Potato is the third most important staple food crop globally following rice and wheat. In the United States, potato is grown on approximately 410,000 ha with a farm-gate value of US$1,032 million. In Canada, potato is grown on approximately 134,000 ha with a farm-gate value of US$235 million. The objective of this manuscript, compiled by the Weed Science Society of America Weed Loss Committee, was to estimate potato yield loss caused by weed interference. Potato yield data from weedy and weed-free plots (or plots with >95% weed control) was obtained from researchers working on weed management in potato in the United States and Canada or from published manuscripts from 2000 to 2018. Potato yield loss from weed interference was 12% to 61% when no weed management tactics were implemented. The average yield loss for all states/provinces (where data was obtained) due to weed interference was 44%. Weed interference would cause a farm-gate loss of approximately US$465 million and US$61 in the United States and Canada, respectively, if weeds are not controlled. These results indicate that weed management is critical for successful potato production, and that an ongoing need for research exists on weed management in this crop.

Tolpyralate is a new 4-hydroxyphenyl-pyruvate dioxygenase (HPPD)–inhibiting herbicide for weed control in corn. Previous research has reported efficacy of tolpyralate + atrazine on several annual grass and broadleaf weed species; however, no studies have evaluated weed control of tolpyralate + atrazine depending on time-of-day (TOD) of application. Six field experiments were conducted over a 2-yr period (2018, 2019) near Ridgetown, ON, to determine if there is an effect of TOD of application on tolpyralate + atrazine efficacy on common annual grass and broadleaf weeds. An application was made at 3-h intervals beginning at 06:00 h with the last application at 24:00 h. There was a slight TOD effect on velvetleaf, pigweed species, and common ragweed control with tolpyralate + atrazine; however, the magnitude of change throughout the day was ≤3% at 2, 4, or 8 wk after application (WAA). There was no effect of TOD of tolpyralate + atrazine on the control of lambsquarters, barnyardgrass, and green foxtail. All weed species were controlled ≥88% at 8 WAA. There was no effect of TOD of tolpyralate + atrazine application on corn yield. Results of this study show no evidence of a TOD effect on weed control efficacy with tolpyralate + atrazine.

Cover crop residue can act as a mulch that will suppress weeds, but as the residue degrades, weed suppression diminishes. Biomass of cover crop residue is positively correlated to weed suppression, but little research is available regarding the composition of cover crop residue and its effect on weed suppression. Field experiments were conducted to determine the impact of cover crop residue properties (i.e., total carbon, total nitrogen, lignin, cellulose, and hemicellulose) on summer annual weed suppression and cash crop yield. Cover crop monocultures and mixtures were planted in the fall and designed to provide a range of biomass and residue properties. Cover crops were followed by corn (Zea mays L.) or soybean [Glycine max (L.) Merr.]. At termination, cover crop biomass and residue components were determined. Biomass ranged from 3,640 to 8,750 kg ha−1, and the carbon-to-nitrogen (C:N) ratio ranged from 12:1 to 36:1. As both cover crop biomass and C:N ratio increased, weed suppression and duration of suppression increased. For example, a C:N ratio of 9:1 is needed to suppress redroot pigweed (Amaranthus retroflexus L.) 50% at 4 wk after termination (WAT), and that increases to 16:1 and 20:1 to have 50% suppression at 6 and 8 WAT, respectively. Similarly, with biomass, 2,800 kg ha−1 is needed for 50% A. retroflexus suppression at 4 WAT, which increases to 5,280 kg ha−1 and 6,610 kg ha−1 needed for 50% suppression at 6 and 8 WAT, respectively. In general, similar trends were observed for pitted morningglory (Ipomoea lacunosa L.) and large crabgrass [Digitaria sanguinalis (L.) Scop.]. Corn and soybean yield increased as both cover crop biomass and C:N ratio increased where no weed control measures were implemented beyond cover crop. The same trend was observed with cash crop yield in the weed-free subblocks, with one exception. This research indicates that cover crop residue composition is important for weed control in addition to biomass.

Tolpyralate is a new Group 27 pyrazolone herbicide that inhibits the 4-hydroxyphenyl-pyruvate dioxygenase enzyme. In a study of the biologically effective dose of tolpyralate from 2015 to 2017 in Ontario, Canada, tolpyralate exhibited efficacy on a broader range of species when co-applied with atrazine; however, there is limited published information on the efficacy of tolpyralate and tolpyralate+atrazine relative to mesotrione and topramezone, applied POST with atrazine at label rates, for control of annual grass and broadleaf weeds. In this study, tolpyralate applied alone at 30 g ai ha−1 provided >90% control of common lambsquarters, velvetleaf, common ragweed, Powell amaranth/redroot pigweed, and green foxtail at 8 weeks after application (WAA). Addition of atrazine was required to achieve >90% control of wild mustard, ladysthumb, and barnyardgrass at 8 WAA. Tolpyralate+atrazine (30+1,000 g ai ha−1) and topramezone+atrazine (12.5+500 g ai ha−1) provided similar control at 8 WAA of the eight weed species in this study; however, tolpyralate+atrazine provided >90% control of green foxtail by 1 WAA. Tolpyralate+atrazine provided 18, 68, and 67 percentage points better control of common ragweed, green foxtail, and barnyardgrass, respectively, than mesotrione+atrazine (100+280 g ai ha−1) at 8 WAA. Overall, tolpyralate+atrazine applied POST provided equivalent or improved control of annual grass and broadleaf weeds compared with mesotrione+atrazine and topramezone+atrazine.

Tolpyralate is a new 4-hydroxyphenyl-pyruvate dioxygenase (HPPD)-inhibiting herbicide for POST weed management in corn; however, there is limited information regarding its efficacy. Six field studies were conducted in Ontario, Canada, over 3 yr (2015 to 2017) to determine the biologically effective dose of tolpyralate for the control of eight annual weed species. Tolpyralate was applied POST at six doses from 3.75 to 120 g ai ha−1 and tank mixed at a 1:33.3 ratio with atrazine at six doses from 125 to 4,000 g ha−1. Regression analysis was performed to determine the effective dose (ED) of tolpyralate, and tolpyralate+atrazine, required to achieve 50%, 80%, or 90% control of eight weed species at 1, 2, 4, and 8 wk after application (WAA). The ED of tolpyralate for 90% control (ED90) of velvetleaf, common lambsquarters, common ragweed, redroot pigweed or Powell amaranth, and green foxtail at 8 WAA was ≤15.5 g ha−1; however, tolpyralate alone did not provide 90% control of wild mustard, barnyardgrass, or ladysthumb at 8 WAA at any dose evaluated in this study. In contrast, the ED90 for all species in this study with tolpyralate+atrazine was ≤13.1+436 g ha−1, indicating that tolpyralate+atrazine can be highly efficacious at low field doses.

The nonionic surfactants Tween 20 [oxysorbic (20 POE) polyoxyethylene sorbitan monolaurate] and Tween 80 [oxysorbic (20 POE) polyoxyethylene sorbitan monooleate] at concentrations of 0.05, 0.1, 0.2, and 0.4% v/v stimulated germination of barnyardgrass [Echinochloa crus-galli (L.) Beauv. # ECHCG] seeds in petri dishes. Stimulation occurred under both 30/20 and 25/15 C diurnal temperature cycles whether or not light was supplied during the 8-h high-temperature portion of the cycle. Barnyardgrass responded most to treatment under environmental conditions in which untreated seeds germinated the least. A 0.1% surfactant concentration often stimulated germination of weed seeds as effectively as did higher concentrations. Germination of redroot pigweed [Amaranthus retroflexus L. # AMARE] seeds was inhibited by Tween 80 in the light under both the low- and high-temperature regimes and by Tween 20 at low temperature in the light. Germination of tumble pigweed (Amaranthus albus L. # AMAAL) seeds was inhibited by both surfactants in the high-temperature regime whether or not light was supplied. Common purslane (Portulaca oleracea L. # POROL) seeds were insensitive to treatment.

Field studies were conducted from 1986 through 1989 to determine the feasibility of using clomazone in a winter wheat-corn-fallow rotation. Clomazone at 1.1 kg ai/ha did not control emerged downy brome in autumn nor did it control redroot pigweed and tumble pigweed that emerged after a spring application of paraquat at 0.6 kg ai/ha. However, clomazone plus atrazine at 0.6 plus 2.2 kg ai/ha provided weed control in winter wheat stubble from October to corn harvest 12 mo later. Corn yields from this treatment were equal to or greater than the conventional practice of applying atrazine plus glyphosate at 2.2 plus 0.4 kg ae/ha in the fall followed by a spring application of metolachlor plus dicamba at 2.8 ai plus 0.3 kg ae/ha. When kochia densities were high, clomazone plus atrazine controlled triazine-resistant kochia better than metolachlor plus dicamba. Atrazine at 2.2 kg/ha applied in autumn followed by clomazone at 0.8 kg/ha in April generally caused more corn injury than when the same rate of clomazone was applied in autumn with the atrazine. The percent of bleached corn plants varied with planter, year, time of application, and rate of clomazone applied. The optimum time to apply clomazone at 0.6 kg/ha was in autumn with atrazine. This treatment caused some corn bleaching but produced the highest corn yield over 3 yr. In a dry autumn and spring, less clomazone injury occurred with a planter equipped with row cleaners than those without. The injury pattern associated with planters was inconsistent when precipitation was average and no injury occurred in a wet autumn and spring.

Perception of light by phytochrome is a mechanism that triggers weed seed germination in response to soil disturbance. Photoconversion of phytochrome from the red light absorbing form to the active far-red absorbing form depends on hydration of phytochrome. This research was conducted to determine the soil water threshold for the photoinduction of germination by the brief exposure of light that occurs during soil disturbance, and to determine how this threshold is affected by the fluence of the light stimulus and fluence sensitivity of the seed population. Redroot pigweed seedling emergence and germination response to red light (R) was studied for a range of water potentials. Water potential gradients were established by incubating seeds in soils wetted to various water contents, or in polyethylene glycol 8000 (PEG) solutions. After imposing the light treatments, seeds were returned to a fully hydrated state. Seedling emergence in response to R increased as the volumetric water content (θv) of soils increased. At volumetric water contents of 4.0%, R-induced seedling emergence was inhibited 50% compared to photoinduced seedling emergence at the highest soil water contents tested. Attenuation of photoinduction was more pronounced at low vs. high R fluences in freshly imbibed seeds, but was unaffected in seeds that exhibited enhanced fluence sensitivity. In ecosystems where seasonal soil moisture extremes are prevalent, the photoinduction of seed germination may be limited in dry microsites such as surface crusts or under extreme drought conditions.

Percent germination and germination rates of seeds of atrazine (6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine)-resistant and -susceptible biotypes of redroot pigweed (Amaranthus retroflexus L. # AMARE) and Powell amaranth (A. powellii S. Wats. # AMAPO) from southern Ontario were compared in petri dishes using a 100-cell germination plate. The plate provided 100 separately controlled, alternating temperature combinations ranging from 0 to 45 C in increments of 5 C. The experiment was conducted under alternating light and dark conditions for 21 days and under constant dark conditions for 7 days. Initial seed viabilities exceeded 90% for all four biotypes. Under alternating light and darkness, the resistant biotype of Powell amaranth had significantly lower total germination and a slower rate of germination than the susceptible biotype, whereas the biotypes of redroot pigweed did not differ in germination response. Germination rate and total germination were positively correlated for all four biotypes. In constant darkness, the atrazineresistant biotypes of both species had significantly lower germination percentages compared to the susceptible biotypes. In general, percent germination of Powell amaranth was greater than that of redroot pigweed for day and night temperatures <25 C, whereas redroot pigweed had greater germination at higher temperatures. Greater seed dormancy of the resistant biotypes under some light and temperature combinations may have implications for the persistence of resistant populations in the field.

The performance of a mechanistic simulation model of crop-weed competition was tested with data on the critical period of weed competition in sugarbeets and both seeded and transplanted tomatoes. In general, there was good agreement between simulated and observed yields for different periods of weed interference in each crop. The model was then used to evaluate the influence of weed density, weed height, and weather conditions on timing of the critical period. Simulations suggested that the greater the weed density, the shorter the period of time that the crop could tolerate early-season competition, and the longer the period of time that the crop must be kept weed free to prevent yield losses. Simulations also suggested that the length of time that a crop can tolerate early-season weed competition is related more to the availability of soil moisture, or possibly essential nutrients, than to light limitations.

Imazethapyr, alone and in combination with other herbicides, was applied PPI, PRE, and POST to pinto beans to determine weed control and selectivity to the crop. All of the herbicides improved pinto bean yield as compared with the unweeded control. Imazethapyr applied PPI and POST provided excellent control of black nightshade, kochia, Russian thistle, prostrate pigweed, and redroot pigweed. Barnyardgrass control with imazethapyr ranged from 58 to 96% and increased to 98% or greater when imazethapyr was combined with metolachlor, pendimethalin, trifluralin, or EPTC. Pinto bean yield was not reduced from any herbicide treatment compared to the handweeded control.

Organic soil amendments are known to affect the composition and density of annual weed communities. The objective of this research was to measure the effect on emergence and growth of redroot pigweed seedlings when soil was amended with composted dairy manure at 18, 36, and 54 T/ha, or with raw dairy manure at 41, 82, and 123 T/ha. Data recorded (1) seedling emergence over 12 days, (2) number of leaves and total leaf area, (3) shoot and root dry weight, and (4) seed number. Maximum seedling emergence (32%) occurred in nonamended soil (the control). Emergence declined in a linear fashion when soil was amended with manure or with compost. Compost additions affected seedling emergence more severely than did manure additions. For every measure of redroot pigweed growth except seed production, amendment with manure at 123 T/ha retarded growth compared to soil alone or compost-amended mixes. Manure applied at 82 T/ha reduced leaf area and plant height relative to other treatments. Growth of redroot pigweed in soil amended with compost at 36 and 54 T/ha was always equal to or greater than growth in soil that was not amended. Seed production in one of two runs of the experiment was more than double in soils amended with compost at 36 and 54 T/ha compared to the nonamended soil. These results suggest that amending soils with raw dairy manure may decrease the competitiveness of redroot pigweed, whereas amending with composted manure is likely to increase competitiveness.

Tenuazonic acid (TeA), a naturally occurring product of Alternaria alternata, a pathogen to croftonweed, was discovered to be a novel natural photosystem II (PSII) inhibitor. However, herbicidal activity of AAC-toxin, a metabolite of this fungus containing TeA as the main active ingredient, has not been evaluated systematically. In this study, we conducted activity-evaluation experiments in the laboratory, greenhouse, and field trials to assess the herbicidal potential of this fungal metabolite. AAC-toxin had high herbicidal activity on all species tested: croftonweed, large crabgrass, barnyardgrass, redroot pigweed, and eclipta. The AAC-toxin caused brown, leaf spot symptoms and leaf necrosis, subsequently killing the seedlings. When AAC-toxin was applied POST at 83 ml ai/ha, more than 95% of large crabgrass, barnyardgrass, and redroot pigweed plants were controlled 2 d after treatment in field trials. It can be concluded that AAC-toxin has broad-spectrum, rapid, and high herbicidal activity similar to that of paraquat and may have the potential to be developed as a microbe-based herbicide.

Glyphosate-resistant (GR) Palmer amaranth has become a serious pest in parts of the Cotton Belt. Some GR cotton cultivars also contain the WideStrike™ insect resistance trait, which confers tolerance to glufosinate. Use of glufosinate-based management systems in such cultivars could be an option for managing GR Palmer amaranth. The objective of this study was to evaluate crop tolerance and weed control with glyphosate-based and glufosinate-based systems in PHY 485 WRF cotton. The North Carolina field experiment compared glyphosate and glufosinate alone and in mixtures applied twice before four- to six-leaf cotton. Additional treatments included glyphosate and glufosinate mixed with S-metolachlor or pyrithiobac applied to one- to two-leaf cotton followed by glyphosate or glufosinate alone on four- to six-leaf cotton. All treatments received a residual lay-by application. Excellent weed control was observed from all treatments on most weed species. Glyphosate was more effective than glufosinate on glyphosate-susceptible (GS) Palmer amaranth and annual grasses, while glufosinate was more effective on GR Palmer amaranth. Annual grass and GS Palmer amaranth control by glyphosate plus glufosinate was often less than control by glyphosate alone but similar to or greater than control by glufosinate alone, while mixtures were more effective than either herbicide alone on GR Palmer amaranth. Glufosinate caused minor and transient injury to the crop, but no differences in cotton yield or fiber quality were noted. This research demonstrates glufosinate can be applied early in the season to PHY 485 WRF cotton without concern for significant adverse effects on the crop. Although glufosinate is often less effective than glyphosate on GS Palmer amaranth, GR Palmer amaranth can be controlled with well-timed applications of glufosinate. Use of glufosinate in cultivars with the WideStrike trait could fill a significant void in current weed management programs for GR Palmer amaranth in cotton.

The difficulty of measuring fecundity of weeds with shatter-prone seeds can be overcome by postanthesis application of nontoxic glues to seedheads. Such glues reduced seed losses of redroot pigweed to <5% of total seed production compared with 12 to 14% for nontreated plants.

Field experiments were conducted in grain sorghum at five locations in Kansas in 2009 and 2010, to evaluate the efficacy and crop safety of early- to mid-POST (EMPOST) and late-POST (LPOST) applications of premixed pyrasulfotole and bromoxynil (PYRA&BROM) in tank mix combinations with atrazine or atrazine plus 2,4-D ester or dicamba compared to bromoxynil plus atrazine. PYRA&BROM at 244 or 300 g ai ha−1 plus atrazine at 560 g ai ha−1 applied EMPOST controlled pigweed species (Palmer amaranth, tumble pigweed, and redroot pigweed), kochia, velvetleaf, common sunflower, ivyleaf morningglory, and common lambsquarters 93% or greater. Puncturevine control among three locations ranged from 85 to 99%. Control of most weed species was not improved by increasing PYRA&BROM rate from 244 to 300 g ha−1 or by tank mixing 2,4-D or dicamba with PYRA&BROM plus atrazine. However, ivyleaf morningglory control was improved at the LPOST timing by adding 2,4-D or dicamba at 140 g ae ha−1. In no instance did any PYRA&BROM treatment provide greater weed control than bromoxynil plus atrazine at 281 + 560 g ha−1 when applied EMPOST, but in most instances PYRA&BROM treatments were more effective than bromoxynil plus atrazine when applied LPOST. Generally, PYRA&BROM treatments were more effective when applied EMPOST than LPOST, especially when 2,4-D or dicamba was added. PYRA&BROM plus atrazine treatments caused foliar bleaching in sorghum within 7 ± 3 d after treatment, but recovery was complete within 3 to 4 wk and grain yields were not reduced. Tank mixing dicamba with PYRA&BROM and atrazine occasionally reduced visible crop response compared to PYRA&BROM plus atrazine. Our results indicate that PYRA&BROM plus atrazine with or without 2,4-D or dicamba selectively controls several troublesome broadleaf weeds in grain sorghum. Foliar bleaching of sorghum leaves can occur but the symptoms are transient, and grain yields are not likely to be reduced.

Competition between crops and weeds may be stronger at the root than at the shoot level, but belowground competition remains poorly understood, due to the lack of suitable methods for root discrimination. Using a transgenic maize line expressing green fluorescent protein (GFP), we nondestructively discriminated maize roots from weed roots. Interactions between GFP-expressing maize, common lambsquarters, and redroot pigweed were studied in two different experiments with plants arranged in rows at a higher plant density (using boxes with a surface area of 0.09 m2) and in single-plant arrangements (using boxes with a surface area of 0.48 m2). Root density was screened using minirhizotrons. Relative to maize that was grown alone, maize root density was reduced from 41 to 87% when it was grown with redroot pigweed and from 27 to 73% when it was grown with common lambsquarters compared to maize grown alone. The calculated root : shoot ratios as well as the results of shoot dry weight and root density showed that both weed species restricted root growth more than they restricted shoot growth of maize. The effect of maize on the root density of the weeds ranged from a reduction of 25% to an increase of 23% for common lambsquarters and a reduction of 42 to 6% for redroot pigweed. This study constitutes the first direct quantification of root growth and distribution of maize growing together with weeds. Here we demonstrate that the innovative use of transgenic GFP-expressing maize combined with the minirhizotron technique offers new insights on the nature of the response of major crops to belowground competition with weeds.

Field experiments were conducted at Oakes, Absaraka, and Tappen, ND, in 2006 and repeated at Oakes and Absaraka, ND, in 2007 to evaluate early season weed control of common lambsquarters and redroot pigweed in onion with POST herbicides applied at multiple reduced rates (microrates) and to determine whether microrate herbicide treatments effectively reduced early season broadleaf weed competition, caused crop injury, or affected yield. Application rates of bromoxynil, oxyfluorfen, metribuzin, and acifluorfen were reduced to 0.25, 0.13, and 0.06× of their lowest labeled rate and applied in sequential applications (every 7 d) either two or three times. The 0.25× rate of bromoxynil (70.1 g ae/ha) provided the greatest control of common lambsquarters (95%). The 0.25× rates of bromoxynil and oxyfluorfen (70.1 g ai/ha) provided the greatest control of redroot pigweed (93 and 85%, respectively). Microrate applications of metribuzin or acifluorfen did not effectively control common lambsquarters or redroot pigweed. In 2006, no onion injury was observed. However, in 2007, applications of oxyfluorfen resulted in approximately 15% injury, regardless of the herbicide rate or the number of applications. Plants outgrew symptoms by 4 wk after treatment and were similar to the untreated plants. Onion treated with oxyfluorfen had the greatest total yield, followed by onion treated with bromoxynil. Onion treated with acifluorfen had a greater total marketable bulb yield than onion treated with metribuzin, but yield was considered poor compared to the other herbicide treatments. Three microrate applications provided greater weed control and increased yield compared with two applications across herbicides and rates. Results suggest that microrate applications of bromoxynil and oxyfluorfen will provide early season broadleaf weed control in onion.

Weed management could be more efficient and require less herbicide if growers could afford to estimate the composition, density, and distribution of weed seed banks. Spatial distribution of a weed seed bank will affect the accuracy of both mean estimates and interpolated maps of density. Consequently, information about the general characteristics of spatial distributions of seeds in a seed bank is needed to identify the most efficient strategies for sampling. Seed banks were sampled on 8.4-m square grids in eight irrigated corn fields to identify the common features of distributions of seed banks of annual weeds. Spatial dependence was described with correlograms for four to eight species in each field. Spatial dependence was detected for 36 of 45 distributions, and seed counts were correlated to an average distance of 25 to 150 m for a distribution. Seed banks of different species and fields had common features of spatial correlation: spatial pattern accounted for less than half of the total variability of seed counts, spatial correlation decreased rapidly over short distances, and ranges of spatial dependence varied with direction. For half of the distributions, the maximum range of spatial dependence was at least twice as long as the minimum range. Seed counts were correlated for the longest distances in the direction of the crop row for 16 distributions, and the distance was longer in the direction of the crop row than across rows for 26 of the 36 samples. Researchers should be able to design more efficient sampling plans for growers if the common features of spatial dependence are considered. For seed banks like these, the accuracy of maps and estimates of seed bank density may be improved by collecting multiple cores around each sampling location to mitigate the effect of short-scale spatial variability. In addition, sampling may be more efficient with grids and interpolation methods that account for ranges that are 1.5 to 2 times longer in the direction of the crop row than perpendicular to the row. With a 55- by 30-m sampling grid, adjacent observations would be correlated, and maps could be made for 80% of these seed banks. More closely spaced observations would be needed to describe the rapid decline in spatial correlation with distance for a more accurate or finer-scale map. Whether sampling seed banks for making management decisions will be cost-effective is not clear. However, potential methods to sample and map seed bank distributions more efficiently have not been exhausted.