Competitive and economic thresholds are distinctive and important concepts to weed scientists. The producer is most concerned about the economic threshold because of its importance in production decisions. The three additive research techniques to determine competitive weed thresholds are removal or plant-back, paired plot, and area of influence. Each method has advantages and disadvantages and must be conducted in a specific manner in the field. The area of influence technique is the most accurate for determining the influence of a single weed. Data must be reported in a usable form such as percent yield reduction. An example calculation for determining the two weed thresholds is illustrated with common cocklebur in soybeans. Generally, the economic threshold is one-half to one-third the competitive threshold.

Members from all four Weed Science Society of America (WSSA) regions in the United States ranked the research need to “develop new methods for controlling the movement of herbicides and their metabolites into ground water, surface water, and air” first of six major weed science research needs. Canadian members ranked the need to “devise more efficient and less costly weed control technology for conservation-tillage crop-production systems” first; but they also gave high ratings to “improve base knowledge of weed science, improve applicator training, and transfer of information to Extension Service personnel, farm producers, and administrators” and to “discover new ecological, biological, and non-chemical methods of weed control.” The needs to “develop improved methods of increasing the tolerance of crops to herbicides” and to “develop new technology for control of perennial weeds of crops and rangeland” were ranked low. The WSSA Research Committee, at the request of WSSA Presidents J. D. Riggleman (1985) and O. C. Burnside (1986), asked 977 members to rank weed science research needs. The members (ca 50% of the active membership in North America) were selected at random from every other state, federal, industry, and “other” member of each state or province from the up-to-date list of the WSSA business office. Within the highest ranked priority research need, the 422 U.S. and Canadian respondents consistently ranked the research areas (a) to “develop new application techniques that minimize or eliminate herbicides and their residues in air and water”, and (b) to “conduct research to regulate movement of herbicides through the soil profile to avoid contamination of ground water” high, regardless of the type of employment. They emphasized increasing research on the morningglory (Ipomoea spp. # IPOXX) complex, yellow nutsedge (Cyperus esculentus L. # CYPES), quackgrass [Agropyron repens (L.) Beauv. # AGREE], and several other weeds. More members, regardless of region or type of employment, ranked conservation tillage the most important crop or situation that needed new and improved weed control technology.

Multiflora rose is a serious weed pest in West Virginia. There are cultural and chemical means for control, but expense and environmental consequences often preclude their use. Limited information is available concerning biotic agents with potential for multiflora rose biocontrol. A literature search, contacts with scientists in Asia where multiflora rose originated and apparently is not a pest, and disease and insect surveys in West Virginia indicate this plant is healthy and, generally, pest free. However, three insects in West Virginia and one disease occurring in the midwestern United States were of special interest. The rose hip borer, Grapolita packerdi Zeller, which consumes the hypanthium tissues of the hips; the rose seed chalcid, Megastigmus aculeatus var. nigroflavus Hoffmeyer, which destroys the seeds within the achenes; and the raspberry cane borer, Oberea bimaculata Oliv., which kills canes, were found in West Virginia. The disease, rose rosette, not found in West Virginia, is killing multiflora rose in Arkansas, Indiana, Kansas, and Missouri. There is little information on the biology and/or host ranges of these insects or the disease. However, finding three insects in West Virginia adversely affecting growth and/or reproduction of multiflora rose and learning of a disease that kills this plant suggest there are biotic agents that might help manage multiflora rose.

Knowledge of interference thresholds, biology, and growth habits of weeds of rice is essential to implement timely, effective, economical weed control technology for profitable rice production. Regression analyses were used to determine threshold levels for weed densities and durations of interference for major weeds of rice. Weed threshold information for individual species or combinations of species can be used to develop models for initiating control inputs and determining their costs and returns. Among the grass weeds in density experiments, red rice reduced rice grain yields the most followed by barnyardgrass, beared sprangletop, and broadleaf signalgrass. Among broadleaf/aquatic weeds in density experiments, hemp sesbania reduced rice grain yields the most followed by northern jointvetch, ducksalad, spreading dayflower, and eclipta. Barnyardgrass, broadleaf signalgrass, and ducksalad interfered with rice the most during early season, whereas eclipta, hemp sesbania, northern jointvetch, red rice, and spreading dayflower caused greater interference during mid- to late-season.

Woollyleaf bursage [Ambrosia grayi (A. Nels.) Shinners # AMBGR] and Texas blueweed (Helianthus ciliaris DC. # HELCI) increasingly are weed problems on the Texas High Plains. Field experiments were established to evaluate dimethylamine, potassium, sodium, calcium, and aluminum salts of dicamba (3,6-dichloro-2-methoxybenzoic acid) applied at 1.1, 2.2, and 4.4 kg ai/ha to infestations of each species. Initial woollyleaf bursage control was better than initial Texas blueweed control. Dimethylamine and potassium salts provided highest control levels, and increasing rates improved control. One-year control was higher for Texas blueweed than woollyleaf bursage, and 1-yr control of both species improved with increased rates.

Herbicides were applied as single, sequential, or tank mixture treatments at different growth stages and rates and were compared with standard treatments for control of barnyardgrass, bearded sprangletop, elipta, spreading dayflower, and aquatic weeds in water- or dry-seeded rice. In water-seeded rice, thiobencarb granules applied alone or sequentially selectively controlled bearded sprangletop and aquatic weeds in the 1- to 4-leaf stages better than standards of propanil alone or mixed with bentazon. Fenoxaprop plus bentazon selectively controlled 1- to 4-leaf bearded sprangletop but failed to control 1- to 4-leaf aquatic weeds. In dry-seeded rice, treatments of fenoxaprop applied to tillering rice after earlier applications of preemergence thiobencarb or postemergence applications of propanil selectively controlled barnyardgrass, bearded sprangletop, eclipta, or spreading dayflower better than standards of propanil alone or mixed with thiobencarb. Fenoxaprop alone or in tank mixtures with other herbicides applied early postemergence either injured rice or failed to control weeds. Esprocarb applied preemergence controlled weeds but injured rice while early postemergence treatments of propanil plus esprocarb performed better than standard herbicide treatments.

Greenhouse and field experiments were conducted at Athens, GA, during 1984 and 1985 to evaluate the interference of several soybean cultivars with sicklepod. The 12 soybean cultivats evaluated in the field were commonly used from 1907 to the present. No correlation of soybean cultivar maturity group or date of introduction with interference on sicklepod was observed. ‘Essex’ and ‘Bragg’ soybean reduced sicklepod dry weight in the field; however, only Essex was unaffected by sicklepod. In the greenhouse experiments, root and shoot growth of sicklepod was less than the cultivars ‘CNS', ‘Peking’, and ‘Forrest’. Sicklepod produced the least above- and below-ground growth while ‘CNS' produced the most of the three cultivars. When grown together, the combined total root growth of each cultivar plus sicklepod was less than calculated expected values. The genetic development of soybean cultivars with improved tolerance to sicklepod infestations possibly could reduce weed control inputs.

Weed population models can serve as a framework to organize weed biology information and to develop weed control strategies. Models help to identify information gaps, to set research priorities, to develop hypotheses pertinent to weed population regulation, and to suggest control strategies. A population simulation model of leafy spurge (Euphorbia esula L. # EPHES) was used to demonstrate the applicability of population models to weed science. Sensitivity analysis of an existing leafy spurge model indicated that transition from basal buds to vegetative shoots, survival of vegetative shoots, and survival of basal buds over winter were important transition parameters influencing population growth of this weed species. Possible mechanisms (intraspecific competition and environmental factors) that influence the transition from basal buds to vegetative shoots were shown. Intraspecific density effects on basal bud transition and production were included to show model refinement and second-generation model development. Four control strategies were simulated and were compared to field studies to show the predictive and management potential of the modeling approach. Simulations of population response to foliage feeding herbivores was highly correlated (r = 0.98) with field data for sheep grazing on leafy spurge. Simulation of picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid) applied to leafy spurge also was correlated (r = 0.97) with field results.

The compatibility of rhodamine B dye {N-[9-(2-carboxyphenyl)-6-(diethylamino)-3H-xanthen-3-ylidene]-N-ethyl ethanaminium chloride} with picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid) or dicamba (3,6-dichloro-2-methoxybenzoic acid) was evaluated in the laboratory and field. Rhodamine B dye formulation (with or without a surfactant) did not affect dry weight of residue precipitated when mixed with dicamba. Formulated dye precipitated 11% more residue than nonformulated dye when mixed with picloram. Common crupina (Crupina vulgaris Cass. # CJNVU) field plant density was higher after applying a picloram spray solution with nonformulated rhodamine B dye compared to picloram alone. Common crupina density and biomass were higher after application with nonformulated dye and dicamba compared to dicamba alone.

Glyphosate was evaluated as a preharvest treatment for enhancing corn kernel dry down and quackgrass control. Glyphosate at 0.4, 0.8, 1.3, and 1.7 kg ae/ha was applied to corn before physiological maturity (kernel moisture 44 to 47%) and at physiological maturity (kernel moisture 35 to 39%). Three weeks after treatment, grain moisture of plants treated at physiological maturity was 2.3 to 6.9% less than that of plants treated before physiological maturity which was 2.2 to 5.5% less than that of untreated plants. Glyphosate did not increase corn kernel drying effectively under high humidity conditions. Glyphosate applied to pre-physiologically-mature corn controlled quackgrass 49 to 64% in the fall of 1984 and 69 to 91% in the fall of 1985. None of the glyphosate treatments reduced corn yield. Glyphosate applied preharvest above the corn canopy may increase the rate of corn kernel drying and may control fall quackgrass in the northern corn belt.

Persistence of chlorsulfuron and other sulfonylurea herbicides, measured by sorghum injury, was determined on a Pullman clay loam (fine, mixed, thermic family of Torrertic Paleustolls) with soil pH varying from 6.5 to 8.1 in a 3-yr winter wheat-sorghum-fallow crop rotation. As pH increased from 6.5 to 8.0, chlorsulfuron persistence increased greatly. With soil pH of 6.5, chlorsulfuron at 34 g ai/ha used on growing wheat did not injure sorghum planted 16 months later. If pH was 7.5 or above, chlorsulfuron persisted and injured sorghum planted within 25 months after herbicide application. Persistence of sulfometuron and metsulfuron was about the same as chlorsulfuron, all of which persisted longer than metribuzin applied at normal use rates. Four annual applications of chlorsulfuron at 71 g/ha did not accumulate when soil pH was 6.5.

Japanese honeysuckle (Lonicera japonica Thunb. # LONJA) vines can smother young trees, presenting problems in nursery, parkland, and woodlot management. The tardy-deciduous nature of honeysuckle provides an application window for its selective control with glyphosate [N-(phosphonomethyl)-glycine] or dichlorprop [(±)-2-(2,4-dichlorophenoxy)propanoic acid] plus 2,4-D [2,4-dichlorophenoxy)acetic acid] immediately after fall defoliation of hardwood species. Glyphosate at 1.5% v/v (5.4 g ae/L) applied in December killed mature, woody honeysuckle vines and eliminated most regrowth from basal and subterranean buds 28 months after treatment. Dichlorprop plus 2,4-D at 1.5% v/v (3.6 g ae/L of each herbicide), when applied shortly after the first freezing temperatures in October, was as effective as glyphosate but was less effective when applied in December. Tuliptree (Liriodendron tulipifera L.), American beech (Fagus grandifolia Ehrh.), and Scotch pine (Pinus sylvestris L.) suffered minimal and temporary injury from these herbicides.

In field experiments over a 3-yr period, quizalofop {(±)-2-[4-[(6-chloro-2-quinoxalinyl)oxy)phenoxy] propanoic acid} at 100 to 150 g ae/ha effectively controlled several annual grass weeds and volunteer barley (Hordeum vulgare L. # HORVX) in flax (Linum usitatissimum L. ‘Linott’). Quizalofop applied when flax was 10 cm tall generally was more effective on wild oats (Avena fatua L. # AVEFA) and green foxtail [Setaria viridis (L.) Beauv. # SETVI] than when flax was 4 cm tall. Volunteer barley was controlled at both stages of flax growth. Weed interference in untreated plots adversely affected flax quality factors such as seed size, seed weight, percent oil, and iodine value of the oil but not in quizalofop-treated plots.

Alternaria cassiae Jurair and Khan applied as conidia at 1 × 105 spores/ml controlled 100% of sicklepod, and 1 × 106 spores/ml controlled coffee senna in the greenhouse. In the field, sicklepod control did not differ between one or two fungal applications, but coffee senna control increased following two applications. Although showy crotalaria resisted A. cassiae infection more than the other two weeds, total biomass was reduced significantly in both greenhouse and field tests.

An undesirable bermudagrass [Cynodon dacytlon (L.) Pers. # CYNDA] often must be removed before converting to an improved bermudagrass turf or to a different turfgrass species. Because bermudagrass control with glyphosate [N-(phosphonomethyl)glycine] has been inconsistent, an experiment was initiated to determine the necessary application rates and frequency for glyphosate and SC-0224 (trimethylsulfonium carboxymethylaminomethylphosphonate) to control bermudagrass. Both glyphosate and SC-0224 applied in three applications (May, June, and August) had controlled three bermudagrass hybrids [Cynodon transvaalensis Burtt-Davy # CYNTR x C. dactylon (L.) Pers. Tifway’, Tifgreen’, and ‘Tifdwarf’] and ‘Ormond’ bermudagrasses when evaluated the next June. None of the cultivars were controlled effectively with either herbicide applied in one (May) or two (May and June) applications regardless of rate. Control with glyphosate and SC-0224 was similar.

Field experiments were conducted at two locations during 1984 and 1985 to determine how time of wild oats and wild mustard control influenced spring wheat yield and net economic return. Wheat yield and economic return were greatest when both weed species were controlled with diclofop plus bromoxynil at the 2-leaf wheat stage. Delaying the application of herbicides or controlling only wild oats with diclofop or only wild mustard with MCPA reduced wheat yield and economic benefit from the herbicide treatment. Wild oats and wild mustard competed equally for limiting growth factors in a greenhouse experiment.