz 5088 Roundup Ready cultivar had positive net returns of $35.11/ha and $39.49/ha.

Field studies evaluated wild radish control in oat with thifensulfuron plus tribenuron in 1995 and 1997. Thifensulfuron plus tribenuron at 16 or 21 g ai/ha controlled wild radish completely in both years. In tolerance studies conducted in 1995 and 1996, thifensulfuron plus tribenuron reduced vigor of ‘Bay,’ ‘Belle,’ ‘Dane,’ ‘Ensiler,’ ‘Gem,’ ‘Horicon,’ ‘Ogle,’ ‘Porter,’ and ‘Prairie’ oat varieties and Wisconsin test selection oat ‘X6396-1’ up to 25% 7 d after treatment (DAT), but all chlorotic symptoms had disappeared 28 DAT. Thifensulfuron plus tribenuron reduced oat height 13 to 19% in 1995 and 21 to 24% in 1996 7 DAT. Bay, Belle, Dane, Ensiler, Horicon, Prairie, and X6396-1 recovered from height reductions by 28 DAT in 1995; however, in 1996 only Horicon and Porter recovered. Thifensulfuron plus tribenuron did not reduce 1995 grain yields, but 1996 Bay and Porter grain yields were reduced 29 and 15%, respectively. Dane and Prairie seed weights were reduced 7 and 14%, respectively, in 1995, whereas no seed weight reduction occurred in 1996.

Field experiments were conducted in 1995 and 1996 at DeKalb and Urbana, IL, to evaluate weed management systems in glyphosate-resistant soybean planted in rows 76 cm wide. These experiments compared weed control using preemergence (PRE) herbicides followed by glyphosate or postemergence (POST) tank-mix combinations of glyphosate and acetolactate-synthase-inhibiting herbicides with glyphosate applied alone at 0.63 kg ae/ha in single or sequential applications. Overall, the use of a tank-mix partner or a PRE herbicide followed by glyphosate improved weed control compared to a single application of glyphosate. However, weed control with these treatments was not better than with sequential applications of glyphosate. Control of giant foxtail exceeded 90% for single applications of glyphosate except at DeKalb in 1995 when late emergence of giant foxtail occurred after POST applications had been made. A PRE grass herbicide or a late postemergence (LPOST) application of glyphosate was necessary for season-long control of late-emerging giant foxtail. Tank-mixing glyphosate with imazethapyr, cloransulammethyl, and CGA-277476 or applying glyphosate LPOST following these herbicides improved giant foxtail control compared with these herbicides applied alone. A single application of glyphosate controlled common lambsquarters 88% or greater in two of three trials. At Urbana in 1995, a single application of glyphosate controlled common lambsquarters 78% compared to 88 to 96% control with PRE herbicides followed by glyphosate or sequential applications of glyphosate. Velvetleaf control with a single application of glyphosate ranged from 55 to 78%. A PRE application of chlorimuron + metribuzin, cloransulammethyl, or sulfentrazone followed by glyphosate POST, as well as sequential applications of glyphosate, consistently improved velvetleaf control compared to a single application of glyphosate. In some cases, adding glyphosate to a POST application of imazethapyr or CGA-277476 improved control of velvetleaf but decreased velvetleaf control when added to cloransulammethyl.

Two field studies, a seeded study and a golf course study, were conducted to compare competition among creeping bentgrass, annual bluegrass, and roughstalk bluegrass when subjected to common weed control practices and foliar applications of iron and magnesium. A research site was selected for the seeded study and divided into 10 whole plots receiving irrigation at either 50 or 100% evapotranspiration deficit. Each whole plot was further divided into subplots receiving one of seven treatments: bensulide, ethofumesate, trinexapac-ethyl, foliar Mg, foliar Fe, foliar Mg plus foliar Fe, and control. The site was seeded to a mixture of creeping bentgrass, annual bluegrass, and roughstalk bluegrass in September 1995, and treatments began in March 1996. Annual bluegrass was reduced 29% in plots treated with foliar Fe and 65% in plots treated with foliar Fe plus foliar Mg. Roughstalk bluegrass was significantly reduced in seeded plots treated with foliar iron (50%), plant growth regulator (75%), and foliar iron plus foliar magnesium (100%). Annual bluegrass and roughstalk bluegrass proportions were not affected by irrigation regime. In a second study, the most effective treatment, foliar magnesium plus foliar iron, was tested on a working golf course fairway and on a practice putting green beginning April 1997 and ending November 1997. Treatments on the golf course fairway and practice putting green were ineffective due to the established, perennial nature of the annual bluegrass biotypes on these sites. Further research is required to improve the efficacy of nutritional treatments on these perennials.