A 2-yr study was conducted near Mead, NE, to determine the interaction of six corn (Zea mays) hybrids having different heights and leaf angles and four herbicide rates on velvetleaf (Abutilon theophrasti) and green foxtail (Setaria viridis) biomass rating and seed production. Corn hybrid grain yields averaged over herbicide rates differed by up to 1,100 kg/ha in 1993 and 3,000 kg/ha in 1994, yet similar grain yield reductions occurred for all hybrids as herbicide rate decreased. Corn hybrids differed in suppression of weed growth and seed production, but suppression differences among hybrids were not consistent across years. Plots with the very tall, erect-leaf hybrid had 7.4 velvetleaf plants/m2, whereas other hybrids had 8.6 to 10.1 plants/m2. Plots with the very tall, erect-leaf hybrid also produced the lowest quantity of velvetleaf seeds of 23,100 seeds/m2 versus 25,100 to 30,700 seeds/m2 for plots with other hybrids. Erect-leaf hybrids suppressed weeds more effectively than horizontal-leaf hybrids. Corn hybrids had less effect on early- Fand late-season weed densities and biomass ratings and on seed production than herbicide application. Selection among current corn hybrids as a tool in integrated weed management will have a small or inconsistent influence on weed suppression.

Nicosulfuron efficacy varies with surfactant, natural salts in the spray water carrier, and added nitrogen fertilizer salts. Scanning electron micrographs (SEM) were taken of nicosulfuron spray droplet residue on large crabgrass in the greenhouse. Spray residue characteristics differ for nicosulfuron applied with surfactants alone and with specific salts. Uniform deposits with close contact to the leaf epicuticular surface generally related positively to nicosulfuron efficacy. Ammonium salt enhancement of nicosulfuron phytotoxicity when with surfactant X-77® related to a change from a ring to a uniform deposit. Spray mixtures containing Tween 20 or Atplus 300F surfactants that gave distinct dark amorphous deposits over anticlinal cell walls generally related to effective nicosulfuron treatments. Salts that were antagonistic to nicosulfuron phytotoxicity left a large amorphous deposit, including ammonium nitrate antagonism of nicosulfuron applied with Pluronic® P85 surfactant and general antagonism from sodium bicarbonate. The SEM information indicates that the effect of surfactants and salts on spray deposit characteristics influence nicosulfuron efficacy.

Greenhouse experiments were conducted in Greece to evaluate the level of propanil resistance in three barnyardgrass biotypes collected from rice fields where propanil had been used for around 25 yr. Suspected resistant biotypes were not controlled by propanil at 2.6, 5.2, 10.4, and 20.8 kg ai/ha, with the highest rate five times greater than the recommended rate. Control was reduced even more when propanil application was delayed until the three- to five-leaf stage. The susceptible barnyardgrass biotype collected from a nontreated propanil area was controlled with 2.6 kg/ha propanil. All barnyardgrass biotypes were effectively controlled by quinclorac (0.75 kg ai/ha) applied either alone at the two- to three-leaf stage or in mixture with propanil (3.5 kg/ha) at the three- to five-leaf stage. Only two of the resistant barnyardgrass biotypes were effectively controlled by azimsulfuron (0.02 kg ai/ha) applied alone at the two- to three-leaf stage, but all biotypes were controlled with the addition of propanil (3.5 kg/ha) at the three- to five-leaf stage. The resistant barnyardgrass biotype, which required the highest rate of propanil to reduce biomass by 50%, also tended to exhibit the most growth potential 7 to 22 d after emergence when compared with the other resistant biotypes or the susceptible biotype. Results clearly show that varying levels of barnyardgrass resistance to propanil exist but that effective control alternatives are available.

A 4-yr field study was conducted to evaluate yellow nutsedge suppression in ‘Tifway’ bermudagrass. Herbicide programs included preemergence (PRE) applications of metolachlor (3.4 kg ai/ha) and postemergence (POST) applications of imazaquin (0.28 kg ai/ha) plus MSMA (2.2 kg ai/ha) or halosulfuron (0.07 kg ai/ha) plus MSMA (2.2 kg/ha). Herbicides were applied to the same plots each year. Yellow nutsedge shoot suppression and tuber numbers were determined each year. Suppression of yellow nutsedge shoots increased over the 4-yr period from <74% in 1993 to >83% by 1996 with two annual applications of imazaquin plus MSMA or halosulfuron plus MSMA. PRE metolachlor applications did not suppress shoot production in any year; nor did they enhance suppression from POST treatments. Sequential applications of halosulfuron plus MSMA and imazaquin plus MSMA increased shoot suppression by 17 to 24% at 3 mo after initial treatment (MAIT) compared to single applications. All treatments reduced tuber numbers (>60%) after 4 years compared to untreated plots.

Field research was conducted to determine effects of application rate, spray adjuvants, and spray carriers on visible control of downy brome, jointed goatgrass, and cheat and potential injury to wheat by MON 37500. MON 37500 at 24 and 46 g/ha with and without a methylated seed oil or nonionic surfactant and carriers of water, urea ammonium nitrate (UAN), and a 1:1 water/UAN combination were applied to the weeds at the one- to four-leaf stage. Cheat was the most susceptible weed to MON 37500, with control consistently above 87% with all treatments except MON 37500 at 24 g/ha in water 26 weeks after treatment. Downy brome control was more variable, with ratings ranging from 50 to 99% among treatments. For jointed goatgrass, only moderate stunting was observed from all MON 37500 applications. Both wheat varieties showed early season injury after MON 37500 was applied with UAN and either adjuvant; however, no visible injury or yield reduction to either wheat variety was noticed at harvest.

Experiments were conducted to investigate the possibility that rhizobacterial nodulation in legumes influences parasitic attack by broomrape. Small broomrape (Orobanche minor), which is commonly associated with swellings of the host root, was studied on red clover (Trifolium pratense) in the presence or absence of Rhizobium leguminosarum bv. trifolii inoculum and was compared to Egyptian broomrape (Orobanche aegyptiaca), which is not associated with host root swellings. Small broomrape showed greater percent seed germination and formed a greater number of attachments on red clover inoculated with rhizobacteria in comparison with noninoculated plants. In contrast, inoculum did not appear to enhance Egyptian broomrape seed germination or the number of its attachments on the host roots compared to noninoculated controls. In aseptic culture, the addition of Rhizobium increased the success of small broomrape in parasitizing clover but was not essential for parasitization. No rhizobacteria were detected in cultures derived from tubercles under septic or aseptic conditions. Morphological observations of small broomrape attachments on red clover suggest that parasitic attachments were not situated over the bacterial nodules but may involve parasite-induced protrusion of host plant root cortex. These results indicate that although rhizobacterial nodulation is not required for parasitization, the presence of nodules facilitates small broomrape germination and attachment to red clover.

Sicklepod (Senna obtusifolia), hemp sesbania (Sesbania exaltata), and pitted morningglory (Ipomoea lacunosa) seed production and viability in early-maturing soybean (Glycine max) were evaluated following preharvest desiccation. The herbicide combinations 1.1 kg ai/ha glyphosate plus 6.7 kg ai/ha sodium chlorate, 2.2 kg/ha glyphosate plus 3.4 kg/ha sodium chlorate, and 0.3 kg ai/ha paraquat plus 6.7 kg/ha sodium chlorate reduced sicklepod germination, emergence, and seedling growth in most cases. Oxyfluorfen at 0.3 kg ai/ha plus 3.4 or 6.7 kg/ha sodium chlorate also reduced sicklepod germination and growth. Glufosinate from 0.8 to 1.4 kg ai/ha reduced sicklepod germination but did not have a consistent effect on other seedling growth parameters. Bromoxynil at 1.1 kg ai/ha plus 3.4 kg/ha sodium chlorate reduced sicklepod germination but did not have a consistent effect on most growth parameters. A wide range of treatments reduced hemp sesbania germination, emergence, and growth to very low levels. All desiccant applications reduced pitted morningglory seed production compared to the untreated check. Glyphosate at 1.1 kg/ha plus 6.7 kg/ha sodium chlorate, 0.3 kg/ha oxyfluorfen plus 3.4 kg/ha sodium chlorate, and 0.8 or 1.1 kg/ha glufosinate reduced pitted morningglory seed production and desiccated weeds effectively. Most treatments reduced emergence in 1996, when seeds were smaller and less mature than those collected in 1995. When application volume of paraquat alone or in tank mixture decreased, germination and growth of sicklepod was reduced, indicating increased paraquat efficacy.

An experiment was conducted at six locations in North Carolina to compare weed-management treatments using glufosinate postemergence (POST) in glufosinate-resistant soybean, glyphosate POST in glyphosate-resistant soybean, and imazaquin plus SAN 582 preemergence (PRE) followed by chlorimuron POST in nontransgenic soybean. Prickly sida and sicklepod were controlled similarly and 84 to 100% by glufosinate and glyphosate. Glyphosate controlled broadleaf signalgrass, fall panicum, goosegrass, rhizomatous johnsongrass, common lambsquarters, and smooth pigweed at least 90%. Control of these weeds by glyphosate often was greater than control by glufosinate. Mixing fomesafen with glufosinate increased control of these species except johnsongrass. Glufosinate often was more effective than glyphosate on entireleaf and tall morningglories. Fomesafen mixed with glyphosate increased morningglory control but reduced smooth pigweed control. Glufosinate or glyphosate applied sequentially or early postemergence (EPOST) following imazaquin plus SAN 582 PRE often were more effective than glufosinate or glyphosate applied only EPOST. Only rhizomatous johnsongrass was controlled more effectively by glufosinate or glyphosate treatments than by imazaquin plus SAN 582 PRE followed by chlorimuron POST. Yields and net returns with soil-applied herbicides only were often lower than total POST herbicide treatments. Sequential POST herbicide applications or soil-applied herbicides followed by POST herbicides were usually more effective economically than single POST herbicide applications.

Experiments were conducted in 1996 and 1997 in water-seeded and drill-seeded production systems to determine rice response to clomazone applied at rates ranging from 0.28 to 2.2 kg ai/ha on silt loam and silty clay soils. Clomazone-induced bleaching of rice seedlings was 15% or less when clomazone was applied at 0.28 or 0.56 kg/ha. Increasing the rate of clomazone increased bleaching in most experiments. Clomazone at 0.84 kg/ha or higher delayed seed head emergence in five of eight experiments. Clomazone at lower rates did not delay seed head emergence. Although clomazone at 0.28 and 0.56 kg/ha did not reduce grain yield, clomazone at 0.84 and 1.1 kg/ha reduced grain yield in one experiment while clomazone at 1.7 and 2.2 kg/ha reduced yield in two experiments. The influence of soil type and seeding method on rice's tolerance to the microencapsulated formulation of clomazone could not be demonstrated in these experiments. Clomazone was more injurious in drill-seeded production than in water-seeded production.

Applying glyphosate relative to the growth stage of soybean is important for maximizing weed control and profits in glyphosate-resistant soybean under no-till systems. A study was conducted in Ontario for 4 yr to evaluate the effectiveness and gross return on the timing and sequence of applications of glyphosate in glyphosate-resistant no-till soybean. Percent control of various weed species varied among years due to environmental conditions. Timing of glyphosate was critical relative to weed emergence and determined the success of the treatment in terms of optimum soybean yield and gross return. Soybean yield and gross return approximated that the critical period for weed control in glyphosate-resistant no-till soybean was the unifoliolate to the one- to three-trifoliolate stage. Sequential applications of glyphosate provided higher soybean yield and gross return than a single preplant application of glyphosate. Glyphosate applied preplant or at the unifoliolate stage followed by a second application at the one- to three-trifoliolate stage consistently provided maximum average soybean yield and gross return. Gross return of the sequential glyphosate treatments was also more consistent across variable soybean price scenarios. Competition from uncontrolled later emerging weeds resulted in soybean yield loss with the single preplant application of glyphosate. Competition from uncontrolled early-emerging weeds reduced soybean yields when glyphosate was applied only at the one- to three-trifoliolate stage of soybean. Overall, two weed control strategies were identified: (1) two applications of glyphosate, the first at preplant to the unifoliolate stage, followed by a second application at the one- to three-trifoliolate stage of soybean, (b) first application of glyphosate at the unifoliolate stage followed by a second application at the one- to three-trifoliolate stage of soybean if later emerging weeds exceeded threshold densities.

Digital imagery from satellites and airborne remote sensing offer an opportunity to accurately detect weed infestations. Image resolution and plant growth stage are critical factors for maximum weed detection with low errors. Data analysis in traditional image assessment has relied on agreement measures, such as Cohen's kappa and asymptotic procedures, that compare what is on the image but not on the ground and what is on the ground but not on the image. Statistical comparisons of multispectral images, however, require some knowledge of the variability of the image classification results to determine significant differences among agreement measures. Bayesian methods were used to develop probability distributions for an agreement measure, conditional kappa, and were then subsequently applied to assess and compare image resolutions and plant growth stages. Results showed that images of a study site known to have yellow starthistle populations could identify the noninfested areas with greater accuracy than infested areas at spatial resolutions of 0.5, 1.0, 2.0, and 4.0 m. The detection accuracy of yellow starthistle in the images taken either prebloom or at flowering with 4.0-m spatial resolution usually was equal to or better than spatial resolutions of 0.5, 1.0, and 2.0 m for the cover classes that were not, moderately (31 to 70%), and highly (71 to 100%) infested. The 0.5-m resolution was better than 4.0-m spatial resolution when detecting the moderate cover class, but both resolutions had high omissional and commissional errors. Contrasting the best detection resolution for finding yellow starthistle colonies across flight times indicated that flying at flowering stage with the 4.0-m spatial resolution provided the best detection of the yellow starthistle cover classes considered. In the cases where different spatial resolutions resulted in equal detection accuracy, the larger spatial resolution was selected because of lower costs of acquiring and processing the data.

A field study was conducted in 1992 and 1993 to identify the spray volume and droplet size combinations to optimize control of common cocklebur (Xanthium strumarium) from acifluorfen by maximizing target deposition. In many instances, acifluorfen controlled common cocklebur better using either small (250 µm) or large (450 µm) spray droplets when applied at the lower carrier volumes of either 56 or 112 L/ha. When sprays were applied at 169 L/ha, there was little difference in control between droplet sizes. Deposition of acifluorfen was determined in 1993. Stepwise regression indicated that acifluorfen deposition amount is less important than environmental conditions for common cocklebur control. Relative humidity was the most significant variable for determining common cocklebur control with acifluorfen.

Leaching and runoff losses of the postemergence-applied herbicide dicamba were evaluated over a 3-yr period (1989 to 1991). Dicamba was applied at the recommended rate (0.56 kg ai/ha) to conventional and mulch tillage planted corn fields on Hagerstown silty clay loam (fine, mixed, mesic Typic Hapludalf). Mulch tillage followed several years of no-tillage corn. Root zone leachates were collected utilizing pan lysimeters placed 1.2 m below the soil surface. Surface runoff was monitored and collected with an HS-flume and automated sampling equipment. Leaching was greatest during 1989, and runoff events were recorded only during this season. Leachate samples containing measurable levels of dicamba were obtained within 21 d of herbicide application or within slightly more than one soil half-life of this chemical. More dicamba leached under mulch tillage than conventional tillage management. Tillage rotation (no tillage to mulch tillage) did not alter the leaching loss potential of dicamba beneath the minimally tilled soil surface, as postulated, compared with the previous untilled surface. The mulch tillage surface reduced runoff water losses compared with conventional tillage, but early-season leaching activity, coupled with the minimal persistence of dicamba in soil, negated runoff transport of this herbicide from either tillage system when the first runoff event occurred 12 d after its application.

A field study was conducted to determine the combined effects of row spacing, plant density, and herbicides on weed management and dry bean (Phaseolus vulgaris) yield. In weed-free dry bean, a reduction in row spacing from 69 to 23 cm increased yield by 19% and an increase in density from 20 to 50 plants/m2 increased yield by 17%. In the presence of weeds, narrow rows and high plant densities increased dry bean yield, but without herbicides, yields remained low. However, when combined with herbicides, narrow-row and high-density production practices resulted in better weed control and higher dry bean yield than that attained in a wide-row and low-density production system. Herbicide combinations, often at reduced rates, controlled weeds as well or better than the full rate of any individual herbicide. Ethalfluralin applied preplant incorporated followed by reduced rates of imazethapyr or bentazon postemergence (POST) consistently controlled weeds. Imazamox exhibited the potential to provide a total POST weed control option in dry bean production. Information gained in this study will be used to develop improved weed management programs appropriate for either wide- or narrow-row dry bean production systems.

Field trials were conducted with spring-sown rye and field pea cover crops to determine the effect of five rye–pea proportions and three seeding rates (high, medium, and low) on weed suppression during cover crop growth. Measurements on weed and cover crop growth were taken approximately 2 mo after seeding when cover crops were killed. Cover crops were killed by mowing in 1996 and by undercutting in 1997 and 1998. Cover crop biomass, averaged over rye–pea proportion, was highest in 1998 at 4.3 million tons (MT)/ha (high seeding rate) and lowest in 1997 at 1.5 MT/ha (low seeding rate). Cover crops of pure rye or rye–pea mixes suppressed weeds more effectively than did pure pea. Dominant weeds were ladysthumb, smooth pigweed, smallflower galinsoga, and common lambsquarters. Ground cover by weeds ranged from a low of 2% (rye–pea mixes) to a maximum of 73% (pure pea). Cover crop mixes of 50% or more rye seeded at the high rate gave the best weed suppression.

The economic threshold is a concept strongly embedded within the weed management literature. There are some theoretical concerns with applying a static approach such as the economic threshold to weed management decision making. An improvement is to adopt a population management approach where the intertemporal effects of decisions are taken into account. The focus should be on managing weed populations through time rather than minimizing the yield effect of weeds in a single season or year. Rather than viewing weeds as an annual production problem, the weed seed bank can be considered a renewable resource stock, and the management goal is to deplete this resource stock through time. The principles of natural resource economics illustrate that including the intertemporal effects of weed control will, for a given size of a seed bank, result in a greater level of weed control and a higher economic benefit than if control decisions were based solely on the current period effects. A dynamic economic model was developed of an extensive Australian spring wheat (Triticum aestivum) cropping system to test these principles using wild oat (Avena fatua and A. ludoviciana) as an example. The model was solved for a 20-yr time horizon for a population management approach and the traditional static economic threshold. The economic benefits from a population management approach were significantly greater than those generated by the economic threshold, and the final seed bank was considerably lower. This result suggests that a paradigm shift from thresholds to longer term population management is warranted.

Adjuvants have proven very useful materials when added to a herbicide formulation or added to the spray tank to improve herbicidal activity of application characteristics. Adjuvants are so closely linked to the agrochemical markets that an understanding of the trends occurring in those markets is critical to understanding trends occurring in the adjuvant markets. The worldwide crop protection market is estimated to be about $31 billion. Herbicides are estimated to represent about 50% of the total, with insecticides (25%), fungicides (20%), and other (5%) making up the balance. The U.S. crop protection market is estimated to be in the $7 to 8 billion range. Herbicides are estimated to represent about 70% of the total, with insecticides (17%), fungicides (9%), and other (4%) making up the balance.

Competition from large crabgrass is often responsible for the establishment failure of spring-seeded cool-season turfgrasses. The objective of this study was to determine how soon after seedling emergence dithiopyr could be applied without causing unacceptable damage to Kentucky bluegrass. Dithiopyr at 0.28 and 0.56 kg/ha was applied at various times prior to seeding and after emergence of Kentucky bluegrass in the field and greenhouse. Dithiopyr damaged Kentucky bluegrass seedlings in the greenhouse when applied earlier than 10 d after emergence (DAE) at 0.28 kg/ha and 14 DAE at 0.56 kg/ha. In the field study with heavy large crabgrass infestation, no significant phytotoxicity or long-term thinning occurred with any application of dithiopyr after emergence of Kentucky bluegrass. Dithiopyr applied as early as 3 DAE improved cover of Kentucky bluegrass by reducing competition from large crabgrass. When seeding is done early in spring before large crabgrass germination, dithiopyr could be applied at 0.28 kg/ha 10 DAE or at 0.56 kg/ha 14 DAE without risk of injury. Dithiopyr could be applied as early as 3 DAE on sites with heavy large crabgrass pressure or to late spring seedings when large crabgrass is germinating because the risk of turf thinning is warranted given the benefit of reduced large crabgrass competition for the desired turf.

Sulfentrazone dissipation in soil was examined in field experiments in 1995, 1996, and 1997 at Knoxville, TN, on a Sequatchie loam soil. Sulfentrazone 50% disappearance time (DT50) varied from 24 to 113 d. Cotton injury was observed the year following sulfentrazone application when half-lives were ≥85 d. Sulfentrazone degradation under controlled laboratory conditions was slower in autoclaved soil than in nonautoclaved surface soil and subsurface soil, with DT50 of 198, 93, and 102 d, respectively. The difference due to autoclaving the soil implied that sulfentrazone degradation was influenced by both microbial and chemical mechanisms.