The escalating evolution of weed species resistant to acetolactase synthase (ALS)-inhibitor herbicides makes alternative weed control strategies necessary for field crops that are dependent on this herbicide group. A fully integrated strategy that combined increased crop seeding rates (2X or 4X recommended), mechanical weed control with a minimum-tillage rotary hoe, and reduced-rate non–ALS inhibitor herbicides was compared with herbicides, rotary hoe, and seeding rates alone as a method of controlling ALS inhibitor–tolerant Indian mustard as a model weed. The full-rate herbicide treatment had the lowest weed biomass (98% reduction) and the highest yield of all treatments in 3 of 4 site-years, regardless of seeding rate. The fully integrated treatment at the 4X seeding rate had weed suppression rates equal to the full herbicide treatment at the recommended seeding rate. The fully integrated and reduced-rate herbicide treatments at the 4X seeding rate reduced weed biomass by 89% and 83%, respectively, compared with the control at the recommended seeding rate. The rotary hoe treatment alone resulted in poor weed control (≤38%), even at the highest seeding rate. Fully integrated and reduced-rate herbicide treatments at 2X and 4X seeding rates had yields equal to those of the full herbicide treatment at the recommended seeding rate. Partially or fully integrated weed control strategies that combine increased crop seeding rates and reduced-rate non–ALS inhibitor herbicides, with or without the use of a rotary hoe, can control weeds resistant to ALS-inhibitor herbicides, while maintaining crop yields similar to those achieved with full-rate herbicides. However, combining increased seeding rate, reduced-rate herbicides, and mechanical rotary hoe treatment into a fully integrated strategy maximized weed control, while reducing reliance on and selection pressure against any single weed control tactic.

Synthetic auxin herbicides are commonly used in forage, pasture, range, and turfgrass settings for dicotyledonous weed control. Aminocyclopyrachlor (AMCP) is a newly developed pyrimidine carboxylic acid with a chemical structure and mode of action similar to the pyridine carboxylic acids—aminopyralid, clopyralid, and picloram. Injury to sensitive dicotyledonous plants has been observed following exposure to monocotyledonous plant material previously treated with pyridine compounds. The absorption, translocation, and metabolism of AMCP has been documented in susceptible broadleaf weeds; however, no information is available, to our knowledge, regarding AMCP fate in tolerant Poaceae, which may serve as the vector for off-target plant injury. Based on this premise, research was conducted to characterize absorption, translocation, and metabolism of AMCP in tall fescue. 14C-AMCP was applied to single tiller tall fescue plant foliage under controlled laboratory conditions at North Carolina State University (Raleigh, NC). Radiation was quantified in leaf wash, treated leaf, foliage, crown, roots, and root exudates at 3, 12, 24, 48, 96, and 192 h after treatment (HAT). 14C-AMCP was rapidly absorbed by tall fescue, reaching 38 and 68% at 3 and 48 HAT, respectively. Translocation of 14C-AMCP was limited to the foliage, which reached maximum translocation (34%) at 96 HAT. Most of the recovered 14C-AMCP remained in the leaf wash, treated leaf, or foliage, whereas minimal radiation was detected in the crown, roots, or root exudates throughout the 192-h period. No AMCP metabolism was observed in tall fescue through the 192 HAT. These data suggest AMCP applied to tall fescue can remain bioavailable, and mishandling treated plant material could result in off-target injury.

Seed deterioration, and therefore seed germination potential, are highly influenced by relative humidity and temperature. However, limited species-specific information is available about the effect of long-term soaking in water on seed germination potential. Knowing the potential fate of a creeping bentgrass seed that falls in an irrigation canal is important for the study of transgene flow in this species at the landscape level. The objectives of this study were to evaluate the effect of soaking time and water temperature on germination of creeping bentgrass seed and to assess how fast a panicle could be moved in an irrigation canal. Germination was determined for seeds from panicles of three cultivars of creeping bentgrass that were soaked in water for up to 17 wk at two water temperatures, 4 and 20 C. Creeping bentgrass seeds did not lose their ability to germinate after 17 wk in water at 20 C and, although reduced, germination was still 46% after 17 wk in water at 4 C. The reduction in germination in seeds from panicles kept in water at 4 C was due to the induction of secondary dormancy, which was overcome by dry seed storage at room temperature. We quantified that a panicle that falls in an irrigation canal has the potential to travel downstream at an average rate of 19 m min−1 and move seeds that could potentially establish seedlings elsewhere. Therefore, movement of creeping bentgrass seed by water has to be considered as a means of gene flow.

Saltcedar (Tamarix pentandra Pall.) and Woods rose (Rosa woodsii Lindl. var. ultramontana [Wats.] Jeps.) were sprayed with systemic herbicides at frequent intervals over a 2-year period. Root samples were collected at each spray date and analyzed for “total available carbohydrates.” Food reserve levels in the roots and response to herbicides were not correlated.

It is generally expected that, in the case of multiple herbicide resistance, different resistance mechanisms within a weed will follow Mendel’s law of independent assortment. Research was conducted to investigate anecdotal observations suggesting that target site–based resistances to inhibitors of acetolactate synthase (ALS) and protoporphyrinogen oxidase (PPO) did not follow independent assortment in common waterhemp. Cosegregation of the two resistances was observed in backcross lines (population sensitive to both herbicides as recurrent parent). Specifically, whereas 52% of backcross plants were resistant to a PPO inhibitor, this percentage increased to 92% when the backcross plants were preselected for resistance to an ALS inhibitor. Molecular marker analysis confirmed that the corresponding genes (ALS and PPX2) were genetically linked. When data from all plants analyzed were pooled, the genetic distance between the two genes was calculated to be 7.5 cM. The two genes were found to be about 195 kb apart in the recently published grain amaranth genome, explaining the observed genetic linkage. There is likely enough recombination that occurs between the linked genes to prevent the linkage from having significant implications in terms of resistance evolution. Nevertheless, documentation of the happenstance linkage between target-site genes for resistance to ALS and PPO inhibitors in waterhemp is a reminder that one should not assume distinct resistance mechanism will independently assort.

Plants of Sumatran fleabane [Conyza sumatrensis (Retz.) E. Walker] were identified in a field with an unusual rapid leaf-injury herbicide symptoms after application of 2,4-D in mixture with glyphosate. The objectives of this study were to confirm the occurrence of resistance to 2,4-D herbicide and to characterize the occurrence of rapid necrosis as the mechanism associated with the herbicide resistance in C. sumatrensis. The studies performed were an initial screening, effect of 2,4-D alone and associated with glyphosate, cross- and multiple-resistance evaluation, effect of commercial formulation and analytical product, and rate of H2O2 evolution. The Marpr9-rn accession was identified with rapid necrosis symptoms and survival to 804 g ae ha−1 of 2,4-D. The resistance factor to the herbicide 2,4-D was 18.6 at 49 d after spraying. The analytical product 2,4-D and the commercial formulation resulted in similar symptoms of rapid necrosis. This symptom did not occur for the six other auxinic herbicides (dicamba, florpyrauxifen-benzyl, fluroxypyr, halauxifen-methyl, picloram, and triclopyr), indicating absence of cross-resistance. Multiple resistance to the herbicides paraquat, saflufenacil, and ammonium glufosinate was not identified in the Marpr9-rn population. However, survival following treatment with the herbicides glyphosate and chlorimuron-ethyl occurred. The evolution of H2O2 began at 15 min after application and was less pronounced in low light. These results indicate the first case of resistance to 2,4-D and occurrence of rapid necrosis in C. sumatrensis.

Esters of the chlorophenoxyacetic acids are generally considered to be more phy to toxic than the acid or acid salts. Hence the esters have found wide use in herbicidal formulations. Much research has been done on the mode of action of plant growth regulators, but the fate of neutral derivatives of active acids has received little attention, even though an acid side chain is one of the molecular requirements for growth substance activity.

A somewhat modified version of Lambert's quantitative treatment of the influence of soil-moisture content on herbicidal activity has been applied to 4-amino-3,5,6-trichloropicolinic acid (picloram) behavior in soil. The bioactivity of picloram decreased as the soil-moisture content was increased. This was due to the effect of varying soil-moisture levels on the concentration of picloram in the soil-water phase.

Field studies were conducted during 1967 and 1968 at the University of Wisconsin Experimental Farm, Arlington, Wisconsin, to determine the comparative effectiveness of combinations of row spacings and rates of 3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea (linuron) for control of yellow foxtail (Setaria glauca (L.) Beauv.) and barnyardgrass (Echinochloa crusgalli (L.) Beauv.) in soybeans (Glycine max (L.) Merr.). Soybean yields and weed control increased as row spacing decreased. Lower rates of linuron were required for comparable weed control as row spacing decreased. Yield increases with decreasing row width were much greater in 1968 than in 1967, probably because of better rainfall during July, August, and early September in 1968. Improved weed control in the narrow rows may have been due partially to absorption by soybean leaves of light wavelengths that may be most favorable for photosynthesis and vegetative growth of some weed species.

This paper presents a framework in which the spatial and temporal domain of weed populations can be analyzed using geographically referenced information. The regional framework for analysis is based on the premise that the domain of a weed species or an assemblage of species can be described in terms of the space and time in which they survive. Published maps of the spatial distribution of individual weed species were converted to digital records defining their geographic domain. Digital records were imported into a geographically referenced data system. Assemblage maps were produced by intersecting domains of individual species. The assemblage maps show areas with similar intensity of the occurrence of species selected for analysis. The most intense occurrence of summer annual species, for example, exists in a relatively small area of the Midwest. The framework presented in this paper is a concept for a tool to evaluate, manage, and manipulate weed distribution data at the regional scale. Analysis at the regional scale is a perspective substantially different from the plot or field-scale analysis traditionally used to study and manage weed populations. Regional analysis has application to weed science by increasing the understanding of regional patterns of weed infestation and the factors that regulate them. This information will be useful to the herbicide industry, plant ecologists, resource managers, and agricultural policy makers.

Clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) was superior to 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid], dicamba (3,6-dichloro-o-anisic acid), picloram (4-amino-3,5,6-trichloropicolinic acid), and triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid} for control of honey mesquite (Prosopis glandulosa Torr. ♯ PRCJG) in the field. However, equal ratio mixtures of clopyralid + picloram or triclopyr (0.28 + 0.28 or 0.56 + 0.56 kg ae/ha) were as effective for honey mesquite control as clopyralid at rates of 0.56 and 1.12 kg/ha. Equal mixtures of picloram + 2,4,5-T, picloram + triclopyr, picloram + dicamba, triclopyr + 2,4,5-T, or triclopyr + dicamba were less effective in controlling honey mesquite than clopyralid alone or clopyralid + picloram or triclopyr in 1:1 mixtures. In the greenhouse, mixtures of clopyralid + 2,4,5-T, picloram, or triclopyr were highly effective for the control of honey mesquite. Mixtures of 2,4,5-T + picloram, 2,4,5-T + triclopyr, or triclopyr + picloram were also effective when applied at 0.28 + 0.28 kg/ha.

Economical combinations of herbicides and hand labor for weeding sugarbeets (Beta vulgaris L.) in high and low weed densities were determined for irrigated production in the southern Great Plains. Ethofumesate [(±)-2-ethoxy-2,3-dihydro-3,3-dimethyl-5-benzofuranyl methanesulfonate] preplant incorporated, a 50:50 mixture of phenmedipham (methyl m-hydroxycarbanilate m-methylcarbanilate) and desmedipham (ethyl m-hydroxycarbanilate carbanilate (ester)] sprayed postemergence on small weeds, and trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) incorporated at layby controlled pigweed (Amaranthus spp.) and kochia [Kochia scoparia (L.) Schrad.] with acceptable selectivity in sugarbeets. Sucrose yield was identical for all methods of weed control. Using herbicides did not lower, and in some cases doubled, the total cost of weed control in an area with a low weed density. On the other hand, effective herbicide combinations saved $298 to $966/ha compared to total reliance on hand labor in an area with a high weed density. Elaborate combinations of herbicides were not justified even with heavy weed density. The last few weeds remaining in a field can be most economically removed by hand labor.

A survey was conducted in Mississippi from 1976 and in Arkansas and Louisiana from 1977 through 1991 to determine annual variation of johnsongrass infestation in cotton and soybean fields. The survey route was 1534 km long. Levels of johnsongrass infestation were estimated in 752 ± 296 cotton fields and 884 ± 407 soybean fields in each year of the survey. The area of fields surveyed annually was 47 000 ± 21 000 ha of cotton and 52 000 ± 19 000 ha of soybeans. Johnsongrass was present in 55 to 90% of the cotton fields. The percent of cotton fields with johnsongrass was about the same in 1991 as in 1976–77. About 90% of the soybean fields in Mississippi and Louisiana had johnsongrass, but only 70 to 80% of soybeans fields in Arkansas had johnsongrass. Soybean fields with 1 to 5% levels of infestation increased slightly in Mississippi but remained about the same in Arkansas and Louisiana. Soybean fields with infestations of 6% or more slightly decreased in Arkansas and Louisiana but not in Mississippi. It was estimated that johnsongrass reduces the average annual value of harvested cotton $5.8 ± 1.9 million and soybeans $23.7 ± 0.6 million in the three states.

Four synthetic multiring analogs of strigol (0.1 mM) were effective as germination stimulants of dormant, unchilled seeds of shepherdspurse (Capsella bursa-pastoris (L.) Medik. # CAPBP). The effect was concentration dependent and exceeded that produced by 0.1 mM gibberellic acid (GA). The analog treatments resulted in normal, unpigmented seedlings after 7 days, whereas GA treatment led to distorted, nongeotropic, yellow seedlings. The strigol analogs were also more effective in inducing shepherdspurse germination than 10 mM nitrate or 5.0 mM thiourea (ineffective in the dark) and 5.0 mM thiourea plus 4.5 mM ascorbate (germination stimulated, but seedling growth, inhibited). Chilling at 4 C for 11 days reduced the effectiveness of the strigol analogs, but not that of gibberellic acid and thiourea plus ascorbate. Ascorbate acts synergistically with thiourea in promoting shepherdspurse germination. Strigol and epistrigol had no effect on either chilled or unchilled shepherdspurse seeds. The stimulation of dormant shepherdspurse seeds by the synthetic strigol analogs is the first demonstration that these compounds have bioregulatory activity in dormant seeds of genera other than Striga.

Field experiments were conducted from 1985 to 1987 to evaluate herbicide systems for minimum-tillage and conventional-tillage peanut production. While acceptable weed control could be achieved in both tillage systems, minimum-tillage systems generally had to be more herbicide intensive. Preemergence or preplant-incorporated within-the-row applications of either ethalfluralin or pendimethalin plus postemergence applications of paraquat and sethoxydim provided Texas panicum control equivalent to preplant-incorporated applications of ethalfluralin or pendimethalin. Early-postemergence paraquat applications improved Florida beggarweed and pitted morningglory control in conventional-tillage systems at least 15% compared to the same systems without paraquat Control of bristly starbur and sicklepod in conventional-tillage systems did not increase with paraquat application. Broadleaf weed control did not differ between tillage systems, except pitted morningglory control was lower in the minimum-tillage system. Conventional-tillage peanuts produced yields 800 to 1900 kg/ha higher, depending on herbicide system, and also provided greater net returns than minimum-tillage peanuts. The greater yield and net returns in conventional- versus minimum-tillage systems were not attributed to weed control or disease problems.

Differences were shown to exist in photosynthetic rate, transpiration rate, and carbon dioxide leaf diffusive resistance between atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] susceptible (S) and resistant (R) plants of redroot pigweed (Amaranthus retroflexus L.). Chlorbromuron [3-(4-bromo-3-chlorophenyl)-1-methoxy-1-methylurea] and diruon [3-(3,4-dichlorophenyl)-1,1-dimethylurea] were the only herbicides tested that controlled both biotypes, but all of the herbicides except norea [3-(hexahydro-4,7-methanoindan-5-yl)-1,1-dimethylurea] controlled the S biotype. Although photosynthetic activity and transpiration were reduced in both biotypes by atrazine at 50 and 70 ppm, the decline was much greater in the S biotype than in the R biotype and persisted a longer time in the S biotype. Leaf CO2 diffusive resistances of the biotypes were increased by atrazine applications. Mesophyll resistance was increased to a greater extent than stomatal resistance suggesting that reduction of photosynthesis is due to a greater effect of atrazine on the mesophyll tissue than on the guard cells.

Tebuthiuron {N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N,N′-dimethylurea} in various formulations and placements was applied to honey mesquite (Prosopis glandulosa Torr. # PRCJG), huisache [Acacia farnesiana (L.) Willd. # ACAFA], live oak (Quercus virginiana Mill. # QUEVI), Macartney rose (Rosa bracteata J. C. Wendl. # ROSBC), and Texas whitebrush [Aloysia gratissima (Gillies & Hook.) Troncoso # ALYLY]. There was little difference in a species response to the various tebuthiuron formulations. Pellets (20%) at 2 g ai/tree base reduced the canopy 85% and killed 50% of the honey mesquite. At 1 g ai/plant, two briquettes at 0.5 g ai each or pellets (5.14 g, 20%) killed 37 and 80% of the huisache, respectively. Pellets were no more effective at 2 g than at 1 g ai on huisache. Basal treatments of 0.25, 0.5, 0.5, 1, and 2 g ai tebuthiuron/tree killed from 8 to 22, 48 to 62, 62 to 85, and 92% of the live oak, respectively. Pellets at 2 g ai placed at the base reduced the canopy 83% and killed 59% of the Macartney rose. On Texas whitebrush, 0.25, 0.5, 1, and 2 g ai tebuthiuron/tree killed from 68 to 85, 75 to 100, 90, and 100% of the plants, respectively. Tebuthiuron persisted mainly in the upper 30 cm of soil for at least 15 months.

Aseptic cultures of isolated corn (Zea mays L. ‘North Dakota KE47101′) shoots and roots and soybean [Glycine max (L.) Merr. ‘Hawkeye’] seedlings absorbed and metabolized atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] similarly to plants grown and treated in nutrient solution under non-sterile conditions. Atrazine injured susceptible soybean seedlings more rapidly than can be explained on the basis of starvation alone. It is postulated that the herbicidal activity of atrazine is partly due to toxic secondary factor(s) which may be of biochemical or photochemical origin. The effect of the toxic secondary factor(s) was reversed by added sucrose independent of light.

Single topical applications of monosodium methanearsonate (MSMA) affected cotton (Gossypium hirsutum L.) growth more than did similar disodium methanearsonate (DSMA) applications. There was usually no difference in yield among rates and dates with single applications of DSMA. Single applications of MSMA at later dates and higher rates reduced yields. Repeated topical applications of MSMA reduced yields and affected boll weight, lint percentage, number of seed per boll, and fiber fineness as compared with these properties of cotton treated once with MSMA. Response of cotton to MSMA was related to the number of applications and rate of herbicide. Repeated applications of DSMA affected cotton yield, lint percentage, and number of seed per boll less than did MSMA.

ED50 values (the dosage required to reduce the posttreatment gain in dry weight by 50%) for diclofop applied to green and yellow foxtail [Setaria viridis (L.) Beauv. ♯3 SETVI and S. lutescens (Weigel.) Hubb. ♯ SETLU] at the two- and four-leaf stages were calculated from linear regression equations derived by plotting the logarithm of the applied dosage against the relative growth reduction expressed as a probit value. At the two-leaf stage, green foxtail was more susceptible to the chemical than yellow foxtail but at the four-leaf stage there was no significant difference in response of the two species. At least part of the difference in sensitivity of the two species at the two-leaf stage could be accounted for by differences in spray retention, with green foxtail retaining 1.8 times (expressed as μL/cm2) or 3.5 times (expressed as μL/g dry weight) as much spray as yellow foxtail, despite the fact that yellow foxtail had a greater projected leaf area. At the four-leaf stage, the amount of spray retained by both species calculated on a per gram dry-weight basis was significantly less than at the two-leaf stage, accounting in part for the higher ED50 values obtained at the later growth stage. However, there appeared to be no direct relationship between the amount of spray retained and the comparative growth response of the two weeds at the later stage of application.