Weed removal experiments in dry edible beans were conducted during 1992 and 1993 at Crookston and Staples, MN. Ten manual weed removal treatments were studied to determine when a natural infestation of weeds first reduced dry bean yield, and when weed removal could be discontinued without further loss of seed yield. Major weeds in order of average biomass production on weedy check plots at dry bean harvest over locations and years were wild mustard, foxtail spp., redroot pigweed, common ragweed, wild buckwheat, hairy nightshade, and common lambsquarters. Hairy nightshade also emerged late in the growing season and could negatively affect harvest efficiency and stain navy beans. Weed removal treatments had little effect on dry bean stands or 100-seed weights of harvested dry bean seed. The critical period for weed control in dry beans was 3 to 5 or 6 weeks after planting (WAP). Thus, weed control practices should begin no later than 3 WAP and continue until at least 5 or 6 WAP for maximum dry bean yields.

The influence of relative planting date on the growth of common cocklebur (Xanthium pensylvanicum Wallr. ♯ XANST), common ragweed (Ambrosia artemesiifolia L. ♯ AMBEL), sicklepod (Cassia obtusifolia L. ♯ CASOB), and redroot pigweed (Amaranthus retroflexus L. ♯ AMARE) grown in competition with soybean [Glycine max (L.) Merr. ‘Bragg’] was studied in the greenhouse. Increases in dry matter and height were slower for the five weed species than for soybean throughout the period of the study. The root: shoot ratio of soybean was the highest of any plant in the study, while common ragweed, common cocklebur, common lambsquarters, and sicklepod were intermediate, and redroot pigweed was the lowest. Soybean dry weight was always reduced when grown in competition with a weed. Soybean dry-matter production was reduced most when weeds were planted 2 weeks before soybean, especially with common cocklebur and common lambsquarters. Weed dry-matter content was severely reduced when the weed seed were planted simultaneously with or following soybean. Soybean height was usually reduced by competition with the weeds. The height of common ragweed was increased, however, when planted simultaneously with soybean. Common lambsquarters, redroot pigweed, and common ragweed heights were increased when planted 2 weeks prior to soybean.

Linuron, metribuzin, oryzalin, and dinoseb were evaluated for weed control in conventional (CT) and rye-stubble reduced-tillage (RT) systems for potatoes. Early season control of weeds was equivalent for all herbicides in both tillage systems. In 1985, redroot pigweed populations were significantly greater in RT than in CT control plots. Populations of common lambsquarters were equivalent, regardless of the tillage system. In 1986, the rye mulch reduced early emergence of both species in untreated RT plots when compared to the untreated CT controls. During both seasons, potato stands decreased 16% with RT. In 1985, total yields did not differ between tillage systems; however, in 1986, total potato yields in RT plots decreased an average of 22% compared to yields in CT plots.

Interference within a mixture of equal densities of common lambsquarters (Chenopodium album L.), kochia [Kochia scoparia (L.) Schrad.], and redroot pigweed (Amaranthus retroflexus L.) in sugarbeets (Beta vulgaris L. ‘Mono Hy D2’) was determined in a 3-yr field study. Yield of sugarbeet roots and sucrose per hectare decreased as intensity of competition from equal populations of these three weeds increased. At densities of 6, 12, 18, and 24 broadleaf weeds per 30 m of row, root yields were reduced 13, 24, 33, and 39%, respectively. Sucrose yields were reduced similarly. Fewer than three weeds per 30 m of row did not significantly reduce root yield. Reduction in root yield (Y) of sugarbeets caused by specific densities (X) of the three broadleaf weeds was predicted by using the linear equation Y = 1.64 + 1.88 X. The actual reductions in yield were always less than the predicted reductions when this equation was tested against 36 weed control systems because the competitive ability of broadleaf weeds that were treated with herbicides, but not killed, was suppressed during the growing season. By harvest, broadleaf weeds present in weed-control-system plots weighed an average of 75 to 85% less than broadleaf weeds present in nontreated plots.

Field studies were conducted in 1992 and 1993 to evaluate AC 263,222 applied postemergence (POST) alone and as a mixture with atrazine or bentazon for weed control in imidazolinone-resistant corn. Nicosulfuron alone and nicosulfuron plus atrazine were also evaluated. Herbicide treatments were applied following surface-banded applications of two insecticides, carbofuran or terbufos at planting. Crop sensitivity to POST herbicides, corn yield, and weed control was not affected by insecticide treatments. AC 263,222 at 36 and 72 g ai/ha controlled rhizomatous johnsongrass 88 and 99%, respectively, which was equivalent to nicosulfuron applied alone or with atrazine. AC 263,222 at 72 g/ha controlled large crabgrass 99% and redroot pigweed 100%, and this level of control exceeded that obtained with nicosulfuron alone. AC 263,222 at 72 g/ha controlled sicklepod and morningglory species 99 and 98%, respectively. Nicosulfuron alone or with atrazine controlled these two species less than AC 263,222 at 72 g/ha. Addition of bentazon or atrazine to AC 263,222 did not improve control of any species compared with the higher rate of AC 263,222 at 72 g/ha applied alone. Corn yield increased over the untreated control when POST herbicide(s) were applied, but there were no differences in yield among herbicide treatments.

Barnyardgrass and many broadleaf weeds were severely stunted by POST applications of isoxaben. Most grasses were only moderately affected by isoxaben at rates up to 1 kg/ha. POST activity was enhanced by soil interception, but evident with foliar interception alone. Cell swelling in stems and in interveinal leaf lamina was observed after foliar application. A radiotracer study in redroot pigweed showed that only a small amount (3.3% of applied) of isoxaben entered the leaf following foliar application, and that little (0.08% of applied) isoxaben migrated beyond the point of application. The potential of isoxaben as a POST herbicide may be limited, in part, by poor absorption and translocation.

The effect of rainfall on the performance of a tank mixture of desmedipham [ethyl m-hydroxycarbanilate carbanilate(ester)] and phenmedipham (methyl m-hydroxycarbanilate m-methylcarbanilate) applied postemergence to redroot pigweed (Amaranthus retroflexus L. ♯ AMARE), wild mustard (Sinapsis arvensis L. ♯ SINAR) and sunflower (Helianthus annuus L.) was evaluated in field studies. The occurrence of 1 mm of rain immediately after herbicide application significantly decreased the control of redroot pigweed and wild mustard. A rainfall quantity of 1 mm also reduced injury symptoms on sunflower. Simulating a 12.7-mm rain less than 18 h after desmedipham and phenmedipham application effectively reduced toxicity to redroot pigweed and sunflower. Toxicity to these two species increased at a lesser rate than for wild mustard as the time interval prior to rain was increased. A rain-free period of 6 h was predicted for near-maximum control of wild mustard with these herbicides.

Information on the effects of multiple weed management tactics in corn is needed to develop integrated weed management systems. The effectiveness and compatibility of an in-row cultivator as compared to a standard interrow cultivator used with reduced rates of a soil-applied herbicide, rotary hoeing, and/or a bioeconomic model for POST herbicide selection was examined. Weed control with a single rotary hoeing at corn emergence controlled annual weeds similarly to two rotary hoeings. One-third recommended use rate of alachlor controlled weeds similarly to a two-thirds rate. Reduced rates of alachlor controlled more weeds than rotary hoeing over 2 yr. The in-row cultivator required early-season weed control (rotary hoeing or reduced alachlor rate) for optimum efficacy. The in-row cultivator provided better weed control than the standard cultivator while the cost of operating the two cultivators was similar. Thus, the in-row cultivator was more efficient than the standard cultivator. Furthermore, less intensive early-season weed control was required with the in-row cultivator for maximum weed control as compared to the standard cultivator. Rotary hoeing plus the in-row cultivator provided similar weed control to other weed management tactics that required both soil-applied and POST herbicides. Gross margin was influenced more by corn yield than cost of weed management tactics.

The effects of soybean row width and redroot pigweed density on growth of crop and weed were studied in field trials in 1983, 1984, and Structural relationships within the canopies of soybean and redroot pigweed in relation to row width and weed density were studied in 1984 and 1985 to assess canopy geometry in relation to intra- and interspecific competition. Early in the growing season, soybean biomass was reduced in the presence of both high and low densities of pigweed. Pigweed biomass was also reduced in the presence of soybeans, especially when grown in narrow rows (25 cm). By midseason, pigweed's contribution to total biomass had reached 43% in wide-row (76-cm) stands and 24% in narrow rows. Soybean produced two to four times more leaf area than pigweed during the first half of the growth season. Narrow-row planting favored soybean leaf area production. Soybean LAI values from weedy stands were reduced compared to those from weed-free stands. Pigweed's contribution to total leaf area averaged 29% in wide-row spacing and 15% in narrow rows. Pigweed leaf area was concentrated in the upper strata of the canopy and thus reduced light available to soybean leaves lower in the canopy. Leaf area distribution patterns suggested that soybean and pigweed were competing for light even though soybean had produced more leaf area than pigweed. These relations were consistent from year to year in spite of variable water conditions.

Sweet corn was grown with a living mulch of white clover, a dead mulch of rye, and without mulch, in both till and no-till conditions. Unplanted controls were also included in the experimental design. Corn yields were highest in clover treatments early in the experiment but lowest in later years. The declining yields in the clover living mulch were related to the strip application of glyphosate which allowed establishment of perennial and biennial weeds, notably dandelion and horseweed. These overwintering weeds apparently prevented effective control of summer annuals, especially redroot pigweed, common lambsquarters and large crabgrass, by atrazine and metolachlor. Presence of a rye mulch decreased weed biomass and had no detrimental effect on corn yield. In general, corn yield was not affected by tillage, although the number of marketable ears was reduced in the no-till treatments during the drought year of 1988. The much greater weed biomass in the unplanted control treatments showed the importance of crop competition for weed control in sweet corn cropping systems.

Field research was conducted during the summers of 1981 and 1982 in order to determine relative infection and population increase of lesion nematodes (Pratylenchus spp.) on seven weed species that commonly occur in field-bean (Phaseolus vulgaris L.) fields in western Nebraska. Weeds were grown at three densities with and without fieldbeans. A representative sample of the root systems from plants in each plot was removed in August and the nematodes were extracted and counted. No difference in nematode infection rate was found among weed population levels. Nematodes per gram of dry root were not different in weeds grown with or without fieldbeans. Weeds grown with fieldbeans had smaller root systems, and consequently total nematodes per root system were less than in weeds grown in the absence of fieldbeans. There was a significant difference among most weed species when nematodes per gram of dry root were estimated. Hairy nightshade (Solanum sarachoides Sendt. ♯3 SOLSA) and barnyardgrass [Echinochloa crus-galli (L.) ♯ ECHCG] supported the highest numbers of nematodes per g oven-dry roots, redroot pigweed (Amaranthus retroflexus L. ♯ AMARE) and common cocklebur (Xanthium pensylvanicum Wallr. ♯ XANPE) had least numbers of nematodes/g oven-dry roots, and infestation levels on other weed species were variable but generally intermediate.

Spring-applied herbicides were evaluated in the field in 1982 through 1984 in western Nebraska for selective weed control in irrigated seedling alfalfa (Medicago sativa L. ‘Appollo’). Weed densities were least in plots treated preplant with benefin [N-butyl-N-ethyl-2,6-dinitro-4-(trifluoromethyl)benzenamine] in combination with postemergence applications of 2,4-DB [4-(2,4-dichlorophenoxy) butyric acid]. Combinations of sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} with bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) and fluazifop-butyl {(±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid} with bromoxynil provided less grass control than sethoxydim or fluazifop-butyl applied alone. Annual weeds did not reduce alfalfa stand density.

Field experiments were conducted in 1979, 1980, and 1981 to determine the level of interference five broadleaf weed species exert on soybean [Glycine max (L.) Merr.]. Weed species studied were common cocklebur (Xanthium pensylvanicum Wallr. ♯ XANST), common ragweed (Ambrosia artemesiifolia L. ♯ AMBEL), common lambsquarters (Chenopodium album L. ♯ CHEAL), sicklepod (Cassia obtusifolia L. ♯ CASOB), and redroot pigweed (Amaranthus retroflexus L. ♯ AMARE). The following soybean seed yield reductions were observed with a density of 16 weeds/10-m row: redroot pigweed 22%, common lambsquarters 15%, common ragweed 12%, and sicklepod 5%, respectively. At a density of 8 weeds/10-m row, common cocklebur reduced soybean yield 11%. No single weed growth parameter predicted soybean seed yield reduction for all weed species. Soybean height was reduced by sicklepod competition; was not affected by competition from common lambsquarters, common ragweed, or common cocklebur; and was increased in the presence of redroot pigweed at 12 weeks after planting, when measured 30 cm from the weeds. Leaf area of soybean was higher at greater distances from the weed for all weed species. The range of soybean leaf area reductions occasioned by proximity to individual weed species corresponded fairly well with differences in soybean yield reduction.

Field studies were conducted for 3 yr to determine weed and potato response to preemergence (PRE) and postemergence (POST) applications of rimsulfuron, metribuzin, and rimsulfuron plus metribuzin. Preemergence applications of rimsulfuron at 27 g ai/ha and POST applications at 18 g/ha controlled barnyardgrass, redroot pigweed, and wild buckwheat. Common lambsquarters was controlled by PRE or POST applications of metribuzin or a tank mixture of 18 g/ha rimsulfuron plus 140 g ai/ha of metribuzin. ‘Russet Burbank’ potato was relatively tolerant to all rimsulfuron, metribuzin, and rimsulfuron plus metribuzin applications, and potato yield was not reduced compared to the hand-weeded control.

The absorption, translocation, and metabolism of 14C-ethofumesate [(±)-2-ethoxy-2,3-dihydro-3,3-dimethyl-5-benzofuranyl methanesulfonate] in sugarbeet (Beta vulgaris L.), common ragweed (Ambrosia artemisiifolia L.), redroot pigweed (Amaranthus retroflexus L.), and common lambsquarters (Chenopodium album L.) were studied as possible bases for selectivity of preemergence-applied ethofumesate. The sensitive redroot pigweed and common lambsquarters plants translocated more 14C-ethofumesate from nutrient culture to the leaf tissue than did the tolerant sugarbeet and common ragweed. The radioactivity was more highly concentrated in sugarbeet and common ragweed roots. The rapid metabolism of ethofumesate by sugarbeet and common ragweed, particularly that which accumulated in the leaf tissue, appeared related to tolerance.

The influence of various factors on the tolerance of corn and selected weed species to thifensulfuron was studied in greenhouse experiments. Corn fresh weight was reduced by postemergence application of thifensulfuron when applied at 180 g ai ha−1 without surfactant or at 18 g ha−1 with a nonionic surfactant. Corn was more susceptible to root exposure whereas velvetleaf was more susceptible to foliar exposure. Velvetleaf was most susceptible when plants were young, when a nonionic surfactant was added, and at 20 rather than 30 C. Simulated rainfall 8 h after application reduced velvetleaf injury by thifensulfuron at 18 g ha−1 without surfactant; however, with addition of a nonionic surfactant, velvetleaf injury was reduced only if rainfall occurred less than 2 h after application. Soil moisture level did not affect velvetleaf susceptibility.

In order to find the most effective and economical method of incorporation, six preplant herbicides for cotton (Gossypium hirsutum L.) were incorporated with a tandem disk or field cultivator, or sprayed on unbedded soil prior to bedding with a disk bedder. These treatments were compared to incorporating sprayed beds with a rolling cultivator and no incorporation. A mixed population of pigweed (Amaranthus hybridus L. # AMACH and Amaranthus retroflexus L. # AMARE) was present each year. There was no significant difference in pigweed control among incorporation with a disk, field cultivator, or rolling cultivator. Puncturevine (Tribulus terrestris # TRBTE) and barnyardgrass [Echinochloa crus-galli (L.) Beauv. # ECHCG] were controlled best following rolling cultivator incorporation. Cotton lint yields were not affected by incorporation methods or herbicides. Economic analysis and weed control indicate that the field cultivator was the best way of incorporation.

The effect of sequential hexane, ethyl acetate, and aqueous methanol extracts of ‘Regal’ sweetpotato periderm on seed germination of sweetpotato, proso millet, and seven weed species was studied. The hexane extract, which contained the nonpolar components of the periderm tissue, was least inhibitory. It inhibited velvetleaf, proso millet, black nightshade, and redroot pigweed germination, and maximum inhibition was 56% for black nightshade at 200 mg of periderm extracted ml–1. The ethyl acetate fraction was inhibitory to proso millet, velvetleaf, black nightshade, goosegrass, tall morningglory, coffee senna, and redroot pigweed. The estimated I50 for ethyl acetate ranged from 17 mg periderm extracted ml–1 for black nightshade to 201 mg ml–1 for coffee senna. Sweetpotato, tall morningglory, and eclipta germination was not inhibited by this extract at the concentrations tested. The aqueous methanol extract was much more inhibitory than the hexane or ethyl acetate extracts, and there was considerable variation between species in response to this extract The I50 estimates for the aqueous methanol extract were 0.5, 0.6, 2.8, 4.4, 5.1, 9.6, 15.7, 21.0, and 25.8 mg ml–1 for velvetleaf, proso millet, black nightshade, goosegrass, sweetpotato, tall morningglory, eclipta, coffee senna, and pigweed, respectively.

The critical weed-control period in field seeded cabbage using natural weed stands was 2 wk in 1988 and 4 wk in 1989. Regression analysis showed that the length of weed-free period, weed density, and ambient light reduction by competing weeds were factors affecting cabbage yield. Weeds that emerged after the initial weed-free period shaded the soil and subsequently lowered soil surface temperatures in 1989. Velvetleaf densities of 1.2 and 3.6 plants per m2 that competed all season with cabbage reduced cabbage yields 52% and 71% of the weed-free treatment in 1988 and 76% and 92% in 1989, respectively. Cabbage yields were reduced at all velvetleaf densities seeded 0, 1, and 2 wk after cabbage, but cabbage yield was not reduced when velvetleaf was seeded 4 or 6 wk after cabbage. Cabbage yield was not different between parallel and staggered velvetleaf configurations at 0.6 plants per m2, but was reduced with longer periods of competition. Velvetleaf seed production was prevented with 6 wk weed-free period after cabbage emergence and subsequent destruction of emerging velvetleaf after cabbage harvest.

Triazine-resistant (TR) biotypes of weeds normally susceptible (S) to triazine herbicides first were documented in Ontario in 1974. By 1988, at least nine weed species had TR biotypes, and at least 50% of the corn-producing areas were infested. Corn has been grown the longest in the southwestern Ontario, but TR weeds are only a minor problem there. Predominant agronomic practices in the southwest include crop rotation, atrazine use on 60% of corn land, use of other postemergence herbicides, interrow cultivation, little silage corn, and little manure returned to the land. Corn, particularly grain corn, is new as a large hectareage crop in eastern Ontario. However, more than 75% of corn land in that area is infested with two or more TR weed species. Predominant agronomic practices in the east include continuous corn, nearly all fields are atrazine treated, use of postemergence herbicides is not common, cultivation is rare, 25% of the corn is for silage, and manure from corn silage is returned to all cultivated land. Adequate herbicide programs exist to control TR weeds in corn and other crops. However, the spread from field to field and from farm to farm is often rapid, particularly in eastern Ontario.