Rationally higher population density is crucial for seeking a balance that meets lodging resistance and maximizes seed yield in mechanized direct-seeded winter canola. In this study, a split-plot experiment with two cultivars (Huayouza9 and Zhongshuang11) and eleven planting densities (12–105 plants m-2) was conducted in a two-season field experiment to evaluate the high planting density in this cropping system and improve its production efficiency. Seed yield noticeably increased in planting density up to 80 plants m-2 in Zhongshuang11 (2187 kg hm-2) and 60 plants m-2 in Huayouza9 (2943 kg hm-2). The seed yield of Huayouza9 did not differ significantly from the local target seed yield. Higher plant density curtailed the luxurious vegetative growth of individual canola plants at the density of no less than 60–80 plants m-2, and high seed yield was derived from the increased ratio of main raceme and branch seed weight in winter canola. An increase in plant densities contributed to the reinforced sunlight interception at the pod-filling stage, providing a larger canopy photosynthetic area for the rapid growth of more canola pods at higher densities (60–105 plants m-2). Lodging resistance and breaking resistance decreased sharply with the plant density increasing from 12 to 60 plants m-2 while remaining almost steady as it further increased from 60 to 105 plants m-2 for Huayouza9 and Zhongshuang11. Hence, the population density of 60 plants m-2 reached a balance between lodging resistance and maximized seed yield in mechanized direct-seeded winter canola in China.

This study explores how higher temperatures influence corn yield response to planting density. Using 1990–2010 field trial data from Wisconsin and econometric models with a variety of specifications, we find that higher temperatures reduce the yield benefits of increasing planting density. However, these adverse high-temperature effects are smaller for genetically modified corn varieties, especially those with rootworm-resistant traits. Consistent with previous studies, these results support the notion that varietal improvements through genetic modification may have paved the way for higher planting densities in US corn production. Moreover, our results imply that expected in-season temperatures are important considerations when making planting density decisions.

The development of early-maturing soybean [Glycine max (L.) Merr.] varieties has led to an increase in soybean production in canola (Brassica napus L.)-dominant crop rotations in western Canada. Herbicide-resistant (HR) volunteer B. napus can be difficult to manage in HR soybean using herbicides alone. In 2013 and 2014, four field experiments were conducted in Manitoba, Canada, to evaluate soybean row spacing, seeding density, nitrogen supply, and interrow tillage as candidate nonchemical weed management tools for an integrated program to manage volunteer B. napus in soybean. Among treatments and sites, volunteer B. napus produced about 830 seeds plant−1 and resulted in large seedbank inputs (averaging about 20,300 seeds m−2). Volunteer B. napus seedling recruitment differed among sites, and resulted in two distinct classes of sites based on average seedling densities of 39 and 89 plants m−2. Weed management tools were more effective at the sites with higher volunteer B. napus densities. At these sites, soybean yield was greater when using an increased soybean-seeding density (44% greater yield using a seeding density of 682,500 vs. 455,000 seeds ha−1) or interrow tillage (36% greater yield with vs. without using interrow tillage). Soybean row spacing (19 vs. 38 vs. 76 cm) did not affect soybean yield, unless the reduction in row spacing was combined with an increased seeding density (65% greater yield with narrow-row soybean seeded at 682,500 vs. wide-row soybean seeded at 455,000 seeds ha−1). At the sites with higher volunteer B. napus densities, seed production of canola volunteer B. napus was greater when nitrogen fertilizer was applied to simulate an environment with greater nitrogen supply (77% greater number of volunteer B. napus seeds produced with vs. without broadcast application of 23 kg N ha−1 urea). In northern climates, seeding soybean at increased densities using narrow-row spacing in fields with limited soil inorganic nitrogen and using interrow tillage in wide-row production systems are effective strategies that could augment chemical weed management in an integrated program for management of volunteer B. napus, and perhaps also other competitive early-season weeds.

Good canopy structure is essential for optimal maize (Zea mays L.) production. However, creating appropriate maize canopy structure can be difficult, because the characteristics of individual plants are altered by changes in plant age, density and interactions with neighbouring plants. The objective of the current study was to find a reliable method for building good maize canopy structure by analysing changes in canopy structure, light distribution and grain yield (GY). A modern maize cultivar (ZhengDan958) was planted at 12 densities ranging from 1.5 to 18 plants/m2 at two field locations in Xinjiang, China. At the silking stage (R1), plant and ear height increased with plant density as well as leaf area index (LAI), whereas leaf area per plant decreased logarithmically. The fraction of light intercepted by the plant (F) increased with increasing plant density, but the light extinction coefficient (K) decreased linearly from 0.61 to 0.39. Taking the optimum value of F (95%) as an example, and using measured values of K for each plant density at R1 and the equation from Beer's law, the corresponding (theoretical) LAI for each plant density was calculated and optimum plant density (9.72 plants/m2) obtained by calculating the difference between theoretical LAIs and actual observations. Further analysis showed that plant density ranging from 10.64 to 11.55 plants/m2 yielded a stable GY range. Therefore, taking into account the persistence time for maximum LAI, the plant density required to obtain an ideal GY maize canopy structure should be increased by 10–18% from 9.72 plants/m2.

From 1976 to 1981, emergence date, height, and dry weight per plant were taken for eight weed species grown at Bushland, TX, at two stand densities. Height and dry weight per plant were related to precipitation and degree-days for air and soil temperatures using base temperatures of 10, 15.5, or 21 C. Kochia [Kochia scoparia (L.) Schrad. ♯ KCHSC] emerged early in March and was followed by poison suckleya [Suckleya suckleyana (Torr.) Rydb. ♯ SUKSU], sunflower (Helianthus annuus L. ♯ HELAN), and common cocklebur (Xanthium pensylvanicum Wallr. ♯ XANPE) in early April. Buffalobu(Solanum rostratum Dunal. ♯ SOLCU), puncturevine (Tribulus terrestris L. ♯ TRBTE), lanceleaf sage (Salvia reflexa Hornem. ♯SALRE), and barnyardgrass (Echinochloa crus-galli L. Beauv. ♯ECHCG) emerged in late April. The weeds began to increase in height during May but plants did not start increasing in weight until June. Kochia, common cocklebur, and sunflower grew tallest, produced most dry matter, made most efficient use of water, and weighed most per plant. Heat unit accumulations were strongly correlated with plant height.

Field experiments were conducted at Vegreville, Alberta, in 1990, 1991, and 1992 to investigate the influence of canola plant density and row spacing (1991 and 1992 only) on interference from Tartary buckwheat. Nonlinear regression equations incorporating both canola and Tartary buckwheat density provided good descriptions of the data. Parameter estimates for both weed and crop density were significant (P < 0.05) but did not differ between row spacings. Where no Tartary buckwheat plants were present, canola yield was little affected by canola plant density. At a given Tartary buckwheat density, canola seed yield increased as canola plant density increased, while Tartary buckwheat shoot weight and seed yield decreased. None of the factors significantly affected canola 1000-seed weight, or oil or protein levels in the canola seed. The results suggest that seeding canola to achieve a density of approximately 200 plants m–2 will significantly reduce the impact of weeds on canola yield, as well as reduce weed biomass and seed yield.

Purple nutsedge (Cyperus rotundus L.) and yellow nutsedge (C. esculentus L.) appeared to be equally acceptable for oviposition by caged Bactra verutana Zeller, but purple nutsedge was significantly more suitable as a host: 90% of the larvae survived to maturity on purple nutsedge compared with 65% on yellow nutsedge. Responses of the plant species to both larval feeding injury and plant density were similar but purple nutsedge tended to be injured more than yellow nutsedge. At a high shoot density (nine shoots per pot), production of tubers by purple nutsedge was more adversely affected by feeding of five larvae per shoot than was production by yellow nutsedge: tuber dry weights were reduced 93 and 80% and numbers of tubers per pot were reduced 77 and 62%, respectively. Production of inflorescences was greatly reduced in both species. The effect of B. verutana on inflorescences may be more important for yellow nutsedge, which is generally considered to reproduce freely by seeds. Both species of nutsedge probably would be about equally affected by augmentation of B. verutana populations as a method of biological control.

Greenhouse studies were conducted to determine the influence of plant density and spray volume on the retention, spray deposition, efficacy, and translocation of the amine salt of triclopyr in purple loosestrife. More spray solution was retained on leaves at 935 Lha−1 than at 94 Lha−1 at populations of 0, 4, or 8 nontarget neighbors. Spray coverage decreased with decreasing height within the plant canopy when spray cards were placed in the top, middle, and soil surface adjacent to the central target plant. Within a population, spray card coverage generally increased as spray volume increased. Regrowth from the crown was affected by spray volume, and uniform spray coverage of the plant was required for adequate control of vegetative regrowth and was achieved with spray volumes of 374 and 935 L ha−1 spray volume. Regrowth of purple loosestrife was greater at 94 Lha−1 at all three plant populations indicating that less herbicide penetrated the canopy to reach the basal portion of the plant. A laboratory experiment was conducted to investigate the translocation of radiolabelled triclopyr to roots and crowns of purple loosestrife. Only 0.3 to 1.4% of absorbed 14C-labelled material was translocated to roots and crowns. Low spray volumes and dense stands of purple loosestrife would likely result in poor control because inadequate amounts of triclopyr reach the basal portion of the plant and translocate to vegetative propagules.

Yellow starthistle is one of the most important alien invasive weeds in the western United States. It has been targeted for biological control based on the assumption that its abundance is limited by natural enemies in its native region but not in the United States. The geographic center of diversity for yellow starthistle appears to be in Turkey. This region is being explored to discover potential biological control agents; however, there is no quantitative information regarding the population density or dynamics of the plant in this region. Such information could help determine which natural enemies help suppress the plant in its land of origin. We measured densities of yellow starthistle plants and seeds during 2 yr at three locations in central Turkey. Densities of mature plants were about 4% of those measured at sites in California. Densities of capitula and seeds produced were about 60 and 65%, respectively, of those measured in California. The greatest difference between the two regions appears to be the densities of mature plants, which indicates the importance of focusing research on natural enemies that reduce plant survival.

Annual bluegrass is a cosmopolitan, cool-season grass that adapts to different environmental conditions. It is one of the main weeds of turfgrass, and its control is mainly achieved through chemical and cultural practices, although little importance is given to the competitive ability of turfgrass species or cultivars. A field study was performed in northeastern Italy from October 2012 to June 2014 to evaluate the competitive ability of selected perennial ryegrass cultivars to suppress emergence and establishment of annual bluegrass seedlings. Two creeping perennial ryegrasses (‘CSI' and ‘PPG-PR171’) and two traditional cultivars (‘Azimuth' and ‘Presidio') were compared. In late autumn in both years, annual bluegrass was sown in a marked area (30 cm by 30 cm) in the turfgrass. Emerged seedlings were counted weekly, and emergence dynamics were modeled. In the second year, final density of annual bluegrass allowed to grow in turfgrass was also evaluated in summer. Results showed that both seedling emergence and density of annual bluegrass were significantly reduced by creeping perennial cultivars, supporting the hypothesis that these cultivars may reduce annual bluegrass establishment in turfgrass. Seedling emergence pattern of annual bluegrass did not differ in creeping and traditional cultivars of perennial ryegrass, suggesting the possibility of creating a general emergence model suitable in any turfgrass to help turfgrass managers in the control of this weed species.

Although some herbicides are available for control of broadleaf weeds on rangeland, currently no herbicides are registered for selective control of weedy annual grasses in perennial forage grasses. Field experiments were conducted to determine the tolerance of several perennial forage grass species to preemergence (PRE) and postemergence (POST) applications of imazapic, a herbicide that controls certain weedy annual grasses. In PRE studies, perennial forage grasses were seeded 1 d after spraying with several rates of imazapic. The grass species by herbicide rate by location and the grass species by herbicide rate interactions were not significant for plant height and biomass 395 d after treatment (DAT). Expressed as a percentage of the untreated control, imazapic applied at 18 to 140 g/ha reduced height of all grass species 10 to 18%, whereas 280 g/ha of imazapic reduced height 39%. Imazapic applied at 18 to 70 g/ha reduced biomass 12 to 26%. Biomass was reduced 51 and 63% when imazapic was applied at 140 and 280 g/ha, respectively. Thus, rates of imazapic required to control downy brome likely will excessively injure perennial forage grass seeded 1 DAT. In POST studies, imazapic was applied to 1-yr-old stands of perennial forage grass. A dose– response model provided a good fit for grass species biomass and height data. In year 1, biomass and height of orchardgrass, smooth brome, and meadow brome were reduced 14 to 29% more than those of bluebunch, crested, intermediate, and western wheatgrass as imazapic rate increased, which implies that the wheatgrasses were more tolerant to imazapic. However, in year 2, slope of regression lines did not differ among grass species, implying that all forage grass species responded the same to increasing rates of imazapic. Plant height of all grass species decreased 25 to 56% when compared with the untreated control, and biomass decreased 28 to 59% as imazapic rate increased from 18 to 280 g/ha. As discussed previously, rates of spring-applied imazapic required for downy brome control severely injured perennial forage grasses whether applied PRE or POST. The level of tolerance of perennial forage grasses to imazapic depended on herbicide dose and perhaps environmental differences between years.

A field study was conducted to determine the combined effects of row spacing, plant density, and herbicides on weed management and dry bean (Phaseolus vulgaris) yield. In weed-free dry bean, a reduction in row spacing from 69 to 23 cm increased yield by 19% and an increase in density from 20 to 50 plants/m2 increased yield by 17%. In the presence of weeds, narrow rows and high plant densities increased dry bean yield, but without herbicides, yields remained low. However, when combined with herbicides, narrow-row and high-density production practices resulted in better weed control and higher dry bean yield than that attained in a wide-row and low-density production system. Herbicide combinations, often at reduced rates, controlled weeds as well or better than the full rate of any individual herbicide. Ethalfluralin applied preplant incorporated followed by reduced rates of imazethapyr or bentazon postemergence (POST) consistently controlled weeds. Imazamox exhibited the potential to provide a total POST weed control option in dry bean production. Information gained in this study will be used to develop improved weed management programs appropriate for either wide- or narrow-row dry bean production systems.

Yellow starthistle, spotted knapweed, and meadow hawkweed are three competitive noxious weeds that collectively infest about 640,000 ha of range and noncropland in Idaho. The purpose of this study was to determine the control of these three noxious weeds and undesirable annual grasses with imazapic applied at different rates and growth stages. Imazapic applied fall preemergence, spring postemergence (POST), or fall plus spring POST and picloram applied fall and spring POST reduced yellow starthistle plant density 75 to 100% compared with the untreated control, at about 100 d after spring treatment (DAST). However, by 440 DAST, yellow starthistle plant density did not differ from the untreated control in plots treated with imazapic, whereas picloram reduced plant density 82 to 99%. Imazapic did not control meadow hawkweed or spotted knapweed, whereas picloram reduced plant density of both species 91 to 100% 60 DAST compared with the untreated control and 71 to 100% 440 DAST. Imazapic did not affect weedy annual grass plant density compared with the untreated control at the yellow starthistle or the spotted knapweed sites. However, at the meadow hawkweed sites, imazapic applied spring POST at 210 g/ha and fall plus spring POST at 70 plus 140 g/ha reduced grass plant density 77 to 84% 60 DAST. All treatments that contained spring-applied imazapic reduced grass plant density 46 to 92%, 440 DAST at the meadow hawkweed site. At all locations, picloram increased weedy annual grass plant density two- to eightfold 440 DAST compared with the untreated control. Imazapic offers only moderate control of weedy annual grasses for a brief period after application and does not provide control of these three major noxious weed species in the West. Unfortunately, it eliminates a fairly new rangeland herbicide from the list of potential tank mix partners for control of auxinic herbicide–resistant weed species.

Greenhouse and field experiments were conducted to investigate the effect of glyphosate rate and degree of glyphosate spray coverage on pitted morningglory control. Pitted morningglory in the two-, four-, and six-leaf growth stages were treated with the isopropylamine salt of glyphosate at 0.28, 0.56, 0.84, 1.12, 1.40, and 1.68 kg ai/ha. Two- and four-leaf plants were controlled 98% with 1.68 kg/ha glyphosate, whereas six-leaf plants were controlled 68%. Control of two-, four-, and six-leaf plants with the commonly used field rate of 1.12 kg/ha was 68, 60, and 50%, respectively. In a separate greenhouse study, four-leaf pitted morningglory plants with 0, 33, 66, or 100% of their total leaf area exposed to herbicide spray were treated with 0.84, 1.68, or 3.36 kg/ha glyphosate. Increasing glyphosate rate from 0.84 to 3.36 kg/ha increased control from 36 to 88%. In contrast, increasing percent leaf exposure to glyphosate from 0 to 100% increased control from 57 to 75%. Increasing glyphosate rate from 0.84 to 1.68 kg/ha always improved control. However, increasing glyphosate rate from 1.68 to 3.36 kg/ha was beneficial only when no leaves were exposed to the spray solution. In the field, glyphosate spray coverage decreased from 85 to 40% as plant density increased from 1 to 32 plants/m2. However, control decreased only 11% (90 to 79%) between the highest and lowest levels of glyphosate spray coverage. These results demonstrated that inadequate control of pitted morningglory with glyphosate was more related to tolerance than glyphosate spray coverage. Glyphosate rates higher than 1.68 kg/ha may be beneficial when spray coverage is severely limited or when plants are beyond the four-leaf growth stage.

Field studies were conducted in 2005 and 2006 to determine the most effective chemical options within three individual herbicide-based burndown programs, glyphosate, paraquat and glufosinate, for controlling glyphosate-resistant horseweed in Mississippi. Burndown treatments were applied April 5, 2005 and March 15, 2006 to horseweed plants 15 to 30 cm in height. Glyphosate at 0.86 kg ae/ha alone provided 60 to 65% horseweed control 4 wk after treatment (WAT). Control 4 WAT ranged from 73 to 74% when the glyphosate rate was increased to 1.25 kg/ha. Glyphosate at 0.86 kg/ha applied in combination with 2,4-D at 0.84 kg ae/ha or dicamba at 0.28 ae/ha maximized control of horseweed (≥ 90%) 4 WAT and soybean yield. Horseweed control 4 WAT with paraquat alone at 0.84 kg ai/ha ranged from 55 to 63% and control did not improve by increasing the rate to 0.98 kg/ha. Addition of 2,4-D or dicamba to paraquat maximized horseweed control both years (78 to 89%), whereas soybean yield was maximized with addition of dicamba or metribuzin at 0.42 kg ai/ha. Glufosinate applied alone at 0.47 kg ai/ha resulted in at least 88% control of horseweed and maximized soybean yield. Results indicate that effective management of glyphosate-resistant horseweed can be obtained in glyphosate-resistant soybean in glyphosate-, paraquat-, and glufosinate-based preplant weed control programs.

Variation in herbivory is often associated with plant density and light environment. The effect of these variables was studied on leaf production and herbivory of Manilkara bidentata in Puerto Rico. The major herbivore of M. bidentata is the leaf miner Acrocercops sp. To determine the effect of plant density, twenty-four 20-m × 20-m plots were established and the densities of saplings, juveniles and adults were determined. Leaf production, herbivory and growth were measured on saplings. Plant density was determined in eight 20-m × 20-m plots surrounding the focal plots. The effect of light was determined by comparing leaf phenology, leaf quality and herbivory in vertical and horizontal profiles. Sapling density in 60-m × 60-m plots was associated with herbivory. In the vertical profile, leaf production was continuous only for the canopy: herbivory increased from the canopy (1.3%) towards the understorey (35.6%). In the horizontal profile leaf production was related to light. Saplings in low light environments had narrow peaks in leaf production compared with saplings in high light environments. Differences in leaf phenology did not result in differences in herbivory possibly because of variation in herbivory among leaves. Although many saplings lost more than 80% of new leaf area, there was no detectable effect on growth. High levels of intra-specific variation in herbivory suggest that to better understand plant/herbivore interactions it is necessary to consider all size classes in a population. It is also important to document these pattern across different spatial scales.