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.

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.

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.

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.

Metolachlor, oryzalin, and napropamide when chemigation-applied controlled large crabgrass more effectively than when spray applied in container-grown azalea, liriope, and Japanese holly. Oxadiazon and oxyfluorfen were equally effective as chemigation or spray applications. Azaleas were generally more susceptible to contact injury from sprays of oxadiazon and oxyfluorfen and large crabgrass interference than were holly and liriope.

Two years of field research were conducted to determine the additive effects of weed cover and insects on pepper production. When weeds covered less than 10% of test plots, the natural insect populations reduced the marketable fruit yield of bell peppers 32% in 1983 and 22% in 1984. When weeds covered 72 or 94% of the test plots, foliage damage due to insects was from 5.8 to 12.1%, respectively. The high percent weed cover and insects reduced yield 99%. The average fresh weight of pepper foliage approached zero as weed cover approached maximum. Foliar levels of iron and aluminum were reduced greatly in peppers grown in competition with a 100% weed cover. The level of boron, copper, phosphorus, and potassium in pepper foliage increased as percent weed cover increased.

Velvet lupine planst were defoliated (50% of upper foliage) in 1983 and 1984 to determine total alkaloid concentration (TAC), percent crude protein (CP), and fiber in regrowth foliage. TAC in regrowth leaves was 15% lower than in the original leaves removed at the beginning of the study. Percent CP of regrowth leaves was 22% lower than in original defoliated leaves. Alkaloids and CP did not increase in response to defoliation stress from simulated herbivory as plant defense theories predicted.

Field experiments were conducted to determine the effects of tank-mix and sequential postemergence (POE) applications of the insecticides carbaryl and dimethoate with sethoxydim on large crabgrass control and cotton tolerance. Large crabgrass control was similar with sethoxydim and sethoxydim-dimethoate tank mixtures. Although tank mixing carbaryl with sethoxydim reduced control, it was not reduced when carbaryl was applied 2 days before or 2 days after applying sethoxydim. Neither the insecticides nor sethoxydim applied alone or as tank mixtures injured cotton or affected lint yield, percent lint, fiber length, fiber length uniformity, fiber strength, or micronaire.

Laboratory experiments with 14C-herbicides were conducted with grain sorghum as an indicator species to determine the effects of imposed moisture stress on absorption, precent recovery, and acropetal and basipetal translocation of the butyl ester of fluazifop, the methyl ester of haloxyfop, the ethyl ester of quizalofop, and sethoxydim. Haloxyfop was the only herbicide where recovery decreased between the 3-and 48-h interval. All plants absorbed more of the herbicide at the 48-h interval than at the 3- or 6-h interval under both stressed and non-stressed conditions. Increased drought stress caused more acropetal movement with fluazifop and sethoxydim and less acropetal movement with quizalofop at the 3-h interval. Basipetal transloation, although different among herbicides, responded similarly to imposed moisture stress, which decreased basipetal translocation approximately 19%.

Multiflora roses showing symptoms of rose rosette were found in nine counties in southern Indiana and two counties in northern Kentucky. The eriophyid mite, Phyllocoptes fructiphilus Kiefer (Acari: Eriophyidae), implicated as the vector of the rose rosette agent, occurred on most symptomatic material. Another eriophyid mite, P. rosarum Liro, was found on symptomatic material collected in Kentucky. Transmission of the causal agent into multiflora rose by shield budding and by P. fructiphilus was successful. The rose rosette agent appears to be spreading east and is established on multiflora rose in the Ohio Valley.

Effects of repeated late-fall applications of the isopropylamine salt of glyphosate at 1.7 kg ae/ha plus 0.5% (v/v) surfactant on adventitious root buds, thickened propagative roots (> 1.3 mm diam), and shoot density of Canada thistle were studied in continuous hard red spring wheat over a 4-yr period. Glyphosate suppressed Canada thistle shoot density more quickly and to a greater extent than thickened root fresh weight or root bud number. A single fall application of glyphosate drastically decreased Canada thistle shoot density for 1 yr after treatment. However, shoot density was the same as the untreated control by 2 yr after a single fall treatment. Two consecutive late-fall applications of glyphosate in 2 yr decreased Canada thistle shoot density 94% in the fall 1 yr after the last treatment. Glyphosate reduced Canada thistle thickened root fresh weight 70% in the first fall 1 yr after a single fall treatment. However, 2 yr after a single fall application of glyphosate, root fresh weight equalled the controls. Two consecutive fall applications of glyphosate reduced thickened root fresh weight 77% 1 yr after the second treatment.

Greenhouse experiments were conducted to determine effects of AC 94,377, seed depth, and commercially applied seed protectants on emergence and early growth of grain sorghum. Mean height of grain sorghum was increased by 500 to 2000 ppmw AC 94,377 but not by 250 ppmw. Emergence and fresh weight of grain sorghum was not affected. Plants arising from seeds without protectants were taller and heavier than those with protectants intact. Soybean sown in soil containing AC 94,377 at 0.6 to 2.5 kg ai/ha was injured; however, soybean grew normally if seeds were not sown until at least 1 week after applying AC 94,377. Cotton was not injured by AC 94,377.

Locoweed poisoning of livestock is the most widespread poisonous plant problem in the western United States. This paper presents a historical review of control methods to reduce locoweed plant density. Hand grubbing, the most common method of control before phenoxy herbicides, was practical in small pastures considering the number of livestock saved. Trials in the 1950s indicated that 2,4-D and 2,4,5-T generally controlled locoweeds effectively if applied when plants were actively growing and soil moisture was not limiting plant growth. Picloram, dicamba, clopyralid, and triclopyr provide more consistent control than 2,4-D and 2,4,5-T when applied at less than optimum environmental conditions.

The productivity of herbicides used in corn and soybeans was calculated from field data collected in Ontario from 1967 to 1985. Combinations of preplant incorporated and preemergence treatments were selected and were evaluated to determine their effect on crop yield. Corn and soybean yields increased from herbicide use, thereby resulting in a positive net benefit to growers. Benefit/cost ratios for herbicide use in corn and soybeans were calculated to be 2.8/1 and 2.6/1, respectively, at an average price of $132/1000 kg for corn and $275/1000 kg for soybeans. The benefit/cost ratios varied with test location, method of herbicide application, and prevailing market price.

The plant pathogenic fungus Colletotrichum coccodes (Cc) and the plant growth regulator thidiazuron (TDZ) were evaluated in Maryland and Quebec for velvetleaf control in ‘Williams' and ‘Maple Arrow’ soybean. TDZ was applied at 0, 0.1, 0.2, 0.3, and 0.4 kg ai/ha alone or was combined with Cc at 109 spores/m2 when velvetleaf was at the 1- to 2-leaf (Trial 1) or 4- to 6-leaf (Trial 2) growth stages. Velvetleaf control increased with TDZ rate, and TDZ combined with Cc further increased control. TDZ reduced velvetleaf biomass and height, and Cc increased velvetleaf mortality. In Quebec, Cc also reduced the biomass of velvetleaf treated in Trial 1 and interacted positively with TDZ at this growth stage. Cc nearly halved the rates of TDZ required for 90 and 75% mortality of velvetleaf treated at the 1- to 2-leaf and 4- to 6-leaf stages to 0.09 and 0.12 kg/ha, respectively, in Quebec. Cc similarly lowered the rate of TDZ required for 75% stand reduction of velvetleaf in Trial 1 to 0.17 kg/ha in Maryland. Cool wet weather in Quebec contrasted with warm, dry weather in Maryland. Soybean biomass and yield were increased significantly by treatment with TDZ plus Cc in Trial 1 at both locations.

Metribuzin at 0.42 kg ai/ha and metribuzin plus terbutryn at 0.28 plus 0.67 kg/ha were applied in the fall and spring to control downy brome in winter wheat from 1981 through 1985. Downy brome control averaged 88% from the fall-applied treatments and 63% from the spring-applied treatments. Yields from the fall- and spring-applied treatments averaged 136% and 132% of the check, respectively. Metribuzin plus terbutryn fall-applied controlled downy brome best (93%), and wheat yields were significantly higher than the checks in all experiments. Moreover, as downy brome density among locations increased, wheat yields decreased.

Tebuthiuron was evaluated at two Wyoming locations for its effects on big sagebrush control and associated vegetation, Big sagebrush was controlled with tebuthiuron at 0.6 to 1.1 kg ai/ha. Green needlegrass, prairie junegrass, blue grama, and Douglas rabbitbrush were unaffected by tebuthiuron treatments. Western wheatgrass and downy brome increased at different study locations and were dominant grass species at the end of 7 yr.

Herbaceous weed control was studied on a loblolly pine planting site in central Louisiana. Pine growth was enhanced without eradicating weeds; reducing weed biomass about 50% increased the mean inside bark volume of loblolly pine saplings 53% on the weeded treatments compared to the untreated control after five growing seasons in the field. Pines receiving both preplant weed control with glyphosate or disking and postplant weed control with a series of yearly treatments (1982, atrazine plus simazine; 1983, atrazine plus oxyfluorfen; 1984, hexazinone; and 1985, hexazinone) had 62% greater volume than pines on the preplant-only treatments. So, the best gains in loblolly pine volume required postplant weed control.

In continuous wheat or barley or in a canola/barley rotation, wild oat control every year over 4 yr maintained wild oat seedling populations at 3 plants/m2 or less. Failure to control wild oats annually increased wild oat populations (>200 plants/m2 by the fourth year) in continuous wheat dramatically, while in the other two cropping systems, populations increased to only 40 plants/m2 or less by the fourth year. In the continuous wheat and in the canola/barley rotation, wild oat control every year generally provided the best economic returns when prices and costs were averaged over 4 yr; in continuous barley, the average return was better when wild oats was controlled only in the second or third years rather than every year.