Crop Population Densities Research Articles

Integrating Automated Labeling Framework for Enhancing Deep Learning Models to Count Corn Plants Using UAS Imagery.

Plant counting is a critical aspect of crop management, providing farmers with valuable insights into seed germination success and within-field variation in crop population density, both of which are key indicators of crop yield and quality. Recent advancements in Unmanned Aerial System (UAS) technology, coupled with deep learning techniques, have facilitated the development of automated plant counting methods. Various computer vision models based on UAS images are available for detecting and classifying crop plants. However, their accuracy relies largely on the availability of substantial manually labeled training datasets. The objective of this study was to develop a robust corn counting model by developing and integrating an automatic image annotation framework. This study used high-spatial-resolution images collected with a DJI Mavic Pro 2 at the V2-V4 growth stage of corn plants from a field in Wooster, Ohio. The automated image annotation process involved extracting corn rows and applying image enhancement techniques to automatically annotate images as either corn or non-corn, resulting in 80% accuracy in identifying corn plants. The accuracy of corn stand identification was further improved by training four deep learning (DL) models, including InceptionV3, VGG16, VGG19, and Vision Transformer (ViT), with annotated images across various datasets. Notably, VGG16 outperformed the other three models, achieving an F1 score of 0.955. When the corn counts were compared to ground truth data across five test regions, VGG achieved an R2 of 0.94 and an RMSE of 9.95. The integration of an automated image annotation process into the training of the DL models provided notable benefits in terms of model scaling and consistency. The developed framework can efficiently manage large-scale data generation, streamlining the process for the rapid development and deployment of corn counting DL models.

Drought risk assessment considering ecosystem resilience: A case study in the Huang-Huai-Hai Plain, China

Drought caused by global warming has a profound impact on the stability of terrestrial ecosystems and agricultural production. As one of the key indicators to measure the response of ecosystems to climate change, the spatial difference of ecosystem resilience will also affect the accuracy of regional drought risk assessment. In this study, the 12-month standardized precipitation evapotranspiration index (SPEI-12) was used to identify drought characteristics and combined with meteorological elements to characterize drought hazard. Drought vulnerability was characterized using crop and environmental sensitivity, disaster prevention, and mitigation capacity. Specifically, ecosystem resilience was also considered in assessing vulnerability. And drought exposure was investigated by considering crop sown area and population density. Based on this, a drought risk assessment framework was constructed using the random forest algorithm and applied to the Huang-Huai-Hai (HHH) Plain, China. The results showed that: (1) approximately 66.34 % of the HHH Plain had a drought hazard level above moderate; among them, southern Hebei, northern Anhui, northern Henan, and scattered small areas in Shandong were extreme high hazard areas. (2) The northeast Anhui, central and southern Shandong, and northeast Jiangsu were extreme high drought vulnerability areas, accounting for 10.34% of the total area. (3) The drought risk in the HHH Plain was low in the south and north, and high in the center. Among these, the drought risk in the border regions of Shandong and Anhui, the northeast part of Jiangsu, as well as central Henan were extreme high, accounting for 11.23%. (4) The drought risk assessment framework constructed by considering ecosystem resilience was more suitable for HHH Plain. This study result could provide scientific reference for managing water resources and preventing drought.

Optimal ranges of social-environmental drivers and their impacts on vegetation dynamics in Kazakhstan

Kazakhstan is part of the Eurasian Steppes, the world's largest contiguous grassland system. Kazakh grassland systems are largely understudied despite being historically important for agropastoral practices. These grasslands are considered vulnerable to anthropogenic activities and climatic variability. Few studies have examined vegetation dynamics in Central Asia owing to the complex impacts of moisture, climatic and anthropogenic forcings. A comprehensive analysis of spatiotemporal changes of vegetation and its driving factors will help elucidate the causes of grassland degradation. Here, we investigated the individual and pairwise interactive influences of various social-environmental system (SES) drivers on greenness dynamics in Kazakhstan. We sought to examine whether there is a relationship between peak season greenness and its drivers – spring drought, preceding winter freeze-thaw cycles, percent snow cover and snow depth – for Kazakhstan during 2000–2016. As hypothesized, snow depth and spring drought accounted for 60 % and 52 % of the variance in the satellite-derived normalized difference vegetation index (NDVI) in Kazakhstan. The freeze-thaw process accounted for 50 % of NDVI variance across the country. In addition, continuous thawing during the winter increased vegetation greenness. We also found that moisture and topographic factors impacted NDVI more significantly than socioeconomic factors. However, the impacts of socioeconomic drivers on vegetation growth were amplified when they interacted with environmental drivers. Terrain slope and soil moisture had the highest q-values or power of determinant, accounting for ~70 % of the variance in NDVI across the country. Socioeconomic drivers, such as crop production (59 %), population density (48 %), and livestock density (26 %), had significant impacts on vegetation dynamics in Kazakhstan. We found that most of the pairwise interactive influences of the drivers exhibited bi-factor enhancement, and the interaction between soil moisture and elevation was the largest (q = 0.92). Our study revealed the optimal ranges and tipping points of SES drivers and quantified the impacts of various driving factors on NDVI. These findings can help us identify the factors causing grassland degradation and provide a scientific basis for ecological protection in semiarid regions.

Response of Maize, Cotton, and Soybean to Increased Crop Density in Heterogeneous Planting Arrangements

The reduction of row spacing and increase of crop population density are important tools for maximizing crop yield. For this strategy to be effective, the crop population should not create intraspecific crop competition that penalizes yield. Thus, planting arrangements that increase light interception throughout the canopy without increasing row spacing might be needed to maintain yield. In this study, heterogeneous planting arrangements on evenly spaced rows were analyzed for maize (Zea mays L.), cotton (Gossypium hirsutum L.), and soybean (Glycine max (L.) Merr.). Each crop had four planting arrangements: (1) normal density in all rows, considered the control, (2) doubled density in all rows, (3) a sequential arrangement of normal and tripled densities (each in every other row; NTNT), and (4) normal-tripled-tripled-normal (NTTN). Maize and cotton did not exhibit changes in growth and architecture when comparing uniform and variable planting arrangements. Soybeans were more adaptable and increased biomass production by 44% to 45% in variable arrangements. None of the crops showed differences in yield due to planting arrangement, so the use of rows with different densities might not be needed when using high densities to maximize yield.

Definition of Optimal Maize Seeding Rates Based on the Potential Yield of Management Zones

The decision on crop population density should be a function of biotic and abiotic field parameters and optimize the site-specific yield potential, which can be a real challenge for farmers. The objective of this study was to investigate the yield of maize hybrids subjected to variable rate seeding (VRS) and in differentiated management zones (MZs). The experiment was conducted between 2013 and 2015 in a commercial field in the Central-West region of Brazil. First, MZ were delineated using the K-means algorithm with layers involving soil electrical conductivity, yield maps from previous years, and elevation. Seven maize hybrids at five seeding rates were evaluated in the context of each MZ and the cause-and-effect relationship with soil attributes was investigated. Optimal yields were obtained for crop population densities between 70,000 plants ha−1 and 80,000 plants ha−1. Hybrids which perform well under higher densities are key in achieving positive results using VRS. The plant population densities that resulted in maximum yields were obtained for densities at least 27% higher than the recommended seeding rates. The yield variance between MZs can be explained by the variance in soil attributes, while the yield variance within MZs can be explained by the variance in plant population densities. The study shows that on-farm experimentation can be key for obtaining information concerning yield potential. The management by VRS in different MZs is a low-cost technique that can reduce input application costs and optimize yield according to the site-specific potential of the field.

Relationship of population migration, crop production pattern, and socioeconomic development: evidence from the early 21st century

Global crop production and population distributions have undergone great changes under climate change and socioeconomic development, and have drawn considerable public attention. How to explain the similarity of the migration patterns of crop yield and population density for different countries/regions is still uncertain and worth studying. Here, we estimated the similarity between migrations of main crop caloric yield (i.e. maize, rice, wheat, and soybean) and population density using Fréchet distance, and investigated the regression relationship between Fréchet distance and related climatic and socioeconomic variables for countries/regions with different economic development stages. The results indicated that different countries/regions showed different Fréchet distances during 2000–2015, with a maximum value of 24.44 for Russia and a minimum value of 0.11 for Georgia. For countries/regions with different economic development stages, the built regression models can explain 39%–93% of the variability in the Fréchet distance. Log(land area), log(GDP), and log(land area under cereal production) were always included in regression models and had higher importance in explaining the variability of Fréchet distance. For the model for all countries/regions, both the log(land area) and log(GDP per capita) may positively link to the Fréchet distance. Possible reasons for these results are that countries/regions with high GDP (or GDP per capita) may ease the conflict of land resources between humans and crops to achieve agricultural industrialization, which causes the far connection of the migrations for crop caloric yield and population density. The complicated interactions of crop production, population dynamic, and socioeconomic development should be given greater attention in the future.

Mung bean cover crop improved soil organic carbon and maize yield in a semiarid area

This study aimed to know the potential use of mung bean (Vigna radiata L. Wilzeck) as a cover crop and its effect on soil organic carbon and maize yield were studied. Six cover crop treatments; 250,000, 375,000 and 500,000 plants/ha combined with desiccation times at 28 and 35 days after sowing (DAS) and one control treatment (without cover crop), were tested in a semi-arid area of North Lombok. Maize at a density of 98,000 plants/ha was planted two weeks following the desiccation time. The treatments were arranged in a Randomized Block Design with three replications. The results revealed that the higher cover crop population density with later desiccation time, produced higher above ground biomass, being the highest at 8.3 Mg/ha and the lowest at 2.6 Mg/ha. The highest cover crop biomass improved soil organic carbon by 60%, nitrogen, phosphorous and potash concentrations in plant tissue by 54%, 54% and 63%, respectively, compared to the one in the control treatment. Maize yield also increased by 24% at the highest cover crop density desiccated at 35 DAS. The use of mung bean as a cover crop sounds promising and further studies are needed to explore more of its potential benefits.

Mechanical properties of soybean plants under various plant densities

Lodging can reduce grain yield and quality, especially in crops planted at high densities. Force analysis, which relates stem mechanical properties and morphological characteristics under high densities, was used to study lodging in soybean (Glycine max (L.) Merr.). We applied mechanical lodging and examined the relationships between stem bending moment of the breaking force and morphological characteristics in two soybean varieties of contrasting heights grown at four crop population densities (200000, 300000, 400000 and 500000 plants ha–1). The experiment was performed in a split-plot design during 2015–16. Measurements were made during the R1–R8 growth stages. Results showed that the full seed stage was the most sensitive period for lodging. Stem strength of the soybean plants increased with growth and development; however, during the R5–R6 stages, the weakened degree of stem strength was greater than the gravity moment. This was the primary reason for the soybeans tending to lodge during this period. In addition, increasing the planting density weakened the mechanical properties significantly. During breeding efforts to improve the soybean lodging resistance, more attention should be paid to strengthening soybean stems. This study also showed that the ratio of stem diameter to plant height could be a new quantitative index for evaluating the lodging resistance of soybeans.

Exploring agronomic strategies to improve oat productivity and control weeds: leaf type, row spacing, and planting density

The trade-off between crop production and weed control is a fundamental scientific issue, as it is frequently influenced by individual crop competitive ability, population density, and planting pattern. A 2 yr field study was conducted to examine the relationship between planting density and row spacing, using two contrasting oat varieties. On average, high planting density (480 plants m−2) reduced weed biomass at oat maturity by 59% in 2012 and by 56% in 2013, when compared with a low density (120 plants m−2). The droopy-leaf variety suppressed weed biomass by up to 69% and weed density up to 72%, compared with the erect-leaf variety. In a drier year, the greatest grain yield was achieved with the droopy-leaf variety under the intermediate density, while in 2013, the erect-leaf variety under the high density had similar yield to the droopy-leaf variety at the intermediate density. A general trend was that increasing plant density suppressed weed infestation, and promoted crop biomass and yield. The droopy-leaf variety exhibited a strong competitive ability under the intermediate planting density, while the erect-leaf variety had a strong competitive ability under the high density. Taken together, there was a complex variety-by-environment interaction to achieve the balance between crop production and weed suppression, which was mediated by growing-season conditions.

Effects of ethephon on anatomical changes in sunflower (Helianthus annuus L.) stems associated with lodging

Stem lodging causes significant losses in crops of cereals and oilseeds. The aim of the present study was to identify the anatomical causes that generate differences in response to stem lodging in sunflower. Two sunflower hybrids (Stay-Green, resistant to stem lodging; Zenit, susceptible to stem lodging) were grown at three crop population densities and artificially lodged at two advanced ontogeny stages (R7 and R8), which were preceded by ethephon application near the flower button stage (R1). Measurements included stem failure moment of force (Bs), thickness of primary and secondary structures in the stem lodging zone (t), diameter of the stem lodging zone (di), sclerenchyma packages area (sp), secondary xylem tissue area (xt) and yield. Stay-Green had significantly higher values for Bs, t, di, sp and xt. At higher crop densities and more advanced ontogeny stages these parameters were reduced, favouring stem lodging, although the effects were ameliorated by ethephon application through anatomical modifications. Zenit exhibited the greatest responses to ethephon application. The present study is the first field study identifying anatomical changes causing stem lodging and intraspecific variability in sunflowers. The information provided can be used by geneticists in selection programs for stem lodging tolerance in the context of increasing crop population densities to improve sunflower yield.

Managing Weeds in Commercial Edamame Production: Current Options and Implications

Edamame, a specialty food-grade soybean popular among health-conscious consumers, is growing in popularity worldwide. Despite a well-developed soybean industry, most edamame consumed in the United States is imported from Asia. Considerable interest exists in growing edamame domestically; however, weed interference is a major problem, and until recently, only a single herbicide was registered for use on the crop. The objectives of this work were (1) to compare effectiveness of weed management treatments that utilize herbicides currently registered for use on edamame or that may be registered in the near future, (2) to determine the significance of edamame cultivar on performance of these treatments, and (3) to identify potential relationships between the crop and weed. Ten different weed management treatments were tested in three edamame cultivars over a 3-yr period. All weed management treatments increased marketable pod yield relative to the nontreated control, but only treatments with saflufenacil orS-metolachlor combinations were comparable to the hand-weeded weed-free treatment. Of the treatments studied,S-metolachlor followed by imazamox was among the greatest yielding, had the least weed density and biomass, and did not reduce crop population density. Also, cultivars differed in their weed-suppressive ability. Path analysis indicated certain relationships were consistent across cultivars, such as weed population density having a direct negative association with crop biomass; however, other edamame–weed interactions were not identical across cultivars. Although more improvements are needed, the vegetable industry is beginning to have nascent weed management options in edamame, which will likely reduce reliance on hand weeding and result in crop-production costs that are more competitive in the global market.

Root lodging tolerance in Helianthus annuus (L.): associations with morphological and mechanical attributes of roots

Background and aims The objectives of this study were to quantify the morphological and mechanical properties of the root-plate within two sunflower hybrids of contrasting susceptibility to root lodging; and to evaluate the effects of crop population density on these properties at two different development stages.

Comparison of Rye and Legume–Rye Cover Crop Mixtures for Vegetable Production in California

Rye (Secale cereale L.) is an important cover crop in high‐value vegetable production in California. A 2‐yr winter study on organic farms in Salinas and Hollister, CA evaluated cover crop population densities, ground cover, aboveground dry matter (DM), and N content of rye and five legume–rye mixtures. Mixtures had 60 or 90% legumes by seed weight and included two or more of the following legumes: faba bean (Vicia faba L.), vetches (V. benghalensis L., V. dasycarpa Ten., V. sativa L.), and pea (Pisum sativum L.). Seeding rates were 90 (rye) and 140 (mixtures) kg ha−1, and densities were 142 to 441 plants m−2 Early‐season ground cover was usually greater in monoculture rye and the 60% legume mixtures than the 90% legume mixtures. Total DM, and legume and rye DM in mixtures differed by year, site, harvest, and cover crop. Total DM was usually at least two times higher at season end than mid‐season. The 90% legume mixtures generally produced more legume DM than the 60% legume mixtures, but legume DM usually declined after mid‐season. Rye DM increased with rye density. Total cover crop N uptake was greater in Hollister than Salinas; however, legume DM and legume N uptake were greater in Salinas. Interactions between site, year, cover crop, and harvest illustrate the complex growth dynamics of legume–rye mixtures. The 90% legume mixtures appear most suitable for vegetable production in California because they had a better balance of legume and rye DM at season end.

Canopy stay-green and yield in non-stressed sunflower

Delayed leaf senescence during the grain filling phase, or stay green (SG), may be functional or cosmetic; the first being considered a valuable trait in breeding of many crop species. To establish whether canopy senescence patterns exhibited by two sunflower ( Helianthus annuus L.) hybrids visually selected for slow post-anthesis canopy leaf area index (LAI) loss reflected functional, rather than cosmetic, SG, LAI dynamics and total biomass increase between anthesis and physiological maturity were followed in three separate experiments in which the putative SG hybrids were compared with standard (i.e., non stay green, NSG) hybrids exhibiting normal rates of canopy senescence under non- or minimal-water stress conditions. In two experiments, pairwise (i.e., one SG vs. one NSG) comparisons were made at two crop population densities. In the third experiment, grown at a single crop population density, four NSG hybrids were contrasted with the two SG hybrids. Canopy senescence dynamics were well described by fitted bilinear functions which discriminated between an initial, slow phase of leaf area loss and a second phase of rapid canopy senescence. No differences between hybrids in the rate of senescence during the first phase were found, but the putative SG hybrids exhibited a significantly slower rate of senescence during the second phase and a significantly higher LAI at physiological maturity (as % of LAI at anthesis). One NSG hybrid showed a greater rate of second-phase senescence than the remaining three hybrids in this category. Across experiments, the anthesis-physiological maturity increment in total oil-corrected biomass and radiation use efficiency (RUE) were significantly greater in the SG hybrids. Cluster analysis based on these four attributes, using data from Exp. 3, clearly discriminated between SG and NSG hybrids. Examination of specific leaf N (SLN) dynamics suggests that the lower RUE values observed in one of the NSG hybrids could be attributable to lower SLN values, but this was not the case for the remaining NSG hybrids. It is also possible that very rapid canopy LAI loss in another NSG hybrid may underlie its lower RUE. The failure of SG hybrids to translate greater post-anthesis biomass increment into grain yield across experiments was associated with their slightly shorter times to anthesis and their significantly lower biomass at anthesis. Grain number, the component of yield most strongly associated with yield, was associated with biomass at anthesis. A further contributing factor was that the duration of grain-filling tended to be shorter in SG hybrids. In the one experiment in which this variable was measured, resistance to stalk breakage was greater in the SG hybrids than the NSG one to which they were compared. We conclude that the SG observed in the hybrids with slower canopy senescence is functional and not cosmetic, and that for this to be translated into a yield advantage all hybrids need to reach anthesis at the same time and grain-filling duration also has to be the same across hybrids. Stay green is an interesting secondary trait to select for and should lead to higher and more stable yields in environments in which stem breakage (lodging) is a problem.

Modelling root and stem lodging in sunflower

Stem and root lodging constitute significant adversities to sunflower ( Helianthus annuus L.) cropping in Argentina. We have adapted previously developed models of the lodging process in cereals to the particularities of sunflower by using functions, developed using data obtained by mechanical lodging, for the [root failure moment/plant anchorage] and [stem failure moment/thickness of stem wall] relationships, and estimates of plant area loaded by wind gusts. The model uses this information to estimate wind failure speed (i.e., the wind speed at which lodging [stem or root, as appropriate] is expected to occur). The model was tested against information (plant and soil characteristics, measured wind gust velocity immediately preceding rainfall) obtained in 26 naturally lodged plots (6 stem-lodged, 20 root-lodged) which occurred across a network of trials (2 seasons, 4 sites, 3 hybrids, 4 crop population densities, all experiments fitted with automatic meteorological stations). Lodging events took place over a range of crop developmental stages between visible capitulum and harvest maturity. Lodging index (proportion of plants lodged) against the difference between observed and model-estimated wind failure speeds showed that the model had good predictive skill across the range of conditions explored in these experiments and was able to distinguish between hybrids of differential susceptibility to lodging within the same experiment. Sensitivity analyses showed that the principal determinants of lodging susceptibility were root plate diameter, stem wall thickness, and the area of the plant loaded by wind gusts. Within the observed ranges (almost twofold) of stem height and stem natural frequency, these two variables had little influence on lodging susceptibility. We conclude that the model, despite the simplifications incorporated into its structure, provides an effective and useful tool for the integration of the complex factors that determine lodging susceptibility in this species.

Stem lodging in sunflower: Variations in stem failure moment of force and structure across crop population densities and post-anthesis developmental stages in two genotypes of contrasting susceptibility to lodging

Stem lodging is a risk to sunflower production in Argentina, and may contribute to fixing the upper limit to commercially viable crop population density, since yield is known to increase up to densities higher than those currently used. Reputedly, crops are particularly susceptible to stem lodging during grain filling and at harvest maturity, but the temporal and spatial unpredictability of lodging events under field conditions has hampered systematic research on this issue. In this study we used mechanical lodging to examine the relationships between stem failure moment of force and stem structure in plants of two sunflower hybrids of contrasting susceptibility to stem lodging grown at each of three crop population densities (5.6 plants m −2, 10 plants m −2 and 16 plants m −2). Measurements were performed at mid- and near-end of grain filling and at harvest maturity in crops grown in three separate seasons at two locations. Stem failure moment of force at all three developmental stages was significantly ( p < 0.05) greater in the lodging-resistant hybrid than in the lodging-susceptible hybrid used in these experiments at 5.6 plants m −2, and fell with increasing crop population density and between 90% grain filling and harvest maturity in both hybrids. At harvest maturity differences in stem failure moment of force between hybrids were significant ( p < 0.05) at all three crop population densities. Stem flexural rigidity (ability of the stem to resist bending) and stem deformation (horizontal displacement from the vertical of the stem immediately prior to breakage) exhibited responses to hybrid, crop population density and developmental stage that were broadly consistent with those of stem failure moment of force. Stem diameter at the breakage point fell with crop population density, but there was little difference between hybrids except at the harvest maturity, when the susceptible hybrid at the two highest crop population densities had smaller diameters than the resistant hybrid. Measured at the breakage point, the thickness of the epidermis plus cortex tissues (i.e., the effective thickness of the stem wall) which surround the friable stem pith fell with crop population density and between end of grain filling and harvest maturity in both hybrids. Differences in effective thickness of the stem wall between hybrids were small and often not significant, but there was a tendency for the susceptible hybrid to exhibit greater thickness of the effective stem wall, particularly at the two higher population densities. Stem failure moment of force was linearly related to thickness of the effective stem wall in both hybrids, the slope of the relationship being significantly greater for the resistant hybrid. We conclude that, within limits, the thickness of the effective stem wall may prove to be useful as a guide to genotype susceptibility to stem lodging in breeding programs and may offer a simple approach to modelling susceptibility. This work has also served to highlight the need to investigate the origins (presumably related to stem anatomy and/or cell wall properties) of the genotypic effects on the stem failure moment of force/the effective stem wall relationship. Finally, the demonstration of the existence of genotype differences in tolerance to stem lodging should encourage the execution of a broader survey aimed at identifying sources of tolerance to stem lodging at high crop population densities in sunflower.

Root lodging in sunflower. Variations in anchorage strength across genotypes, soil types, crop population densities and crop developmental stages

Root lodging is an important adversity affecting sunflower ( Helianthus annuus L.) production in Argentina under current husbandry practices, and may limit progress towards the achievement of higher yields via increased plant population density. Although there are perceptions that lodging susceptibility varies across developmental stages, crop population densities, genotypes and soil types, these perceptions have not been tested for sunflower using a standardized experimental protocol. This study aimed at: (1) identifying the sources of the variation in root lodging susceptibility in response to variations in crop population density in two genotypes of reputedly different susceptibility; (2) detecting the crop developmental stages most susceptible to root lodging; and (3) examining the relationships between root failure moment, root plate diameter and soil shear strength. We mechanically induced lodging at three developmental stages in plants rooted in pre-wetted plots. The crops were grown at 5.6 plants m −2 over 3 years on either Typic Argiudoll or Typic Hapludoll soils and at 3, 5.6, 10 and 16 plants m −2 on a Typic Argiudoll. The force needed to induce root lodging (root failure moment) and root plate diameter varied across genotypes, plant densities and developmental stages. Root failure moment and root plate diameters were greater ( p < 0.05 for both variables) in the resistant hybrid across the three development stages and almost all crop population densities. For both hybrids, the most susceptible development stage was R2, and root failure moment and root plate diameter diminished ( p < 0.05) as crop population density increased. Although root failure moment did not differ between soil types, root plate diameter was greater ( p < 0.0001) in the coarser soil. The relationship between root failure moment and the product of root plate diameter cubed by soil shear strength (a measure of plant anchorage strength) for both hybrids, both soil types, and all crop population densities could be described by a single linear relationship ( y = 0.2382 x; R 2 = 0.812; p < 0.025).

Effects of fertilization and cropyears on maize (Zea mays L.) yields in different crop rotations

Four basic elements of crop management of maize production — crop rotation, fertilizer application, irrigation and population density — were studied on chernozem soil in long-term experiments. The weather in all the three years (2004, 2005 and 2006) studied was basically favourable both as regards water supply and temperatures. In the critical phenophase (tasseling and silking in July), however, due to the precipitation amounts differences could be found. In years with favourable water supplies it was the crop rotation and fertilizer application that basically determined maize yields whereas irrigation and population densities did not have significant effects. According to our long-term experimental results we can state that we have to harmonize the key-agrotechnical elements (crop rotation, fertilization, irrigation, plant density) to realize sustainable maize production.

Far‐red enrichment and photosynthetically active radiation level influence leaf senescence in field‐grown sunflower

Basal leaves frequently senesce before anthesis in high population density crops. This paper evaluates the hypothesis that quantitative and qualitative changes in the light environment associated with a high leaf area index (LAI) trigger leaf senescence in sunflower (Helianthus annuus L.) canopies. Mean leaf duration (LD, time from achievement of maximum leaf area) of leaf 8 was significantly (P &lt; 0.05) reduced from 51 to 19 days as crop population density was increased from 0.47 to 4.76 plants m−2. High compared to low plant population density was associated with earlier reduction in the photosynthetically active radiation (PAR) and red/far‐red ratio (R/FR) reaching the target leaf. However the changes in R/FR preceded those in PAR. When the light environment of individual leaves of isolated plants growing under field conditions was manipulated using filters and FR‐reflecting mirrors, LD was positively and linearly related with the mean daily PAR (MDR) received in the FR‐ (no FR enrichment) treatments. FR enrichment of light reaching the abaxial surface of the leaf significantly (P &lt; 0.05) reduced LD by 9 days at intermediate PAR levels with respect to FR‐controls, but did not affect LD at the maximum PAR used in these experiments. However, when light reaching both leaf surfaces was enriched with FR, LD (for leaves receiving maximum PAR) was 13 days shorter than that of the FR‐ control. These results show that basal leaf senescence in sunflower is enhanced both by a decrease in PAR and by a decrease in R/FR.

Critical period of weed competition in three vegetable crops in relation to management practices

SummaryThe critical period of weed competition was determined in three vegetable crops: early cabbage (Brassica oleracea var. capitata L.), pickling cucumbers (Cucumis sativus L.), and field‐seeded processing tomatoes (Lycopersicon esculentum L.). There were significant interactions between weed‐removal treatments, year, and row width. Cabbage yields were reduced if plots were not kept weed‐free for at least 3 weeks after transplanting or if weeds which emerged with the crop were allowed to remain longer than 4–5 weeks, Cucumber yields were reduced if plots were not kept weed‐free for up to 4 weeks after seeding or if plots remained weed‐infested longer than 3–4 weeks. Higher crop population densities (narrower row widths) in cabbage and cucumbers resulted in smaller plants, earlier competition from weeds, and therefore a shorter period that the crop could remain weed‐infested without suffering reduced yields. Yields of direct‐seeded tomatoes were reduced if plots were not kept weed‐free for up to 9 weeks after seeding or if weeds which emerged with the crop were allowed to remain longer than 5 weeks. In each crop the timing of the critical period of competition was verified by weed removal only during this interval. There was a true critical period in direct‐seeded tomatoes, but not in cabbage or cucumbers where a single weeding was sufficient to prevent yield losses.