Emergence characteristics, including initial time of emergence, magnitude of emergence, and mean time of emergence, ofAmaranthus rudisSauer,Setaria faberiHerrm.,Eriochloa villosa(Thunb.) Kunth, andAbutilon theophrastiMedik. were studied in central Iowa during the first 3 yr following burial of seed collected and buried in the fall of 1994 or 1995. Although the initial emergence date varied among years, the emergence sequence among species was consistent.Eriochloa villosaand A.theophrastiwere the first species to emerge, with initial emergence occurring between April 28 and May 10. Initial emergence dates for these species were the same, except for 1995 whenA. theophrastiemerged 4 d prior toE. villosa. Amaranthus rudiswas the last species to emerge, with initial emergence ranging from 5 to 25 d afterA. theophrasti.First-year emergence in 1995 was 8% forA. theophrasti, 7% for A.rudis, 41% forE. villosa, and 33% forS. faberi, based on the number of buried seed. Proportional emergence during the first year following burial in 1996 was similar to 1995 forA. theophrastiandS. faberi, but greater emergence was observed in 1996 forE. villosaandA. rudis.During the 3 yr of both studies, cumulative emergence of the two grass species (43 to 71%) was higher than for the broadleaf species (13 to 35%). A high percentage of the total annual emergence ofE. villosaoccurred within the first 2 wk of initial emergence, whereas a high percentage ofA. rudisemergence occurred late in its emergence period. Emergence characteristics of the four species were consistent among years and seed sources.

Also known as great ragweed, horseweed, horse-cane, richweed, bitterweed, bloodweed, blood ragweed, tall ragweed, palmate ragweed. Classification and Description:Giant ragweed is an erect summer annual that is native to the U.S. and it can be commonly found throughout many parts of the country. It can reach heights from 3 to more than 16 feet. Giant ragweed is a member of the Asteraceae, or sunflower, family of plants. Seedling giant ragweed has a purple hypocotyl and cotyledons that are round to oblong and thick. The first true leaves do not have lobes but do have toothed margins and are lanceolate (long and thin) in shape. Subsequent leaves are opposite, blades simple, hairy and large (4-10 inches long and up to 8 inches wide). Leaves occur on petioles and most often have three prominent lanceolate-shaped lobes, although they can occasionally have five lobes. The lobes originate from the same point (palmate). These large, three-lobed leaves make giant ragweed a very distinctive plant. Leaf margins are serrated. Stems can be reddish and are erect, branching above, rough and sometimes hairy. Stems can be reddish. Giant ragweed has separate male and female flowers. Male flowers occur in slender racemes (columns) in the upper terminals. Female flowers occur in clusters in leaf axils below the male flowers. All flowers are small and greenish-yellow. Fruit is a large, black, woody achene that is egg-shaped, except the widest part is towards the end instead of in the middle. The widest end has one single short beak and other shorter projections, which make it resemble a crown. Seed is small and enclosed in the fruit. Reproduction is by seeds. Weed Status and Injury:Giant ragweed can readily be found along fence rows of agronomic crop fields and pastures in Tennessee. Increasingly, it is becoming established in agronomic crop fields. Herbicides commonly used on agronomic crops, like glyphosate, only provide partial control, and so giant ragweed is becoming an increasing problem in row crops. It can also be found in pastures, low woods and young Seedling giant ragweedGiant ragweed in a fence row W119Programs in agriculture and natural resources, 4-H youth development, family and consumer sciences, and resource development. University of Tennessee Institute of Agriculture, U.S. Department of Agriculture and county governments cooperating. UT Extension provides equal opportunities in programs and employment.

Management of perennial weeds is a major concern in reduced-tillage cropping systems. Field research was conducted at Nashua, IA, from 1977 through 1990 to evaluate the long-term impacts of tillage and cropping patterns on perennial weed populations in corn and soybean production. Continuous corn and a corn/soybean rotation were conducted utilizing moldboard plow, chisel plow, ridge tillage, and no-tillage systems. The research area was free of established perennial weed species at the initiation of the experiment in 1977. Hemp dogbane was observed by 1980, with the greatest densities in no-tillage. By 1990, continuous corn had greater hemp dogbane densities with no-tillage than other tillage system by crop rotation treatments. American germander densities were not affected by tillage systems in 1980 and 1981, but by 1990, corn/soybean rotations had greater densities in moldboard plow than other tillage systems. Field bindweed developed primarily in the corn/soybean rotations with the greatest densities occurring in no-tillage. Greater and more diverse populations of perennial weeds developed in reduced-tillage systems than in the moldboard plow system. However, practices used to control annual weeds and environmental factors interacted with tillage to regulate perennial weed populations.

A major challenge that organic grain crop growers face is weed management. Th e use of a rye (Secale cereale L.) cover crop to facilitate no-tillage (NT) organic soybean [Glycine max (L.) Merr.] production may improve weed suppression and increase profitability. We conducted research in 2008 and 2009 to determine the eff ect of rye management (tilling, crimping, and mowing), soybean planting date (mid-May or early June), and soybean row width (76 or 19 cm), on soybean establishment, soil moisture, weed suppression, soybean yield, and profi tability. Soybean establishment did not diff er between tilled and NT treatments; and soil moisture measurements showed minimal risk of a drier soil profi le in NT rye treatments. Rye mulch treatments eff ectively suppressed weeds, with 75% less weed biomass than in the tilled treatment by mid-July. However, by this time, NT soybean competed with rye regrowth, were defi cient in Cu, and accumulated 22% as much dry matter (DM) and 28% as much N compared to the tilled treatment. Soybean row width and planting date within NT treatments impacted soybean productivity but not profi tability, with few diff erences between mowed and crimped rye. Soybean yield was 24% less in the NT treatments than the tilled treatment, and profi tability per hectare was 27% less. However, with fewer labor inputs, profi tability per hour in NT rye treatments was 25% greater than in tilled soybean; in addition, predicted soil erosion was nearly 90% less. Although soybean yields were less in NT rye mulch systems, they represent economically viable alternatives for organic producers in the Upper Midwest.

The influence of secondary soil disturbance on the emergence pattern and seed bank depletion of an annual weed community in a long-term, no-tillage corn cropping system was determined in 1992 and 1993. As a component of this research, the seed bank was characterized prior to implementation of soil disturbance treatments. The seed bank was initially composed of common lambsquarters, redroot pigweed, and giant foxtail, with approximately 55, 36, and 8% of the total viable seeds, respectively. The remaining 1% was comprised of five other species in 1992 and eight in 1993. The spatial distribution of viable seeds of each species, except common lambsquarters and redroot pigweed, was described by a negative binomial distribution. Three dispersion indices indicated that seeds of individual and total weed species were aggregated and that the level of aggregation of viable seeds of a species was associated with seed density; at lower seed densities, the level of aggregation was greater. Soil disturbance increased common lambsquarters emergence 6-fold in 1992 relative to nondisturbed soil, but did not influence emergence in 1993. Rainfall was about 50% less in 1993. In contrast, soil disturbance increased giant foxtail and redroot pigweed emergence approximately 6- and 3-fold in 1992 and 1993, respectively. Seedling emergence associated with soil disturbance, relative to nondisturbed soil, increased seed bank depletion of common lambsquarters 16-fold in 1992, and giant foxtail and redroot pigweed and average of 6- and 3-fold in 1992 and 1993, respectively. These results indicated that soil disturbance increased seedling emergence and seed bank depletion of the predominant species in the weed community of a long-term, no-tillage system, but that this response was dependent on rainfall for common lambsquarters.

The distribution and dissipation of alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl) acetanilide], atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5 triazine), and metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] in soil were studied in 1990, 1991, and 1992. Crop management practices included four tillage methods-chisel plow, moldboard plow, no-till, and ridge-till-and two crop rotations-continuous corn (Zea mays L.) and a corn-soybean [Glycine max (L.) Merr.] rotation. All herbicides were broadcast-spray applied with no incorporation. No-till plots had the smallest amounts of alachlor and metribuzin, whereas ridge-till plots had the smallest amounts of atrazine. Moldboard-plow plots usually contained the highest amounts of all three herbicides, although ridge-till plots had the highest metribuzin levels in 1992. These differences were seldom significant at the 0.05 level of probability, however. Throughout the growing season, 50 to 84% of the alachlor and metribuzin were retained in the top 10-cm layer of soil, and at least 68% of the atrazine was retained in the top 20 cm. From 84 to 98% of the herbicide applied was lost each year, probably by microbial degradation and, for alachlor, by volatilization after application. First-order half-lives were 36 d for alachlor, 55 d for atrazine, and 32 d for metribuzin. A two-compartment model better fitting the alachlor data returned a half-life of 24 d for that herbicide.

Field studies were conducted in 1995 and 1996 to determine the rate response of velvetleaf seedling survival, seed production, and shoot biomass to postemergence herbicides in corn and soybean. Dicamba and imazethapyr were applied to corn and soybean, respectively, at 1, ½, ¼, ⅛, 1/16, 1/32, and 0× labeled rates. Velvetleaf mature plant density was linearly related to seedling density, thus indicating that seedling survival was not density dependent, even after seedling densities exceeded 150 plants m−2. Seedling survival as influenced by herbicide was described by a dose–response curve in corn and soybean. In corn, seedling survival ranged from 0 to 48% across herbicide treatments and years. Seedling survival was greater at the ½× or lower herbicide rates than at the 1× rate. In soybean, maximum seedling survival was 61 and 14% in 1995 and 1996, respectively, and minimum seedling survival was less than 2% in each year. Seedling survival was less in 1996 than in 1995 because velvetleaf was infected with wilt in 1996. In soybean, seedling survival was 20 times greater when treated with herbicides at the ½× rate than when treated at the 1× rate in 1995, but seedling survival was similar when herbicides were applied at 1, ½, ¼, and ⅛× rates in 1996. Velvetleaf fecundity (seeds per plant) was dependent on mature plant density in 1995 but was density independent in 1996. Fecundity as influenced by herbicide was described by dose–response curves in corn each year and in soybean in 1995. In 1995, velvetleaf treated with herbicides at ½× and ¼× rates produced 20 to 30 times more seed per square meter than when treated with herbicides at the 1× rate. Differences in seed per square meter were exaggerated by high densities of velvetleaf. Seed per square meter did not differ between velvetleaf treated with herbicides at 1× or ½× rates in corn or soybean in 1996. Wilt infection of velvetleaf in 1996 was the likely cause of differences in herbicide performance between years. Herbicides at reduced rates were not effective at limiting seedling survival and seed production compared to 1× rates in the absence of wilt. As a result, long-term management of velvetleaf with herbicides at reduced rates likely will be difficult, especially in areas with high densities, unless integrated with other management practices.

Intermediate wheatgrass [IWG; Thinopyrum intermedium (Host) Barkworth & D.R.Dewey] is a cool-season perennial forage grass bred for higher seed yield. It is the first perennial grain crop in the United States, commercialized as Kernza since 2015. Managing IWG as a dual-use grain and forage crop could provide several ecosystem services including conserving soil and clean water while increasing economic income to growers. However, little is known about the weed management risks associated with IWG. Therefore, we studied weed community composition, biomass, IWG grain, and aboveground biomass in a factorial experiment with two weed management treatments, two nitrogen fertilization rates, and four forage harvest schedules (no harvest, summer only, summer + fall, and spring + summer + fall).Over three production years, weed biomass decreased by 88% regardless of treatment, and the weed community composition changed from predominantly winter annual to perennial species. In the second and third production years the weed community composition remained relatively stable. Grain yield was 16% greater with 135 kg N ha −1 than 90 kg N ha −1 but was not affected by in-season forage harvest or weed management treatments in the second and third years. Grain yield decreased from 763 to 371 kg ha −1 over three years, while aboveground biomass remained stable. Weed presence did not affect yields in second and third years. Dual-use IWG cropping systems effectively suppressed weeds and IWG is a promising grain crop alternative for farmers interested in diversifying their cropping systems under similar conditions. Abbreviations: AIC, Akaike's Information Criterion; IWG, intermediate wheatgrass.This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.