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.

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.

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.

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.

Chloramben effectively controlled weeds in lettuce and endive grown on muck soils better than thiobencarb and chlorpropham. Chloramben at 1.1 kg ai/ha controlled two weeds economically important on muck soils, livid amaranth and oakleaf goosefoot. Chloramben applied preemergence at 1.1 kg/ha and higher caused early stunting of lettuce which the lettuce outgrew by harvest. Field-seeded endive was stunted severely at these same rates, severely reducing yield. Transplants improved tolerance of all crops. Effective use rates on muck soils were 25% to 50% of those typically used on mineral soils.

Multiflora rose is a major weed in pastures, highway embankments, and recreational areas. A chalcid wasp, Megastigmus aculeatus (Swederus) (Hymenoptera: Torymidae), which feeds on the multiflora rose seeds, is a potential biological control agent for this weed. Distribution of multiflora rose and the seed wasp was studied from 1983 to 1986 in Virginia. Multiflora achenes collected from 58 sites in 51 counties showed that M. aculeatus was present in all but one of the counties surveyed; female:male sex ratio was high (94:1), indicating parthenogenesis. Infestation rates of the achenes averaged 26.5% in 1985 and 23.9% in 1986. Severe winter temperatures appear to affect the wasp population adversely.

Conservation tillage systems for cotton production were monitored for their influence on horseweed populations. Without fall tillage, horseweed became established and was present the following spring. Disking in the fall or spring controlled horseweed for the subsequent cotton crop.

The soil-borne fungus, Fusarium solani (Mart.) App. & Wr. f. sp. cucurbitae, has shown potential as a mycoherbicide to control Texas gourd. Formulations and times of application that effectively controlled Texas gourd were determined in field studies. Aqueous conidial suspensions (1 × 108 microconidia/ml) were applied preplant incorporated, postemergence, or both at 280 L/ha alone or combined with trifluralin at 1 kg ai/ha. Alginate granules containing fungal spores and hyphae and amended with 2% (w/v) soyflour were applied pre- or postemergence at 220 kg/ha alone or sequentially with conidial applications. Mortality of 70% or greater was achieved with eight of nine fungal treatments. The fungus applied as conidia combined with trifluralin, as well as trifluralin alone, significantly reduced Texas gourd emergence.

Field studies on time of emergence, influence of planting date on growth and reproduction, and winter survival of rhizomes were conducted on sorghum-almum grown in corn and crop-free environments. In 1985, peak emergence of sorghum-almum occurred during early May in crop-free plots and mid-May in corn. In 1986, two peaks of emergence, one in early June and one in late June, were noted in both crop-free and corn plots. Emergence after mid-July was 4% or less of the total emerged in 1985, and no sorghum-almum emerged after mid-July in 1986. In planting date studies, sorghum-almum was seeded alone or in corn at 2-week intervals. Corn competition reduced sorghum-almum shoot, rhizome, and root growth at all planting dates. Maximum sorghum-almum seed production was 43 110 seed/plant when grown without competition but only 1050 seed/plant when grown with corn competition. When grown with corn competition, no seed developed on sorghum-almum seeded 6 or more weeks (mid-June or later) after corn planting. Shoot dry weight of sorghum-almum grown with corn competition was 3 g/plant or less for plants seeded 4 or more weeks (early June or later) after corn planting. Therefore, controlling sorghum-almum in corn through mid-June should prevent seed production and corn yield losses due to sorghum-almum competition. Rhizomes produced by sorghum-almum grown alone or with corn competition did not survive the winter; therefore, in Minnesota, sorghum-almum survival from one growing season to the next depends on seed production.

Field experiments were conducted to determine the effect of selected rice pesticides on the mycoherbicide Colletotrichum gloeosporioides (Penz.) Sacc. f. sp. aeschynomene (designated as C.g.a.) for northern jointvetch control in dry-seeded rice. Sequential treatments of pencycuron at 0.56 kg ai/ha, SN-84364 at 0.40 kg ai/ha applied 7 and 14 days after C.g.a., or pencycuron sprayed sequentially after a tank mixture of C.g.a. plus actifluorfen did not reduce infectivity of C.g.a. Also, propiconazol at 0.60 kg ai/ha or pencycuron applied 7 days before and after C.g.a. did not reduce fungus development on northern jointvetch. Sequential treatments of benomyl at 0.56 kg ai/ha or propiconazol applied 7 and 14 days after C.g.a. reduced pathogen activity on northern jointvetch.

The tolerance of several field pea cultivars to glyphosate was compared in the laboratory and greenhouse, using root, foliar, and tissue culture exposure techniques. Pea cultivar response among glyphosate exposure techniques did not always agree. However, the cultivar ‘Alaska’ was consistently one of the most susceptible cultivars regardless of exposure technique. ‘Melrose’ was one of the most tolerant cultivars, especially when its roots and cells were exposed to glyphosate. The response of ‘Frogel’, ‘Glacier’, and other cultivars varied among glyphosate-exposure techniques.

A modified nutrient solution, used to study lettuce cultivar response to thiobencarb, provided an excellent medium for lettuce growth and is an alternative to other nutrient solutions. Leaf dry weight was inhibited 50% by 3- and 5-μM thiobencarb in ‘Dark Green Boston’ and ‘Great Lakes 366’, respectively, while ‘Bibb’, ‘Slobolt’ and ‘Valmaine’ responded intermediately. Growth inhibitions and herbicide injury symptoms, which included abnormal growth and stunted secondary leaves, were similar in both nutrient solution and soil assays. Due to its ease of preparation and elimination of the variability associated with soil assays, the nutrient solution may be considered a superior alternative to soil assays.

Diuron, fluorochloridone, hexazinone, metribuzin, norflurazon, oryzalin, prodiamine, simazine, and terbacil controlled seedling asparagus planted in sprinkler-irrigated plots similarly (>89%). In furrow-irrigated plots, cinmethylin, diuron, fluorochloridone, hexazinone, napropamide, oryzalin, and prodiamine provided similar control (>86%) of planted seedling asparagus. Herbicides with high water solubilities that depended upon root uptake for maximum activity tended to control seedling asparagus better in sprinkler-irrigated tests than in furrow-irrigated tests. Fluorochloridone, norflurazon, and prodiamine provided partial control of volunteer asparagus (48 to 84%) in an established stand of furrow-irrigated asparagus.

Puccinia canaliculata (Schw.) Lagerh. from Salisbury, MD, identified in greenhouse tests as specific to yellow nutsedge, was field evaluated for biological control of yellow nutsedge over a 2-yr period. Treatments included all combinations of P. canaliculata and 0.3 or 0.6 kg ai/ha benzaton. Disease development was reduced 50% by treatment with bentazon, compared with P. canaliculata alone. P. canaliculata reduced tuber recovery by 44%. Phytotoxicity was significantly higher from the rust treatments than controls. Treatment with P. canaliculata did not affect the number of shoots/m2.

A model and a proposed method for testing herbicide interactions were modified from an analysis of variance (ANOVA) model for a 2 by 2 factorial experiment. Statistical tests for either synergism, antagonism, or additivity of herbicide combinations were developed through transforming growth data to logarithms followed by significance tests of 2 by 2 contrasts of the form μii - μi0 - μ0i + μ00 with respect to the log-transformed data. Using actual experimental data, heterogeneity of variance was less severe on the log scale compared to the original measurement scale. An expedient SAS(R) program for obtaining the desired significance tests was developed.

Field experiments were conducted to evaluate the effect of application time on common waterplantain control in wild rice with MCPA and 2,4-D. Common waterplantain control was greatest when MCPA or 2,4-D were applied at 1.1 or 1.7 kg ai/ha at the two-aerial leaf stage. The best late-season control was MCPA applied at the scape elongation growth stage. Common waterplantain was controlled adequately when 0.6 to 0.8 kg/ha of MCPA were applied at the scape elongation or early flowering stage. However, because of common water plantain interference and sensitivity of wild rice to late herbicide applications, the best treatment stage for wild rice yield was when MCPA was applied at 0.6 kg/ha to common waterplantain at the two-aerial leaf stage. Wild rice at this time is at the more tolerant one-aerial leaf growth stage.

Experiments were conducted in south Texas from 1983 to 1987 to evaluate clopyralid for huisache control using a carpeted roller. Huisache control with clopyralid applied as broadcast sprays or with a carpeted roller generally equaled that from the same rates of picloram or of 1:1 mixtures of clopyralid plus picloram. Applications of herbicides in the fall generally were more effective than in the spring, regardless of application method. In most cases, the herbicides at 48 to 60 g ae/L fall applied with a carpeted roller killed 70% or more of the huisache. Herbicide efficacy was not reduced when all huisache top growth was removed within 1 week after roller application of a 1:1 mixture of clopyralid and picloram at 60 g ae/L. The treat-and-cut sequence should improve the quality of hay from huisache-infested pastures.

Field trials were conducted in 1985 and 1986 to determine the effect of incorporation on volatilization of clomazone from soil. Volatilization was detected up to 2 weeks after surface-applied or soil-incorporated treatments of clomazone at 1.1 kg ai/ha. The amount of volatilization detected was greatest following rainfall and varied between years. More clomazone volatilized after surface application than after incorporation, regardless of the climatic conditions present. Clomazone volatilization detected was in the order of no-tillage > minimum tillage > conventional tillage.

Field experiments were established to evaluate the effect of herbicide treatments on control of tall fescue before spring or fall no-till alfalfa planting and to determine the effect of fescue control on alfalfa seedling establishment, forage yield, and forage quality. No treatment provided significantly greater tall fescue control than the standard two applications of paraquat at 13- to 21-day intervals. Fescue control values with this treatment ranged from 86 to 99% over experiments and were associated with maximum values for alfalfa ground-cover and alfalfa yield percentage parameters, 52 to 83% and 64 to 96%, respectively. Glyphosate and SC-0224 at 2.2 kg/ha and greater generally gave tall fescue control equivalent to the paraquat treatment in both spring and fall seedings as did sethoxydim at 0.6 kg/ha in spring seedings. Applications of fluazifop or HOE-0661 did not result in satisfactory tall fescue control or alfalfa establishment. Adding nonionic surfactant to glyphosate or SC-0224 did not increase tall fescue control. There was a direct linear relationship between tall fescue control and both alfalfa seedling establishment and yield percentage.