Greenhouse experiments were conducted in 2016 at Pontotoc and Verona, MS. On March 3 (Pontotoc) and March 7 (Verona), landscape fabric was placed in the bottom of polyethylene lugs, each 0.22 m2, then approximately 5 cm of a 1:1 (v/v) blend of soilless potting media and masonry sand was added. ‘Beauregard’ sweetpotato [Ipomoea batatas (L). Lam.] storage roots weighing between 85 and 227 g, and several with emerging sprouts ≤1 cm, were placed longitudinally in a single layer on the substrate, then covered with an additional 3 cm of the substrate. Sprouted yellow nutsedge (Cyperus esculentus L.) tubers were transplanted equidistantly into sweetpotato-containing lugs at six densities: 0, 18, 36, 73, 109, and 145 m−2. Trials were terminated 55 and 60 d after planting at Pontotoc and Verona, respectively. Predicted total sweetpotato stem cuttings (slips) decreased linearly from 399 to 312 m−2 as C. esculentus density increased from 0 to 145 m−2. Predicted total slip dry weight at a C. esculentus density of 145 m−2 was reduced 21% compared with 0 m−2. Predicted rotten sweetpotato storage roots increased from 2.6 to 11.3 m−2 as C. esculentus density increased from 0 to 145 m−2. In response to increasing C. esculentus density, sweetpotato seed roots exhibited increased proximal-end dominance.

Weed management during spring crop production in eastern Washington presents many challenges. Many spring crops are weak competitors with weeds. In May of 2010 and 2011, two spring crop trials were initiated near Pullman, WA, to compare the relative competitiveness of barley (Hordeum vulgare L.), wheat (Triticum aestivum L.), lentil (Lens culinaris Medik.), and pea (Pisum sativum L.) using cultivated oat (Avena sativa L.) as a surrogate for wild oat (Avena fatua L.) competition. The experiment was arranged as a split-block split-plot design with four replications. One set of main plots included three oat density treatments (0, 63, and 127 plants m−2), while a second set included each crop species. Crop species main plots were then split into subplots of two different seeding rates (recommended and doubled). Crop populations decreased as oat density increased and increased as crop seeding rate increased. As oat density increased, preharvest crop biomass decreased for all crops, while oat biomass and yield increased. Oat biomass and yield were greater in legume plots compared with cereal plots. Increasing oat density decreased yields for all crops, whereas doubling crop seeding rate increased yields for barley and wheat in 2010 and barley in 2011. Compared with legumes, cereals were taller, produced more biomass, and were more competitive with oat.

Wild oat (Avena fatua L.) is one of the most problematic weed species in western Canada due to widespread populations, herbicide resistance, and seed dormancy. In wheat (Triticum aestivum L.), and especially in shorter crops such as lentil (Lens culinaris Medik.), A. fatua seed panicles elongate above the crop canopy, which can facilitate physical cutting of the panicles (clipping) to reduce viable seed return to the seedbank. However, the viability of A. fatua seed at the time of panicle elongation is not known. The objective of this study was to determine the viability of A. fatua seed at successive time intervals after elongation above a wheat or lentil crop canopy. A 2-yr panicle clipping and removal study in wheat and lentil was conducted in Lacombe, AB, and Saskatoon, SK, in 2015 and 2016 to determine the onset of viability in A. fatua seeds at successive clipping intervals. Manual panicle clipping of A. fatua panicles above each crop canopy began when the majority of panicles were visible above respective crop canopies and continued weekly until seed shed began. At the initiation of panicle clipping, A. fatua seed viability was between 0% and 10%. By the last clipping treatment (approximately 6 to 7 wk after elongation), 95% of the A. fatua seeds were viable. Seed moisture and awn angle were not good predictors of A. fatua viability, and therefore were unlikely to provide effective tools to estimate appropriate timing for implementation of A. fatua clipping as a management technique. Based on A. fatua seed viability, earlier clipping of A. fatua is likely to be more effective in terms of population management and easier to implement in shorter crops such as lentil. Investigations into long-term effects of clipping on A. fatua populations are needed to evaluate the efficacy of this management strategy on A. fatua.

Harvest weed seed control (HWSC) technology, such as impact mills that destroy weed seeds in seed-bearing chaff material during grain crop harvest, has been highly effective in Australian cropping systems. However, the impact mill has never been tested in soybeans [Glycine max (L.) Merr.] and weeds common to soybean production systems in the midwestern and mid-Atlantic United States. We conducted stationary testing of Harrington Seed Destructor (HSD) impact mill and winter burial studies during 2015 to 2016 and 2017 to 2018 to determine (1) the efficacy of the impact mill to target weed seeds of seven common weeds in midwestern and five in the mid-Atlantic United States, and (2) the fate of impact mill–processed weed seeds after winter burial. The impact mill was highly effective in destroying seeds of all the species tested, with 93.5% to 99.8% weed seed destruction in 2015 and 85.6% to 100% in 2017. The weak relationships (positive or negative) between seed size and seed destruction by impact mill and the high percentage of weed seed destruction by impact mill across all seed sizes indicate that the biological or practical effect of seed size is limited. The impact mill–processed weed seeds that retained at least 50% of their original size, labeled as potentially viable seed (PVS), were buried for 90 d overwinter to determine the fate of weed seeds after winter burial. At 90 d after burial, the impact mill–processed PVS were significantly less viable than unprocessed control seeds, indicating that impact mill processing physically damaged the PVS and promoted seed mortality overwinter. A very small fraction (<0.4%) of the total weed seed processed by the impact mill remained viable after winter burial. The results presented here demonstrate that the impact mill is highly effective in increasing seed mortality and could potentially be used as an HWSC tactic for weed management in this region.

Before any late-season weed control operations are planned to manage herbicide-resistant weeds, it is essential to evaluate the plants’ maturity and shattering potential. Our goal was to assess the seed-shattering phenology of common ragweed (Ambrosia artemisiifolia L.) using pollination bags as seed traps. A secondary goal was to evaluate the efficiency of these traps. Trials were conducted from 2014 to 2017 at two locations in eastern Canada (Saint-Jean-sur-Richelieu, QC, and Harrow, ON). At each location, three adjacent fields were seeded with spring wheat (Triticum aestivum L.), soybean [Glycine max (L.) Merr.], or corn (Zea mays L.). Each field was divided into four replicate blocks that included two treatment plots with 5 weeds m−2 planted on the same date as the crop or when crop plants had two leaves (early or late emergence). To evaluate shattering in time, the experiment included up to 12 weekly collection dates (subplots). In each subplot, weeds were individually bagged at flowering (using mesh bags) until collection, when the number and viability of shattered and retained seeds per plant was recorded. Weather data as well as crop and weed stages were recorded. The effect of the pollen bags on seed retrieval and viability was evaluated by installing open and closed bags in corn and uncropped (bare) plots at a single location. Ambrosia artemisiifolia seed biomass was equivalent or higher in closed bags, and seed viability was equivalent or slightly reduced. No seeds were produced before harvest in spring wheat, as dispersal started in September. The percentage of seeds retained on the plant decreased linearly (1 site-year) or followed a logistic equation (4 site-years) with day of year or growing degree days. Dispersal in time was similar between early- and late-emerging weeds and similar in both corn and soybean. On average, more than 50% of A. artemisiifolia seeds were dispersed before harvest in corn and soybean.

Scotch broom [Cytisus scoparius (L.) Link] is a large nonnative, leguminous shrub that threatens native plant communities by rapidly invading recently disturbed sites, competing vigorously for soil water and nutrients, and imparting soil legacy effects that inhibit native plants. In the Pacific Northwest, logging debris retention after forest harvesting prevents or slows C. scoparius invasions. A series of studies were conducted to determine potential mechanisms by which logging debris modifies the light environment to limit germination and growth of C. scoparius. In laboratory studies, seed germination did not vary significantly (P > 0.05): (1) between presence and absence of light for several temperature regimes, (2) when exposed to red (660-nm wavelength) versus far-red (730-nm wavelength) light, and (3) across a range of red/far-red light (R/FR) ratios. These results indicate that modification of the light environment by logging debris or plant canopies has little or no influence on C. scoparius germination. In a study to simulate effects of variable mass of logging debris, “heavy” debris (2 kg m−2) caused biologically relevant reductions in photosynthetic photon flux density (PPFD) and R/FR relative to conditions under “light” debris (1 kg m−2). Cytisus scoparius germination did not differ significantly between simulated heavy and light debris; however, values of seedling root and shoot biomass under heavy debris were 16% and 71% of those observed under light debris, respectively. These results indicate that heavy debris limits biomass of C. scoparius seedlings, particularly roots, by reducing both PPFD and R/FR, which increases seedling vulnerability to summer drought or other stressors. Retention of heavy logging debris after forest harvesting has potential application on sites likely to be invaded by C. scoparius, as well as those sites with seedbanks containing C. scoparius.

North African knapweed (Centaurea diluta Aiton) is an annual weed that is widespread in southern Spain and is of increasing concern in dryland cropping systems. Despite its expanding range in Spain, there is limited information on the emergence timing and pattern of this species, knowledge of which is critical for developing more timely and effective management strategies. Therefore, there is a need to develop simple and reliable models to predict the timing and emergence of this annual weed under dryland conditions. A multi-location field experiment was established across Spain in 2016 to 2017 to assess the emergence of C. diluta. At each of 11 locations, seeds were sown in the fall, and emergence was recorded. Overall emergence averaged 39% in the first year across all sites and 11% in the second year. In both years, the main emergence flush occurred at the beginning of the growing season. A three-parameter Weibull function best described seedling emergence of C. diluta. Emergence models were developed based on thermal time (TT) and hydrothermal time (HTT) and showed high predictability, as evidenced by root mean-square error prediction values of 10.8 and 10.7, respectively. Three cardinal points were established for TT and HHT at 0.5, 10, and 35 C for base, optimal, and ceiling temperatures, respectively, while base water potential was estimated at −0.5 MPa.

Some well-managed organic soils are known to have higher crop yield potential than conventionally managed soils due to the greater soil quality and the ability to tolerate weed competition. However, low available soil mineral N and P in some organic systems may mask such soil quality–related benefits. We hypothesize that when plant-available N and P are not limiting, tillage-based highly diverse organic crop rotations have less yield loss (better crop tolerance) due to weed competition and higher crop yields than no-till conventional systems with low-diversity rotations. A greenhouse study was carried out in Saskatoon, Canada, using long-term (18-yr) organically managed soils (ORG) and no-till conventional soils (CONV) with three crop rotation diversities (LOW, MEDIUM, and HIGH) to compare the crop tolerance to weed competition under standard soil nutrient management conditions and under excess supply of mineral N and P. Under fertilized conditions, crop biomass increased by 50% and 69% in ORG and CONV systems, respectively. Weed biomass was similar between ORG and CONV systems under nonfertilized conditions but was 14% greater in CONV when excessive N and P were supplied. Crop biomass loss (crop tolerance) was not different among cropping systems under excess fertilizer or under standard fertilizer levels. Even with greater weed biomass under fertilized conditions, the CONV system showed crop tolerance similar to that of the ORG system. Under nonfertilized conditions, the crop biomass yield was 43% lower in ORG compared with CONV, and even after mineral N and P were applied, ORG systems showed less (17%) crop biomass than CONV. Further, differences in crop tolerance were not identified among crop rotations under both fertilizer levels. Overall, this study revealed that there were no yield benefits or better crop tolerance to weed competition in organically managed soils compared with no-till conventional soils, even under nonlimiting soil macronutrient conditions.

Hairy fleabane [Conyza bonariensis (L.) Cronquist] is a problematic weed in Australian no-till cropping systems. Consequently, a study was conducted to examine the effect of temperature, light, salt stress, osmotic stress, burial depth, and sorghum crop residue on germination and emergence in two populations (C and W: collected from chick pea [Cicer arietinum L.] and wheat [Triticum aestivum L.] fields, respectively) of C. bonariensis. Both populations were able to germinate over a wide range of alternating day/night temperatures (15/5 to 35/25 C); however, the C population had optimum (and similar) germination over the range of 20/10 and 30/20 C, while the W population showed maximum germination at 25/15 C. A negative relationship was observed between osmotic potential and germination, with 31% and 14% germination of the C and W populations at −0.6 MPa, respectively. These observations suggest that population C was more tolerant to higher osmotic potentials than population W. Seeds of both populations germinated when exposed to a wide range of sodium chloride levels (NaCl, 0 to 200 mM); however, beyond 200 mM NaCl, no germination was observed in either population. Maximum germination of the C (70%) and W (41%) populations was observed on the soil surface with no emergence from a burial depth of 1 cm. The application of sorghum residue at an amount of 6,000 kg ha−1 reduced emergence of the C and W populations by 55% and 58%, respectively, compared with the no-residue treatment. Knowledge gained from this study suggests that the following strategies could be used for more efficacious management of C. bonariensis: (1) a shallow-tillage operation to bury weed seeds in conventional tillage systems, and (2) retention of sorghum residue on the soil surface in no-till systems.

Annual sowthistle (Sonchus oleraceus L.) is a broadleaf weed that is increasing in prevalence in the northern cropping regions of Australia. Being a member of Asteraceae family, this weed possesses many biological attributes needed to thrive in varying environments and under differing weed management pressures. Interference of this weed in a wheat (Triticum aestivum L.) crop was examined through field studies in 2016 and 2017. Different densities of S. oleraceus were evaluated for their potential to cause yield loss in wheat: 0.0 (weed-free), low (9 to 15 plants m−2), medium (29 to 38 plants m−2), and high (62 to 63 plants m−2). Based on the exponential decay model, 43 and 52 plants m−2 caused a yield reduction of 50% in 2016 and 2017, respectively. Yield components such as panicles per square meter and grains per panicle were affected by weed density. At the high weed infestation level, S. oleraceus produced a maximum of 182,940 and 192,657 seeds m−2 in 2016 and 2017, respectively. Sonchus oleraceus exhibited poor seed retention at harvest, as more than 95% of seeds were blown away by wind. Adverse effects on crop, high seed production, and wind-blown dispersal may lead to an increased prevalence of this weed in the absence of an integrated weed management strategy utilizing both herbicides and nonchemical options.

Itchgrass [Rottboellia cochinchinensis (Lour.) W.D. Clayton] is among the most troublesome weeds in subtropical climates where sugarcane (Saccharum spp. interspecific hybrids) is cultivated. Two R. cochinchinensis biotypes commonly infest sugarcane in Louisiana. The Louisiana-1 biotype is daylength neutral, but Louisiana-2 flowered when daylength decreased to 13 h. Coupled with biotype diversity, seedling emergence has been reported to occur earlier in the growing season, as sugarcane emerged from winter dormancy. Both R. cochinchinensis biotypes were established in a common garden experiment in Louisiana during periods of sugarcane development and field preparation to simulate discontinuous emergence. Plant height and raceme production were recorded weekly for each biotype and establishment timing; aboveground biomass was harvested in autumn. Louisiana’s subtropical humid climate stimulated rapid plant growth that typically began in May and persisted through September. Without sugarcane competition, maximum R. cochinchinensis heights for Louisiana-1 and Louisiana-2 were 206 and 179 cm and growing degree days to 20-cm height in 2017 ranged from 546 to 832 and 865 to 1,160, respectively. Slower initial growth reported with Louisiana-2 would allow more time for growers to treat escaped plants with POST herbicides. Total raceme production, by autumn, was zero for Louisiana-2 established in June or later, but Louisiana-1 established in June produced up to 202 racemes. The present study demonstrated the importance of managing the Louisiana-2 biotype in March and April to limit seed production, but fields infested with Louisiana-1 were at greater risk for potential crop yield loss, because plants produced high quantities of seed when established over a wide period of time.

Current recommendations for the control of glyphosate-resistant horseweed [Conyza canadensis (L.) Cronquist var. canadensis] in soybeans [Glycine max (L.) Merr.] consist of comprehensive herbicide programs, which often include herbicide applications outside the soybean growing season. Integration of cover crops with herbicides could potentially improve C. canadensis control and allow for a reduction in herbicide inputs. Two separate field studies were conducted from 2016 through 2018 with the objectives of: (1) determining the effect of planting date and seeding rate of a cereal rye (Secale cereale L.) cover crop on C. canadensis population density and control in the subsequent soybean crop; and (2) determining whether the cover crop could replace a fall herbicide treatment or allow for a reduction in the use of spring-applied residual herbicides. There was no effect of rye planting date, late September versus late October, on C. canadensis density in either study. In 2016 to 2017, C. canadensis density was greater in the absence of a rye cover crop in both studies, but otherwise not affected by seeding rates of 50 versus 100 kg ha−1. In the 2017 to 2018 season, the presence of rye resulted in an increased C. canadensis density in the spring residual herbicide study (Study I), and had no effect in the fall herbicide study (Study II). Conyza canadensis densities were lowest in the treatments where a comprehensive spring residual or fall herbicide treatment had been applied, averaged over rye planting date and seeding rate. Earlier-planted rye at a higher seeding rate produced the most biomass but did not result in lower C. canadensis densities. These results suggest that cereal rye planted at a density of 50 kg ha−1 as a cover crop before no-till soybeans may be sufficient to reduce glyphosate-resistant C. canadensis plant density, but cannot be relied upon to reduce the need for fall herbicide treatments and spring residual programs.

A field experiment was conducted at the research farm of the University of Tehran to evaluate the effect of cover crops and herbicide (nicosulfuron) dosage on corn (Zea mays L.) productivity. Cover crops suppressed weed biomass and density during the fallow period; however, there was no significant effect on weed biomass or weed density in the subsequent corn crop. A mixture of cereal rye (Secale cereale L.) and hairy vetch (Vicia villosa Roth.) was the most weed-suppressive cover in the fallow period. Nicosulfuron applied at different amounts suppressed weed biomass and density in corn when compared with weedy plots. Application of 40 g ai ha−1 of nicosulfuron was as effective on weed control as the recommended application. Cover crops did not affect corn growth; however, corn yield improved with herbicide application.

Understanding the effects of crop management practices on weed survival and seed production is imperative in improving long-term weed management strategies, especially for herbicide-resistant weed populations. Kochia [Bassia scoparia (L.) A.J. Scott] is an economically important weed in western North American cropping systems for many reasons, including prolific seed production and evolved resistance to numerous herbicide sites of action. Field studies were conducted in 2014 in a total of four field sites in Wyoming, Montana, and Nebraska to quantify the impact of different crop canopies and herbicide applications on B. scoparia density and seed production. Crops used in this study were spring wheat (Triticum aestivum L.), dry bean (Phaseolus vulgaris L.), sugar beet (Beta vulgaris L.), and corn (Zea mays L.). Herbicide treatments included either acetolactate synthase (ALS) inhibitors effective on non-resistant B. scoparia or a non–ALS inhibiting herbicide effective for both ALS-resistant and ALS-susceptible B. scoparia. Bassia scoparia density midseason was affected more by herbicide choice than by crop canopy, whereas B. scoparia seed production per plant was affected more by crop canopy compared with herbicide treatment. Our results suggest that crop canopy and herbicide treatments were both influential on B. scoparia seed production per unit area, which is likely a key indicator of long-term management success for this annual weed species. The lowest germinable seed production per unit area was observed in spring wheat treated with non–ALS inhibiting herbicides, and the greatest germinable seed production was observed in sugar beet treated with ALS-inhibiting herbicides. The combined effects of crop canopy and herbicide treatment can minimize B. scoparia establishment and seed production.

More than 80% of soybean [Glycine max (L.) Merr.] in Brazil is cultivated in no-till systems, and although cover crops benefit the soil, they may reduce the amount of residual herbicides reaching the soil, thereby decreasing herbicide efficacy. The objective of this study was to evaluate sulfentrazone applied alone, sequentially after glyphosate, and in a tank mixture with glyphosate before planting no-till soybean. Experiments were performed in two cover crop systems: (1) pearl millet [Pennisetum glaucum (L.) R. Br.] and (2) forage sorghum [Sorghum bicolor (L.) Moench ssp. bicolor]. The treatments tested were: glyphosate (720 g ae ha−1) at 20 d before sowing (DBS) followed by sulfentrazone (600 g ai ha−1) at 10 DBS; glyphosate + sulfentrazone (720 g ae ha−1 + 600 g ai ha−1) for cover crop desiccation at 10 DBS; and sulfentrazone alone at 10 DBS without a cover crop. The accumulation of straw was 31% greater using sorghum rather than pearl millet. In the sorghum system, the concentration of sulfentrazone at 0 to 10 cm was 57% less with sequential application and 92% less with the tank mixture compared with the treatment without cover crop straw at 1 d after application (DAA). The same occurred in the pearl millet system, where the reduction was 33% and 80% for the sequential application and tank mixture, respectively. The absence of a cover crop resulted in greater sulfentrazone concentrations in the top layer of the soil when compared with the sequential application or tank mixture. At 31 and 53 DAA, the concentration of sulfentrazone at 10 to 20 and 20 to 40 cm did not differ among treatments. Precipitation of 90 mm was enough to remove the herbicide from the cover crop straw at 31 DAA when using sequential application. An additional 90-mm precipitation was necessary to promote the same result when using the tank mixture.

Weeds remain the foremost production challenge for organic small grain farmers in the northeastern United States. Instead of crops sown in narrow, single-line rows, band sowing offers a more uniform spatial arrangement of the crop, maximizing interspecific while reducing intraspecific competition. Weeds in the inter-band zone are controlled by cultivating with aggressive sweeps; tine harrowing can target weeds in both intra- and inter-band zones. Field experiments in Maine and Vermont in 2016 and 2017 evaluated band sowing for improved weed control, crop yield, and grain quality in organic spring barley (Hordeum vulgare L. ‘Newdale’). Specifically, we compared: (1) the standard practice of sowing 16.5-cm rows at a target crop density of 325 plants m−2, (2) narrow-row sowing with increased crop density, (3) wide-row sowing with interrow hoeing, and (4) band sowing both with and (5) without inter-band hoeing. Mustard (Sinapis alba L. ‘Ida Gold’) was planted throughout the experiment as a surrogate weed. Compared with the standard practice, band sowing with hoeing reduced surrogate weed density on average by 45% across site-years. However, effects on weed biomass and yield were inconsistent, perhaps due to suboptimal timing of hoeing and adverse weather conditions. In 1 out of 4 site-years, band sowing with hoeing reduced surrogate weed biomass by 67% and increased crop yield compared with the standard treatment. Results also indicate that band sowing with hoeing may improve 1,000-kernel weight and plump kernel grain-quality parameters.

Estimates indicate that 30% of land surface globally is affected by soil acidity, influencing agricultural production. Application of lime increases soil pH and improves crop growth. We tested the hypothesis that liming will reduce rigid ryegrass (Lolium rigidum Gaudin) growth by improving the competitive ability of the crop. Experiments at Merredin and Wongan Hills in Western Australia indicated that application of lime in previous years reduced L. rigidum density, biomass, and seed production in wheat (Triticum aestivum L.) crops in 2018. At Merredin, L. rigidum seed production in 2018 was reduced from 9,390 to 2,820 seeds m−2, and wheat tiller number and yield was increased, following lime application of 0 to 6,000 kg ha−1 in 2016. At Wongan Hills, lime application of 4,000 kg ha−1 in 1994 reduced seed production in the 2018 wheat crop from 4,708 to 1,610 seeds m−2, and application of 3,000 kg ha−1 of lime in 2014 reduced seed production from 3,959 to 921 seeds m−2 in 2018. Again, lime increased wheat tiller number, but not yield. A screen house experiment (in controlled conditions) indicated that lime application increased the initial growth of both L. rigidum and wheat seedlings. This supports the conclusion that reduced L. rigidum growth and seed production in the field resulted from increased competitive ability of the crop, rather than any direct and detrimental impact of lime on L. rigidum growth. Incorporation of lime reduced initial emergence of L. rigidum in controlled conditions, with L. rigidum seeds at a uniform depth, and in the field experiments in situations of high weed density, with seeds buried by the incorporation process. Nationally, the revenue loss from residual L. rigidum in crop is A$93 million per year. The current research confirms that application of lime will increase the competitive ability of crops growing in regions with acidic soils.