In field studies in 1991, 1992, and 1993, rimsulfuron at 26 and 35 g ai/ha, sequentially at 26 g/ha, at 26 g/ha plus metribuzin at 280 g ai/ha, and metribuzin at 280 g/ha were evaluated POST for weed control in transplanted ‘Agriset’ tomato. Common lambsquarters was controlled by rimsulfuron at 35 g/ha. Rimsulfuron plus metribuzin gave consistent control of common ragweed, but jimsonweed control was inconsistent and goosegrass control was generally low. Rimsulfuron treatments caused slight (< 12%) temporary injury to new terminal growth of tomato. Yield of tomato fruit was consistently high in the metribuzin, metribuzin plus rimsulfuron, and rimsulfuron sequential treatments. In greenhouse studies, giant foxtail and large crabgrass control by rimsulfuron was above 95 and 85%, respectively, but goosegrass was not controlled. Height of four tomato cultivars was not reduced, but dry weight of ‘Floradade’ and ‘Sunbeam’ was reduced by rimsulfuron.

Little information is published related to seed germination and seedling emergence of Japanese foxtail, a troublesome annual grass weed widely distributed in winter wheat fields in China. Three Japanese foxtail populations were studied under laboratory and greenhouse conditions, to determine the effects of different environmental factors on seed germination or seedling emergence. Chemical control is absolutely necessary in integrated management, and efficacy of POST herbicides against different growth stages of Japanese foxtail was evaluated. Germination rate was 90% or more when temperature ranged from 5 to 25 C, with germination onset shortened as temperature increased. Light was not required for germination to occur. For pH values ranging from 5 to10 there was no effect on seed germination. Japanese foxtail seed germination was sensitive to osmotic stress and completely inhibited at an osmotic potential of -1.1 MPa. The ‘1513’ population of Japanese foxtail demonstrated tolerance to soil salinity, with 98% germination at 80 mM NaCl compared with 25 and 40% germination for populations ‘1532’ and ‘1544’, respectively. High amounts of crop residue (10 t ha−1) suppressed Japanese foxtail emergence 38 to 55%. Germination of seeds placed at 160 C for 5 min was completely inhibited for dry seeds, with a similar effect at 130 C for pre-soaked seeds. Seed burial in the soil from 0 to 4 cm had no effect on seedling emergence, but burial at 7 cm completely inhibited seedling emergence. POST herbicides mesosulfuron-methyl (13.5 g ai ha−1), clodinafop-propargyl (67.5 g ai ha−1), pyroxsulam (13.5 g ai ha−1), pinoxaden (67.5 g ai ha−1) and isoproturon (1125 g ai ha−1) reduced plant dry weight 80% or more when applied at three- to seven-leaf stage, but control declined with application at later growth stages. The information from this study helps to develop an integrated approach to Japanese foxtail management.

Cover crops (CCs) play an important role in integrated weed management. Data necessary to evaluate the role of CCs in weed management at the watershed scale with topographic positions are lacking. We evaluated the effects of cereal rye and hairy vetch CCs on weed suppression at different topographic positions (shoulder, backslope, and footslope) at a watershed scale. Watersheds with a CC treatment followed a crop rotation of corn–cereal rye–soybean–hairy vetch, whereas watersheds without a CC (no-CC) had a crop rotation of corn–winter fallow–soybean–winter fallow. A negative relationship was present between CCs and weed biomass at the shoulder, backslope, and footslope topographic landscape positions, with R2 values of 0.40, 0.48, and 0.50, respectively. In 2016, a cereal rye CC reduced weed biomass 46% to 50% at footslope and shoulder positions compared to no CC. In 2018, a cereal rye CC reduced weed biomass between 52% and 85% at all topographic positions in CC treatment watersheds compared to no-CC watersheds. Hairy vetch in 2017 reduced weed biomass 62% to 72% at footslope and shoulder topographic positions in CC watersheds compared to no-CC. The C:N ratio of weed biomass in CC treatment watersheds was generally higher compared to watersheds without CCs. In this study, several significant interactions were found between the topographic positions and CC treatments. Cover crop–induced weed suppression at different topographic positions can lead to developing better site-specific weed control strategies. Therefore, CC interactions with topography, weed germination potential, and the role of soil moisture at the watershed scale should be further evaluated.

Evolution of multiple herbicide–resistant Palmer amaranth warrants the development of integrated strategies for its control in the southcentral Great Plains (SGP). To develop effective control strategies, a better understanding of the emergence biology of Palmer amaranth populations from the SGP region is needed. A common garden study was conducted in a no-till (NT) fallow field at the Kansas State University Agricultural Research Center near Hays, KS, during the 2018 and 2019 growing seasons, to determine the emergence pattern and periodicity of Palmer amaranth populations collected from the SGP region. Nine Palmer amaranth populations collected from five states were included: Colorado (CO1, CO2), Oklahoma (OK), Kansas (KS1, KS2), Texas (TX), and Nebraska (NE1, NE2, NE3). During the 2018 growing season, the CO1 and KS1 populations displayed more rapid emergence rates, with greater parameter b values (−5.4, and −5.3, respectively), whereas the TX and NE3 populations had the highest emergence rates (b = −12.2) in the 2019 growing season. The cumulative growing degree days (cGDD) required to achieve 10%, 50%, and 90% cumulative emergence ranged from 125 to 144, 190 to 254, and 285 to 445 in 2018; and 54 to 74, 88 to 160, and 105 to 420 in the 2019 growing season across all tested populations, respectively. The OK population exhibited the longest emergence duration (301 and 359 cGDD) in both growing seasons. All tested Palmer amaranth populations had a peak emergence period between May 11 and June 8 in 2018, and April 30 and June 1 in the 2019 growing season. Altogether, these results indicate the existence of differential emergence pattern and peak emergence periods of geographically distant Palmer amaranth populations from the SGP region. This information will help in developing prediction models for decision-making tools to manage Palmer amaranth in the region.

A three-year experiment was conducted near Scottsbluff, NE, to evaluate the selectivity of POST-applied imazethapyr for weed control in alfalfa, birdsfoot trefoil, cicer milkvetch, red clover, sainfoin, and yellow sweetclover. Imazethapyr injured all legumes 15 DAT. Imazethapyr also reduced the height of birdsfoot trefoil, cicer milkvetch, red clover, and yellow sweeclover 28 DAT. Legume first cutting yields were not reduced by imazethapyr and with alfalfa, birdsfoot trefoil, cicer milkvetch, and sainfoin, first cutting yields were increased by imazethapyr. Imazethapyr reduced weed biomass in all legume seedings. Weed biomass in new seedings of alfalfa was reduced more than that of the other legumes.

Field experiments were conducted at three locations in 2000 and 2001 to evaluate what effect soil characteristics have on sensitivity of two corn hybrids to soil-applied isoxaflutole plus flufenacet. Soil types were a Flanagan silt loam with 3.9% organic matter (OM), a Drummer silty clay loam with 5.0% OM, and a Cisne silt loam with 2.1% OM. Soil pH was adjusted to four target levels, < 6.0, 6.0 to 6.5, 6.6 to 7.0, and > 7.0, at each location. Isoxaflutole plus flufenacet treatments consisted of the recommended rate (1 ×) and two times (2 ×), and four times (4 ×) the recommended rates based on soil type. Additional treatments included 1 × and 2 × isoxaflutole plus flufenacet with 1.1 kg/ha atrazine and S-metolachor plus atrazine as a control treatment. Corn hybrid ‘Garst 8366’ (G8366) was more sensitive to soil-applied isoxaflutole plus flufenacet compared with ‘Garst 8600’ (G8600). Corn injury occurred only at two of three locations. Addition of atrazine to the 1 × and 2 × isoxaflutole plus flufenacet treatments showed a trend (P = 0.15) toward increased injury and reduced yield only for the G8366 on the Cisne soil. The 4 × rate of the premixture reduced yield 9 to 49% at two locations. Yields were reduced at the low-OM (2.1%) location, regardless of hybrid or application rate. At two locations, there was a negative linear relationship between soil pH and corn grain yield, particularly at the 4 × rate. Corn visual injury and yield reductions were greatest from herbicide applications to low-OM soils with high pH levels.

At 14 d after treatment (DAT), glufosinate-resistant CPRS PB-13 rice was injured with early- and late-season glufosinate applications in 1998 and 1999. At 35 DAT, injury was less than 5%. Glufosinate delayed CPRS PB-13 heading by 7 to 15 d and reduced plant height at harvest but did not affect grain moisture and yield compared with the nontreated CPRS PB-13. But yield was reduced compared with conventional Cypress. At 14 DAT, glufosinate-resistant BNGL HC-11/62 rice was injured with early- and late-season applications. Treated BNGL HC-11/62 reached 50% heading 3 to 5 d later than did the nontreated. An application of glufosinate reduced plant height and increased grain moisture compared with the nontreated. Rice treated at the three- to five-leaf, preboot, and boot timings resulted in reduced yields compared with the nontreated BNGL HC-11/62.

Cloransulam postemergence (POST), diclosulam preemergence (PRE), and flumetsulam preplant incorporated (PPI) and POST were evaluated in six trials at two locations for control of sicklepod and pitted morningglory in soybean. Sicklepod control with cloransulam plus flumetsulam POST was equivalent to chlorimuron POST in seven of eight comparisons both 3 and 6 wk after treatment. Sicklepod control with cloransulam POST was equivalent to chlorimuron in only three of eight comparisons 3 wk after treatment, but late-season control was equivalent in five of six comparisons. Pitted morningglory control with cloransulam alone or in tank-mixture with flumetsulam POST was equivalent to chlorimuron in all comparisons. Control of sicklepod and pitted morningglory was greater in most comparisons when a POST application followed flumetsulam plus trifluralin PPI compared to only trifluralin PPI. In a total PRE stale seedbed system, where all treatments were tank-mixed with pendimethalin plus glyphosate, sicklepod control with all rates of flumetsulam and 26 or 35 g ai/ha diclosulam was equivalent to the standard treatment of imazaquin or metribuzin plus chlorimuron. None of the total PRE programs controlled sicklepod as well as when glyphosate was applied sequentially POST. Pitted morningglory was controlled 83 to 93% with 26 or 35 g/ha diclosulam, equivalent to imazaquin, metribuzin plus chlorimuron, or sequential glyphosate applications 8 wk after the PRE application. Increasing flumetsulam rate increased pitted morningglory control early season, but flumetsulam was not as effective as the other herbicides.

Field mint is widespread on Prince Edward Island and has become a problem in the production of cereal grains, potatoes, and other crops. Studies were conducted to determine if deep burial of the rhizome could be used as a control method. Rhizome pieces measuring 2.5, 5.0, or 10.0 cm in length were hand-buried in soil at depths of 2.5, 5.0, 10.0, or 20.0 cm in the spring. As planting depth increased, shoot emergence was delayed and fewer shoots emerged throughout the season. The smallest rhizome of 2.5 cm produced the fewest shoots during the season, but larger rhizomes (5.0 or 10.0 cm long) produced greater amounts of shoot and rhizome dry weights than smaller rhizomes. Deep burial of field mint to 20.0 cm by inversion plowing could be used as a means of field mint control, especially in early planted competitive crops that close canopy quickly.

In the summer of 1995, green foxtail seed samples were collected from 17 fields in southwestern Manitoba where trifluralin resistance was first identified in 1988. Since 1988, six of these fields had not been treated with dinitroaniline herbicides (DNAHs), four had been treated once, and seven had been treated two or more times. The proportions of resistant (R) and susceptible (S) seeds in each sample were determined using a petri-dish seed bioassay. Among the samples collected from fields that had not been treated between 1988 and 1995, two contained > 99% R seeds. The others ranged from 40 to 83%. Among the four samples collected from fields that were treated once, two contained > 99% R seeds and the other two contained 59 and 67% R seeds. Five of the seven samples collected from fields treated two or more times contained > 97% R seeds and the other two approximately 89 and 68% resistant seeds. There was no statistical correlation between the proportion of R seeds and the frequency of DNAH use since 1988. The proportion of R seeds present in 1988 and 1995 could not be compared because of the way in which fields were treated prior to sampling in the 2 yr; however, the results clearly indicate that trifluralin resistance can persist in green foxtail populations for at least 7 yr. By inference, the results indicate that trifluralin-resistant green foxtail plants are almost, if not equally, as fit as susceptible plants. Consequently, once resistance is present in a green foxtail population, DNAHs cannot be used for its control, even in the absence of additional selection pressure, for at least 7 yr and probably much longer.

Differential response of wild carrot to 2,4-D was found in seeds collected from 10 locations in Michigan, three in Ohio, one in Illinois, and one in Ontario, Canada. Greenhouse studies were conducted on plants grown from the collected seeds to confirm resistance of wild carrot to 2,4-D, and to study variations among and within populations. The differential response of wild carrot to 2,4-D in field research was due to resistant individuals. Among the 14 locations, wild carrot control with 2,4-D ranged from 18 to 91%. Wild carrot varied in its response to 2,4-D among and within populations as well as within individual umbels. In 69% of the tested samples, at least one wild carrot plant was resistant to 2,4-D.

Field experiments were conducted in 1999 and 2000 to evaluate early POST (EPOST) and late POST (LPOST) control of common ragweed and giant foxtail with mesotrione at 70, 105, and 140 g ai/ha alone and in mixtures with glufosinate at 300 g ai/ha in glufosinate-resistant corn. Glufosinate-resistant corn injury was frequently higher with mixtures of mesotrione plus glufosinate than with mesotrione applied alone. Mixtures of mesotrione with glufosinate applied EPOST injured corn 6 to 21% in 1999, but in 2000, injury from mixtures was 23 to 30% from LPOST applications. Common ragweed control was above 77% with all treatments, which included 105 g/ha mesotrione. Giant foxtail control was higher at 76 to 78% by mixtures of mesotrione with glufosinate applied LPOST than by mesotrione alone. Corn yields were highest when glufosinate was included in treatments at either application timing. In the greenhouse, mixtures of mesotrione with glufosinate-injured glufosinate-resistant corn 11% or less, but corn biomass was reduced by 25% for the mixture of mesotrione at 105 g/ha plus glufosinate at 350 g/ha. Mixtures of mesotrione with glufosinate can be more effective than mesotrione alone but control of common ragweed and giant foxtail might not be commercially acceptable.

Researchers are currently developing predictive weed management models to aid producers in maintaining or improving economic profitability of peanut production while minimizing herbicide inputs and reducing environmental impact. HERB (Version 2.1.P), a computer decision model, has recently been developed for peanut and is now awaiting validation of weed control decisions before being released to the public. Field validation trials in 1994 and 1995 indicate that the current competitive index parameters in the HERB model are invalid, and statistically estimated competitive indices were generated. Estimating new parameters improved R 2 values from 0.37 to 0.61. New competitive index parameters allow the HERB model to more accurately predict the level of yield loss at a given weed density.

Three Group V soybean cultivars were grown in the greenhouse to quantify interference when planted at various intervals with sicklepod, and the effects of chlorimuron and imazaquin POST on this interspecific interference. Soybean shoot height or fresh weight was not affected by sicklepod interference when sicklepod was planted 10 d after soybean in most cases. ‘Asgrow 5979’ soybean shoot height was not reduced when planted with sicklepod, but was reduced when planted 10 d after sicklepod and no herbicide treatment was added. When sicklepod was planted 10 d before Asgrow 5979 and ‘9592 Pioneer’ soybean, shoot height was reduced more than ‘Hutcheson,’ regardless of herbicide treatment. The cultivar 9592 Pioneer was more effective in reducing sicklepod shoot height than Asgrow 5979 when no herbicide treatment was used. Chlorimuron or imazaquin had little or no effect on sicklepod interference with soybean under these conditions.

Experiments were conducted in North Carolina during 2005, 2006, and 2007 to determine peanut and weed response when peanut was planted in strip tillage after desiccation of cereal rye, Italian ryegrass, oats, triticale, wheat, and native vegetation by glyphosate and paraquat before planting with three in-season herbicide programs. Control of common ragweed and yellow nutsedge did not differ among cover crop treatments when compared within a specific herbicide program. Applying dimethenamid or S-metolachlor plus diclosulam PRE followed by imazapic POST was more effective than a chloroacetamide herbicide PRE followed by acifluorfen, bentazon, and paraquat POST. Incidence of spotted wilt in peanut (caused by a Tospovirus) did not differ when comparing cover crop treatments, regardless of herbicide program. Peanut yield increased in all 3 yr when herbicides were applied POST, compared with clethodim only. Peanut yield was not affected by cover crop treatment. Response to cover crop treatments was comparable, suggesting that growers can select cereal rye, Italian ryegrass, oats, or triticale as an alternative to wheat as a cover crop in peanut systems without experiencing differences associated with in-season weed management.

Glyphosate and adjuvant combinations were applied to rhizome johnsongrass at vegetative and reproductive growth stages to evaluate control and rainfastness in field studies. Using a rainfall simulator delivering 1.3 cm of water in 15 min, plots received either no rainfall or rainfall 15 or 60 min after glyphosate was applied at 2.1 kg ai/ha in combination with the nonionic surfactants Kinetic® HV at 0.25% (v/v) or Induce® at 1.0% (v/v) or the silicone surfactant Break-Thru® at 0.125% (v/v). Regardless of adjuvant, rainfall 15 or 60 min after application reduced johnsongrass control compared with no rainfall. Johnsongrass control 14 d after treatment at the reproductive stage was at least 89% with no rainfall, but no more than 53 and 65% with rainfall at 15 and 60 min, respectively. Based on initial weed control, adjuvants did not consistently improve rainfastness. Johnsongrass regrowth did not occur when glyphosate was applied with either adjuvant. In contrast, for glyphosate applied to johnsongrass in the vegetative stage, addition of Break-Thru improved control over Induce at both 15- and 60-min rainfall timings in one of two experiments. With no rainfall, addition of Kinetic HV and Break-Thru increased johnsongrass control in only one experiment. For application at the vegetative stage, johnsongrass regrowth averaged across rainfall timings was no more than 10%. In other field experiments, glyphosate at 1.4 kg/ha plus nonionic surfactants, silicone surfactant, crop oil concentrate, methylated seed oil, or a blend of silicone surfactant and methylated seed oil were equally effective in reducing johnsongrass regrowth when applied after seedhead emergence. Improved control of vegetative johnsongrass with some adjuvants was not reflected in decreased regrowth.

A general life cycle model was modified to demonstrate how agronomic practices and weed biology factors affect the rate of appearance of herbicide-resistant downy brome, jointed goatgrass, and wild oat in Pacific Northwest wheat cropping systems. The model suggests herbicide rotation strategies for cropping systems that include imidazolinone-resistant wheat as a weed management tool. Simulation of continuous annual imidazolinone-resistant winter wheat and imazamox herbicide use resulted in the resistant soil seed banks of downy brome, jointed goatgrass, and wild oat surpassing their susceptible soil seed banks in 5, 7, and 10 yr, respectively. Reducing the initial seed bank density of downy brome before beginning a rotation that includes imidazolinone-resistant winter wheat reduces the likelihood of selecting for herbicide-resistant biotypes. The best simulated management option for reducing the total jointed goatgrass soil seed bank in low-precipitation areas is an imidazolinone-resistant winter wheat–fallow rotation. Rotations that include winter and spring crops and rotations that include non–group 2 herbicides minimize herbicide resistance selection pressure and reduce the wild oat soil seed bank.

Field trials were established in orchardgrass and tall fescue grass pastures and hay fields during 2000 and 2001 to evaluate wild carrot, broadleaf plantain, buckhorn plantain, bladder campion, and yellow crownbeard response to a variety of herbicides and prepackaged herbicide combinations. Picloram plus 2,4-D at 0.23 plus 0.84 kg/ha and higher, triclopyr plus clopyralid at 0.96 plus 0.32 kg/ha and higher, and 2,4-D plus triclopyr at 2.20 plus 1.12 kg/ha provided greater than 84% wild carrot, broadleaf plantain, and buckhorn plantain control at 4 mo after treatment (MAT) in both years. Metsulfuron, picloram plus 2,4-D at 0.30 plus 1.12 kg/ha, and 2,4-D plus triclopyr at 2.20 plus 1.12 kg/ha provided from 56 to 61% control of bladder campion 4 MAT, which was the highest bladder campion control observed in these trials. Greater than 80% yellow crownbeard control was achieved during both years with picloram plus 2,4-D at 0.15 plus 0.56 kg/ha and higher and with triclopyr plus clopyralid at 0.96 plus 0.32 kg/ha and higher. Results from these experiments reveal that good broad-spectrum control of many biennial and perennial broadleaf weeds can be achieved with higher rates of the picloram plus 2,4-D and triclopyr plus clopyralid prepackaged combinations.

Field tests showed that the lawn spray-gun with a 4 gpm lawn tip reduced the percentage of application volume deposited 90 cm to 210 cm downwind from the spray swath edge when compared with XR8004 flat-fan or RA-6 wide angle hollow cone applications at wind speeds between 4.7 and 14.4 km/h. The percentage of applied volume collected at 210 cm downwind from the XR8004 flat-fan applications was 5 and 16 times greater than the percentage from the RA-6 Raindrop nozzle and lawn spray-gun applications, respectively. Visible injury alone with height increases and fresh weights from tomato plants located downwind from the applications concur with spray-drift data for all nozzle types. Triclopyr injury decreased as the distance from the swath edge increased. All tomato plants located downwind up to 210 cm from the XR8004 flat-fan applications were visibly injured (15 to 40%); whereas, only plants less than 150 cm downwind from the RA-6 Raindrop applications and less than 90 cm downwind from the lawn spray-gun applications were injured (2 to 8%).

Lack of consistent weed control has been a major limiting factor in the adoption of no-till soybean production. Field studies were conducted at Waseca, MN from 1987 through 1990 to evaluate the efficacy of imazethapyr applied either alone or in combination with other herbicides for weed control in no-till soybean. Fall applications of imazethapyr did not provide acceptable weed control. Imazethapyr applied 2 to 4 wk before planting provided a weed-free seedbed whereas burndown treatments applied 1 to 3 d before planting failed to do so. Early preplant imazethapyr applied during the second week of April did not control weeds as well as imazethapyr applied during the last week of April. Imazethapyr applied alone PRE failed to control weeds adequately. A split application of early preplant plus PRE imazethapyr resulted in excellent weed control, especially when metribuzin was added with each application. Imazethapyr is a promising herbicide for weed control in no-till soybean production.