Postemergence treatments utilizing various combinations of fluazifop-P, paraquat, and 2,4-DB were compared to a preplant-incorporated (PPI) application of benefin followed by a ground-cracking application of alachlor and dinoseb plus naptalam and a postemergence application of 2,4-DB for weed control, peanut yield, and net economic return to land, overhead, and management. The greatest peanut yields (3-yr average of 4510 kg/ha) and net returns (3-yr average of $521/ha) were provided by a postemergence system that utilized one ground-cracking and one postemergence application of paraquat and one postemergence application of fluazifop-P and 2,4-DB. Seven postemergence systems provided equivalent or greater yield and net returns than the PPI and dinoseb plus naptalam system. Fresh weight reductions of Texas panicum, sicklepod, Florida beggarweed, and pitted morningglory from postemergence weed control systems were equivalent to reductions obtained from the PPI and dinoseb plus naptalam system. The addition of paraquat and 2,4-DB to the PPI and dinoseb plus naptalam system improved the 3-yr average peanut yield and net economic return by 510 kg/ha and $136/ha, respectively, compared to the same system without paraquat and 2,4-DB.

Effects of soil pH on growth, competitive ability, and leaf nutrient content of corn (Zea mays L.), Powell amaranth (Amaranthus powellii S. Wats. ♯ AMAPO), velvetleaf (Abutilon theophrasti Medic. ♯ ABUTH), and green foxtail [Setaria viridis (L.) Beauv. ♯ SETVI] were measured in the field. Corn yields were significantly reduced by weed competition at all pH levels, but leaf nutrient content, as a percentage of dry weight, was not affected. Aboveground dry weights of Powell amaranth and velvetleaf were significantly lower at pH 4.8 than at pH 6.0 or 7.3, whereas growth of green foxtail was greater at pH 4.8 than at pH 7.3. Weed competitive ability, as evidenced by reductions in dry weight, varied with soil pH and companion plant. Powell amaranth and velvetleaf had higher levels of S, Zn, and especially Mn, at pH 4.8 than at pH 7.3. N and K in the leaf tissue were greater in the weed species than in corn at all soil pH levels. The dicot species had higher percentages of Ca and Mg in leaf tissue at all soil pH levels and accumulated higher percentages of Mn at low pH than the monocot species.

Greenhouse and laboratory studies were conducted to determine the physiological basis for selectivity of nicosulfuron and primisulfuron in 5 plant species. Differential sensitivity of the species was quantified by determining GR50 values (herbicide rate required to reduce plant growth 50%) for each species/herbicide combination. GR50 data indicated the following levels of sensitivity: corn—tolerant to both herbicides; seedling johnsongrass—sensitive to both herbicides; barnyardgrass—sensitive to nicosulfuron and tolerant to primisulfuron; giant foxtail—sensitive to nicosulfuron and tolerant to primisulfuron; and eastern black nightshade—tolerant to nicosulfuron and sensitive to primisulfuron. Studies using 14C-radiolabeled herbicides were conducted to determine whether differential herbicide absorption, translocation, or metabolism contributed to whole plant responses. Nicosulfuron and primisulfuron selectivity in corn, johnsongrass, barnyardgrass, and giant foxtail was primarily due to differential herbicide metabolism rate. Tolerant species metabolized the herbicide more rapidly and extensively than sensitive species. Differential herbicide absorption, translocation, or metabolism did not explain differential sensitivity of eastern black nightshade to the herbicides. Further studies indicated that the tolerance of eastern black nightshade to nicosulfuron and its sensitivity to primisulfuron was directly related to lower sensitivity of the acetolactate synthase (ALS) to nicosulfuron than to primisulfuron. Eastern black nightshade translocated very little (3%) of the nicosulfuron applied. The ALS sensitivity of johnsongrass and eastern black nightshade was similar in the presence of nicosulfuron. A combination of a higher ALS level and less herbicide translocation contributes to tolerance of eastern black nightshade and to sensitivity of johnsongrass to nicosulfuron.

Studies were conducted to examine over time the effects of propachlor (2-chloro-N-isopropylacetanilide) on the growth of cucumber (Cucumus sativus L. ‘Straight Eight’) roots and associated biosynthetic reactions. Complete inhibition of root elongation occurred within 16 hr after exposure to propachlor. Inhibition of growth was not found to be a result of an effect on ATP formation or respiration. Protein biosynthesis was reduced several hours before the observed inhibition of growth therefore implicating it as the causal factor. Inhibition of protein synthesis occurred prior to an observed reduction in RNA synthesis suggesting that the primary effect of propachlor is on protein biosynthesis and that its effect on nucleic acid synthesis is secondary. It is concluded that the primary mechanism of action of propachlor is its effect on nascent protein biosynthesis.

Field research was conducted from 1987 to 1991 to evaluate the influence of four weed management systems on weed population and species dynamics in conventional-tillage and no-tillage grain sorghum production. These weed management systems included zero, low, medium, and high input systems. The weed seedbank increased faster in zero and low input weed management systems than in the high input weed management systems because of differences in weed control. Tillage influenced weed seed densities as well. Common ragweed, common lambsquarters, horseweed, and sicklepod seed densities often were greater in no-tillage than conventional-tillage plots. Common cocklebur and large crabgrass seed densities were usually greater in conventional-tillage than no-tillage plots. Smooth pigweed seed densities were not affected by tillage. Increasing weed management inputs diminished differences between tillage systems. Smooth pigweed dominated the weed populations after 4 yr in both tillage systems even in high input systems.

An overreliance on herbicides in several important grain- and cotton-producing regions of the world has led to the widespread evolution of herbicide-resistant weed populations. Of particular concern are weed populations that exhibit simultaneous resistance to multiple herbicides (MHR). Too often, herbicides are the only tool used for weed control. We use the term herbicide-only syndrome (HOS) for this quasi-addiction to herbicides. Growers and their advisers focus on herbicide technology, unaware of or ignoring basic evolutionary principles or the necessary diversity provided by other methods of weed control. Diversity in weed control practices disrupts resistance evolution. Significant challenges exist to implementing diversity, including how to address information so that producers choose to alter existing behaviors (HOS) and take calculated risks by attempting new and more complex strategies. Herbicide resistance management in the long term will require creativity in many sectors, including roles for growers, industry, researchers, consultants, retailers, and regulators. There can be creativity in herbicide registration and regulation, as exemplified by the recent U.S. Environmental Protection Agency program that encourages herbicide registrants to register products in minor crops. We propose one idea for a regulatory incentive to enable herbicide registrants in jurisdictions such as the United States to receive an extended data exclusivity period in exchange for not developing one new herbicide in multiple crops used together in rotation, or for implementing stewardship practices such as robust mixtures or limitations on application frequency. This incentive would provide a mechanism to register herbicides in ways that help to ensure herbicide longevity. Approaches based only on market or financial incentives have contributed to the current situation of widespread MHR. Our suggestion for regulatory creativity is one way to provide both financial and biological benefits to the registering company and to the overall stakeholder community by incentivizing good resistance management.

Competition of downy brome (Bromus tectorum L.) with seeded perennial grasses is an important factor in the lack of success of many seedings on sagebrush conversion areas in the Great Basin. The seriousness of downy brome competition was emphasized by Stewart and Hull. Their work in Idaho indicated repeated failure in seeding of crested wheatgrass (Agropyron cristatum [L.] Gaertn.) on areas where downy brome was not eliminated or reduced. Hull and Pechanec pointed out that many studies indicate that reduction of dense stands of downy brome is essential for the early establishment of seeded species. They also note that in some cases seeding of perennials has been successful without reduction of this weedy annual grass.

Palmer amaranth is a troublesome weed in cotton production. Yield losses of 65% have been reported from season-long Palmer amaranth competition with cotton. To determine whether water is a factor in this system, experiments were conduced in 2011, 2012, and 2013 in Citra, FL, and in Tifton, GA. In 2011, infrequent rainfall lead to drought stress. The presence of Palmer amaranth resulted in decreased soil relative water content up to 1 m in depth. Cotton stomatal conductance (gs) was reduced up to 1.8 m from a Palmer amaranth plant. In 2012 and 2013 higher than average rainfall resulted in excess water throughout the growing season. In this situation, no differences were found in soil relative water content or cotton gs as a function of proximity to Palmer amaranth. A positive linear trend was found in cotton photosynthesis and yield; each parameter increased as distance from Palmer amaranth increased. Even in these well-watered conditions, daily water use of Palmer amaranth was considerably higher than that of cotton, at 1.2 and 0.49 g H20 cm−2 d−1, respectively. Although Palmer amaranth removed more water from the soil profile, rainfall was adequate to replenish the profile in 2 of the 3 yr of this study. However, yield loss due to Palmer amaranth was still observed despite no change in gs, indicating other factors, such as competition for light or response to neighboring plants during development, are driving yield loss.

Resistance to aryloxyphenoxypropionate and cyclohexanedione herbicides (ACCase inhibitors) was characterized in five green foxtail populations from western Canada. Field histories indicated that these populations were exposed to up to seven applications of diclofop-methyl and sethoxydim over the previous ten years. Major differences occurred among the populations in their levels of resistance and patterns of cross-resistance. With one exception, R/S ratios calculated from GR50 values derived from growth room experiments varied from 2 to 54 depending on the population and herbicide. The exception was UM131 which was at least 75 times more resistant to sethoxydim (R/S > 2981) than the other populations. UM59 was not resistant to sethoxydim. It was also the least resistant to the other herbicides with R/S ratios ranging from 2 for fenoxaprop-p-ethyl to 8 for diclofop-methyl. In comparison, UM8 was resistant to diclofop-methyl, fenoxaprop-p-ethyl, sethoxydim, and tralkoxydim with R/S ratios of 11, 5, > 39, and 27, respectively. In field experiments, shoot dry weights of UM8 treated at two times the recommended rates of these herbicides were reduced by 9, 45, 74, and 49%. In contrast, the susceptible population, UM7, was completely controlled. UM7 and UM8 did not differ in their response to trifluralin, ethalfluralin, quinclorac, propanil and TCA.

Two DNA molecular marker techniques were used to evaluate genetic diversity in 58 accessions of jointed goatgrass and 6 accessions of the related wild species barb goatgrass. Random amplified polymorphic DNA (RAPD) assays were performed on 8 U.S. and 50 Eurasian jointed goatgrass accessions using 30 random decamer primers. The frequency of scorable polymorphic bands within jointed goatgrass was 6 out of 90 (6.7%). Cluster analysis of RAPD data showed small genetic distances (values of 0.005 or less) among jointed goatgrass accessions. To validate the effectiveness of RAPD techniques to detect genetic diversity in tetraploid Aegilops species, six accessions of barb goatgrass were assayed using a subset of 20 decamer primers (from the original 30). RAPD data for barb goatgrass were pooled with jointed goatgrass data from the same primers. A total of 63 scorable bands were generated, of which 27 (43%) were polymorphic between two or more accessions. RAPD analysis readily distinguished between the two species and detected much greater levels of genetic diversity within barb goatgrass than between the jointed goatgrass accessions. Amplified fragment length polymorphism (AFLP) assays were performed on a subset of the 58 jointed goatgrass accessions, 3 U.S. and 13 Eurasian. These accessions were selected to represent a range in geographic diversity within our collection. Ten primer combinations generated 560 scorable bands of which 28 (5%) were polymorphic. Cluster analysis of AFLP data showed a slightly smaller range in genetic distance (0.0002 to 0.0022) among accessions compared with RAPD results; however, AFLPs distinguished among all but 2 of the 16 accessions surveyed. Although AFLP produced more scorable bands than RAPD did, both methods revealed limited genetic diversity in jointed goatgrass.

Nascent research on the profitability of site-specific weed management has focused on reduced herbicide use, ignoring significant information costs for scouting, making treatment maps, and patch herbicide application. Including these information costs results in few, if any, studies, fully covering added costs with herbicide savings. Real-time, sensor-driven site-specific herbicide management promises to overcome many of the scouting and map-making costs so long as weed species recognition accuracy continues to improve and commercial capital costs to decline. Two means to increase revenues are possible from site-specific weed management: value-added crop products (e.g., herbicide-free, nontransgenic crops) and environmental stewardship payments from government programs. Site-specific ecological weed management offers a more fruitful means to garner both higher prices from “green” consumers and revenues for sound environmental stewardship. It entails using spatial tools to enhance habitat for weed predators and diseases and their vectors, as well as to grow crops that are better able to compete with weeds. The most economically promising areas for future weed management will draw upon both spatial and developmental management tools.

In several European nations, including France and Germany, atrazine has been banned because of environmental concerns. However, in Canada, atrazine remains an important component of modern weed control in corn. The objectives of this study were to determine the value of atrazine to corn producers by examining weed control efficacy, yield of corn, adjusted gross return, and the variability associated with PRE and POST herbicides applied alone or in combination with atrazine. A randomized complete-block design experiment was conducted at two locations for 3 yr to evaluate the performance of selected PRE and POST herbicides with and without atrazine. The addition of atrazine to PRE herbicides increased weed control (25%), improved herbicide performance consistency, increased corn yields (8%), increased adjusted gross return (Can$59 ha−1), and reduced risk ($30 ha−1) over sites and years. Although improving weed control, the addition of atrazine to POST herbicides increased the risk of return compared with treatments without atrazine by about $20 ha−1 because the increased cost of atrazine was not always offset by higher corn yields. Our results clearly demonstrate a value of atrazine for broadleaf weed control in corn, both in terms of efficacy and economic return. From our findings, we estimated that the economic benefit of atrazine to Ontario, Canada, corn producers to be at least $26.1 million in 2004. Under current economic pressures facing agricultural producers, our findings show that a balance between the environmental effects and the benefits of atrazine to corn producers must be found because no alternative herbicide with equal economic and agronomic attributes is available at this time. To meet this balance, research on further reducing atrazine use rates while maintaining effective weed control in corn and on developing a sustainable stewardship plan is warranted.

Broadleaf signalgrass is sensitive to nicosulfuron and resistant to primisulfuron, but corn is resistant to both. Research was conducted to determine the effect of varying light level and air temperature on absorption, translocation, and metabolism of nicosulfuron and primisulfuron in broadleaf signalgrass and corn. Corn absorbed between 60 and 85% of the applied nicosulfuron and primisulfuron within 72 h after treatment (HAT), depending on environmental treatment. Absorption, translocation, and metabolism all tended to be more rapid at higher temperature and light intensity. Nicosulfuron and primisulfuron translocation out of the treated leaf was < 4.5% of herbicide absorbed through 72 HAT. Corn rapidly metabolized both herbicides in both environments. However, primisulfuron was metabolized more rapidly (high = 99%, low = 92%) than nicosulfuron (high = 95%, low = 78%). Broadleaf signalgrass absorbed 20% more nicosulfuron than primisulfuron through 72 HAT. Nicosulfuron translocation out of the treated leaf in broadleaf signalgrass was ≤ 15% absorbed through 72 HAT, while primisulfuron translocation was ≤ 4% during the same time period. Primisulfuron metabolism was more rapid than nicosulfuron in broadleaf signalgrass. During the first 4 HAT, broadleaf signalgrass metabolized > 20 times more primisulfuron than nicosulfuron. By 72 HAT, broadleaf signalgrass under conditions of high light and temperature had metabolized nearly 90% of the primisulfuron absorbed but ≤ 7% of the nicosulfuron absorbed was metabolized during the same time. These results suggest that differential activity of nicosulfuron and primisulfuron on broadleaf signalgrass may be based on differential rates of metabolism to nonphytotoxic compounds; uptake and translocation differences agree with the differential broadleaf signalgrass activity. Additionally, environment has the potential to affect rates of sulfonylurea absorption, translocation, and metabolism.

The relative adsorption of trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine), triallate [S-(2,3,3-trichloroallyl)-diisopropylthiocarbamate], and diallate [S-(2,3-dichloroallyl)diisopropylthiocarbamate] was studied on several adsorptive surfaces. The herbicides were strongly adsorbed, with the following general trend: activated charcoal > peat moss > wheat straw (Triticum aestivum L.) ∼ cellulose triacetate > cation exchange resin > anion exchange resin > silica gel ∼ cellulose powder > kaolinite ∼ montmorillonite. Trifluralin and triallate were readily desorbed from montmorillonite by water, with miniminal desorption for peat moss, cellulose triacetate, and wheat straw.

The increasing occurrence of herbicide resistance, along with no new herbicide modes of action developed in over 30 yr, have increased the need for nonherbicidal weed management strategies and tactics. Harvest weed seed control (HWSC) practices have been successfully adopted in Australia to manage problematic weeds. For HWSC to be effective, a high proportion of weed seeds must be retained on the plant at crop maturity. This 2-yr (2014, 2015) study evaluated seed shatter of wild oat, green foxtail, wild mustard, and cleavers in both an early (field pea) and late (spring wheat) maturity crop in field experiments at Scott, Saskatchewan. Seed shatter was assessed using shatter trays collected once a week during crop ripening stage, as well as at two crop maturation or harvest stages (swathing, direct-combining). Seed shatter differed among weed species, but was similar between crops at maturity: ca. 30% for wild oat, 5% for cleavers, < 2% for wild mustard, and < 1% for green foxtail. Overall, seed shatter of wild oat occurred sooner and at greater levels during the growing season compared with the other weed species. Viability of both shattered and plant-retained seeds was relatively high for all species. The small amount of seed shatter of cleavers, wild mustard, and green foxtail suggests that these species may be suitable candidates for HWSC. Due to the amount and timing of wild oat seed shatter, HWSC may not reduce population abundance of this grassy weed.

Yellow nutsedge (Cyperus esculentus L.) and purple nutsedge (C. rotundus L.) were grown in clay pots in the field to investigate the effect of sunlight intensity, planting date, and harvesting date on growth and development. Reducing the length of the growing season by delayed planting or early harvesting reduced the growth (dry matter accumulation) and tuber production of both species. Purple nutsedge growth (dry matter accumulation) was linearly reduced at 30 and 73% shade, but yellow nutsedge growth at 30% shade was not different from that at full sunlight. Tuber production in both species began about August 1, with slight delays in the initiation of tuber growth as planting date was delayed. At the end of the growing season a significant number of tubers were formed in both species even at the latest planting date under 73% shade.

Results from the “Potato Ecosystem Project,” a cropping systems study in northern Maine, were used to test the hypothesis that greater reliance on organic nutrient sources and less reliance on synthetic fertilizer sources can benefit weed management efforts. ‘Atlantic’ potato was grown in a 2-yr rotation within a factorial arrangement of three pest management systems, two soil management systems, and both rotation entry points. Weed control in the conventional (CONV) pest management system relied on full rates of herbicides, whereas the biointensive (BIO) system relied exclusively on cultivation. The reduced input (RI) pest management system relied on cultivation in 1991 and 1992 and on 50% of standard herbicide rates plus cultivation from 1993 to 1995. The two soil management systems, unamended (barley/red clover rotation crop; 1× synthetic fertilizer for potato) and amended (pea/oat/hairy vetch green manure rotation crop; manure, compost, and 0.5× synthetic fertilizer for potato) contrasted practices typical for the region to those designed to achieve rapid improvements in soil quality. Midseason weed biomass in potato was dominated by common lambsquarters. In 1991 and 1992, weed biomass in potato was least in the CONV system and did not differ between the RI and BIO systems. In 1993, weeds in both RI and CONV potatoes were effectively suppressed below the level measured in the BIO system. Soil management had no effect on weed biomass from 1991 to 1993 but became an important factor affecting weeds in the BIO system in 1994 and 1995. Weed biomass was 77% lower in 1994 and 72% lower in 1995 in the amended soil management system than in the unamended system. No significant yield loss due to weeds was detected in the 1994 BIO system, but in 1995 yield loss due to weeds was 37% in the unamended system compared to 12% in the amended system. Soil management effects on weeds in the 1994 BIO pest management system carried through to the following season's germinable seed bank. Density of germinable common lambsquarters seed (0 to 10 cm soil depth) in the 1995 BIO system was 4,082 m−2 in the unamended soil management system compared to 1,280 m−2 in the amended soil management system. We suggest that organic amendments and green manure promote a potato crop better able to compete with weeds and that these inputs be considered as potentially important components of integrated weed management systems that have minimal reliance on herbicides.

Greenhouse studies were conducted to determine the response of purple and yellow nutsedge to selective placement of 5 cm of soil treated with AC 263,222 above and/or below nutsedge tubers. AC 263,222 applied early POST (EPOST) or POST at 71 g ai ha-1 as a foliar, soil, or foliar + soil application also was evaluated. AC 263,222 applied below nutsedge tubers did not reduce purple or yellow nutsedge shoot number, shoot dry weight, shoot regrowth dry weight, or root-tuber dry weight. However, when it was applied above purple nutsedge tubers, shoot dry weight was reduced; and when it was applied above yellow nutsedge tubers, shoot dry weight and root dry weight were reduced. AC 263,222 applied 5 cm above + 5 cm below purple nutsedge tubers reduced shoot number, shoot dry weight, shoot regrowth dry weight, and root dry weight to 9, 4, 10, and 16% of the control, respectively, in purple nutsedge and to 23, 16, 9, and 15% of the control, respectively, in yellow nutsedge. AC 263,222 applied EPOST or POST reduced shoot dry weight, shoot regrowth dry weight, and root dry weight of purple nutsedge to less than or equal to 11, 7, and 27% of the control, respectively, and to less than or equal to 10, 16, and 29% of the control, respectively, for yellow nutsedge with no differences between application methods. The foliar and soil activity of AC 263,222 for purple and yellow nutsedge control provides an advantage over currently registered peanut herbicides.

Showy crotalaria (Crotalaria spectabilis Roth) seed dormancy was due to seed coat impermeability to water. The seed coats became impermeable during later stages of maturation on the plant. When incubated immediately after harvest, 9% of the mature, black seeds (11% water content) imbibed water and germinated. The remaining 91% had impermeable coats and did not germinate. After dry storage for 3 months at 23 C, 24% of the seeds imbibed water and germinated. Seeds of another seed lot, which contained seeds of different maturity stages, attained 47% imbibition and germination after storage for 1 yr. The dry, less mature green seeds had a higher percentage (51%) of permeable seeds than did the black seeds (31%) from the same lot. Several seed coat treatments induced imbibition of water and germination of previously impermeable seeds. Effective treatments included piercing of seed coats, scarification with sandpaper, soaking in 70 C water, and simply applying pressure on the strophiole area. Covering the pressed strophiole with petroleum jelly significantly blocked imbibition and indicated that water entered the seeds at only the pressed area. However, studies indicated natural loss of impermeability in showy crotalaria seeds may occur at other sites as well.

Weed seeds can require an exposure to light for induction of germination. Conducting tillage operations at night and thus preventing the photoinduction of germination has been proposed as a means to reduce weed emergence in agricultural systems. This research was conducted to evaluate night tillage as a weed management option and to determine which tillage operations have the greatest effect on light-mediated recruitment. Weed emergence was evaluated after conducting factorial combinations of day and night moldboard plowing and disking in the springtime from 1992 through 1995. The light environment during disking generally had a slightly greater effect on emergence than the light environment during plowing. Emergence of pigweed species and giant foxtail was, at most, 30 to 55% higher following day vs. night disking. Emergence of other weeds was not affected by the light environment during tillage. We conclude that night tillage may not be a viable approach to weed management due to insufficient reductions in weed emergence associated with night tillage and the high degree of variability in the recruitment response to light conditions during tillage.