The effect of common cocklebur interference on snap beans was investigated at Bridgeton, NJ, in 1987 and 1988. Snap bean yields were reduced by increasing weed densities. Yields were reduced 8 to 44% in 1987 and 2 to 55% in 1988 by full-season in-row weed densities ranging from 0.5 to 8/m2. One weed and 4 weeds/m2 were the damage-threshold populations of common cocklebur with full-season interference in 1987 and 1988, respectively. The weed-free requirement, after planting, was to the unifoliolate stage of snap beans. Common cocklebur at 4 weeds/m2 did not affect yield when snap beans were maintained weed free until the unifoliolate stage of snap beans. The critical duration of interference for common cocklebur emerging with snap beans was between the emergence and full-bloom stage of snap beans. Snap bean yield was not reduced with weed removal at full bloom.

Metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one], when applied pretransplant at 0.28 kg ai/ha, killed peppers (Capsicum annuum L.) that were transplanted bare root or in plugs. Activated carbon applied at 1 g/plant effectively reduced injury of such peppers in soils treated with up to 0.56 kg ai/ha metribuzin. The increase in crop tolerance to metribuzin was dependent on the amount of activated carbon added. The level of crop protection for plug transplants depended on the amount of activated carbon per plant rather than on the percentage in the greenhouse mix by dry weight. One gram was required to protect plug transplants grown in cells that held 10 or 20 g of greenhouse mix.

Experiments were conducted in pots and in the field to determine the influence of seed depth on seedling emergence pattern and control of Mexican sunflower with or without imazethapyr plus pendimethalin. Mexican sunflower seeds were placed at 0-, 2.5-, 5-, 7.5-, and 10-cm soil depths, and imazethapyr–pendimethalin mixture was applied PRE at 0, 0.05 + 0.65, 0.09 + 1.31, and 0.18 + 2.62 kg ae (+ ai)/ha. Mexican sunflower emergence was 57, 30, 14, 3, and 2% from 0-, 2.5-, 5-, 7.5-, and 10-cm seed depths, respectively. Seedling emergence started 2 to 3 d after planting and continued for 4 to 5 d. At 0- to 5-cm seed depths, 84 to 100% of total seedling emergence occurred within 3 d after the first seedling emerged, but emergence rate was less from 7.5- and 10-cm seed depths. Imazethapyr plus pendimethalin at 0.05 + 0.65 and 0.09 + 1.31 kg/ha had no effect on seedling emergence in Mexican sunflower, but subsequent growth was severely inhibited regardless of seed depth. The herbicide treatments were more injurious to Mexican sunflower seedlings that emerged from 5- to 10-cm depths than to those that emerged from 0 and 2.5 cm. Burying seeds 7.5 cm deep or more is a potential control measure for Mexican sunflower, and the deeper the emergence depth, the more susceptible the seedling is to imazethapyr plus pendimethalin.

Oxyfluorfen (0.28, 0.42, 0.56, and 0.84 kg ai ha−1) under clear polyethylene film was evaluated for weed control, crop injury, and effects on yields in transplanted muskmelon, cucumber, and summer squash. Numerous narrowleaf and broadleaf weeds were effectively suppressed by 0.42 ha−1 of oxyfluorfen. Crop injury, occurring soon after transplanting, was reported in New York and North Carolina. Injury was usually transient, and injured crops frequently grew more vigorously than those grown on untreated black polyethylene mulch. Muskmelons were consistently the most tolerant of the three crops. At high rates, yields of squash and cucumber in 1988 were reduced in New York and North Carolina, respectively. In greenhouse studies, positioning the cotyledons under the polyethylene film caused greater injury in all three crops than when cotyledons remained above the plastic.

Field experiments were established in 1978 and repeated in 1979 to monitor quackgrass [Agropyron repens (L.) Beauv. ♯3 AGRRE] rhizome growth through a growing season. Quackgrass shoots grew most vigorously when temperatures were moderate. They elongated and differentiated an inflorescence in June. Rhizome apical tips remained belowground in late spring, early summer, and early fall. Quackgrass rhizome tips grew vertically and emerged from the soil in midsummer and late fall. Experiments conducted in growth chambers revealed that photoperiod, temperature, and nitrogen level affected quackgrass shoot and rhizome growth. Shoots grew vigorously at 21C and elongated to differentiate an inflorescence when exposed to a photoperiod of 14 h or longer. Rhizome apical tips remained belowground when plants were grown at 21 C and received 16 or 12 h of light, but the tips curved upward and emerged from the soil when quackgrass was grown at 32 or 10 C, or when the plants received 14 or less than 10 h of light. The temperature and photoperiod responses coordinated to partition the growing season into primary rhizome and rhizome-tiller growth periods.

Diethatyl [N-(chloroacetyl)-N-(2,6-diethylphenyl) glycine] applied posttransplant in peppers (Capsicum annuum L.) but preemergence to hairy galinsoga [Galinsoga ciliata (Raf.) Blake # GASCI] controlled the weed at 1.7 to 2.2 kg ai/ha. No serious crop injury or yield reduction was observed when diethatyl was applied to peppers at 9 kg/ha posttransplant or at bloom. Soil ridging, practiced to improve drainage and control the crown rot phase of Phytophthora blight (Phytophthora capsici L.), limited the duration of weed control by disrupting the herbicide-treated surface soil and exposing untreated soil. Diethatyl applied at 2.2 kg/ha posttransplant and again at pepper bloom, after the soil ridges were formed, controlled hairy galinsoga throughout the entire growing and harvest season.

A bioassay using red beet root length indicated an increase in imazethapyr bioactivity in Berryland sand (BLS) as soil pH increased from 3.7 to 6.5. Increasing pH above 6.5 had no effect on imazethapyr bioactivity. The lowest imazethapyr concentrations detected by the bioassay at pH 6.5 were 0.5, 1, 2.5, and 5 to 10 μg kg−1 in acidwashed quartz sand, BLS, Aura loamy sand, and muck soil, respectively. After application of 0.07 kg ae ha−1 with 12.5 mm of simulated rainfall at 3-d intervals, imazethapyr remained in the 0 to 15 cm soil in BLS soil columns. Under this rainfall regime, the herbicide moved 30 cm deep 5 mo after application, but residues decreased significantly in surface soil. Imazethapyr bioactivity was highest and mobility was lowest at low soil temperature (10 C) and low moisture (4 to 6% w/w). Imazethapyr movement was monitored successfully using the red beet root length bioassay, and an herbicide residue map was developed to show the pattern of imazethapyr bioactivity in soil columns.

The phytotoxicity of three herbicides commonly used on peaches was evaluated in the greenhouse and in the field. Peaches grown using standard sand culture techniques in the greenhouse were treated with diuron, terbacil, and simazine at 0, 0.12, 0.25, 0.5, 1.0, and 5.0 ppmw for 4 weeks. Peaches were most sensitive to diuron and least sensitive to terbacil. Field studies showed peaches grown in a sandy loam soil low in organic matter were more likely to be injured by terbacil than diuron or simazine. Soil mounded around the trunk to prevent winter injury did not affect peach tolerance to diuron or simazine but increased terbacil injury in one year.

Greenhouse studies were conducted with soil columns to determine the influence of rainfall amount, intensity, and time of application on the bioactivity of 0.07 kg ae ha−1 imazethapyr in three soils of New Jersey: the Berryland sand (BLS) (sandy, siliceous, mesic Typic Haplaquod), Aura sandy loam (ASL) (mixed, mesic Typic Hapludult), and Quakertown silt loam (QSIL) (mixed, mesic Typic Hapludult). The Brockman-Duke simulated rainfall machine was used for rainfall application and smooth pigweed as an imazethapyr bioactivity indicator. As rainfall amount increased from low to high (75 to 300 mm mo−1 equivalent) imazethapyr bioactivity declined significantly in BLS and ASL but was unaffected in QSIL. Rainfall intensity from 6 to 25 mm h−1 had no effect on imazethapyr bioactivity in the three soils. A delay in rainfall application up to 14 d after herbicide application significantly reduced the bioactivity of imazethapyr in BLS but had no effect in ASL and QSIL.

The role of preplant glyphosate applications and residual herbicides in the efficacy of glyphosate for weed management in double-crop no-till glyphosate-resistant soybean (GRS) was investigated in the coastal plains of Mid-Atlantic United States. The experiment had a two- by two- by five-factorial treatment structure laid out in three or four randomized complete blocks at research centers in Delaware and New Jersey. The factors investigated were preplant weed management: preplant or no preplant glyphosate applications; postemergence (POST) herbicide treatments: 0.8 kg ae/ha glyphosate alone or 0.8 kg/ha glyphosate tank-mixed with 0.6 kg ai/ha clomazone plus 0.07 kg ai/ha imazethapyr; and GRS growth stage at herbicide application which ranged from cracking, 5 to 8 d after planting, (DAP) to the V6 stage (35 DAP). Preplant glyphosate applications did not influence the efficacy of POST glyphosate applications alone or with the residual herbicides. Glyphosate alone or with clomazone plus imazethapyr provided excellent control of horseweed and fall panicum irrespective of the time of herbicide application from GRS at cracking to the V6 stage. With other weed species, residual herbicide influence varied with year, weed species, and GRS growth stage at herbicide application. Generally, glyphosate alone was most effective when applied at the V2 to V6 stages (16 to 35 DAP). A tank-mix of glyphosate with clomazone plus imazethapyr extended this window to include applications at GRS cracking and the V1 stage. Herbicide treatments were safe on GRS at all stages of application up to the V6 stage (35 DAP).

Giant sensitiveplant interference at different population densities in cassava established at 10,000 plants ha−1 was investigated on a Ferric Luvisol in a humid tropical environment. Interference for 12 mo was compared at 0, 10,000, 20,000, 30,000, and 40,000 plants ha−1 and at natural populations (averaging 630,000 plants ha−1) in four randomized complete blocks. Results showed that the order of cassava growth parameter response to giant sensitiveplant interference for 12 mo was leaf number > height > stem girth > leaf size = petiole length. The natural population density of giant sensitiveplant reduced growth faster and more than populations of 10,000 to 40,000 plants ha−1 in cassava. All giant sensitiveplant populations from 10,000 plants ha−1 and higher reduced storage root yield in cassava 12 mo after planting. Yield reduction increased as giant sensitiveplant population increased and the highest reduction of 85% occurred in the natural population of giant sensitiveplant.

The optimum weeding regime for thorny mimosa control in cassava established at 10,000 plants/ha was studied at Ibadan, Nigeria (7°22½′N, 3°50½′ E), a humid tropical environment. The study compared six weeding regimes, each comprising manual removal of thorny mimosa three times at different intervals within 13 wk after planting (WAP). Cassava vegetative growth recovered from thorny mimosa interference when the first weeding occurred within 5 WAP, but interference for more than 5 WAP reduced storage root yield. Allowing thorny mimosa infestation after 11 WAP had no effect on cassava growth or root yield. Manual removal of thorny mimosa at 4, 7, and 11 WAP consistently gave the highest cassava root yield.

The efficacy of glyphosate applied alone or in combination with residual herbicides in full-season no-till glyphosate-resistant soybean (GRS) was investigated in New Jersey and Delaware on sandy drought-prone soils. Treatments were in a two- by two- by five-factorial arrangement laid out in three or four randomized complete blocks. The factors investigated were—two preplant glyphosate applications: preplant glyphosate applications or no preplant glyphosate applications; two herbicide treatments: 0.8 kg ae/ha glyphosate alone or 0.8 kg/ha glyphosate tank-mixed with 0.6 kg ai/ha clomazone plus 0.07 kg ai/ha imazethapyr; and herbicide application at five GRS growth stages: at cracking or one of the four times between the V1 and V7 stages. Preplant glyphosate application for the control of emerged weeds was essential for satisfactory control of common annual weeds with glyphosate alone or glyphosate combined with residual herbicides when rainfall was high (avg. 120 mm/mo), but less important when rainfall was low (avg. 72 mm/mo). Compared to glyphosate alone, glyphosate plus residual herbicides improved the control of common lambsquarters, fall panicum, and common ragweed, when applied at cracking or at the V1 stage and preceded by preplant glyphosate applications. At all stages of application, satisfactory full-season control of ivyleaf morningglory was achieved only with glyphosate plus residual herbicides. Horseweed, large crabgrass, giant foxtail, or smooth pigweed control varied from good to excellent (80 to 100%) at all stages of application of glyphosate alone or with residual herbicides. Glyphosate applied alone or with residual herbicides was safe on GRS regardless of time of application up to the V7 stage. The highest soybean yield was consistently achieved with preplant glyphosate applications followed by glyphosate alone at the V2 to V4 stages or a preplant glyphosate application followed by glyphosate plus residual herbicides applied from crop emergence to the V4 stage.

A 2-yr experiment repeated at five locations across the northeastern United States evaluated the effect of weed density and time of glyphosate application on weed control and corn grain yield using a single postemergence (POST) application. Three weed densities, designed to reduce corn yields by 10, 25, and 50%, were established across the locations, using forage sorghum as a surrogate weed. At each weed density, a single application of glyphosate at 1.12 kg ai/ha was applied to glyphosate-resistant corn at the V2, V4, V6, and V8 growth stages. At low and medium weed densities, the V4 through V8 applications provided nearly complete weed control and yields equivalent to the weed-free treatment. Weed biomass and the potential for weed seed production from subsequent weed emergence made the V2 timing less effective. At high weed densities, the V4 followed by the V6 timing provided the most effective weed control, while maintaining corn yield. Weed competition from subsequent weed emergence in the V2 application and the duration of weed competition in the V8 timing reduced yield on average by 12 and 15%, respectively. This research shows that single POST applications can be successful but weed density and herbicide timing are key elements.

A 2-yr experiment assessed the potential for using soil degree days (DD) to predict cumulative weed emergence. Emerged weeds, by species, were monitored every 2 wk in undisturbed plots. Soil DD were calculated at each location using a base temperature of 9 C. Weed emergence was fit with logistic regression for common ragweed, common lambsquarters, velvetleaf, giant foxtail, yellow foxtail, large crabgrass, smooth pigweed, and eastern black nightshade. Coefficients of determination for the logistic models fit to the field data ranged between 0.90 and 0.95 for the eight weed species. Common ragweed and common lambsquarters were among the earliest species to emerge, reaching 10% emergence before 150 DD. Velvetleaf, giant foxtail, and yellow foxtail were next, completing 10% emergence by 180 DD. The last weeds to emerge were large crabgrass, smooth pigweed, and eastern black nightshade, which emerged after 280 DD. The developed models were verified by predicting cumulative weed emergence in adjacent plots. The coefficients of determination for the model verification plots ranged from 0.66 to 0.99 and averaged 0.90 across all eight weed species. These results suggest that soil DD are good predictors for weed emergence. Forecasting weed emergence will help growers make better crop and weed management decisions.

Field studies were conducted under full-season conventional tillage in Delaware and New Jersey to determine the critical time to apply glyphosate with or without residual herbicides for optimum weed control in glyphosate-resistant soybean (GRS). The residual herbicides tank-mixed with glyphosate (0.84 kg/ha) were clomazone (0.55 kg/ha) and imazethapyr (0.063 kg/ha). Herbicide application was made at cracking, unifoliate, and one- to six-trifoliate stages of GRS. Weeds varied in growth stages from preemergence (PRE) at cracking to an average height of 30 cm at the six-trifoliate stage of GRS. Herbicide activity varied by year and weed species. Herbicidal action was better under high (>125 mm/mo) than low (<100 mm/mo) rainfall regime. Glyphosate application without residual herbicides was less effective at cracking and unifoliate than at one- to three-trifoliate leaf stages. Mixing residual herbicides with glyphosate at cracking and unifoliate stages enhanced weed control but made no difference when application was delayed until one- to three-trifoliate stages. For optimum weed control in GRS, the window of application for glyphosate alone was between the one- and three-trifoliate leaf stages, approximately 18 to 28 days after planting (DAP). If glyphosate was tank-mixed with residual herbicides, the window of application extended from cracking until the four-trifoliate stage; and weed interference until the four-trifoliate stage (approximately 32 DAP) did not depress GRS yield.

This study compared the effect of weed control and orchard floor management (OFMA) options including organic mulch on summer annual weed interference in a newly established peach orchard. Weed interference where no preemergence (PRE) herbicides were applied, including vole damage, caused 29% peach tree mortality, reduced tree trunk cross-sectional area (TCSA) 62% by the fourth year of orchard establishment, and reduced fruit yield and fruit number in 1999 by 73 and 75%, respectively, but had no effect on fruit size. Compared with a no-till or conventionally tilled orchard floor, the population of grassy weeds within the tree row was greater in killed perennial ryegrass sod (PRS) plus hard fescue residue mulch treatments but was less in killed PRS plus tall fescue residue mulch treatments. Among the no-PRE treatments, the tree row broadleaf weed populations were suppressed in killed PRS with or without the addition of fescue residue mulch to the tree row when compared with the no-till or conventionally tilled orchard floor treatments. PRE herbicide treatments strongly affected peach fruit yield and TCSA but not average fruit size. There was no effect among the killed PRS, with or without hard or tall fescue residue mulch treatments, on peach fruit yield, TCSA, or average fruit size when compared with the no-till or conventionally tilled orchard floor treatment options. All treatments with herbicide had higher yields in 1999 than those without herbicides.