A 3-yr study was conducted in eastern Wyoming from 1995 to 1997 to evaluate the effect of fertilizer placement on jointed goatgrass competitiveness with winter wheat. Fertilizer placement methods consisted of applying 45 kg/ha of nitrogen (50% as urea and 50% as ammonium nitrate) in a deep band 5 cm below and 2.5 cm to the side of the wheat row, broadcasting on the soil surface, or injecting fertilizer by spoke wheel 10 cm deep and 5 cm to the side of the wheat row. Neither fertilizer placement nor jointed goatgrass presence affected winter wheat stand. Wheat yield reductions from jointed goatgrass competition were 7 and 10% higher with the broadcast than deep-band or spoke-wheel injection methods, respectively. Wheat spikes/plant, seeds/spike, 200-seed weight, and plant height were not influenced by fertilizer placement; however, the presence of 35 jointed goatgrass plants/m2 reduced spikes/plant 21%, seeds/spike 12%, and 200-seed weight 6%. Jointed goatgrass populations were not influenced by fertilizer placement method; however, the number of spikes/plant was reduced 8 and 10%, joints/spike 3%, and biomass 15 and 21% by deep band or spoke wheel fertilizer placement.

The commercial release of crops with engineered resistance to 2,4-D and dicamba will alter the spatial and temporal use of these herbicides. This, in turn, has elicited concerns about off-target injury to sensitive crops. In 2014 and 2015, studies were conducted in Tifton, GA, to describe how herbicide (2,4-D and dicamba), herbicide rate (1/75 and 1/250 field use), and application timing (20, 40, and 60 DAP) influence watermelon injury, vine development, yield, and the accumulation of herbicide residues in marketable fruit. In general, greater visual injury and reductions in vine growth, relative to the non-treated check, were observed when herbicide applications were made before watermelon plants had begun to flower. Although the main effects of herbicide and rate were less influential than the timing of applications with respect to plant development, the 1/75 rates were more injurious than the 1/250 rates; dicamba was more injurious than 2,4-D. In 2014, the 1/75 and 1/250 rates of each herbicide reduced marketable fruit numbers 13 to 20%, but only for the 20 DAP application. The 1/75 rate of each herbicide when applied at either 20 or 40 DAP reduced the number of fruit harvested per plot in 2015. Dicamba residues were detected in marketable fruit when the 1/75 rate in 2014 and 2015 and the 1/250 rate in 2015 was applied to plants at 40 or 60 DAP. Residues of 2,4-D were detected in 2015 when the 1/75 and 1/250 rates were applied at 60 DAP. Across both years, the maximum level of residue detected was 0.030 ppm. While early season injury may reduce watermelon yields, herbicide residue detection is more likely in marketable fruit when an off-target contact incident occurs closer to harvest.

Four field studies were conducted at the Peanut Belt Research Station near Lewiston Woodville, NC, in 2000, 2001, and 2002 to evaluate crop tolerance, weed control, grain yield, and net returns in glyphosate-resistant corn with various herbicide systems. Preemergence (PRE) treatment options included no herbicide, atrazine at 1.12 kg ai/ha, or atrazine plus metolachlor at 1.68 kg ai/ha. Postemergence (POST) treatment options included glyphosate at 1.12 kg ai/ha as either the isopropylamine salt or the diammonium salt, either alone or in mixtures with mesotrione at 105 g ai/ha plus crop oil concentrate at 1% (v/v) or halosulfuron at 53 g ai/ha plus 0.25% (v/v) nonionic surfactant. All response variables were independent of glyphosate formulation. Addition of metolachlor to atrazine PRE improved large crabgrass and goosegrass control but did not always improve Texas panicum control. POST control of these annual grasses was similar with glyphosate alone or in mixture with halosulfuron or mesotrione. Glyphosate POST controlled common lambsquarters and common ragweed 89 and 93%, respectively. Glyphosate plus halosulfuron POST provided more effective yellow nutsedge control than glyphosate POST. Atrazine PRE or atrazine plus metolachlor PRE followed by any glyphosate POST treatment controlled Ipomoea spp. at least 93%. Glyphosate plus mesotrione in total POST systems always provided greater control of Ipomoea spp. than glyphosate alone. The highest yielding treatments always included glyphosate POST, either with or without a PRE herbicide treatment. Similarly, systems that included any glyphosate POST treatment had the highest net returns.

This study characterized the emergence pattern of a weed community of 16 species between April 1 and August 31 over a 7-yr period. Weed seedlings were counted weekly in quadrats established in winter wheat stubble within no-till and conventional-till production systems. Weed emergence showed two peaks, the first between April 25 and May 9, and the second between May 30 and June 13. Tillage did not affect the weed community emergence pattern. Knowledge of weed community emergence pattern in conjunction with crop simulation models could be used to suggest cultural practices such as optimal planting dates that favor a crop over weeds, and possibly reduce herbicide use for within-crop weed control.

Lentil and pea are two important crops grown in rotation with winter wheat in the Palouse region of Idaho and Washington. Imazethapyr plus pendimethalin often is used to control weeds in lentil and pea, but the effects of these herbicides on these crops and the subsequently planted winter wheat crop are not well known. The component and combined effects of several rates of imazethapyr and pendimethalin on growth and yield of lentil and pea and the subsequently planted winter wheat crop were measured in 1997 and 1998 field experiments. No herbicide treatment reduced lentil or pea biomass or seed yield compared with the untreated control. Wheat biomass was reduced 35 to 51%, and grain yield was reduced 11 to 17% in all plots treated with 2,240 g/ha pendimethalin at the lentil hilltop site. Imazethapyr at 106 g/ha plus 1,120 g/ha pendimethalin also reduced wheat biomass 24% at the lentil hilltop site. Wheat was not injured at other sites or by other treatments at the lentil hilltop site.

Glyphosate-resistant (GR) horseweed management continues to be a challenge in no-till cotton systems in Tennessee and Mississippi. Field studies were conducted in 2009 and 2010 to evaluate saflufenacil in tank mixtures with glyphosate, glufosinate, or paraquat on GR horseweed prior to planting cotton. Saflufenacil and saflufenacil tank mixtures were applied 7 d before planting (DBP). Three broad spectrum herbicides were tank-mixed with saflufenacil at rates of 0, 6.3, 12.5, 25, and 50 g ai ha−1. Saflufenacil at 25 and 50 g ai ha−1 in tank mixture with all three broad-spectrum herbicides provided similar GR horseweed control when compared to the current standard of glyphosate + dicamba. Across all saflufenacil rates, lint cotton yield among the glyphosate, glufosinate, and paraquat tank mixture treatments did not differ from each other. Control of horseweed with 25 or 50 g ha−1 of saflufenacil across all tank mixtures also was not different from the standard of glyphosate + dicamba. Moreover, saflufenacil, on silt loam soil evaluated in this study, showed no more cotton injury than glyphosate applied 7 d or more before planting. Saflufenacil applied alone at 25 g ha−1 provided lower control of GR horseweed than the standard, which translated to lower lint yield compared to the glyphosate + dicamba treatment or saflufenacil with each tank mixture partner. The 12.5 g ha−1 rate of saflufenacil tank mixed with either paraquat or glufosinate provided less horseweed control (< 85%) than if higher rates of saflufenacil were used (> 95%). However, lint cotton yield was not different between these treatments. This research suggests that saflufenacil at 25 g ha−1 is the most optimal rate for tank mixtures with glyphosate, glufosinate, or paraquat. It also reaffirms earlier research that the 25 g ha−1 saflufenacil rate safely can be applied inside the currently labeled 42-d waiting period between a saflufenacil application and cotton planting.

The effect of variables that influence the retention of spray droplets impacting on leaf surfaces was studied, using a uniform-size droplet generator to produce drops ranging from 63 to 545 μm diam. To observe the impaction of spray droplets more easily and to measure their velocity before and after impaction, high-speed motion photography (6000 frames/sec) was used. Rebound of spray droplets depends on the micro-structure of the target surface. Leaf surfaces of several crops and weeds reflected 63 to 545 μm diam water drops traveling at velocities less than velocities of drops delivered by popular nozzles at commonly used spray pressures. If the concentration is high enough, some surfactants can reduce the rebound of drops. With one surfactant, its concentration had to be much greater than the critical micelle concentration to reduce reflection of spray drops.

The efficacy of imazethapyr in drill-seeded imidazolinone-resistant rice was evaluated in Louisiana in 2000 and 2001. Imazethapyr was applied preemergence (PRE), or no PRE, followed by a postemergence (POST) application of imazethapyr alone, or in a mixture with bensulfuron, bentazon plus aciflurofen, carfentrazone, halosulfuron, propanil plus molinate, triclopyr, or V-10029. Imazethapyr applied PRE followed by a POST application of imazethapyr controlled barnyardgrass equivalent to or higher than other treatments evaluated. Red rice control at 35 days after postemergence treatment (DAT) was 66 to 81% with imazethapyr applied PRE followed by any POST application, but a reduction in control was observed with a POST application of imazethapyr. Although alligatorweed control increased with POST applications, these treatments suggested only suppression. Hemp sesbania control never exceeded 10% with imazethapyr-only treatments and at 35 DAT, all POST applications, except bensulfuron, increased control above 84%. Rice yield increased with treatments receiving a PRE application of imazethapyr compared with no imazethapyr applied PRE.

The initiative to use plant pathogens and allelochemicals from pathogens and other microorganisms as biological weed control agents (bioherbicides) began about 30 yr ago. Since then, numerous plant pathogens (bacteria and fungi) and microbial allelochemicals have been isolated, identified, and tested for their bioherbicidal potential. Pathogens (and in some cases microbial phytotoxins) may be used directly on target weed species, or such allelochemicals may provide unique chemical templates for the synthesis of new herbicide classes with novel molecular modes of action. To date, the most successful microbial products that have led to the development of commercial herbicides are bialaphos (commercially available in Japan) and glufosinate (marketed worldwide). Glufosinate is the ammonium salt of phosphinothricin, which is the active ingredient of bialaphos derived from a nonphytopathogenic Streptomyces species. This overview will examine selected advances in the isolation and identification of novel plant pathogens that have weed hosts, and some microbial allelochemicals with phytotoxic properties. Perspectives on the use of these bioherbicides in weed control, relative to their allelopathic interactions with plants will be discussed.

Two field experiments were conducted in Oklahoma to determine whether changing wheat row spacing, seeding rate, and cultivar would reduce interference from rye. Wheat row spacing did not affect rye seed production. Averaged over row spacing, increasing wheat seeding rate from 67 to 134 kg/ha reduced rye seed production 21 and 25% in two experiments. At one site, grain yield of rye-infested wheat was increased 27 and 23% by doubling wheat seeding rate in 10- and 20-cm rows. Doubling the seeding rate of rye-infested wheat in 30-cm rows did not increase wheat yield. At a second site when data were pooled over row spacings, increasing the wheat seeding rate from 67 to 101 kg/ha increased yield of rye-infested wheat 21%, but yield was still 36% less than for rye-free wheat. In two additional experiments, rye seeds in harvested wheat were reduced 36% by increasing the wheat seeding rate from 60 to 162 kg/ha, whereas yield of rye-infested wheat was increased 82%. Nine cultivars were compared for competitive ability against rye in four experiments. Of 32 possible cultivar location situations, the rye-induced yield loss of ‘Jagger’ wheat was less than 16 other cultivar location situations, whereas the yield loss of ‘Triumph 64’ wheat was less than 10 other cultivar location situations. Mature wheat height, wheat yield, and wheat maturity classification were each negatively correlated to rye yield in infested wheat at two of four sites. Wheat plant density, head density, sunlight interception, and fall cover capability classification were each negatively correlated to rye yield at one of four sites.

Experiments were conducted in 1989 and 1990 to investigate the influence of 220 g ai ha–1 AC 263,222, 840 g ai ha-4 glufosinate, 560 g ai ha–1 glyphosate, and 840 g ai ha–1 paraquat applied at R5, R6, R7, and R8 soybean growth stages as harvest-aid treatments on soybean seed quality. Soybean yield was severely reduced by all herbicides applied at R5. Only paraquat and glufosinate reduced soybean yield when applied at R6, and none of the herbicides reduced yield when applied at R7 or R8. Paraquat and glufosinate reduced seed weight when applied at RS and R6. Glyphosate and AC 263,222 reduced seed germination when applied at RS, R6, and R7 growth stages, and normal seedling percentages were also reduced by glyphosate at these growth stages. Glufosinate and AC 263,222 affected normal seedlings only at R5 and R6. Soybean hypocotyl and primary root lengths were reduced by glyphosate and AC 263,222 applications at R5 and R6, whereas glufosinate and paraquat did not affect these variables. Glyphosate applied at R5 reduced shoot weight in 1-mo-old soybean plants. No effects were observed on soybean yield, seed germination or seedling development when herbicides were applied at R8.

Field experiments were conducted from 1979 to 1989 to determine the influence of conventional, reduced, and no-tillage systems and different herbicide combinations on weed species and population, weed control, and soybean injury, population, and yield. In no-till (NT) non-treated plots, there was an abrupt shift from horseweed as the dominant early spring emerging weed to gray goldenrod in 1985. Following its initial observation, gray goldenrod became the dominant species within 2 yr, with giant foxtail as the only other species observed in these plots. Giant foxtail was the dominant weed species from 1980 to 1989 in conventional till (CT) and reduced-till (RT) plots. There also was a shift in the frequency of occurrence and in density of several broadleaf weed species during the 11-yr study. Most herbicides provided excellent control of all weeds in all tillage systems, especially those that included POST herbicides. There was little difference between glyphosate and paraquat in controlling weeds present at the time of planting in NT. PRE herbicides caused 2 to 9% soybean injury with slightly greater injury occurring in CT and RT than in NT. The POST broadleaf herbicides did not significantly increase soybean injury. There were no differences in soybean population or yield among the herbicide treatments regardless of tillage. There also was no difference in soybean population or yield in NT compared with CT when averaged over all herbicide treatments.

Field experiments were conducted in 1988 and 1989 to evaluate timing of POST herbicide application in peanut for annual broadleaf weed control, peanut yield, and net returns. Bentazon tank-mixtures with either acifluorfen or paraquat controlled common lambsquarters, prickly sida, and morningglory species equally when applied either at ground-cracking (GC) or 2 wk after ground-cracking (2WGC). Paraquat controlled common lambsquarters least (32 to 33%). Bentazon plus acifluorfen plus BCH 81508S controlled 82% common lambsquarters at 2WGC compared with 66% control from bentazon plus acifluorfen plus a crop oil concentrate. Lactofen controlled 86% common lambsquarters when applied at GC compared to 34% at 2WGC. All applications applied at 4 wk after GC (4WGC) controlled essentially no annual broadleaf weeds. Higher yields and net returns were generally obtained with earlier herbicide applications. Bentazon tank-mixtures with acifluorfen provided equivalent peanut yields and net returns for GC and 2WGC applications.

Data from field studies at Fayetteville, AR, were used to predict the herbicide rate needed to provide 70, 80, or 90% control of a weed based upon weed age. Reduced herbicide rates generally needed to be applied within 6 to 12 d after emergence to control weeds 90%. Reduced rates (280 g ai ha–1 or less) of acifluorfen controlled hemp sesbania, smooth pigweed, Palmer amaranth, and pitted and entireleaf morningglory 90%. Bentazon at 350 to 650 g ai ha–1 controlled common cocklebur and prickly sida 90%. Common cocklebur, smooth pigweed, and pitted morningglory were controlled 90% with chlorimuron at 2 to 5 g ai ha–1 and imazaquin at 20 to 80 g ai ha–1. Prickly sida and hemp sesbania were controlled 90% with imazaquin at 70 g ha–1 and chlorimuron at 6 g ha–1, respectively. Barnyardgrass, large crabgrass, red rice, and sicklepod were not controlled with reduced herbicide rates.

The effects of ammonium sulfate and pelargonic acid on weed control with glufosinate and glyphosate and safety to glufosinate-resistant and glyphosate-resistant soybean were investigated in the greenhouse and field. Annual and perennial weeds varied in their sensitivity to the herbicides. Based on fresh weight reduction 10 d after treatment (DAT), common milkweed was more tolerant to glufosinate, and horsenettle was more tolerant to glyphosate. Giant foxtail was highly sensitive to both herbicides. The activity of glufosinate on common milkweed and of glyphosate on horsenettle was enhanced with the addition of 5% (wt/v) ammonium sulfate. The addition of pelargonic acid at 3% (v/v) did not enhance the activity of glufosinate or glyphosate on any weed, and it antagonized common lambsquarters and giant foxtail control with glufosinate and with glyphosate. Glyphosate was more effective than glufosinate in suppressing the regrowth of the perennial weeds horsenettle and common milkweed, but addition of ammonium sulfate and pelargonic acid was not beneficial with either herbicide. Under field conditions, the addition of ammonium sulfate or pelargonic acid to glufosinate or glyphosate did not improve efficacy on annual weeds. The addition of pelargonic acid improved yellow nutsedge control with glufosinate, but only at 6 DAT. Glufosinate and glyphosate applied alone or in combination with ammonium sulfate were safe to transgenic soybeans resistant to the respective herbicide. The addition of pelargonic acid to glufosinate or glyphosate in the greenhouse caused a rate-dependent reduction in soybean fresh weight. In the field, slight soybean injury with the addition of pelargonic acid was evident 6 DAT, but not 23 DAT. Addition of ammonium sulfate can increase the efficacy of glufosinate and glyphosate on perennial weeds without negatively affecting soybean yield.

A two-year herbicide residue study was conducted at a commercial nursery to determine the presence of herbicide residues in pond water and sediment following normal production cycles. The runoff water from irrigation and rainfall was contained on the nursery and reused for irrigation. The nursery site was approximately 60 ha with two interconnected ponds, one for containment of runoff water and one serving as a source of irrigation water. The most commonly used herbicides were oxyfluorfen plus pendimethalin (granular formulation-Ornamental Herbicide 2); approximately twice as much herbicide was applied during the second year of the study. Herbicide levels found in pond water and sediment were approximately two-fold greater during the second year corresponding to the increase in active ingredients applied. The highest concentration of oxyfluorfen found in water and sediment was 0.04 μg/ml and 4.0 μg/g, respectively. The highest concentration of pendimethalin found in water and sediment was 0.008 μg/ml and 14.3 μg/g, respectively. In irrigation water samples, the highest concentration of oxyfluorfen and pendimethalin detected was 0.005 μg/ml and 0.002 μg/ml, respectively. The herbicides did not accumulate in water or sediment over the two-year period. No landscape plant phytotoxicity problems are likely at the herbicide levels detected in irrigation water samples.

Tolerance of eight processing sweet corn hybrids to nicosulfuron was evaluated in field studies conducted at Arlington, Wis. in 1988 and 1989. Magnitude of sweet corn vigor reduction varied between years. Greater vigor reduction occurred to ‘Jubilee’ sweet corn than to the other seven hybrids. Injury to the hybrids increased as nicosulfuron rate increased. In 1988, some of the hybrids were stunted slightly by all of the herbicide rates, but within a hybrid there was no significant difference among herbicide rates. In 1989 none of the hybrids, including Jubilee, was stunted by even 140 g ai ha–1 nicosulfuron compared with the corresponding check. Effects of nicosulfuron on yields of the eight hybrids in 1988 and in 1989 are best described by a quadratic regression equation.

Herbicide-resistant weeds are becoming a major problem in the Midwest, and strategies must be adopted to delay further selection. Strategies of rotating and tank-mixing herbicides with different modes of actions should be effective, but adoption may be limited and certain limitations may exist. Therefore, integrating nonchemical practices that indirectly lower selection pressure or restrict the growth of resistant populations is desirable. Appropriate integration of mechanical weeding, crop rotation, increased crop competition, and decision aids may further delay the development of resistance. Understanding the effect of these practices on weed population dynamics is required to more accurately predict their contributions toward resistance management. This knowledge will aid in justifying the adoption of improved management systems.

Michigan soybean producers have observed that glyphosate efficacy is sometimes reduced in tank mixtures with foliar manganese (Mn) fertilizers. The objectives of this study were to evaluate the effects of Mn formulation, Mn application timing, tank mixture adjuvants, and Mn rate on glyphosate efficacy. Three Mn formulations, manganese sulfate and ethylaminoacetate chelate (Mn-EAA), manganese sulfate and lignin sulfonate chelate (Mn-LS), and manganese sulfate monohydrate (MnSO4) reduced glyphosate efficacy in greenhouse and field bioassays, but Mn ethylenediaminetetraacetate (Mn-EDTA) did not. Mn-EAA applied less than 3 d before glyphosate reduced glyphosate efficacy on velvetleaf but not giant foxtail or common lambsquarters. The antagonism increased as the interval between treatment applications was shortened but did not appear when Mn was applied to velvetleaf 1 d or more after glyphosate. Including the adjuvants ammonium sulfate (AMS), EDTA, or citric acid in the glyphosate–Mn tank mixture increased control of giant foxtail and velvetleaf but only matched the efficacy of the glyphosate plus AMS control in three combinations: AMS with Mn-LS on velvetleaf, citric acid with MnSO4 on giant foxtail, and EDTA with Mn-EAA on giant foxtail. AMS increased the glyphosate efficacy in Mn tank mixtures as much as, or more than, citric acid and EDTA, with two exceptions: EDTA with Mn-EAA on giant foxtail and citric acid with MnSO4 on velvetleaf. Control of velvetleaf declined as the amount of Mn from Mn-EAA, Mn-LS, and MnSO4 in the tank mixture increased.

Field experiments were conducted at Manhattan and Hesston, KS, in 2004, and at Manhattan, KS, in 2005, to evaluate cotton response to seven hormonal-type herbicides. Herbicides 2,4-D amine, 2,4-D ester, clopyralid, picloram, fluroxypyr, triclopyr, and dicamba were each applied at 0, 1/100, 1/200, 1/300, and 1/400 of the herbicide use rates on cotton in the six- to eight-leaf stage. Herbicide use rates were 210 and 280 g ae/ha for fluroxypyr and clopyralid and 561 g ae/ha, for 2,4-D amine, 2,4-D ester, dicamba, picloram, and triclopyr. At 14 d after treatment (DAT), all herbicides caused leaf cupping and epinasty, except triclopyr and clopyralid, which caused severe bleaching and chlorosis. The order of visual injury ratings was 2,4-D ester > 2,4-D amine > picloram > dicamba > fluroxypyr > triclopyr > clopyralid. By 56 DAT, slight injury symptoms were observed on plants treated with all herbicides, except all rates of 2,4-D, from which symptoms were severe. All rates of 2,4-D and the highest rate of picloram caused more than 60% flower abortion. Ranking of fiber yield reduction after herbicide treatment was 2,4-D ester > 2,4-D amine > picloram > fluroxypyr > dicamba > clopyralid > triclopyr. This research demonstrated that cotton is extremely susceptible to simulated drift rates of 2,4-D and picloram, whereas clopyralid and triclopyr caused early injury, with minimal effect on cotton yield.